Electrophotographic process employing image and control grid means

Abstract

An electrographic apparatus and process for producing, on a record medium having a layer of insulating material in contact with an electrically conductive backing member, an electrostatic charge image corresponding to an image to be recorded comprises an image grid and at least one control grid arranged between a corona discharge device and the layer of insulating material. The image grid comprises an electrically conductive core having insulating and conducting areas defining the image to be produced. The control grid is electrically conductive and arranged in spaced and generally parallel relation to the image grid and between the latter and the corona discharge device. When the core of the image grid and control grid are individually biased to a potential for establishing electrical fields of different strengths between the respective areas and the backing member and a flow of ions is directed toward the grids and the record medium, the flow of ions through the grids is modulated by the electrical fields to produce an electrostatic charge image on the layer of insulating material.

Parent Case Text

This is a continuation of application Ser. No. 185,647 filed Oct. 1, 1971, now abandoned, which is a divisional application of Ser. No. 492,988, filed Sept. 27, 1965, now U.S. Pat. No. 3,680,954.

Description

This invention relates to electrographic recording and in particular to the following preferred embodiments: (1) document copying, (2) document copying in color, (3) duplicating, (4) character printing, and (5) multiple copying of documents.

With reference to the "document copying" embodiment of this invention, there are at present two types of commercial electrophotographic document copying processes. One employs a single-use copy paper having a photoconductive coating and the other employs a reusable drum having a photoconductive coating. Both processes employ the image-wise discharge of a uniformly charged photoconductive insulating layer by image-wise exposure to produce an electrostatic charge image. This charge image is developed to a visible image by depositing thereon a finely divided powder or toner. The resultant powder image may be fixed to the photoconductive layer (as is done in the process which employs a single-use copy paper) or it may be transferred to another surface (as is done in the process which employs a reusable drum).

The electrophotographic process employing the reusable drum has the following disadvantages which are overcome by the present invention. The machine is very expensive for the low-volume user; it does not do a good job of copying photographs and large solid areas; the machine is relatively complex and requires more than the usual amount of maintenance; it uses a selenium coated drum which is expensive and fragile and which must be replaced periodically (normally, after about every 40,000 to 50,000 copies); the process requires a developed-image transfer step; and it has a relatively low sensitometric speed. The electrophotographic process which employs a single-use, photoconductor-coated, copy paper has the following disadvantages which are overcome by the present invention. Copies must be made on paper having a coating of photoconductive insulating material; because of this coating, the paper is relatively expensive; the paper is heavier than ordinary paper; copies can be marred by scratching with metals; the process has a relatively low sensitometric speed; and the transparency, luster, color, dullness, etc. of such paper are to a great extent dependent upon the appearance of the photoconductor.

With reference to the "document copying in color" embodiment of this invention, conventional electrophotographic systems, if used for color printing would involve either (1) the transfer of developed images which transfer has inherently associated therewith problems of registration or (2) repeated development on top of the same sensitive surface, with the problems of strong inter-image effects. In addition, in the latter case there would be difficulty in obtaining good whites due to the dye sensitizer in the photoconductive coating.

With reference to the "duplicating" embodiment of the invention, conventional duplicating systems either require large, expensive machines and long "make-ready" times or have the problems of poor quality, limited numbers of copies, and of being messy.

With reference to the "character printing" embodiment of the invention, conventional printers involve the use of mechanical elements to make an impression on the page. These elements consequently become worn and are limited in writing rate by inertia. Xerographic and photographic character printers have a somewhat greater printing rate than mechanical printers but need a special cathode ray tube and the xerographic ones are relatively insensitive. Photographic recording requires a relatively long processing time, and does not provide an inexpensive, real-size (standard type size), quick-access copy. The size of the type in the photographic copy is usually substandard.

With reference to the "multiple copying of documents" this embodiment incorporates the above discussed advantages of both the "document copying" and the "duplicating" embodiments of the invention. The subject embodiment can be used to make either a single copy or a very large number of copies from a single exposure.

It is an object of the present invention to provide an electrographic recording system.

It is a primary object of the present invention to provide an electrophotographic document copying and duplicating system which is free of all of the above-mentioned disadvantages of the present systems.

It is thus an object of the present invention to provide an electrophotographic copying system which is inexpensive, capable of copying onto a large variety of materials, employs an indefinitely reusable photoconductive insulating member, does not require the deposition of toner onto a photoconductive insulating member, and which can be embodied in a small, light and inexpensive machine.

It is a further object of the invention to provide an electrographic system which, in addition to overcoming all of the above disadvantages of the prior systems, exhibits very high sensitometric speeds and provides extreme flexibility.

It is a still further object of the invention to provide an electrographic color recording system which eliminates the above-mentioned problems of prior color systems such as the requirement for transfer with its associated registration problems and the requirement for repeated development on top of the same sensitive surface with the associated strong interimage effects.

It is a further object of the invention to provide an electrographic color recording system which is fast, which allows dyes to be chosen primarily for stability and color and not for their chemical properties, which does not require charge transfer or toner transfer, which provides for adjustment of color balance on a single print rather than only on a series of prints therefore reducing the rejection rate, which allows the use of white light in inspecting the process, which provides for variable contrast by electronic controls and by design of the photoconductive element, which employs low cost equipment, which provides an inherently correct neutral scale rendition, which is subject to color balancing, in which sufficient, deliberately introduced interimage effects are available to accomplish color masking, and which provides greatly decreased color degradation (improved color rendition) compared to color xerography because the sensitive surface is above the previously deposited toner and is not affected by it thereby eliminating autopositive interimage effects.

It is a still further object of the invention to provide an electrographic duplicating system in which the printing-master can be made in any of a number of different ways, in which the printing-master is simple, inexpensive and rugged, and in which the duplicating process can be carried out using the same apparatus used for document copying by simply replacing the photoconductive element with the printing-master.

It is another object of the invention to provide an electrographic character printing system which overcomes the above-mentioned disadvantages of prior systems.

It is another object of the invention to provide an electrographic alphanumeric character printing system which involves no moving parts in the printing head, which has high speed and durability, which allows the making of quick-access, stable, real-size, black-on-white prints, in which a wide range of type sizes can be obtained, which allows selection of arbitrary type fonts, which uses less expensive copy paper than that which the photographic systems use, which provides real-time writing capability for a computer thus eliminating or at least reducing the need for a buffer or printer allocating stage, which has a wide range of applications, such as ticker-tape, radio or wired-teletype, and punched-card printer, etc., and which eliminates the need for an expensive character display cathode ray tube.

It is another object of the invention to provide a process and apparatus for making multiple prints from a single exposure.

These objects are accomplished by the following invention. I have discovered a unique electrographic recording system which comprises directing a flow of ions toward a record medium and imagewise modulating the flow of ions to produce an image on said record medium. The imagewise modulation is accomplished by interposing a grid or an array of grids in the flow of ions.

The primary difference between the several embodiments of the invention is in the nature of the grid or grids used. In the document copying embodiment of the invention, the grid is a photoconductive grid. The color recording embodiment uses the same photoconductive grid but employs color separation filters in the exposing step and differently colored developers in the developing step from the document copying embodiment. The duplicating embodiment employs a grid similar in appearance to the photoconductive grid but different in construction in that it employs an imagewise distributed coating of insulating material on a conductive grid. One character printing embodiment employs conductive grids formed in the shape of the characters to be printed. The multiple copying of documents embodiment employs both photoconductive grids and insulator coated grids.

In the preferred embodiments of the invention a record medium is used which has an insulating surface coating on a relatively conducting support layer. The flow of ions, as imagewise modulated by the grid or grid array, produces an electrostatic charge image on the insulating surface, which charge image can be xerographically developed to produce a visible image.

In the document copying embodiment of the invention, for example, the imagewise modulation of the flow of ions is accomplished by means of a photoconductive grid comprising a biased or grounded electrically conductive core or grid which is, in the preferred embodiments, covered with a layer of photoconductive insulating material.

It has been found extremely important for the operation of this invention that the photoconductor completely cover all of the exposed surface of the conductive grid with a uniform coating. Even microscopic cracks or holes in the photoconductive coating on the grid are detrimental to the operation of the process.

This photoconductive grid is positioned directly in the ion flow and preferably just above the record medium. The photoconductive grid is imagewise exposed to produce a conductivity image in the photoconductive material, while the flow of ions is being directed through the grid and toward the record medium, and, hence, can be designated as an image grid means. The terms "electrically energize" and "imagewise energize" are intended to encompass, for the purpose of this specification and claims, the biased or grounded, electrically conductive areas of the grid. In the areas of the grid which are insulating or non-energized (where the grid is not exposed) the flow of ions will first produce a small surface potential (from a few to a few hundred volts) on the grid surface and will then pass through the grid to charge the underlying insulating surface of the record medium. In the conducting, and thus imagewise energized, areas of the grid (where it is exposed) the ions are captured by the grid and are thus removed from the ion flow, whereby the areas of the record medium underlying the exposed or energized areas of the grid remain uncharged. An electrostatic charge image corresponding to the light image is thus formed on the insulating surface of the record medium. This electrostatic image is then xerographically developed and the developed image fixed to the record medium or alternatively transferred to a final receiving sheet, in which case the record medium can be cleaned and reused.

One of the most startling effects noted with this invention is in connection with the exceptional sensitivity of the process. In xerography, because the process is electrostatic, the maximum operating gain of the system is unity. The operating gain of the system is defined as the number of stored charges removed by one absorbed photon. In the present system, which is electrodynamic, operating gains greater than unity can be achieved. This process has exhibited speeds of up to 300 times the speed of present electrophotographic systems. An additionally startling effect in this connection is that of an increase in effective speed with increased resolution, i.e., a 200-line per inch photoconductive grid gives many times the speed of a 100-line per inch grid. A 300-line per inch grid gives speeds over 100 times faster than previous electrophotographic systems and previously considered unavailable. This simultaneous increase of speed and resolution is opposite to the relationship found in other systems, for example, in photography.

A great degree of flexibility is available in designing an electrophotographic document copying machine to operate on the principles of the present invention, as will be evidenced by the following discussion. Many kinds of photoconductors (both n and p types) having various levels of sensitivity and dark current, can be used. Useful photoconductors are, among others, cadmium sulfide, selenium, selenium and tellurium mixtures, zinc oxide, arsenic trisulfide, cadmium telluride, cadmium selenide, germanium (PN or NP) and organic photoconductors such as triphenylamine in an insulating organic resin vehicle (such as that sold under the trademark Vitel PE-101) sensitized with 2,4-bis(4-ethoxyphenyl)-6-(4-n-amyloxy-styryl) pyrylium fluoroborate. Further it is possible to vary the effective sensitivity of a given photoconductor within the system. For example, electrical bias can be used to employ photoconductors that do not exhibit low resistance upon exposure; increasing the corona current changes the time constant depending on the geometry of the source and the nature of the photoconductor; and increased exposure tends to decrease time constants. It is understoood of course that other materials which exhibit a change in conductivity upon activation can be used in the present invention in place of a photoconductor. Such other materials include photoinsulators, i.e., materials which are normally conductive but which become insulating upon exposure to light, and heat-sensitive materials which exhibit a change in conductivity when heated. Hence, the image grid means can be considered as being coated with a radiation responsive insulating material. A layer of photoconductive insulating material may be formed on the grid, for example, by evaporation techniques or by spray coating. It is necessary, however, to spray or evaporate from a widely diverse number of angles with respect to the grid so that all of the surface of the grid will be completely covered with a uniform layer of photoconductor, including the inside walls of the holes in the grid. Woven mesh is hardly suitable for this process, if coated by evaporation, particularly in the finer weaves, because of the difficulty of completely covering the wire surface in the region where the wires cross each other. Spray coating can be used to totally coat woven mesh for this process, provided the coating has reasonable leveling or wetting action on the mesh. A wide choice is also available in connection with the electrically conductive grid which forms the core of the photoconductive grid and to which the photoconductive material is applied. The choice as to the shape, size, material and method of manufacture is large. It has been found that an etched or electroformed mesh is superior in mechanical properties for one-to-one docment copying; window screening works well for moderate size posters and is stronger and less expensive; and hardware cloth can be used for very large prints. Neither the toner nor any part of the document copying apparatus ever needs to come into contact with the grid; the grid is thus indefinitely reusable. It is also relatively simple to construct and relatively inexpensive. An extremely wide choice of record mediais available, including ordinary paper at low humidities. The record medium can be, for example, a single layer of insulating material, a sheet of paper or other support having a thin insulating coating, a sheet of thermal-deformable plastic or an ion-sensitive silver halide emulsion layer, but is preferably an insulating surface with a relatively large capacitance per unit area. The record medium, after formation of the electrostatic image thereon, can be xerographically developed, for example, by any of the well-known methods and the developed image can be fixed thereon to form the final copy, or the toner can be transferred to a final copy sheet and fixed thereto, in which case the record medium can be cleaned and reused. In this latter case, the record medium can conveniently be in the shape of a rotatable drum; in connection with this embodiment it should be noted that the drum does not have an expensive, fragile, photosensitive coating but rather a simple, rugged, inexpensive, electrically insulating coating. Although the preferred embodiments of the invention utilize a record medium having an insulating surface whereby an electrostatic image is produced thereon which can be xerographically developed, it is noted that the invention is not limited to such record media. For example, a "Berchtold layer" (a mosaic of conducting areas separated by insulating layers as described in U.S. Pat. No. 2,866,903) can be positioned behind the photoconductive grid with an insulating recording sheet positioned behind the Berchtold layer. Further, the record medium can be a conducting sheet which changes color in response to a flow of current therethrough, as is known in photoconductography. If the record medium is replaced with an electroluminescent panel, the electrographic system functions as a light amplifier. Since the electroluminescent panel glows where there is current, the image has a tonal scale reversed from that of the normal (i.e., a negative). When the gain is less than unity, the reversed tonal scale would be useful either in direct viewing of a photographic negative as a positive, or in converting a negative to a positive. In other words, the imagewise pattern of ions coming from the grid can be used to form a record (either permanent or temporary) in various ways.

Further, a wide choice of exposing methods is available, including projection and contact exposing methods with either area exposure or line scanning. The associated benefits of line scanning are usable, i.e., right-reading, wrong-reading and co- and counter-current scanning. Various scanning embodiments useful in the invention include (1) stationary corona charger, grid and lens with moving document and record medium for both co- and counter-current scanning, and (2) stationary document and record medium with (a) moving corona charger, grid and lens system, (b) moving corona charger with stationary grid and lens system, (c) moving corona charger and lens system with stationary grid, and (d) moving corona charger and grid with stationary lens system. The conductivity image produced on the photoconductive grid may thus be spatial (in the case of large area exposure) or temporal (in the case of line scanning) or both in the case of small area scanning.

Many types of ion sources, including corona discharge electrodes such as needles and wires, are well-known in the art and any of such may be used in the present invention.

Certain arrangements of the document copying embodiment of the invention require the use of a photoconductive material which will remain conductive (exhibit persistance of conductivity) for a period of time after the illumination has been turned off and in the presence of a corona discharge. The persistence of conductivity of a photoconductive material is often altered in the presence of a corona discharge. For example, in zinc oxide photoconductive layers, the photoconductivity which would normally persist for many minutes is destroyed in slightly over a second in the presence of a corona discharge even at very low corona levels. Contrarywise, in certain cadmium sulfide photoconductive layers, normally the photoconductivity will decay in less than 0.1 second, but under a corona will exhibit persistence of conductivity for several minutes.

The above description is applicable to the color recording embodiment of the invention since it also employs a photoconductive grid. Many of the unique advantages of this color recording embodiment result directly from the fact that toner is not deposited on the photosensitive layer as it is in the prior systems. Thus there is no need to transfer toner and no associated registration problems. Further, there are no detrimental interimage effects caused by charging, exposing, and developing a photosensitive layer containing previously deposited toner. The dyes to be used can be chosen for their color and stability. Many other advantages of this embodiment are disclosed above in the objects of the invention.

The duplicating embodiment employs a grid comprising a grounded or electrically biased conductive core or grid having an imagewise distribution of insulating surface areas. The insulating and conductive areas of the grid modulate the flow of ions in the same manner as described above with respect to the photoconductive grid. The primary difference between the two embodiments is that the image is effectively permanent (permanent for the duration of the copy run) on the printing-master grid. As stated above this embodiment has many advantages over the known duplicating systems.

The character printing embodiment employs a grid comprising a grounded or biased conductive electrode formed in the shape of a character to be printed. This grid modulates the flow of ions in somewhat the same manner as do the grids of the above embodiments. The grid is energized to either attract or repel the ions in the flow of ions to produce an electrostatic charge image or "shadow" of the grid. A stack of individual electrodes, made of thin wire in the shape of characters, or a grid comprising a plurality of individual electrode segments corresponding to parts of characters, can be moved across the record medium to reproduce a page of type. The corona current is transmitted to the paper only when a character is to be printed. The d.c. corona current can be turned on and off by auxilliary means, the d.c. supply, or extra grids. Each electrode or electrode segment is connected to ground or to a bias potential through a switch. The switch (or switches in the case of the grid employing electrode segments) corresponding to the character to be printed is closed and the remaining electrodes have practically no blocking effect and do not cast a "shadow." Alternative to the above described scanning motion, a complete row of grids with a full set of electrodes for each space in the line can be used to provide printing of a line as a whole. The switches can be photocells to allow the switching to be done by light. In one embodiment, described below, the photocells are arranged in an X-Y array. The light pattern can be simultaneous, as in the case of exposure through a punched card, or sequential, as in the case of the output of a cathode ray tube.

The multiple copying of documents embodiment employs a photoconductive grid or grid array to produce an electrostatic charge image on an insulating grid (or on itself or another photoconductive insulating grid in the dark) and then this grid having the electrostatic charge image is used to modulate an ion flow to imagewise charge an insulating record medium. Many hundreds of copies can be produced from a single exposure. This embodiment employs a different principle of operation from that of the previously described embodiments. In this embodiment ions are not imagewise removed from the ion flow by means of conductive grid areas. The ion flow is modulated by electrostatic fields. The ions are prevented from flowing through the areas of the grid which have the charge image, but flow freely through the remaining areas of the grid.

In the present specification and claims the term "ion flow" or "flow of ions" is employed in describing the step of imagewise charging the record member. Although it is true that the preferred source of charges is a corona discharge electrode and the preferred charges are air ions, it is to be recognized that electrons, other charged subatomic particles, charged particles of matter, etc., can be used as the flow of charges in the present invention which flow is directed toward the record member and imagewise modulated to produce an electrostatic charge image thereon. This charge image can be made visible by any known xerographic developing methods. Various types of charges can be used in the present invention and it is intended that the term flow of ions and ion flow be interpreted to include any of such charges and that it not be limited in meaning strictly to air ions. The grid or conductive mesh in this invention is analogous to the grid in a vacuum tube in which relationship the term grid is generally defined as an electrode having one or more openings for the passage of ions therethrough, which electrode exercises control on the passage of ions without collecting more ions than is necessary. Thus, the use of the term grid for the electrode of the invention which controls the flow of ions to the record medium is consistent with present usage of the term. The term grid, as used in the present specification and claims is intended to encompass any and all electrode configurations which allow for the passage of ions therethrough; the term grid thus encompasses such constructions as are also known by the terms screen, mesh, perforated plate, slot etc. Since the resolution of the ultimate image depends on the number of openings per linear inch in the grid and since this is commonly called lines per inch in halftone production, the same phrase lines per inch will be used in the present specification and claims to define the size of the grid. This term together with information about the percent of open area of the grid adequately defines the size of the grid. It is noted that the resolution of the grid system is determined by the number of holes per unit length only in the case of stationary operation without interaction between the holes. An example of this is the single grid system operated stationary, close to the record sheet. If a scanning system is used, the system in the direction of motion has a resolution equal to the reciprocal of the diameter of the holes, or slot width, if there is no interaction between the holes and the time frequency response of the system is not limiting. The relationships in the scan perpendicular direction are much more complex. There is a marked difference between the stationary and the scanning relationships for cases where only a small portion of the area of the grid is open, or single apertures are used. In these cases the slow scanning resolution is higher than the stationary exposure. The slot is a limiting case in that the resolution in the scanning direction is obtained only by virtue of the scanning operation. With respect to the embodiments of the invention which use photoconductors, the phrase "imagewise exposing the grid" means, of course, imagewise exposing the photoconductor to suitable radiation to which the photoconductor is sensitive. As used in the specification and claims, exposing includes exposing to visible light, x-rays, alpha, beta, and gamma rays, and particulate radiation. Any type of radiation may be employed that will render the photoconductor conductive. In the specification and claims, the term "insulating" as used with respect to certain types of record media is intended to encompass any material which will hold an electrostatic charge image for a period of time long enough to allow for the development thereof. The period of time needed to develop an electrostatic image may be extremely short, as in the case of thermal-deformable or electro-deformable plastic sheet. The term potential or connected to a predetermined potential is intended to include any potential including ground potential. The imagewise modulation of the flow of ions can be either spatial or temporal or both; line scanning would be both, point scanning would be only temporal, and overall exposure would be only spatial.

These and other embodiments of the present invention will be more fully understood by reference to the following detailed description of the invention when read in connection with the accompanying drawings, in which:

FIG. 1 is a schematic illustration of one document copying embodiment of the present invention;

FIGS. 2-6 are enlarged, perspective views of various photoconductive grids useful in the invention;

FIG. 7 is an enlarged cross-sectional view through another photoconductive grid useful in the invention;

FIG. 8 is a greatly enlarged cross-sectional view through a photoconductive grid and a record medium which illustrates certain principles of the invention;

FIGS. 9A and 9B each schematically illustrate a multigrid embodiment of the invention;

FIG. 10 is a schematic diagram of an equivalent circuit of the circuit shown in FIG. 1;

FIG. 11 is a schematic diagram of an equivalent circuit of the circuit shown in FIG. 9A;

FIG. 12A is a schematic illustration of a two-grid system;

FIG. 12B is a graph showing the characteristic output current of the circuit of FIG. 12A;

FIGS. 13-15 each schematically illustrate another multigrid embodiment of the invention;

FIGS. 16-20 each schematically illustrate a document copying embodiment of the invention;

FIG. 21 shows, greatly enlarged, an alternative exposure station for use in the embodiment of FIG. 19;

FIGS. 22A-22B schematically illustrate a simple reflex printing embodiment of the invention;

FIG. 23 is a schematic diagram of a document copying apparatus according to one embodiment of the invention;

FIG. 24 illustrates a lens system useful in the embodiment of FIG. 23; FIG. 25 is a schematic illustration of an embodiment in which the grid is controlled by a photoconductor spaced from the grid;

FIGS. 26-28 schematically illustrate an embodiment of the invention which employs a photoinsulating material;

FIG. 29 schematically illustrates an embodiment employing a layer of cellular material on top of the photo conductive grid for the production of reversals;

FIG. 30 is a schematic illustration of a color reproduction embodiment of the invention;

FIG. 31 is a schematic illustration of a duplicating embodiment of the invention;

FIG. 32 shows a duplicating embodiment of the invention employing multiple grids.

FIG. 33 is a schematic illustration of a character printing embodiment of the invention;

FIG. 34 is a plan view showing a line of character printing grids for use in printing a line at a time;

FIG. 35 is a schematic illustration of a grid composed of electrode segments for use in printing alphanumeric information;

FIG. 36 is a schematic illustration of a alphanumeric character printing embodiment of the invention;

FIG. 37 is a schematic illustration of a modification of the character printing embodiment of FIG. 33;

FIG. 38A schematically illustrates certain principles of operation of the embodiment of FIG. 33 when the grid is biased to attract ions; FIG. 38B is a graph showing the charge density across an electrostatic image produced by the embodiment of FIG. 38A;

FIG. 38C is a graph showing the nature of the toner deposit on the electrostatic image of FIG. 38A;

FIG. 39A schematically illustrates certain principles of operation of the embodiment of FIG. 28 when the grid is biased to repel ions;

FIG. 39B is a graph showing the charge density across an electrostatic image produced by the embodiment of FIG. 39A;

FIG. 39C is a graph showing the nature of the toner deposit on the electrostatic image of FIG. 39A;

FIGS. 40A and 40B schematically illustrate a two-step process using three grids to produce multiple copies from a single exposure;

FIGS. 41A and 41B schematically illustrate a variation of the embodiment shown in FIGS. 40A and 40B employing an integral array of grids using a foraminous insulating spaces;

FIG. 42 schematically illustrates another integral grid construction useful in the process of FIGS. 41A and 41B;

FIGS. 43, 44, and 45 schematically illustrate more complex grid arrays containing larger numbers of grids for use in the multiple copying of documents embodiment of the invention;

FIGS. 46A, 46B, and 46C show a preferred three-step process for making multiple copies from a single exposure;

FIG. 47 schematically illustrates the exposure step of another grid structure useful for reflex printing in the general process of FIGS. 46A, 46B, and 46C; and

FIG. 48 is a schematic illustration of an apparatus employing, for example, the grid of FIG. 47 for making single or multiple copies of documents by reflex exposure.

I DOCUMENT COPYING

FIG. 1 illustrates one embodiment of the present invention. In FIG. 1 a transparency 10, having an image to be reproduced, is illuminated by a light source 12. The image is focused by a lens 14 onto a photoconductive grid 16. The grid 16 consists of a grounded, electrically conductive electrode core or grid 18, for example of metal, completely covered with a layer 20 of photoconductive insulating material. Positioned immediately below the grid 16, is a record medium 22 consisting of an electrically insulating layer 24 on a support 26, such as paper. It should be noted that the insulating layer 24 is not, or at least need not be, a photoconductive insulating material. The record medium 22 can therefore be quite inexpensive. The support 26 is positioned in overlying contact with a grounded field electrode 28 during the step of imagewise exposure of the grid 16, a corona discharge is produced adjacent the grid 16 but on the opposite side thereof from the record medium 22. The corona discharge is produced, for example, by connecting a corona discharge electrode 32 to one terminal of a voltage source 34 by means of a switch 36. The other terminal of the voltage source 34 is connected to ground 38. The corona discharge provides a source of ions, and the electric field produced between the corona discharge electrode 32 and the field electrode 28, directs a flow of ions toward the record medium 22. In the imagewise exposed areas of the grid 16, the photoconductive layer 20 is conducting, and the ions which come into proximity thereto are attracted to the photoconductive layer 20 and pass directly to ground. In the remaining (dark) areas of the grid 16 the photoconductive layer 20 is insulating and the ions, after building up a small potential (from a few to a few hundred volts) on the surface of the grid, pass through the openings in the grid 16 to charge the underlying surface of the insulating layer 24. This modulation or control of the flow of ions from the corona discharge electrode 32 to the record medium 22 by the grid 16 is described in more detail in connection with FIG. 8 below.

FIGS. 2-6 are perspective views showing alternative grid constructions which are useful in the present invention. FIG. 2 is a perspective view of the grid 16 of FIG. 1 showing the photoconductive insulating layer 20 and the core or grid 18. The grid 18 may be formed by etching or electroforming. FIG. 3 shows a grid 40 consisting of a photoconductive insulating layer 42 on an electrically conductive electrode grid which consists of a series of equi-spaced, parallel electrodes 44 connected to a common electrical line 43. FIG. 4 shows a grid 50 formed from a perforated metal plate which forms the electrode core or grid 52. The grid 52 is completely covered with a layer 54 of photoconductive insulating material. FIG. 5 shows a photoconductive insulating layer 56 on an electrically conductive electrode core or grid 57. The core or grid 57 can be formed, for example, by electroforming in which a durable high quality stainless steel plate is covered with a photoresist, exposed to the desired pattern (to harden the exposed areas of the photoresist - the photoresist in the background being subsequently washed away), etched to leave posts and then electroplated with, for example, nickel, copper, gold or silver. The plating is then peeled off of the steel plate resulting in an excellent electrically conductive electrode foil which forms the core or grid 57. FIG. 6 illustrates a grid 60 in which an electrically conductive grid 61 is provided with a single, narrow slot or opening 62 having tapered faces 64. The surfaces of the grid 61 adjacent the opening 62 are covered with a layer 66 of photoconductive insulating material. The opening 62 is of the order of 0.1mm. wide. The grid 60 finds use in connection with scanning processes, as is more fully discussed below. FIGS. 2-6 show examples of the various shapes and constructions which the photoconductive grid of the present invention can take. The resolution of the ultimate image depends on the number of openings or holes in the grid per linear inch, hereinafter referred to as lines per inch. The openings or holes in the grids of FIGS. 2-5 are on the order of about 50 to 500 lines per inch. The process of the present invention has been found to operate very well with 150, 200 and 300 lines per inch grids. It is noted that in scanning, the number of holes per inch does not determine the resolution but the size of the holes or slot, electrically and optically, does. The term lines per inch is only justified for non-scanning operations. In the scanning case resolution is more nearly the reciprocal of the hole diameter in lines per unit length.

FIG. 7 is an enlarged cross-sectional view through a photoconductive grid 70 which is identical to the grid 16 of FIGS. 1 and 2 except for the nature of the layer 72 of insulating material which covers the electrode core or grid 76. The grids of FIGS. 1-6 are well shown as being completely covered with a photoconductive insulating material. In FIG. 7 only a part of the insulating layer 72 is photoconductive. The insulating layer 72 of the grid 70 of FIG. 7 consists of a photoconductive insulating layer 74 on one half of the grid 70 and a non-photoconductive, preferably opaque, insulating layer 78 covering the other half of the grid 70. Alternatively, an opaque insulating coating can be coated over the photoconductor on one side of the grid. The two layers 74 and 78 meet to provide the complete insulating layer 72. In general, the photoconductive insulating layer 74 faces both the corona discharge and the light source during exposure and imagewise charging; however certain embodiments of the invention, to be discussed below, employ different arrangements.

FIG. 8 illustrates how the flow of ions is modulated by the imagewise exposed image grid means or photoconductive grid. The ions are either attracted to the grid in the exposed areas thereof (and thus removed from the flow of ions) or repelled from the grid due to the surface charge thereon in the unexposed areas (and thus pass through the openings in the grid). For the purpose of this description, a grid 80 (similar, for example, to the grid 16 of FIGS. 1 and 2) is shown having a grounded electrode core or grid 82 completely covered with a layer 84 of photoconductive insulating material. The grid 80 is imagewise exposed as indicated by the small arrows 96. Ions, illustrated by the long, open arrows 86, are directed from a corona discharge (not shown) to the field electrode 88 positioned behind the insulating record sheet 90, and through the grid 80. In striking the photoconductive layer 84 in the unexposed areas 92 thereof, the ions produce a small surface charge thereon. This charge will build up to somewhere between a few volts and a few hundred volts which will prevent further charging thereof and which will force the ions which would otherwise hit the unexposed areas of the grid 80 to flow around the grid structure and through the openings in the grid 80. These ions, along with other ions which are flowing through the openings, continue through the grid and impinge upon the insulating record sheet 90 to deposit charges thereon in the underlying areas 94. However, in the imagewise exposed areas 93 of the grid 80, the photoconductor becomes electrically conductive and any ions striking it pass through the photoconductive layer 84 to the electrode 82 and to ground and are thus removed from the flow of ions. Furthermore, the conducting areas 93 of the photoconductive layer 84 have a trapping action extending a certain distance from the photoconductive layer 84. As soon as the photoconductivity reaches a certain value, the distance at which the trapping action is effective extends to and beyond the middle of the openings or holes between the individual elements or wires of the grid 80 and thus any ions 86 directed toward such areas of the grid 80 are essentially completely trapped; that is, the ions are drawn over to the photoconductive layer 84 and pass to ground so that no ions pass through the exposed areas of the grid 80 to charge the insulating record sheet 90. There is, however, some leakage (pass-through of ions) in the exposed areas or light is wasted. This trapping action is substantially the same whether the insulating layer surrounding the electrode grid 82 is all photoconductive or partly photoconductive and partly nonphotoconductive as shown in FIG. 7. The degree of trapping depends not only on the amount of exposure and the degree of photoconductivity of the photoconductor, but also on the potential of the electrode grid 82 relative to that of the field electrode 88.

If the electrode grid 82 has a potential somewhere between that of the corona source and that of the field electrode 88, the trapping action in the illuminated areas is somewhat reduced. If the grid 82 has a potential opposite in sign to that of the corona electrode, the trapping action in the illuminated areas is somewhat increased thus tending to clean-up the background and effectively increase sensitivity. It should be noted that it is the complete coating of all the conductive surface of the grid within the picture area that allows the use of the preferred potential opposite in sign to that of the corona electrode. Even microscopic holes or cracks in the coating of photoconductor on the grid will make the system wholly inoperative with the preferred bias.

It is noted that since the developing station is preferably remote from the charging station, developer powder or toner never needs to touch the photoconductive grid; the photoconductive grid can be roused indefinitely.

FIG. 9A illustrates an embodiment of the invention which employs a control grid means which can take the form of an additional electrode grid 100 between a photoconductive grid 102, which can be, for example, any of those shown in FIGS. 2-7, and an insulating record sheet 104 carried on a field electrode 106. The conductive grid 100 forms an electrostatic shield and does not absorb a majority of the ion flow. By this means the voltage across the photoconductive grid 102 can be decoupled from the voltage across the record sheet 104. The advantage of such decoupling will be more fully understood by reference to FIG. 10 which is an approximate equivalent circuit of the system shown in FIG. 1. A current source 110 supplies current to a current divider circuit consisting of two branches. One branch represents the photoconductive grid 16 of FIG. 1 and is shown in FIG. 10 by a capacitor 112 in parallel with a variable resistor 116 (representing photoconductance). The other branch represents the air resistance between the grid 16 and the record medium 22 of FIG. 1 and is shown in FIG. 10 by a resistor 118 in series with a capacitor 114 (the series capacitance of the paper). Since no current flows from the grid 16 to the record medium 22, or vice versa, when the current source (corona) is shut off, a diode is included in each branch. The transient behavior of this circuit can be analyzed in detail. However, the most significant characteristics involve the terminal voltages on the capacitors. When the system is at equilibrium, there is no current flowing through the capacitor 114, so that it is charged to the potential across the photoconductor represented by variable resistor 116. All the current is flowing through the photoconductor 20 so the final potential across it and the record medium 22 is equal to the current through it times the photoconductor resistance. The record medium 22 potential is then limited in the simple grid system of FIG. 1 to the current times the change in photoconductor resistance. It should be noted that this equivalent circuit implies that the potential deposited on the record medium is limited to the maximum potential that the photoconductor can stand. This establishes a minimum thickness and resistivity in the dark of a given photoconductor in order to provide a developable image for any given development process. It should be noted that at maximum deposited charge on the record medium the total flow of current is through the photoconductor while it has the maximum potential across it, resulting in maximum power dissipation in the photoconductor.

An approximate incremental equivalent circuit for the circuit of FIG. 9A is shown in FIG. 11. FIG. 11 shows a current source 120 (the corona source of FIG. 9A) which supplies current to two branches of a circuit. One branch represents the photoconductive grid 102 of FIG. 9A and is shown by a resistor 126 in parallel with a capacitor 122. The other branch represents the air resistance between the two grids and is shown by a resistor 119. Since there is no capacitor in series with the resistor 119, at equilibrium current will flow through the resistor 119. A dependent current source 121 supplies current to the paper capacitance 123. The current source 121 supplies a current equal to or slightly less than that which flows through the electrical resistance of the air. This means that in the steady state the current to the paper is independent of the charge on it and that the paper can be charged to an arbitrary level extending the charging time. In practice the photoconductive grid 102 delivers its output into the additional grid 100, which grid 100 appears to the grid 102 to be a grounded metal plate, though it is not actually grounded. The major part of the current which is delivered to the grid 100, however, is transmitted through it to the record sheet 104 provided there is a potential of about 300 volts or more (for convenient dimensions) between the record sheet 104 and the additional grid 100 to accelerate the flow of ions. Since the flow of ions is independent of voltages above about 300 volts (for convenient dimensions) it is possible to put a large potential between the additional grid 100 and the record sheet 104.

This potential decreases the transit time of the ions between the additional grid 100 and the record sheet 104 to the point whose diffusion of the image due to kinetic motion is negligible. The main source of diffusion is between the photoconductive grid 102 and the additional grid 100. However, this is between two permanent parts of the apparatus which are stationary relative to each other and the amount of diffusion can be minimized by proper manufacture. Since the transit time is inversely proportioned to the potential, the record sheet 104 can be positioned more distant from the grid 100 by just increasing the potential. In embodiments which do not employ the additional grid 100, ions diffuse at angles of about 45.degree. so that the record sheet 104 should be in virtual contact with the photoconductive grid 102.

FIGS. 12A and 12B give an idea of the limits of the applicability of the incremental circuit model of the double grid. A metal grid system was set up as shown schematically in FIG. 12A. The current to a metal receiving sheet 132 was measured as a function of the voltages on the two metal grids 130 and 131. In the area where the grid 131 -to- receiving sheet 132 voltage (V.sub.2) is above about 300 volts, the delivered current depends only on the intergrid voltage V.sub.1, and can vary between zero and about 4 microamps for the system shown. In this operating area the charging of the paper is dependent on the charging time and the intergrid surface voltage V.sub.1 only, making it possible to charge the paper to hundreds of volts (limited only by breakdown or discharge through the insulator coating on the paper) with an intergrid potential of a few volts.

FIG. 12B shows the characteristic output current of the circuit shown in FIG. 9A as a function of the additional grid 100 to record sheet 104 voltage. The parameter of variation is the potential across the photoconductive grid 102. These curves were actually obtained from the all-metal circuit shown in FIG. 12A (using a 10 KV corona), in order to be able to measure the surface potential on each grid. Note that about 30 volts is all that is required to control a microampere delivered to 1,000 volts. This is approximately the charging current used to charge in 0.1 second over the 3 square inches of the electrode used. The voltage gain is approximately a factor of 10, and is equal to the power gain of the device. In the embodiment shown in FIG. 1, in order to produce an acceptable image, the photoconductive grid has to "stand off" or hold without discharging a surface potential of about 300 volts. However, using the embodiment shown in FIG. 9A, having a second, all-metal grid 100, only a tenth of the voltage is needed at about the same current as before. One-tenth as thick a layer of photoconductor can be used on the photoconductive grid 102. The thickness of the photoconductor layer on the photoconductive grid 102 is the primary limiting factor of the device; therefore, the resolution can be theoretically increased by about a factor of 10.

If a thinner layer of photoconductive material on the photoconductive grid 102 is not desired, it is also possible to use, instead, a photoconductor with a higher dark current than was previously possible. This is particularly useful in extending the response of the system into the infrared, where most of the photoconductors are characterized by high dark currents.

By attaching the two grids 100 and 102 to each other through an occasional intermediary insulating spacer, there are some advantages for large stationary exposures. The definition is no longer strongly dependent on the spacing between the metal grid 100 and the record sheet 104 so that some bowing of the assembly due to gravity and/or electrostatic forces is allowable. The field between the field electrode 106 and the metal grid 100 exerts a force attracting the grid assembly to the record sheet 104. Due to the incremental current source characteristic of the device, this field can be increased to a high enough level to overcome the attractive force of the corona source 108 on the photoconductive grid 102. Thus, the arrangement shown in FIG. 9A, when using such spacers, is suitable for large spans without rigid support. The grid 100 is preferably metallic and will wear well even if it touches the dielectric surface of the record sheet 104. However, at these points, some contact-charged spots will occur in the image and some contour to the surface of the dielectric layer may be needed to minimize the areas of these spots.

The divorcing of the charging rate from the potential on the record sheet 104 causes an increase in the average charging rate, i.e., the charging proceeds at a uniform relatively high rate instead of tapering off. Thus, the additional grid 100 effectively increases the current gain of the system. Any difficulty in employing papers and developers that would work well at low potentials can be eliminated by using the embodiment shown in FIG. 9A. It is possible to develop an insulating record sheet on a temporary conducting backing, such as an insulating paper on a metal sheet, and then remove the record sheet from the backing when the image is developed and fixed.

When using the embodiment of FIG. 9A with a scanning exposure step, a certain amount of difficulty is encountered. Normally, the system appears in either the one or the two grid versions (FIG. 1 and FIG. 9A, respectively), to have a rapid decrease in response between 10 and 100 cycles/second when the current is delivered to a metal plate. In the one grid system (FIG. 1) the interaction between the charge on the record sheet and the rate of current delivery acts like a negative feedback loop and extends the frequency response at the expense of gain, producing acceptable scanned images. There is no evidence of such an occurrence in the two-grid system (FIG. 9A). The primary use of the embodiment shown in FIG. 9A is for relatively high resolution and high sensitivity stationary exposure of images. A typical use would be in making x-ray photographs or electrographs. In such cases some additional sensitivity may be secured by coating the metal grid 100 with an x-ray fluorescent electrically conducting coating. If the dimensions of the grids 100 and 102 are appropriate, they may be held together by a simple, tacky adhesive layer, preferably with an equally perforated interleaving material therebetween to hold the two grids a certain distance apart (usually about twice the distance between the openings in the grid). Moire patterns can then become a problem, but such problems are solvable by the methods used in color half-tone patterns. In the visible spectrum, the two-grid device FIG. 9A is particularly adapted to stationary exposures for moderate-to-high resolution such as is required in a microfilm reader-printer or a hand-held camera.

The two-grid system of FIG. 9A can use any photoconductor that the single grid system of FIG. 1 can use. It is also possible to accomplish some compensation for the electrical properties of some photoconductors that is impossible to do in the single-grid system of FIG. 1, specifically, there are some photoconductors that have sufficient resistance even when well exposed to develop a sufficient surface charge to allow some current to reach the paper, but when given an attractive bias, will attract the current and produce a clean image. Additional bias can compensate for resistance in the exposed areas of the photoconductors, rendering a usable iamge when at lower light levels, thus increasing the effective sensitivity of the system. Alternatively, photoconductors with very high impedance can be used if sufficient bias is used.

FIG. 9B shows an alternative arrangement of a two-grid system having the same electrical characteristics as the arrangement shown in FIG. 9A, but with improved image resolution. A foraminous insulating spacer 107 is coated on its two surfaces with metal electrodes 101 and 103, which thus form metal grids corresponding to grid 100 and the metal core of photoconductor coated grid 102 of FIG. 9A. It is necessary in applying metal electrodes 101 and 103 to ensure that the metal does not coat the inner walls of the foraminous insulating spacer 107. The spacer 107 may be made by drilling a regular array of small holes, typically 0.003 inches in diameter, on 0.005 inch centers, in a sheet of insulating plastic 0.006 inches thick. The thickness of the spacer should optimally be about twice the diameter of the holes. The number of holes per linear inch determines the resolution of the finished print, and the individual holes should have a diameter as large as is mechanically consistent with the center-to-center hole spacing. Metal electrode 103 is then coated completely with photoconductor 105 either by evaporation or spraying, taking care that the holes are not filled, but that the edges of electrode 103 are thoroughly covered. The use of the spacer 107 prevents any migration of charge from one hole to another in the low field region between electrodes 101 and 103 and thus improves the resolution of the finished print. There is no significant sideways migration of charge in the space between electrode 101 and receiving sheet 104 because of the high field in this region.

FIG. 13 shows a 3-grid system which provides a means for correcting frequency response at the expense of current gain and resolution. FIG. 13 shows a corona discharge electrode 140, an image grid means or a photoconductive grid 142, an insulating record sheet 144 on a grounded, conductive field electrode 146 and a metal grid 148 analogous to the metal grid 100 in FIG. 9A. The difference between the embodiment of FIG. 13 and that shown in FIG. 9A is the use of another metal grid 150, placed in front of the 2-grid system of FIG. 9A, the photoconductive grid 142 being closer to the rear grid 148 than to the front grid 150 and the grids 148 and 150 comprising the control grid means. The front grid 150 is provided with a slight repelling bias and acts to limit the current through the system. With a fixed bias on grid 148, the bias on grid 150 is set at such a value that at an input frequency of 100 cps a small change in bias in one direction will not affect the current while a small change in the other direction will affect the current. At lower frequencies, this is also the maximum current that can flow, while at higher frequencies a smaller current will flow for the same input amplitude.

In addition to the improvement in frequency response obtained with the arrangement shown in FIG. 13, this arrangement also has the advantage of limiting the excessive buildup of charge on the photoconductive grid 142. Referring to this advantage, the grid 150 can be referred to as a "limiter grid." For its use as a limiter grid it can be much coarser (be of larger mesh) and have a spacing from the photoconductive grid 142 which is large compared with the spacing of the latter from the record sheet 144 or from the "screen grid" when the limiter grid is used in conjunction with a double-grid system. The limiter grid therefore does not interfere appreciably with the optical image falling on the photoconductive grid 142, and does not require a high degree of mechanical precision in its construction. The purposes of the limiter grid are: (1) to limit the potential on the surface of the unilluminated photoconductive grid 142 and therefore to prevent damage to the photoconductor which might result from exceeding the voltage tolerance of the photoconductor, and (2) to prevent bulging or arching of the center of the photoconductive grid by shielding it from the strong electrostatic field which is generated by the high-voltage corona wire.

The limiter grid is held at a potential, relative to the conductive core of the photoconductive grid 142, which is of the same polarity as the potential on the corona wire and which is of a magnitude roughly equal to the voltage which the photoconductor can withstand. As the surface of the photoconductor builds up potential to approach that of the limiter grid, the electrostatic field between the photoconductor surface and the limiter grid is such as to prevent further charging of the photoconductor. The potential across the photoconductor is therefore held to a safe value. The limiter grid can be used whether the photoconductive grid is followed directly by the record sheet or by other grids.

In some of the other embodiments of the subject invention the high voltage impressed between the photoconductive grid and the corona wire produces a strong electrostatic attraction which tends to bulge or arch the center of the photoconductive grid relative to its supports. This changes the spacing between the photoconductive grid and the record sheet or the following screen grid, which in turn alters the electrical characteristics and tends to degrade the definition in the electrostatic image, differentially across the field.

In this embodiment the limiter grid also acts as a shield between the control grid and the corona wire. The electrostatic attractive force is transferred from the control grid to the limiter grid, and the coarse limiter grid can be allowed to arch under this attraction, without appreciably affecting the control characteristics of the semiconductor grid or the definition in the electrostatic image on the record sheet.

In the above description of the 3-grid embodiment of FIG. 13, the system is primarily intended to operate with the photoconductive grid 142 potential approximately that of the rear grid 148 potential. When the system is operated in this manner, there is a significant current through the photoconductive grid 142, approximately equal to or up to twice the current being delivered at a maximum to the record sheet 144. The system can also be operated with the potential on the photoconductive grid 142 more nearly at the potential of the front grid 150. In this case, increasing the potential on the photoconductive grid 142 decreases the current arriving at the record sheet 144 giving a condition which is contrary to the mode of operation described above with respect to FIG. 13. In this mode of operation the current through the photoconductive grid 142 was much less than that of the plate current (the plate current is the current to the record sheet 144; this current has very nearly the same functional relationship to the grid voltages that the plate current does in a vacuum tube). Thus, there is an equivalent current gain between the photoconductor and the controlled ion flow. The current gain is high enough so that there is insufficient time for the photoconductor on the photoconductive grid 142 to become fully charged during a combined exposing and charging period. Therefore, a charging period preceding the exposure is desirable in this embodiment. It is also desirable to expose without having the ion flow transmitted to either the photoconductive grid 142 or the record sheet 144. Several alternative cycling schemes are available and will be discussed below. An assumption is made that the photoconductive grid 142 is closer to the front grid 150 than to the rear grid 148, this being the reverse of that described above with reference to FIG. 13.

Initially, the charging step comprises making the front grid 150 about 20-40 volts negative, the photoconductive grid 142 several hundred volts negative, e.g., 300 volts, the rear grid 148 about 10-20 volts negative, while the corona discharge is "turned on" in the dark. The charging step is continued until the photoconductive coating on the photoconductive grid 142 is sufficiently charged, and the surface potential approaches asymptotically the surface potential on the front grid 150. At the termination of the charging operation, approximately 230-260 volts potential exists across the photoconductive grid 142, depending on the voltage of the front grid 150. The successive negative potentials of the uncoated grids 150 and 148 prevent charges from reaching the record sheet 144. During exposure, the potential of the photoconductive grid 142 can be raised to ground potential or somewhat higher. This will prevent any ion flow from reaching the surface of the photoconductive grid 142 and cancelling part of the exposure. Alternatively, the corona can be turned off at the high voltage supply. The exposure period is limited only by the time over which the photoconductive grid 142 can reliably hold a charge in the dark. Any predictable dark decay of the potential can be balanced in the next step by adjusting the potential difference between the front grid 150 and the photoconductive grid 142.

During the step of charging the record sheet 144, the rear grid 148 is raised to a potential of about 600 volts, and the front grid 150 to about 200-400 volts. The photoconductive grid 142 is chosen so that the dark areas of the photoconductor have a surface potential slightly (5-10 volts) in excess of the potential of the front grid 150. The discharged areas need only to be discharged 10-20 volts to permit current to flow through to the record sheet 144. There is a negligible current flow to the discharged areas of the photoconductive grid 142 surface, at least during the time needed to charge the record sheet 144, so the charge image remains effectively stable. In this mode of operation a charge image is laid down on the record sheet 144 in the areas where the photoconductive grid 142 was light-exposed; that is, the reverse of the mode of operation described above.

The following cycle can be used to lay down a charge on the record sheet 144 in the areas corresponding to the unexposed areas of the photoconductive grid 142. Essentially, the photoconductor is charged originally with charges of the opposite polarity; then, in the charging step of the cycle, the photoconductive grid 142 potential is adjusted to make the charged areas at a slightly lower potential than that of the front grid 150.

FIG. 14 shows another embodiment of the invention which works on a different principle of operation. In this embodiment, two perforated layers, 160 and 162, one layer being photoconductive and the other layer being semiconductive, are placed face-to-face with the holes in register and with the exterior faces but not the holes being coated with conductive layers 166 and 168, respectively, either opaque or transparent as the case demands. The transmission of ions from a corona source 164 through the holes of the assembly is a maximum when the conductivity of the two layers 160 and 162 is identical. Thus, by choosing the appropriate light level and/or resistance of the semiconductive layer, the system is either an autopositive or a negative-to-positive electrographic system. Since the semiconductive layer functions as a series area load resistor and limits current through the photoconductive layer, the only limit on the photoconductor is that it be able to stand the rather moderate (30 to 100 volts) voltages across it and that it be able to dissipate the internal heating without damage. In a case where heating is a problem, an insulating layer may be substituted for the photoconductive layer provided the surfaces of the holes are coated with a thin layer of photoconductive material and electrical contact with the semiconducting layer is made. A photoconductor having a different photoresponse, or shielded from illumination may be substituted for the semiconducting layer. If two photoconductors are used of approximately the same sensitivity but to different parts of the spectrum, the device will transmit corona only in areas where two exposures have a set ratio to each other, or one exposure can be used as a mask for the other. In all normal operating conditions there is much more current flowing through the holes than is incident on the surface of the photoconductor or semiconductor. Essentially the field across the photoconductor, not the current through it, controls the ion stream. In this approximation it is possible for a small current in the photoconductor to control a large ion current. The photoconductor is not exposed to direct corona effects and does not suffer as severe alteration of characteristics with corona as is common in the single grid system.

The operating conditions of the system shown in FIG. 14 should give very high gain if the layer 160 is photoconductive and is the top layer in the sandwich, and the layer 162 is of intermediate conductivity and has a conductivity equal to or somewhat less than the dark conductivity of the photoconductive layer 160. In this case, there is a maximum flow of ions through the assembly when the photoconductive layer 160 is not illuminated. In the dark state, some of the ion current is absorbed on the conducting upper layer 166 of the assembly but virtually none is absorbed in the rest of the structure of the assembly. When the photoconductor 160 is illuminated and becomes conducting, virtually all of the corona current is absorbed on the front layer 166. In either state there is no reason for the current through the photoconductor to be equal to the corona current, and it can be many times less. The photoconductor when used in an integrated assembly allows the use of photocurrents greater than one electron per photon. The exposure should precede the charging of the record sheet to some extent in order to prevent depositing charge in the areas of exposure before the potentials in the integrated grid system have been stabilized. Whether or not this system can exhibit higher current gain than the aforementioned systems depends on whether or not there can be practical photoconductors with as low a dark current as is required in the systems and still have a photoconductive gain in the high impedance systems.

FIG. 15 shows the use of two grids to perform as a repulsion-type photosystem. FIG. 15 shows a field electrode 180 supporting a record medium 182 of the type, for example, having an insulating surface. A photoconductive grid 184 comprising the image grid means is supported above the record medium 182 and a metal electrode 186 comprising control grid means is positioned above the photoconductive grid 184. A corona discharge electrode 188 is provided as in the previous examples. Each of the grids 184 and 186 and the corona discharge electrode 188 can be connected to any desired potential (not shown) by means of switches 190, 192, and 194, respectively. For simplicity, a two-part operating cycle will be described. Initially, the metal grid 186 voltage is held at about 10 volts negative relative to ground (with the corona defined positive) and the photoconductive grid 184 is held at several hundred volts (e.g., 300 volts) negative relative to ground. The corona is turned on and the photoconductor surface is charged to very nearly the potential of the metal grid 186 (the limiting voltage to which the surface of the photoconductor grid 184 can be charged is determined very nearly by the potential of the metal grid 186), so that there is a potential of approximately the difference in potential between the metal grid 186 and the photoconductive grid 184, across the photoconductor (e.g., 290 volts). In order to imagewise charge the insulating record sheet 182, the metal grid 186 is made positive to about 300 volts and the photoconductive grid 184 is made positive to a potential of slightly less than the surface potential of the photoconductor, for example, in this case about 580 volts. The photoconductor is then imagewise exposed sufficiently to reduce the surface potential to about 10 volts below that of the metal grid 186 in the illuminated areas (e.g., a surface potential of 570 volts). A change in surface potential of about 10-20 volts is sufficient. The corona current can flow to the record sheet 182 in the areas that were exposed and the electrostatic charge image can be subsequently xerographically developed. In this version, charge is deposited in the exposed areas and thus the exposure may take place either before or during the step of charging the record sheet 182.

Alternatively, an opposite polarity of corona can be used in the charging and exposing steps. In this case the metal grid 186 potential would have to be about 500 volts or more and the photoconductive grid 184 potential would have to be adjusted so that the surface potential in the unexposed areas is about 490 volts, i.e., 10 volts fewer than the voltage of the metal grid 186 during the exposing step. For example, a negative charge would exist on the photoconductor during the charging step. In this case, the charge would be deposited on the record sheet 182 in the unexposed areas. If the exposure period is shorter than the charging time for the record sheet 182, then there is no need to turn off the corona. For a time exposure, the exposure should be made with the corona off. Both of the two grid systems described above have relatively simple cycles and constructions, while maintaining the feature of current gain, which in these cases is not due to photoconductor current gain but to a gain from the gas discharge electrode system itself. The presence of a conducting front grid in any of the grid systems described above increases the life and usefulness of the photoconductive grid by decreasing the current that flows through the photoconductor. A front grid can be added to any of the herein described photosystems to limit the voltage across the photoconductor and thus to prolong the photoconductor life. This two-grid system with the photoconductor on the grid nearest the record sheet will be discussed at greater length in the section of making multiple copies of documents (see FIGS. 46A, 46B, and 46C).

FIG. 16 shows another embodiment of the present invention. A photoconductive grid 190, which can be any of the grids shown, for example, in FIGS. 2-5 or 7, is positioned directly over an insulating record sheet 192 which is in overlying contact with a field electrode 194. The conductive core 196 of the grid 190 and the field electrode 194 are grounded. In this embodiment the grid 190 is imagewise exposed while a corona discharge produced by a shielded corona discharge wire 200 connected to a source of high voltage (not shown) is moved by means of a motor 201, for example, across the top of the grid 190 as indicated by the double arrow 202. This is a simple method of providing a uniform ion source across the entire surface of the photoconductive grid 190. It is noted, however, that there is no uniform charging of any surface; an electrostatic charge image is produced directly on the surface of the record sheet 192 during the charging step.

FIG. 17 illustrates schematically a counter-current scanning system in which a transparent document 210 to be reproduced is moved, by drive rollers 211, for example, continuously past a light source 212 so that a narrow area of the document 210 is focused by a lens system 214 onto a narrow photoconductive grid 216. The grid 216 may be any one of the grids shown in FIGS. 2-7 for example, and consists of an electrode core or grid 215 covered with a photoconductive insulating layer 217. The electrode core or grid 215 is held at a predetermined potential, such as at ground or at a bias potential. In this embodiment the document 210 is shown as a transparency which is back-lighted; however, the document may of course be opaque and front-lighted. The document 210 is continuously moved by drive rollers 211, through the copy plane of the lens system 214. A single reflection is used in the lens system 214 to provide an erect image as is well known in the art. In this embodiment, the photoconductive layer 217 on the grid 216 should have practically no persistence of conductivity and the scanning operation should be performed slowly enough or in steps so that any persistence is ineffective. A record sheet 218 consisting of an insulating layer 220 on a backing 222, such as paper, is moved continuously immediately behind and parallel to the grid 216 synchronously with the movement of the document 210. The record sheet 218 is accurately positioned with respect to the grid 216 by a grounded field electrode 224. The spacing between the grid 216 and the record sheet 218 is not too critical, provided the grid 216 is within one hole diameter of the record sheet 218, since the resolution depends primarily on the number of apertures per linear inch or lines per inch in the grid 216 and only very slightly on the spreading of the charges between the grid 216 and the insulating layer 220. The charges tend to spread to fill the openings through which they pass in the grid 216 even when the insulating layer 220 is very close to the grid 216. A corona discharge electrode 226 is provided which extends across the length of the grid 216 and is maintained at a high potential by a voltage source 228. The electrode core or grid 215 is maintained at or slightly above ground potential by adjusting the contact point 230 on the resistance element 232.

FIG. 18 shows an embodiment of the present invention in which a document 250 to be copied is held stationary and parallel to a stationary record sheet 252, immediately over which is positioned a photoconductive grid 251 which can be any one of the grids shown in FIGS. 2-5 or 7, for example. The record sheet 252 consists of an insulating layer 254 carried on a support 256 and is supported on a grounded field electrode 258. In this embodiment the source of illumination, the scanning optics, and the source of ions are moved, by means of a motor 265, for example, across or optionally back and forth across and immediately over the grid 251 and between the document 250 and the record sheet 252, as indicated by the double-headed arrow 260. The light source, the scanning optics and the ion source in this embodiment are all incorporated into a movable carriage 262. A light source 264 which consists of two tubular lamps extending across the width of the document 250 to be reproduced is positioned on top of the carriage 262. Light from the document 250 is reflected by a mirror 266 into a narrow section of a reflecting lens system 268 which focuses the light and directs it back to a mirror 270 and into focus at a line on the grid 251. As the carriage 262 moves, a right-reading record of the document 250 is produced on the grid 251 and therefore a right-reading electrostatic charge image is produced on the insulating layer 254 after the charging step. The carriage 262 carries with it, in the path of the light beam from the mirror 270 to the grid 252, a corona discharge electrode 272 connected to a high voltage source 267. The corona discharge electrode 272 produces ions which are directed toward the grid 251 by the field between the electrode 272 and the grounded field electrode 258. The ions are trapped at the grid in the exposed areas but pass through the grid 251 in the unexposed areas thereof as described above with reference to FIG. 8. To minimize astigmatism or distortion with respect to the images formed, it is preferable to employ a moderately well-corrected 2- or 3-element lens system so that the full width of the document being copied is within a relatively broad angular zone of good resolution. In this embodiment, the carriage 262 extends approximately the width of the document 250 and the grid 251.

FIG. 19 shows an embodiment of the invention which employs a small-area photoconductive grid 350 adapted for start-stop motion across the insulating surface of a record sheet 352 carried on a grounded field electrode 354. A movable carriage 356 carries both the grid 350 and a corona discharge electrode 358 which is connected through a switch 360 to one terminal of a voltage source 362. The other terminal of the voltage source 362 and the core 353 of the grid 350 are grounded. In this embodiment the switch 360 is open while the carriage 356 is moving and is then closed when the carriage is stopped. The image on an opaque document 364 is focused on the grid 350 (or on the plane through which the grid moves) by means of a light source 366 and an appropriate lens system 368 as is well known in the art. A motor 370 provides the start-stop motion of the carriage 356. The switch 360 can be an automatic switch which cooperates with the motor 370 to open and close the switch 360 synchronously with the movement of the carriage 356. This embodiment is particularly useful with certain currently available photoconductors because the periodically-pulsed corona discharge avoids or at least greatly reduces image storage effects of the photoconductor. The corona electrode 358 should be placed close to the grid 350 in order to provide a feathered edge to the individual pulse charge pattern. The term "feathered edge" is used to describe the gradual transition from full corona current density in the center of the charging zone to zone corona current density outside the corona charging zone; a bell-shaped curve ##SPC1##

would appear to be optimum. The feathered edge provides some tolerance in the cycling of the pulses.

FIG. 20 shows an embodiment of the present invention, adapted for continuous operation, which can employ either contact exposure, projection exposure or reflex exposure. The embodiment is shown in FIG. 20 with contact exposure. It can be arranged for reflex exposure as discussed below and as shown in FIG. 21. A positive transparency 271 passes over a transparent roller 273 and is illuminated by a light source 274. Light passing through the transparency 271 strikes a photoconductive grid 276 which in this embodiment is in the shape of a portion of a drum 280. The photoconductive grid 276 may have any of the constructions shown in FIGS. 2-5 or 7 and consists of a grounded electrode core or grid covered with a photoconductive insulating material. The entire periphery of the drum 280 need not be made up of the grid 276, but may include a more durable section 278 connecting the ends of the grid 276. The drum 280 can also be in the shape of a flexible endless belt. The drum 280 rotates through a charging station 282 positioned after the exposing station 275. The photoconductive coating on the grid 276 must in this embodiment have persistence of conductivity. It is noted that in this embodiment the exposed areas of the grid 276, when it arrives at the charging station 282, face the insulating record sheet 284, which sheet is positioned adjacent the periphery of the drum 280 by means of a grounded field electrode 286 in the form of a roller. A corona discharge electrode 288 is located within the drum 280 and is held at a high potential with respect to the field electrode 286 to direct a flow of ions to the record sheet 284 through the unexposed areas of the grid 276. A motor 285 provides for the rotation of the drum 280.

FIG. 21 shows an embodiment similar to that of FIG. 20 except that a photoconductive grid 290 (which constitutes a portion of the drum 280 of the embodiment shown in FIG. 19) is used which is covered with an insulating layer 294 (which is opaque and preferably black) and with a photoconductive insulating layer 292. In the embodiment the photoconductor must exhibit persistence of conductivity. The document 271 is reflex illuminated by a light source 296. The light, indicated by arrows 298, passes through the grid 290 and is reflected from the document 271 back to the photoconductive side of the grid 290. The light is fairly well sealed from the photoconductor as it passes from the light source 296 to the document 271, but strikes the photoconductor 292 directly after reflection from the document 271. As is well known, most reflex printing systems in which the light passes through a photosensitive layer rely on the differences in exposure between the areas where light passes only once through the layer and where it passes twice through the layer (the second passage being where it is reflected back through the layer). In the present system, the light does not necessarily pass through the photosensitive layer (the photoconductive layer), but passes through the openings in the grid 290. As indicated by the arrow 300, the grid 290 moves synchronously with the document 271 and after exposure moves to a charging station as in the embodiment shown in FIG. 20.

FIGS. 22a and 22b illustrate a very simple embodiment of the invention employing reflex exposure. In this embodiment a photoconductive grid 310, which may have any of the constructions shown in FIGS. 2-5, comprises a metal core 311, which is held either at ground potential or at a bias potential (not shown) and which is coated on the lower side with a photoconductive insulating material 312 and on the upper side with a nonphotoconductive insulating material 314 (which is opaque and preferably black). The grid 310 forms the bottom wall of a housing 316 in which an exposing lamp 318 and an ion source 320 are mounted for back and forth movement (by any conventional track and roller means) across the top of the grid 310. In FIG. 22a a document 322 to be reproduced is placed immediately below the grid 310 and the light source 318 is moved as indicated by the double-headed arrow 324 across the grid 310 and then back again to illuminate the document 322 and by the light reflected therefrom to reflex expose the photoconductive layer 312 of the grid 310. In this embodiment the photoconductive material must be one which exhibits persistence of conductivity. Immediately following this exposure step, the document 322 is removed and a record sheet 326, see FIG. 22b, having an insulating layer 328 on a support 330, and placed on a grounded field electrode 322, is positioned immediately below the imagewise exposed grid 310. An ion source 320, consisting of, for example, a corona discharge electrode connected to a high voltage source (not shown) is then energized and moved across the grid 310 as indicated by the double-headed arrow 334. Thus, according to the invention, the record sheet 326 receives an electrostatic charge image, the charge being in the areas corresponding to the dark areas of the document 322. The record sheet 326 with its electrostatic charge image is then developed by any of the known xerographic developing methods. The embodiment shown in FIGS. 22a and 22b is particularly useful with liquid toners since the insulating layer 328 may be chosen to be compatible with any desired toner. FIG. 22c shows an embodiment in which the record sheet is a thin, transparent, insulating plastic sheet 340 having a transparent conducting backing 342. The sheet 340 is placed on top of the document 322 and simultaneous exposure and imagewise charging can be affected in a single pass by the unit 344 which contains both a light source 346 and a corona discharge electrode 348. A photoconductive grid 341 is used having a conductive core 343 which is held either at ground or at a bias potential (not shown) and which is coated on the top with an insulating layer 347. In this embodiment a photoconductor can be used which does not exhibit persistence of conductivity. A transparent, positive copy is produced directly upon development.

FIG. 23 is a cross-sectional view through a document copying apparatus 400 incorporating an embodiment of the present invention which employs counter-current scanning. The apparatus 400 comprises a cabinet 402 having an entrance slot 404 for the documents to be copied. Feed rollers 412 feed a document 414 through an exposing station 418 to the drive rollers 420 which deliver the document 414 out of the machine at the exit slot 406. A copy-paper supply roll 430 is positioned in the upper part of the housing 402. A pair of drive rollers 434 feed the copy paper 436 past a charging station 438 to a pair of drive rollers 440 which feed the copy paper to a liquid developing station illustrated schematically at 428. Any known liquid developing method can be used. The copy paper 436 can be turned over so that the electrostatic image faces down at the station 428, if desired. The final copy passes out of the housing 402 through an exit slot 410 by means of a pair of drive rollers 444. This embodiment of the invention operates well at paper transport speeds of 3 inches per second. A light source 424 is positioned to front-light opaque originals. The image on the original is focused, by means of a lens system 450, onto a photoconductive grid 452. The grid 452 may consist, for example, of any of the embodiments shown in FIGS. 2-7. The charging station 438 additionally includes a corona discharge electrode 454 which may be, for example, a needle or wire as is well known in the art. The corona discharge electrode 454 is connected to a high voltage supply 456. Immediately below the grid 452 is a grounded field electrode 458. The copy paper 436 having an electrostatic charge image thereon may be developed by any of the well-known methods; however, liquid development is the preferred method for use with this embodiment of the invention. A fixing station and a paper cutter can be employed as is known in the art. If desired, the supply roll 430 can be replaced with a supply of individual sheets of copy paper and a cut-sheet feeder mechanism.

FIG. 24 shows the lens system 450 of the embodiment shown in FIG. 23. The lens system 450 is shown for use in a counter-current scanning system, i.e., the document 414 to be copied moves in one direction as indicated by the arrow 460 and the record sheet 436 moves in the opposite direction as indicated by the arrow 462 with the lens system 450 positioned between the document 414 and the sheet 436. The lens system 450 comprises a set of truncated prisms 464 (commonly called "doves") to provide a single reflection and therefore an erect image as is well known in the art. Each of the prisms 464 is in optical alignment with a single double convex lens 466.

EXAMPLE

An arrangement was used as shown in FIG. 1. A brass grid having 125 lines/inch was coated with cadmium sulfide powder in a binder sold under the trademark "Pliolite S-7" (a 70-30 percent styrene-butadiene copolymer), the mixture having about 2 percent (by weight) of binder. 0.01 foot-candle-second exposure yielded a density of 1.0. The coating of the grid was accomplished by spraying the photoconductor-binder mix with a finely atomizing spray gun (artist's air brush), carefully spraying from a number of very different angles so that the entire surface of the grid, including the inside walls of the holes, was uniformly coated with photoconductor. Periodic microscopic examination during the spraying was used to ensure complete coverage of the entire conducting surface. A 125 line/inch grid was about 10 times as sensitive as a 60 line/inch grid, using as nearly as possible identical coatings. Brass grids having the size of 200 lines/inch have also been used with similar coatings. The latter grids use wires which are 1 mil square and which have a separation between centers of 5 mils. The coating has a thickness of about 1/2 mil.

A similar example in which a zinc oxide-in-resin binder coating was used in place of the cadmium sulfide powder demonstrated a resolving power limited only the grid spacing.

If the distance between the photoconductive grid and the record sheet is 0.1 mm. there is negligible blurring of the outline of openings or perforations. Grids having 75 percent of the area open and with 135 lines per inch, provide a clean differentiation between off and on, (i.e., from no current to no impedance). The width of the openings in the grid should be limited both to keep adequate resolution and to permit complete shut-off of the ion flow. For photoconductors that are slow to become dark adapted again, and where the metal core of the grid is at ground potential and common to all areas of the photoconductor, a heating current can be run through the grid. Such an erase heating current would allow reuse of a zinc oxide photoconductive material in about one minute. A strong corona current erases the zinc oxide conductivity in two seconds in the simple grid system.

Effective sensitivity and contrast of the image can be modified by controlling the timing of the ion flow through the photoconductive grid during and after exposure. If the ion flow is applied immediately after exposure, the conductivity of the photoconductor is at its maximum and an image is produced that corresponds to maximum sensitivity. If an appreciable portion or multiple of the decay period of the photoconductor occurs before the ion flow is applied, then the correspondingly decreased effect of the light is reflected in an effectively lowered sensitivity of the photoconductive grid. By applying the ion flow at a predetermined time pattern, the sum of the currents can be obtained on the record sheet which will represent an altered contrast image due to non-linearities in the photoconductor and the corona current response.

It is noted that an additional advantage of the present invention is that the corona discharge electrode used as the ion source in the above description can be connected to either an a.c. or a d.c. voltage source. When using an a.c. voltage source the flow of ions is rectified in passing through the photoconductive grid of the invention, so that only charges of one polarity will reach the record medium.

FIG. 25 shows another embodiment of the invention in which a flow of ions through a conductive grid interposed between an ion source and a record medium is controlled by a photoconductor. This embodiment differs from the above embodiments in that in this embodiment the photoconductor is separate from and is spaced from the conductive grid rather than being coated on the grid. This embodiment employs a series of parallel, spaced metal electrodes 668 each one of which is connected to a respective one of a series of photocells 666. A bus bar 664 connects all of the photocells 666 to ground potential. The electrodes 668, the photocells 666 and the bus bar 664 as shown in FIG. 25 are all supported by an insulating plate 652 provided with an opening 654 therein to allow for the passage of ions from a corona discharge electrode 656 to a record medium 658. The record medium 658 is preferably in contact with the plate 652 or else the opening 654 should be narrow and deep, preferably the depth should be twice the width (this preferred ratio applies to all embodiments). The corona discharge electrode 656 is connected through a switch 657 to a voltage source 660. A grounded field electrode 662 positioned behind the record medium 658 provides the electric field necessary to direct the flow of ions toward the record medium 658. A document 670 to be copied is illuminated by a light source 672 and the image on the document 670 is projected in focus on the photocells 666 by a lens system 674. A single reflection (not shown) is provided in the lens system 674 as is well know, for the co-current scanning shown. In practice, the illumination on a given photocell 666 determines the amount of current (ion flow) which will flow through the opening 654 on each side of the corresponding electrode 668, which in turn determines the amount of charge the underlying area of the record medium 658 will receive. If an extended image, such as that on the document 670, is desired, the optical image on the photocells and the record medium 658 can be synchronously moved to scan the image. As is usual in line scanning, the motion of the record medium 658 can be co- or countercurrent with respect to the optical image to produce for any given optical system, either a right-or-wrong-reading image. The proportions (ratios of length to width to thickness) of the gap-type photocell (the length is fixed owing to resolution requirements) are almost arbitrary, which provides for considerable accommodation to various photoconductors. The major restriction in the choice of photoconductors is that the time constant should be short enough not to affect the resolution in the scanning direction. It is possible with a careful choice of photoconductor to use a single continuous stripe of photoconductor and rely on geometrical factors to provide effective isolation between adjacent electrodes. When this is done, there is no registration step involved during fabrication.

FIGS. 26-28 illustrate another embodiment of the invention. FIG. 26 shows a grid 902 positioned in an exposure plane by means of a grid support 904 which is, for example, electrically conductive and in electrical contact with the conductive core 906 of the grid 902. The grid 902 consists of a conductive core 906 completely covered with a layer 908 of photosensitive material. The photosensitive material in this embodiment is normally conductive but becomes nonconductive upon exposure to light. An example of such a material is polystyrene sensitized with an aryl azide.

The following is an example of a method for preparing such a material:

(a) Polystyrene pellets 15 gms Toluene 100 ccs Diethylbenzene 5 ccs

To this add 12 ccs of the following solution.

(b) 2, 6 di(4 azido benzal) 2 gms 4 methyl cyclohexanone Toluene 100 ccs

According to the process of this embodiment the grid 902 is imagewise reflex exposed to a document 910 having image areas 912, by uniformly illuminating the grid 902 as shown by the arrows 914. By means of this exposure step the layer 908 becomes nonconducting in the exposed areas. Thus the entire top surface of the grid 902 is rendered totally nonconductive. Light passing through the grid 902 is incident on the document 910 to be copied and is reflected from the document 910 to reflex imagewise expose the grid 902.

The document 910 and the uniform illumination are then removed, and the grid 902 is used (FIG. 27) to produce an electrostatic charge image on an insulating record sheet 916 supported on a conductive backing 922. A corona discharge electrode 918 is connected to a source 920 of corona generating potential. Charge is deposited on sheet 916 opposite only the nonconductive areas of the grid 902, i.e., these areas which were exposed (the background areas). After completion of this imagewise charging step, the sheet 916 is removed and developed. To produce a positive print, the charge image is reversal developed, i.e. developed with a toner having the same polarity as that of the charge image. Since the conductivity image in the layer 908 has persistance, many prints can be made from the original conductivity image without re-exposure. The bias on the core 906 can be varied as desired by means of the electric circuit 924.

FIG. 28 shows an alternative exposure arrangement to that of FIG. 26. In this arrangement the same grid 902 is imagewise projection exposed to a negative transparency by means of a projector 926. The areas of the grid 902 corresponding to the image areas are made insulating, while the areas corresponding to the background areas of the transparency are left conducting. Both sides of the grid 902 must be exposed or the ion flow will not pass through the grid. Several methods may be used to expose the rear surface. A white diffuse sheet 929 may be placed under the grid 902 to uniformly reflect light back to the lower surface of the grid 902. Alternatively, the photosensitive layer 908 or the grid 902 may be made turbid in order to scatter light around to the back. Any of several pigments are suitable, such as zinc oxide or titanium dioxide. As a further alternative, the grid 902 can be exposed uniformly from the rear during manufacture to make the entire rear surface insulating. If none of these procedures is adopted, the exposure time is determined by the time it takes to expose the rear of the grid by normal scattered light, which is about one-fifth as bright as the deliberately scattered light. The grid 902 is then used to imagewise charge a record sheet in the manner shown in FIG. 27. The areas of the grid 902 which have been exposed are made insulating and thus permit charge to reach the record sheet. Thus, the image areas (the letter areas) become charged. Since the areas of the grid 902 corresponding to the background in the original are unexposed, they remain conducting and permit no charge to reach the record sheet. The record sheet is then developed with a direct developer, that is, one having toner polarity opposite to that of the charge image. If, for example, the initial charging is positive, a positive charge is deposited in the letter areas of the record sheet. A negative polarity toner particle is required to develop these areas.

An advantage of the embodiment of FIGS. 26-28 for making prints by means of reflex exposure is that the response of the photosensitive material is not affected by the non-image illumination as it is in conventional reflex copying. The exposure on the upper surface of the grid 902 in FIG. 26 to uniform illumination does not affect the ability of the imagewise exposure of the lower surface of the grid (by reflection from the document) to control the subsequent deposition of charge. The inherent contrast of the system is thus much higher than that of conventional reflex systems.

Another advantage of this embodiment of the invention is that a large number of prints may be made in a short time; the limitation on the rate of making prints is the efficiency of the development station.

FIG. 29 shows another embodiment of the invention. The process described in most of the previously described embodiments is normally a direct positive system, i.e., the illuminated areas of the grid are conductive, preventing charge from being deposited in the corresponding areas of the record sheet which gives an untoned area in the final copy when direct development is used. The embodiment of FIG. 29 provides a process for depositing charge on the areas of the record sheet corresponding to the illuminated areas of the photoconductive grid. An advantage of this process is that there is no need to use a reversal developer in the negative-positive reproduction of documents. Further, the electrode structure of this embodiment is relatively simple and easy to construct.

FIG. 29 shows a photoconductive grid 930, which may have any of the various constructions shown above, a corona discharge electrode 932 supplied with a corona generating potential from a voltage source 934 and a record sheet 936 having a grounded backing 938. In addition to this structure, which is in general identical to that of the above embodiments, FIG. 29 also shows a very thin (about 5 mil) layer 940 of an intermediate resistivity cellular material such as polyurethane foam laid on top of the grid 930 and a voltage source 942 for biasing the conductive core 944 of the grid 930.

The grid 930 is imagewise exposed in any of the ways described above. Upon activating the corona electrode 932, the layer 940 becomes charged, provided that the resistivity is high enough that the charge will not leak off as fast as it is supplied. Where a discharge path is provided, such as where the photoconductive grid is exposed, ions will pass through the grid and impinge upon the record sheet 936. In the unexposed areas, the charge built up across the polyurethane layer 940 will prevent further ions from the corona discharge from approaching the layer 940. Thus, no charge will appear on the record sheet 936 in these areas.

The condition of the layer 940 is very important. The material must be very fine (preferably over 100 pores/linear inch) and should be non-hygroscopic. Certain rubber polyurethane derivatives have been found to be superior from this standpoint.

The shape of the pores has considerable importance regarding the resolution of the system. One known foam has cylindrical pores, which makes it ideal for this application. These prevent the lateral diffusion of ions before they reach the photoconductive grid 930. This particular material, however, is a very good insulator. In order to be useful in this application, it must be either loaded with a semi-conductive material during manufacture or coated prior to use. Organic semi-conductors are preferred, because they are generally applied as a continuous film rather than being particulate in nature.

EXAMPLE

50 grams of Cu-doped cadmium sulfide were milled in 55.6 grams of Pliolite S-7 (sold as a 30 percent solution in toluene manufactured by Goodyear Tire and Rubber Co.) for 24 hours in a ball mill. The dispersion was sprayed onto a 200-line per inch nickel screen and allowed to dry for 12 hours. A piece of polyurethane felt (a compressed polyurethane foam, sold by Scott Paper Co.) was milled to 0.005 inch thickness. Neoprene cement (F-1, made by Carboline Company, St. Louis, Mo.) diluted 5:1 in toluene was sprayed on the screen with an air brush from a distance such that it was almost dry as it reached the screen, and immediately the polyurethane felt was pressed against the screen. This modified screen was used to make good quality reversal prints according to the method described above.

In all the above document copying embodiments of the invention many modifications of the optical system are possible. Any optical system, however convoluted, complex, or folded that will project an image is usable. Further, both continuous and pulsating light sources are usable. In addition, the transconductance of the grid system can be modified by changing one or more of the following: (1) the pattern of the openings in the grid, (2) the thickness of the dielectric coating, (3) the corona current, (4) rate of transport, (5) the developer, (6) use of a facing electrode during development, (7) use of a magnetic brush, (8) dry or liquid developing station, or (9) use of bias during development and/or exposure. It is also noted that if the mechanical problem of registration is non-essential, the record sheet can be moved and scanning-type optical systems (counter-current or co-current) can be used.

Although this embodiment has been entitled "Document Copying", the imagewise exposure of the photoconductive grid can be by other means than by light reflected from or passing through a document to be copied. For example, by suitable use of a light-tight box, lens, and shutter this embodiment can be used as a camera.

II DOCUMENT COPYING IN COLOR

This embodiment of the invention relates to an electrographic color recording system. This system is similar to the "Document Copying" embodiment described above and any of the various embodiments described above can be employed to produce color prints according to this embodiment of the invention. In FIG. 30 a color, positive, transparent original 500 is illuminated by a light source 502 through an appropriate color separation filter 503 from the filter selector 504. A lens system 506 focuses an image of the original on the plane through which a photoconductive grid 522 passes. A grounded field electrode 510 is positioned immediately behind a record sheet 508. A carriage 512 is mounted for back and forth movement (by means of a motor 509) across the record sheet 508 as indicated by the arrow 514. The carriage 512 contains an image charging station 516, a set of developing stations 518, and an infrared heat or visible-light source 520. With the light source 502 turned on and the appropriate color separation filter in the path of the light beam, the carriage 512 is moved once across the record sheet 508. In this one pass the record sheet 508 is sequentially imagewise charged, xerographically developed by a predetermined one of the developing stations 518, and then heated in order to fuse the developer, avoid contamination of subsequent developers, and remove the residual charge. One pass of the carriage 512 is required for each color of developer deposited. The imagewise charging station 516 consists of a photoconductive grid 522, which may be, for example, any of the grids shown in FIGS. 2-7, and a corona discharge electrode 524 connected to a voltage source 526. Three developing stations are shown, labeled C for cyan, M for magenta and Y for yellow; each station preferably consists of a liquid developer tank and a roller applicator.

This color printing system has the following advantages over previous electrophotographic color systems. It is fast; the dyes are chosen primarily for stability and color and not their chemical properties; color balance is adjustable on a single print instead of only on a series of prints, which reduces the rejection rate; white light can be used to inspect the copy at any time; the system possesses an inherently correct neutral scale rendition, which is subject to color balancing; sufficient deliberately-introduced interimage effects are available to accomplish masking; and greatly decreased color degradation (improved color rendition) is obtained compared to previous electrographic color systems since the sensitive surface is separate from the previously-deposited toner and is not affected by it, eliminating autopositive interimage effects.

The following example illustrates the color printing embodiment of the invention.

EXAMPLE

A dielectric record sheet comprising a 4-mil thick paper support coated with a 1-mil layer of polypropylene was held down (by vacuum) on a surface of conducting rubber under an enlarger head fitted with separation filters. On one side of the exposure area was a carriage containing a cadmium sulfide coated grid, an appropriate developing station for each primary color, an infrared light source (visible light can be used) for charge erasure, and an extra charging head. The carriage was arranged so that the entire effective exposure surface of the dielectric sheet was covered by all of the processing stations during one passage of the carriage over the record sheet. In operation, a separation filter, e.g., red was positioned and the carriage was transported across the paper, locally depositing a charge pattern corresponding to the color separation image and developing the image with the appropriate color developer, e.g., a cyan developer composition. This composition was made from cobalt blue oil pigment mixed with a small amount of green pigment and dispersed in a resin binder comprising a polymer of styrene, substituted styrene, and its homologs. Exposure through a red filter of the cadmium sulfide grid to form the red record was accomplished using about 1 foot-candle in the same plane. The charge was deposited through the grid simultaneously to form a red record on the polypropylene-coated paper.

Since xerographic development leaves some residual charge, a 500-watt infrared source (a visible lamp can also be employed) was used to heat the dielectric layer to remove the charge during the return stroke. The heating operation also fixes the developer to the surface and avoids contamination of subsequent developers. Using the heating lamp only during the return stroke makes the use of a light-baffle non-critical; obviously, if shielded well, it can be used during exposure. There are several modifications of the erasure procedure which can be used to introduce interimage effects to provide the equivalent of masking.

After three passes, exposing with about a 0.5 foot candle intensity to form the green record (magenta development) and about 5 foot-candles intensity to form the blue record (yellow development), a full color image was developed and the white light was turned on to inspect and view the print. If the print had been printed too lightly in any one color, the appropriate filter can be positioned, the corona current minimized, some bias voltage placed on the grid, the transport speed raised, the light level increased, and a low contrast overprint of the appropriate color made. It should be noted that this is one of the very few rapid processors where a print can be inspected in final form, then reprocessed to correct the exposure or color balance.

There are at least four principal ways in which interimage effects may be introduced to improve color rendition. It should be noted that each of these methods introduces a negative dependency commonly found in color xerography, which decreases color saturation.

The deposit of toner can be thick enough to decrease the capacitance per unit area of the dielectric coating, which decreases the density of toner deposited in subsequent development.

There is normally some residual charge left in the developed or toned areas. If complementary (positive followed by negative or vice versa) charge patterns and developers are used sequentially, their residual charge pattern discourages the immediately following toner deposit.

If a more drastic effect is wished, the dielectric sheet, after the second charge has been laid down, can be exposed to intense light with a spectral distribution corresponding to the "sensitivity of the mask". The light will heat the dielectric layer where it is absorbed, and remove charge by rendering the dielectric layer conductive. When developed, the heated areas will have less toner deposited on them because of their lower charge.

A bias charge can be laid down over the entire dielectric sheet after the first development and before erasure. Then if visual light is used for erasure at a controlled intensity, the residual charge will be absent where there is toner, and only present in the background. The background charge should be of the same sign as the image charge. Bias will be needed during development.

In some instances it is desirable to employ four-color reproduction as is common in the graphic arts industries. Thus, a fourth step of exposure and development for black may be employed, preferably following the three primary-color developing steps. The needs for such fourth color are in accordance with known graphic arts principles. For the cyan powder a cobalt blue oil paint pigment mixed with a small quantity of green pigment was used. For the magenta powder a vermillion paint powder, which is a substantial match for a standard photographic magenta was used. The materials are capable of being blown into a fine air powder cloud by the action of jet or air on the loose pigment material to form a pigment aerosol or the materials can be used in other forms of development, e.g., liquid, cascade, and magnetic brush.

Coloring agents such as dyes, stains or pigments can be added to the melt to produce powders of a desired color. Examples of suitable coloring agents include:

1. cyan blue toner GT (described in U.S. Pat. No. 2,486,351 to Richard H. Wiswall, Jr.)

2. benzidine yellow

3. brilliant oil blue BMA (color index no. C.I. 61555, National Aniline Division of Allied Chemical and Dye Corp.)

4. Sudan III Red (color index no. 26100, Fisher Scientific Co., Pittsburg, Pennsylvania).

5. oil yellow 2 G (color index no. 11020, American Cyanamid, New York, N.Y.)

6. oil red N-1700 (color index no. 26120, American Cyanamid, New York, N.Y.)

1 member

1 member

American Cyanamid, New York, N.Y.)

American Cyanamid, New York, N.Y.)

These and other suitable coloring agents may be employed singly or in combination to impart to the developer powder a desired color.

A black printer developing station can be added to the printer in order to use the equipment as a compatible autopositive black and white printer. A black printer alone with no filters in the optical system can be used to make a microfilm reader-printer. The sensitivity of the electrographic system is high enough to use a beam-splitter instead of a moving mirror to obtain the image of the print.

III PRINTING MASTER

FIG. 31 shows an embodiment of the invention which is particularly adapted for use in duplicating. Duplicating is usually distinguished from document copying in that a duplicating machine produces a relatively large number of copies from a specially prepared printing master at a lower cost per copy. FIG. 31 shows a printing master grid 600 which is analogous to the photoconductive grids described above. The grid 600 comprises an electrically conductive core or grid 602, which can have any of the configurations shown in FIGS. 2-5, for example, having an imagewise distributed coating 604 of insulating material thereon. The insulating image 604 can be coated or otherwise placed on the grid 602 by any known method. For example, the image to be recorded can be formed as a stencil through which the insulating material can be sprayed or painted onto the grid 602. However, the insulating image must completely cover all of the conducting surface of the grid as explained above in the case of the photoconductor coated grids. The insulating image 604 can be either a positive or a negative and since the electrostatic image to be produced from this printing master cana be xerographically developed with charged toner particles having either the same or the opposite polarity of charge from that making up the electrostatic image, a wide degree of flexibility is available in the process. In the production of the printing master grid 600, a grid can be used which already has one side coated with an insulator, so that spraying or painting only from one side insures that the grid will be completely coated in the insulating areas. The printing master grid 600 is employed, according to the invention in essentially the same way that the above-described image grid means or photoconductive grids are used. The grid 600 is positioned adjacent a record sheet 606 which can be identical to any of the record media described above. A flow of ions is then directed toward the record sheet 606 and through the grid 600 from an ion source such as, for example, a corona discharge electrode 608 connected to one terminal of a high voltage source 610 through a switch 612. A grounded field electrode 614 is positioned immediately behind the record sheet 606 to provide the necessary electric field. The operation of this embodiment differs from that of the above-described embodiments in the elimination of the imagewise exposure step. The printing master grid 600 has a permanent conductivity image. The operation of this embodiment can be carried out in the same apparatus as is used in all of the above-described embodiments, except those employing a scanning grid, by merely replacing the photoconductive grid with the printing master grid 600. This embodiment can be used with the additional grids shown for example, in FIGS. 9A and 13. Further, by using two or more printing master grids of the same image but produced using color separation filters, color prints can be produced by using successive charging and developing steps with appropriately colored developer. Any grid can be used in this process as long as it has a conductivity image which is functionally permanent (i.e., last as long as the copy run); thus any grids which are photosensitive, heat sensitive, etc., and which exhibit a functionally permanent change of conductivity upon energization can be used.

FIG. 32 shows another embodiment of the invention in which arbitrary patterns which have been previously formed on a grid or a succession of grids are printed onto an insulator coated record sheet.

FIG. 32 shows a succession of parallel, co-extensive conductive grids 950-960, which can be considered as an image grid means, located between a corona discharge electrode 970 connected to a suitable voltage source 968, and a record sheet 962 supported on a conductive backing 964. Each of the grids 950-960 is connected to a voltage divider network 966 and can be shorted to the grid next above or below it through one of a succession of switches 971-975.

Coated on each of the grids 950-958 is a pattern of insulating material corresponding to the character to be printed. Either the character area or the background area may be coated, depending on the use, as will be evident from the description below. The pattern on each of the grids 950-958 need not be continuous in nature; that is, it may consist of any number of isolated areas. The field from this charge pattern repels ions from the coated surface of each of the grids 950-958, forcing them to pass through and charge the record sheet 962. The bias potential on the grids 950-958 is adjusted to allow the ions in the uncoated area of the grids 950-958 to pass through unimpeded; it has little effect on the ions passing through the grids 950-958 in the coated areas. The record sheet 962 is thereby uniformly charged.

When it is desired to print the character on the top grid 950, for example, the switch 971 between it and the grid next lower in potential is closed. There is essentially no net effect in those areas of the record sheet 962 which are controlled by the insulating pattern on the top grid 950. In the areas in which neither grid is coated, there is a field-free zone between the two grids, since the potential difference has been made zero. All of the ions are then caused to return to the first grid 950, from which there is a current flow to ground through the network 966. Since there exists a field in areas in which the first grid 950 is uncoated but the second grid 952 is coated, there is a tendency for portions of the pattern on the second grid 952 to print. As the effect of the second grid 952 on the ion stream in these areas is much less (about one-tenth) than that of the upper grid 950, there is some "ghosting" produced, which can be eliminated by making the insulating coating on the second grid 952 of a material having sufficient conductivity that charge will be conducted away to ground as rapidly as it is deposited.

In order to print from the second grid 952, all switches are left open except for the one between the second grid 952 and the third grid 954. Ions will then pass through all areas of the first grid 950 with equal ease, and the action of the second grid 952 becomes in all respects the same as that of the first grid 950 in the example above. The principle can be extended to all character-bearing grids in turn.

If it is desired to print from several grids simultaneously, the "background" area of each grid is coated and the character area is uncoated. In this case, the pattern on the record sheet is a composite of the patterns on all grids which have been short-circuited to the grid next lower in potential. When the corona is turned on, the background area of the record sheet is charged, and the uncharged area corresponds to the pattern resulting from superposition of the character areas of the grids being printed from. This area is then developed by reversal, i.e., with a toner having the same polarity of charge as that of the charge image. Optimum spacing of the grids has been found to be approximately equal to twice the size of the mesh opening.

A field grid or control grid means 960 can be used in the present embodiment in order to minimize the effect on the ion stream of the charge already on the record sheet 962. This relaxes the tolerance on the grid-to-receiver distance. In the absence of a field grid, the bottom grid is shorted to ground in order to print the character on it.

Use of a facing electrode during development, e.g., a magnetic brush, aids in filling in the cross-hatched pattern caused by the screen support of the characters being printed from. As in all prior embodiments in which a grid is in the corona field, the potential of each grid must be adjusted empirically so as to minimize its effect on the field.

IV CHARACTER PRINTER

FIG. 33 shows a printing embodiment of the invention in which the modulation of a flow of ions is accomplished by interposing between an ion source and a record medium a grid comprising a conductive electrode formed in the shape of a character to be printed. The electrode an be biased to either attract or repel ions, as is more fully discussed below with reference to FIGS. 38 and 39. The grid in this embodiment is actually a printing-master grid somewhat similar to that described above with respect to the duplicating embodiment of the invention. The primary difference is that in this embodiment all areas of the grid are conductive as distinguished from the grid in the duplicating embodiment in which a portion of the grid is insulating.

FIG. 33 shows a stack 700 of wire electrodes 702 and 704 in the shape of Arabic numerals. Although a large number of electrodes can be employed in the stack 700, only two numbers are shown for the purpose of clarity. The electrodes 702 and 704 are placed between a corona discharge source 706 and a record sheet 703 having an insulating surface 710. The record sheet 708 can be, for example, polypropylene-coated paper. The wire electrodes 702 and 704 are connected either to ground potential or to an appropriate bias potential through switches 712 and 714 respectively. In operation, the switch 712 or 714, whichever one corresponds to the character to be printed, is closed and the corona discharge electrode 706 is energized by means of a switch 716. The grounded or biased electrode, for example, 702, modulates the corona current within a characteristic radius, thus producing an electrostatic charge image, corresponding to the electrode 702 on the record sheet 708. A relatively wide, electrostatic charge pattern shadow is left on the record sheet 708 by the grounded electrode. The ungrounded electrode 704 has practically no blocking effect and does not cast a shadow. The record sheet 708 is then removed and the electrostatic charge image xerographically developed with toner having the same polarity of charge as that of the electrostatic charge image. A field electrode 718 is provided, as in the above embodiments, behind the record sheet 708 to provide the necessary electric field.

If a page of type is to be printed, either (1) the stack 700 can be moved across the page in a scanning motion to print a line, character by character, as in the typewriter, or (2) a complete row of grid arrays can be made with a full set of electrodes for each space in the line to allow printing of a line at a time. In this case, the paper would be moved past the grid array without reversal of direction as a line is electrostatically recorded on the record sheet 708. The switches 712 and 714 can be any electronic component capable of handling about 300 volts with extremely small leakage currents, for example, vacuum tubes, some transistors, relay switches, and high impedance photoconductors. If there are some resistors added to the circuit, between the wire electrodes 702 and 704 and the corona power supply 720, or if an alternate source of potential is used, low impedance switching devices will be needed. Such resistors lower the impedance of the circuit and also serve to minimize the effects of conductivity of the various mechanical supports of the electrodes 702 and 704.

FIG. 34 shows an embodiment for use in printing a line at a time employing a complete row of grid arrays with a full set of electrodes for each space in the line. The record medium is moved past the printing array as line after line of printing is electrostatically recorded on the record medium. The record medium then passes immediately to a developing station, and if necessary, to a fixing station. FIG. 34 shows a circuit board 730 on which the electrical components for a particular alphanumeric character are supported. Near one end of the support 730 is an elongated, rectangular opening 732 in which the electrode characters 734 are positioned. Each one corresponds to a particular character space on the record medium spaced below the circuit board array. Electrical leads 736 are connected to each of the electrode characters 734. Each lead 736 is connected to ground or to a suitable potential through a switch 738 and an electrical lead 740. In one embodiment the switches 738 are photocells and the switching is done by light. FIG. 34 shows only one of the circuit board elements of a complete array needed to print a line as a whole. FIG. 34 illustrates a photocell row for the alphanumeric character 5. In a complete device, additional circuit board elements, one for each alphanumeric character, are superimposed or stacked above each other and insulated from each other.

FIG. 35 illustrates a group 800 of electrode segments which can be used in place of a stack of individual electrode characters such as is shown in FIG. 33. The group 800 can be positioned in the opening of a circuit board such as is shown in FIG. 34. Each of the electrode segments shown is provided with an electrical connection and each segment is connected to the adjoining segments through insulating members 802. The use of such a group 800 of electrode segments is preferred, for certain applications, over providing a separate electrode for each letter in the alphabet and for each number 0 through 9. In order to print, for example, the letter N, using the group 800 of electrode segments, the electrode segments 804, 806, 808, 810, 812 and 814 are energized by closing the switches in the corresponding electrical lines 816, 818, 820, 822, 824 and 826 to connect the electrode segments to either ground potential or to the desired bias potential.

FIG. 36 shows an embodiment for printing a line of alphanumeric characters at a time. The switching is done by light by means of a photocell array 850 in which the photocells 852 (a few of which are shown) are arranged in an X-Y arrangement. The X direction corresponds to the space to be printed and the Y direction corresponds to the particular alphanumeric character to be printed. The array may comprise, for example, 36 photocells in the Y direction and about 66 in the X direction. The switching by light, can be done, for example by means of a moving light beam, such as in the output of a cathode ray tube, such as is shown, for example, in U.S. Pat. No. 3,111,598, issued Nov. 19, 1963, to Tatham, Jr., or by uniformly illuminating a punched card 854 as shown in FIG. 36. The printing is done by a linear array 857 of grids in which the array is similar to that shown in FIG. 34 and in which the individual grids are of the type shown in FIG. 35. One grid is provided for each space in the line to be printed. Alternatively the photocell array 850 could be connected to a single grid which moves or scans across the record medium to print a space at a time. When a particular photocell in the array 850 is illuminated, a particular character (corresponding to the position of that photocell in the Y-direction) will be printed in a particular space (corresponding to the position of that photocell in the X-direction). When a photocell is illuminated, an electrical signal is carried by one of the electrical lines 858 (only a few are shown) to a decoder or switching matrix 856, which energizes the appropriate electrode segments of the appropriate grid in the array 857. Various types of switching matrices are available for use as the matrix 856, such as a conventional diode matrix. A flow of ions is provided by means of a corona discharge electrode 864 connected to a source 862 of high potential. A field electrode 864 is provided on the other side of the array 857 from the corona discharge electrode 860 to direct the flow of ions through the array 857. A record medium 866 having an insulating surface is positioned adjacent the array 857 by the field electrode 864. The record medium 866 is drawn from a supply roll 868 by appropriate drive rollers past the charging station, through a developing station 870, a fixing station 872 and a sheet cutting mechanism 874.

There can be many variations in the design of these photocell arrays, including logic circuits involving series elements. To print a line, the appropriate light pattern is used to expose the photocells, and a pulse of corona current is used to record the line on the dielectric record sheet. The light pattern may be simultaneous as in the case of an exposure through a punched card, or sequential, as in the case of the output of the cathode ray tube, provided the corona pulse is used within the response time of the photoconductor. In the case of a sequential exposure, the exposure level may be higher for the earlier-exposed photocells in order to allow for the longer decay period before printing. If a sufficiently high corona current is used, the limitation on the rate of printing is the response time of the photoconductor. Response times of a millisecond or less have been reported for cadmium sulfide and selenium, both of which are available in appropriate impedances. This would correspond to printing rates of 1000 lines per second. If these were the standard 120 character lines used in computer printout, this would correspond to a clock rate for the computer of 1 megacycle, assuming full printout is demanded. The rate limiting process in many computer operations is the relatively slow printout involved. This embodiment of the present invention will produce full-size, quick-access copies. If printing from punched cards is desired, the rate limiting step would be the mechanical transport of the cards. The high printing rate may be desirable in teletype operations where a great many stations share a single channel on a time-sharing basis, such as a 90-minute telegram service via a single satellite. There are obviously several advantages to be gained from this simplicity of the equipment and the fact that there are no moving parts in the printing elements. Wear on the printing elements should be negligible as contrasted to the transport of the record medium since nothing but air touches the printing elements. By printing the line in sections, it is possible to maintain a steady flow of information from the source to the printer.

FIG. 37 shows an embodiment of the invention which uses photocells which have a pronounced exposure memory under high electrical fields, but which erase this memory and become insulators quickly, once the field is released. We have discovered this effect in some cadmium sulfide powders; this effect was previously reported for cadmium selenide powders. If such photoconductors are used in an array similar to that described with reference to FIG. 34, with auxiliary electrodes, an arrangement for each character space similar to that shown in FIG. 37 results. FIG. 37 shows a corona discharge electrode 750, a dielectric record sheet 752 positioned on a field electrode 754, an electrode 756 in the shape of one character and an electrode 758 in the shape of another character. A source of potential 760 is connected to the corona discharge electrode 750 through a switch 762. In this embodiment there is a metal grid 764 above the character electrodes and a metal grid 766 below the character electrodes in the path of the corona current from the electrode 750 to the dielectric record sheet 752. When it is desired to illuminate the photocell, for example 769, to print the character 758, the switch 768 is opened and the corona is turned on. The switch 770 is closed to prevent premature recording of an electrostatic image. As long as the corona is on and the switch 768 open, the photocell will remember any exposure that is made, although there will actually be some slight decay of photoconductivity with time. After the appropriate photocell or photocells have been exposed, corresponding to setting a line of type, the lower switch 770 is momentarily opened to record the electrostatic image. After printing the line, the switches 768 and 770 to both grids 764 and 766 respectively are closed, eliminating the field across the photocells and erasing the exposure memory. If the line were printed in segments, a continuous input could be accommodated. Currently available cadmium sulfide has an erase period of about 0.1 seconds or less and a memory of more than 30 seconds. This particular material would also be of some use in reducing variations in printing time due to variations in punch card transport of characters and several can be used at once to form a single character. The photoconductor memory unit is one of the very few memory units that can control continuously a flow of energy and decide at any fractional level of flow instead of all on or all off. This feature can also be of use in analog memory units.

The electrostatic charge image discussed above with respect to this embodiment is produced by local exhaustion of charge by attraction to an electrode character as shown in FIG. 38A FIG. 38A shows a portion of an electrode character 780 and a dielectric recording sheet 782 supported on a field electrode 784. The arrows 786 represent the corona current. As shown in FIG. 38A there is an exhaustion of charge by attraction of ions to the wire electrode 780, the charge being carried to ground through the closed switch. In FIG. 38B the curve 790 represents the charge density resulting from charging in the manner shown in FIG. 38A. FIG. 38C shows a curve 792 which represents the corresponding toner deposit onto the electrostatic charge image of FIG. 38B. FIG. 39A shows that a sharper, higher density, developed image can be produced if a repulsion of charges instead of an attraction of charges is used to form the electrostatic charge image. In the case of attraction, only a smoothly-varying function of the charge density is produced since the deposit of developer depends on the rate of change of charge density with distance in the plane of the paper. This results in a relatively low density developed image of a diffuse nature, as shown in FIG. 39B. If a charge repelling potential is applied to the character wire 794, as shown in FIG. 39A, a pattern of a charge density minimum surrounded by a maximum on both sides is produced as shown by the curve 796 in FIG. 39B. The resultant increase in the derivative of charge density with respect to distance enhances the deposit of toner at the edge of the character and sharply terminates the deposit as shown by the curve 798 in FIG. 39C. The result is a very sharply defined character. The repulsion-formed character holds its sharpness better at increased separation between the electrode and the recording paper. Clean, sharply defined lines have been produced that are about 10 percent as wide as the paper-electrode space. A voltage divider circuit is recommended for driving the repulsion grid from the corona supply. There are several combinations of locking photoconductors (having a memory under an electrostatic field, e.g., cadmium sulfide, cadmium selenide) and unlocking photoconductors (e.g., selenium, and some forms of cadmium sulfide) that can be used to provide a memory for setting up optically a line array of printing elements. Moreover, any relatively high impedance switching device can be used to control the electrodes. In this case the impedance was provided in the circuit by the voltage dropping resistor used from the corona supply; a separate supply or an acceptance of a heating load would allow low impedance circuitry. Compared to the attraction system of electrographic printing, much lower impedance circuitry is used, minimizing any RC time constants. Since the current is repelled from the writing elements, instead of attracted, the average current density on the surface of the electrode character elements is lowered, and so consequently any corrosive tendency is lowered. The increased separation of the printing element from the record sheet will allow simpler and less dimensionally critical assembly of the printing head.

It is noted that this character printing embodiment is not limited to use in the printing of alphanumeric characters. A conductive electrode can be formed in the shape of an entire image to be recorded and then used as a printing master. As described above, the electrode can be grounded or connected to an attractive or a repelling bias potential.

V MULTIPLE COPYING OF DOCUMENTS

This embodiment of my invention provides a simple, fast, inexpensive, and highly flexible system for the multiple copying of documents. As stated above, this embodiment employs a different principle of operation from that of the previously described embodiments. In this embodiment the ion flow is imagewise modulated by electrostatic fields rather than, for example, by means of conductive grid areas which remove ions from the ion flow.

This system employs a two step process. The first step is the production of an electrostatic charge image on an insulator coated grid. The second step is the use of this insulator coated grid in conjunction with a control grid to imagewise modulate a flow of ions directed toward an insulating record member. The image grid means comprises the insulator coated grid having the electrostatic charge image thereon, and the control grid means comprises at least one conductive grid positioned in substantially parallel, adjacent relation to the image grid means. The two grids are each energized to the desired potential, which potential depends on the grid-to-grid spacing, the grid-to-record member spacing, the potential of the charge image, and certain other factors. FIG. 46C shows one set of useful potentials. By properly energizing the two grids, the electrostatic field between the two grids can be used to control the flow of ions therethrough. In one embodiment the insulator coating on the one grid is a photocondutive insulator. To produce the electrostatic charge image thereon this photoconductive insulating coating is uniformly charged and then imagewise exposed. A typical set of values for the various potentials involved are the following. The photoconductor is originally uniformly charged to 300 volts and is then imagewise discharged to about 250 volts in the most highly exposed areas and to about 280 volts in the least exposed areas. The top grid is then connected to a bias supply of 1500 volts, which is appropriate for 1/8 inch spacing from the record member. Since this is the grid that draws current during the step of imagewise charging the record member, it is connected directly to the bias supply, permitting the use of a very low power bias supply for the coated grid, as a matter of personal preference. The voltage difference (300 volts) which was used between the two grids in the step of uniformly charging the coated grid, is reduced by the voltage drop due to photodecay in the white areas (50 volts) in the final image and impressed between the two grids. Thus a voltage difference of about 250 volts is impressed between the two grids. However, due to the charge image on the insulator coated grid there is not a uniform electrostatic field between the two grids. The potential of the top or conductive grid is essentially equal to the potential of the surface areas of the insulator coated grid in the white (250 volt) areas and therefore there is little electric field between the two grids in these areas. There is a potential difference, and therefore an electric field, between the two grids in the remaining areas. A flow of ions is then directed through the control grid array and toward a record member for a long enough period of time to generate a charge image on the record member which will develop to the desired contrast in the desired time. Setting the bias for less photodecay voltage and increasing the charging time can produce an increase in sensitivity without a change in contrast or development ime. Alternatively increasing the charging time or current at constant bias can increase constrast. Thus, there is available an independent control of contrast and an interrelated control of sensitivity and contrast. The spacing between the two grids, or between the grids and the paper, is not critical except that sparking must be avoided. The grids are separated by from 0.005 inch to 0.06 inch (for a 200 line/inch grid, for example) and the distance from the grid to the paper can be anywhere from 0.08 to 0.25 inch.

The sensitivity of the system can be increased by extending the corona current density or the charging time and decreasing the change in intergrid voltage between the first step and the second step of charging the record member. As long as the conductivity of the unexposed photoconductor can be neglected, the effects of the variations in thickness of the photoconductor appear primarily as a random variation multiplying times the exposure effects, or, in other terms, the signal-to-noise ratio due to variations in coating thickness should be independent of exposure, if the leakage current is much smaller than the photocurrent. This should be contrasted to the xerographic process where the initial noise effects, due to variations in coating thickness, are both additive and multiplicative, leading to strong dependencies of signal-to-noise ratio, on exposure. The independence of coating thickness, signal-to-noise effects, and from variations in exposure, leads one to believe that extreme sensitivities can be obtained by extending the paper charging time.

The sensitivity of the system is also dependent on the initial potential to which the photoconductor is charged. Maximum gain is obtained at potentials just short of irreversible break-down of the photoconductor. In the case of one preferred photoconductor this was about 300 volts; while at 360 volts the photoconductor became conductive for a few days after use. At the operating voltage, the photoconductor appears to be indefinitely reusable (55,000 exposures).

Since the record member need not be in position during the exposure step, the photoconductor can be exposed from either side to produce a right-reading copy. If the photoconductor is exposed through the top grid, then moire patterns can be avoided by having the top grid have half as many holes per linear inch as the bottom grid, and be about 80% open when the grid wires of both are parallel.

Since the charge image is stable and can be used for multiple prints, it is possible to correct and compensate for exposure effects by changing the electrical characteristics when making successive prints. For example, a low electrical contrast image may be of use for locating the charge level corresponding to a given exposure, then an expansion of the tone scale or a high-contrast print can be made around that electrical level.

If the same sign of corona is used to charge the paper and the photoconductor, then a negative image results; i.e., charge is deposited in the exposed areas on the record member. If opposite signs of corona are used in the two corona steps, then a positive charge image results, i.e., charge in the dark areas. With a photoconductor which can be used with either sign of charge, positive or negative charge images can be produced in either positive or negative charges on the record member as desired by the operator. Thus, positive or negative images can be produced with available developer of either sign from a single grid array.

FIGS. 40A and 40B show an arrangement of three grids to produce multiple copies from a single exposure. FIG. 40A shows the electrical arrangement during the step of forming a charge image on the insulating grid and FIG. 40B shows the electrical arrangement during the repetitive step of imagewise charging a record sheet. FIG. 40A shows an insulating record sheet 1202 mounted on a conductive base-plate 1200, a metal grid 1204 positioned immediately above the record sheet 1202, an insulator coated grid 1206 above the metal grid 1204, and a photoconductor coated grid 1208 above the insulator coated grid 1206. A corona discharge electrode 1210 is positioned above the set of grids and is connected to a corona generating voltage source 1209 by means of a suitable switch (not shown) to provide a source of ions. Voltage sources 1207, 1205, and 1203 are provided for energizing the grids 1204, 1206, and 1208 to the desired potential by means of suitable switches (not shown). The photoconductor coated grid 1208 is imagewise exposed by means of a projector 1211. The grids 1208 and 1206 consist of a conductive core or grid, which may have any of the configurations shown above in previous embodiments, and a complete coating of photoconductor and insulator respectively.

The photoconductor coated grid 1208 is employed to form an electrostatic charge image on the insulator coated grid 1206, by imagewise exposing the photoconductor coated grid 1208 while the corona electrode 1210 is energized. The record sheet 1202 need not be present at this time. However, if the record sheet 1202 is present, charging of the record sheet 1202 at this time is prevented by setting a potential on the metal grid 1204 to repel the ions from the ion flow, thus preventing the record sheet 1202 from receiving any charge. At the same time, it is desirable, though not necessary, that there be no field between the metal grid 1204 and the record sheet 1202. This may be accomplished by adjusting the bias voltage so that these two elements are at about the same potential.

FIG. 40B shows the electrical arrangement for imagewise charging the record sheet 1202. The photoconductive grid 1208 is flooded with light to make it uniformly conducting and the corona electrode 1210 is energized to charge the record sheet 1202 in an imagewise manner. Since the charge image on the insulating grid 1206 is not substantially affected by this operation, record sheet 1202 may be replaced by a fresh, similar sheet and the process repeated. Many hundreds of prints may be made from a single exposure step. A somewhat simpler construction using only the two coated grids will function very nearly as well as the above system. A slight alteration in the potentials of the two grids during the first corona application is required in order to prevent corona from reaching the paper.

FIGS. 41A and 41B show a variation of the embodiment described above with respect to FIG. 40 in which like reference numerals indicate like elements. FIG. 41A illustrates the exposure step and shows a pair of metal grids 1212 and 1214 which are coated one on each surface of a foraminous insulating spacer 1218. A photoconductor layer 1216 is coated on the grid 1214 so as to completely cover all of the exposed surface thereof. It is necessary in this design that the thickness of the foraminous insulating spacer 1218 be at least three times the diameter of its holes 1217. Useful materials for the spacer 1218 are Teflon, epoxy, Mylar, glass, and many other plastics. Any material of suitable high insulating value, which can be formed into the desired shape is suitable. The presence of the walls 1215 of the holes in the spacer 1218 prevents lateral diffusion of the ions in the ion flow in the volume between the grids 1214 and 1212 and thus improves resolution. The walls of the holes in the spacer 1218 act as the insulating grid 1206 of FIGS. 40A and 40B, and the operation of the embodiment shown in FIGS. 41A and 41B is substantially the operation described above with respect to FIGS. 40A and 40B. That is, a charge image is produced in the spacer 1218 by imagewise exposing the photoconductive layer 1216 by means of the projector 1211 while the corona electrode 1210 is energized.

FIG. 41B shows the step of imagewise charging the record sheet 1202 by means of the charge image on the spacer 1218.

FIG. 42 shows the printing step of an alternative construction of a device using an insulating storage grid. Metal grids 1214 and 1220 are coated on the surface of a foraminous insulating spacer 1224, which in this case should have a thickness of about twice the diameter of the holes. A photoconductor layer 1216 is coated onto the metal grid 1214 in such a way as to completely cover it and an insulating layer 1222 is coated onto the metal grid 1220 in such a way as to completely cover it. In this form of the invention, the insulator layer 1222 receives and holds the charge image formed in an exposure step similar to that described above and this charge image is used in the printing step to control the deposition of charge onto the record sheet 1202 without any substantial change in the charge image. The arrangement of FIG. 42 has an advantage over that of FIGS. 41A and 41B in that the insulating spacer 1224 is much easier to make since the thickness through which holes must be formed is much smaller in relation to hole diameter. The embodiments of FIGS. 40A, 40B, 41A, 41B and 42 produce an increase in gain over that attainable in single stage designs, since there is an effective increase in voltage with the action of each grid-controlled stage. The ultimate limit on gain per stage is determined by the effective capacitance between each grid and the plane of the record sheet, or ground.

Further increases in gain of the system of the subject embodiment can be achieved by the use of more complex systems containing larger numbers of grids. In FIG. 43, an insulating spacer 1236 carries metal grids 1242 and 1244 on its surfaces, and metal grid 1244 is completely covered with a photoconductive layer or grid 1248. An insulating spacer 1234, placed adjacent the spacer 1236, carries metal grids 1238 and 1240 on its surfaces and metal grid 1238 is completely covered with an insulating layer or grid 1246. Preferably the photoconductive grid 1248 is formed of one of the types having a high photoconductive gain such as cadmium sulfide. The fact that the four metal grids are connected to suitable potentials is indicated in the drawing. This electrographic system is operated in two steps. Initially there is no potential between a record sheet 1232, mounted on a grounded conductive base plate 1230, and the insulating grid 1246. There is no potential drop between the photoconductor grid 1248 in the exposed areas thereof and the metal grid 1242. There is a potential difference between the metal grid 1242 and the metal grid 1240, and there is another potential difference between the metal grid 1240 and the metal grid 1238, both potential differences being in a direction to accelerate ions from the corona discharge electrode 1250 toward the insulating grid 1246. Under these conditions, during imagewise exposure of the photoconductor grid 1248 by means of a projector (not shown) and during corona energization the insulating grid 1246 will become charged in the areas corresponding to the unexposed areas of the photoconductive grid 1248. In the second (printing) step, the photoconductive layer or grid 1248 is flooded with illumination and there is a potential difference applied between each successive pair of metal grids, in a direction to move ions toward the record sheet 1232, but no potential between the metal grid 1240 and the charge image areas of the insulating grid 1246. The charge image on the insulating grid 1246 will thus control the current flowing to the record sheet 1232 provided there is a sufficient accelerating field between the surface of the insulating grid 1246 and the record sheet 1232. In the first step the insulating grid 1246 can be charged to a higher voltage relative to its support, the metal grid 1238, than the voltage between the surface of the photoconductive grid 1248 and its support, the metal grid 1244. In the second step charge on the insulating grid 1246 can control approximately 100,000 times as much charge flowing through the holes in the spacer 1234. In other words the use of the insulator coated grid 1246 in this gaseous system increases the amount of charges or ion flow that can be controlled by a single exposure, by a factor of more than 100, 000, which may be termed the "gaseous system charge gains." The total net gain of the system is somewhat less than the photoconductive gain times the "gaseous system charge gain" of the second step. Although this system is basically a multiple print system, to a certain extent the gaseous system charge gain can be used to increase the sensitivity of the system.

FIG. 44 shows another charge image storage system for making multiple prints from a single exposure. In this case a photoconductive layer 1266 is coated to completely cover a metal grid 1260 which has been coated on one side of a foraminous insulating spacer 1254. The spacer 1254 carries a metal grid 1262 on its other surface. The photoconductor in this instance preferably has a moderate to low photoconductive current gain and must be highly insulating in the dark. Selenium or any of a large number of organic photoconductor preparations is suitable. A second foraminous insulating spacer 1252 carries metal grids 1256 and 1258 on its surfaces and the metal grid 1256 is completely coated with an insulating layer 1264. In this system, the photoconductor grid 1266 is initially charged and is then exposed areas just as a selenium plate is in one of the common xerographic processes. The potentials on the metal grids are then altered so that the charge distribution on the surface of the photoconductive grid 1266 can control the current flowing through the holes 1265 to charge the surface of the insulating grid 1264. The potentials on the metal grids are again altered and the photoconductive grid 1266 is flooded with light so that the charge image on the surface of the insulating grid 1264 can control the current flowing through its holes 1267 to charge the record sheet 1232. An alternative to flooding the photoconductive grid 1266 with light is to bias the grid 1266 in such a way as to bias the charge image on the photoconductive grid 1266 out of existence (as far as this process is concerned). In the process of FIG. 44, if the photoconductive gain in the photoconductor is 1, the gain of the system would be the gaseous charge gain in the first step multiplied by the gaseous charge gain in the second step, or about 1,000,000. This figure for the gain assumes that it is possible to uniformly coat and fabricate all of the grid arrays involved. In practice the limiting gain for these systems is reduced because of variations of coating thickness of insulator and photoconductor. These variations produce a mottle or granularity in the image when too much gain is used, resulting in a loss of contrast. With very fine mesh grids, this problem becomes equivalent to the silver halide granularity problem. It should be noted that this is a three-step process which has high system gain, and which can use a low gain, high impedance photoconductor. This process is suitable both for multiple copy use and for relatively high sensitivity production of single copies. In the process of FIG. 44, four separate grids can be used in an alternate construction, without loss of resolution, except for the possibility of increase in moire patterns.

FIG. 45 shows a somewhat more complex system. An insulating spacer 1252, carrying metal grids 1256 and 1258 and an insulating layer or grid 1264 coated on the metal grid 1256 is the same as in FIG. 44. An insulating spacer 1268 carries on its surfaces metal grids 1270 and 1272. An insulating layer or grid 1274 completely covers the metal grid 1270 and a photoconductive layer or grid 1276 is completely coated on the metal grid 1272. In this embodiment the photoconductive grid 1276 need not store charge so it may be a high gain photoconductor of relatively high dark current. In operation, the photoconductive grid 1276 is imagewise exposed by means of a projector (not shown) and imagewise controls the deposition of ions or charges onto the insulating grid 1274. The charge image on the insulating grid 1274 is then used, along with any alterations in the potentials on the various grids which are necessary and with flooding illumination or biasing of the photoconductive layer or grid 1276, to control deposition of another charge image onto the insulating grid 1264. The charge image on the insulating grid 1264 is then used to control deposition of charges onto the record sheet 1232; the potential on each grid being altered as described with respect to FIGS. 43 and 44, for example, to provide the control. During this printing step, the charge image on the insulating grid 1274 is biased out of existence (as far as this process is concerned) and the photoconductive grid is flooded with light. This system has one stage of photoconductive gain and two stages of gaseous charge gain giving an extremely high total gain. This system would be primarily useful for making multiple copies. For single prints, the effective gain would be limited by the precision to which the various bias potentials could be set in the first step of the three-step process.

In the systems of FIGS. 43, 44 and 45, the image is not subject to diffusion because it is confined either by strong electric fields or by the walls of the holes in the insulating spacers. Except as it may be necessary to avoid moire patterns, the alignment between the holes of the two spacers of these systems is not critical. The resolution of the systems, depending on how the moire patterns are eliminated, will either be equal to the number of holes per linear inch or to about one-half that number. In all of these systems, there is at least one stage of voltage gain, and usually two or more, which means that the potential across the photoconductor can be a very small fraction of the voltage needed in the image on the record sheet. If the high gain of these systems is not needed, some mismatch between the impedance of the photoconductor and that of the corona can be compensated for by using the high voltage gain of the systems. In all of these systems the various voltages used in the steps up to and including the printing step are interdependent and over fairly large ranges of variation change in one of the voltages in an early step may be compensated for by other changes in that step or in later steps.

FIGS. 46A, 46B and 46C show the three steps of a preferred variation of the subject embodiment of the invention which is especially useful for making multiple copies of documents from a single exposure. Of course single copies may be made if desired. FIG. 46A shows an insulating record sheet 1280 mounted on a backing electrode 1278, a photoconductive grid or image grid means 1283 comprising a metal core or grid 1282, which can have any of the configurations shown above, and a photoconductor layer or coating 1284 completely covering the grid 1282. Positioned above the photoconductive grid 1283 is a metal grid or control grid means 1286 and above that a corona discharge electrode 1288. FIG. 46A shows the step of charging the photoconductor grid 1283. The photoconductor must be of the variety having very high resistance in the dark so that in the dark it is essentially an insulator and will store charge on its surface for at least the time required to make the desired number of prints. A large number of well known organic photoconductors have this property, and also zinc oxide may be used if the sign of all the potentials shown in FIGS. 46A, 46B and 46C is reversed so that the zinc oxide photoconductor is charged negatively. Most organic photoconductors will operate satisfactorily with either polarity of corona. The photoconductor 1284 must cover all of the surface of the grid 1282, including the inside walls of the holes, to a sufficient thickness to be able to withstand and hold a charge of about 300 volts to operate with the potentials shown in the FIGS. With the grid 1286 grounded as shown, the potential of the grid 1282 will control the potential to which the photoconductor 1284 can be charged by the corona source 1288. In FIG. 46A the backing electrode 1278 and the record sheet 1280 are shown in position, although their presence during this step is purely optional. The record sheet 1280 is not charged during this step with the arrangement shown. If the backing electrode 1278 and the record sheet 1280 are not present during this step, then conductive grid 1282 may alternatively be grounded and a potential of plus 300 volts may be applied to the conductive grid 1286. This will put a charge of nearly 300 volts on the photoconductor 1284 with respect to the conductive grid 1282.

FIG. 46B shows the step of imagewise exposing the photoconductor 1284 to an optical image by means of a projector 1292 to produce an electrostatic charge image on the photoconductive grid 1283. This exposure step corresponds exactly to the exposure step in ordinary selenium or zinc oxide xerography, and the charging step of FIG. 46A together with the exposure step of FIG. 46B taken together constitute an ordinary xerographic process for obtaining an imagewise charge distribution on the surface of the photoconductor 1284 with respect to the conductive grid 1282. In FIG. 46B the corona source 1288, the conductive grid 1286, and the conductive grid 1282 are all shown grounded. This is merely for convenience. All may simply be disconnected from any power source, or may be connected to any arbitrary potential as long as there is no corona produced by any of the elements during the exposing step. In addition, the backing electrode 1278 and the record sheet 1280 are shown in position in this step. This is purely for convenience. Neither is needed during the exposure step and it is not necessary that the backing electrode 1278 be grounded during the exposure step. In fact if the backing electrode 1278 and the record sheet 1280 are either transparent or are not in the position shown during the exposure step it is perfectly practical to expose the photoconductor 1284 from the other side, and this has advantages in some arrangements because it makes it possible to obtain right-reading images with simpler optical systems.

It should be noted that the photoconductor 1284 need not be discharged completely by the exposure step, as is often desirable in xerography. In fact, for the system shown, enough exposure to cause a difference in potential between exposed and unexposed areas of the photoconductor 1284 of about sixty volts is sufficient. Thus, relatively little exposure is needed in this process, and this fact can be stated differently by saying that the process has a high photographic speed. In some cases, as when a very large number of prints is desired from a single exposure, it may be desirable to increase the exposure beyond the amount mentioned above.

FIG. 46C shows the step of imagewise charging the record sheet 1280. Charging is accomplished by energizing the electrode 1288 and keeping the photoconductor 1284 in the dark. A very large number of record sheets may be imagewise charged by simply inserting new record sheets and repeating the simple step of charging the record sheet, keeping, of course, the photoconductor 1284 dark during the entire operation. Thus many copies may be made from a single exposure of the photoconductor 1284. It may be desirable to ground both of the grids 1283 and 1286 during insertion of the subsequent record sheet to avoid handling difficulties due to electrostatic forces. In some cases it may be desirable to change the voltages on the various electrodes systematically during the charging of a large number of record sheets to correct for a slight deterioration which may occur in the charge image on the photoconductor 1284. In the same fashion it may be desirable to change the time cycle of the step of charging the record sheets for the same reason. The potentials shown in FIG. 46C are about optimum for a separation between the photoconductor grid 1283 and the record sheet 1280 of one-eighth of an inch. For larger separations, the voltage between the grid 1282 and the record sheet 1280 may need to be increased to maintain sharpness, and conversely for smaller separations this voltage may have to be decreased to avoid arcing between the electrodes. In any case, the voltage difference between grids 1282 and 1286 should be near the value of 250 volts as shown for the case described. Another way of explaining this is that the potential of the grid 1286 should be very near, preferable slightly more positive than, the potential of those areas of the surface of the photoconductor 1284 which were partially discharged by optical exposure. The separation of the grid 1286 from the photoconductive grid 1283 is not particularly critical. The potentials given are for a separation of about 0.030 inches. Larger or smaller separations may be used, but the separation must not be so small that the grids, which are in some of the steps of the process mutually attracted, can bow together and touch. It is important to note that this separation need not even be very uniform. A separation varying over the area of the grids from 0.030 to 0.060 inches has been successfully used. The number of lines per inch used in the grid 1282 determines the resolution of the final picture, each hole in the grid acting essentially independently of all others. On the other hand, the grid 1286 may be much coarser, and may be chosed to give a minimum shadowing effect, either optical or electrical, caused by its wires. If exposure is made as indicated in FIG. 46B through the conductive grid 1286, moire effects are particularly well minimized by making that grid of a mesh just one half the number of lines per inch as the mesh of the grid 1282, and by aligning the two grids so their wires are essentially parallel.

EXAMPLE

An arrangement was used as shown in FIGS. 46A, 46B, and 46C. A 250 line per inch electroformed nickel grid having 70 percent open area was stretched on a steel frame having an opening 8 1/2 inches square. This grid was completely coated with photoconductor by spraying with a highly atomizing spray gun from a number of highly diverse directions, an organic photoconductor mixture prepared as follows:

A solution containing 11 percent total solids in ethylene chloride was prepared containing 25 percent of 4,4'diethylamino-2,2'-dimethyltriphenylmethane in Vitel PE-101 sensitized with 2 percent by weight of the dye 4, 6 (4-diethyoxyphenyl) 2,(4-amyloxystyril) pyrilium fluoroborate. To 20cc. of this solution was added 0.01 gram of sodium acetate, and the solution was diluted to five times its volume with toluene.

The spraying of the grid was done immediately after preparing the mixture because the material tended to flocculate upon standing for a half hour or more. The coating applied was about 0.0075 inches thick, and was uniform over all surfaces of the nickel mesh, including the inside walls of the holes. After drying for several hours, the photoconductor coated grid was subjected to the radiation of an infra red heat lamp to heat and slightly melt the coating, which was observed under the microscope to become much more uniform and smooth. It was found that a minimum of this heating should be used, for the heating seemed to lower the sensitivity of the coated grid in subsequent use, and it was found possible to spray some grids with sufficient perfection that the heating was not needed. Another grid of 150 lines per inch and 67 percent open area was stretched on another steel frame designed to fit into the first frame without contact between the frames. This grid was not coated in any way. The two grids were mounted 0.030 inch apart and 0.125 inch from a grounded metal plate carrying a commercially available record sheet consisting of a conductive paper coated on one side with a highly insulating polymer layer. A corona assembly consisting of a multiplicity of 0.0015 inch tungsten wires spaced 1/2 inch apart in parallel relation was mounted so that the wires were 3/4 inch from the top, uncoated grid. Alternate wires of the corona assembly were arranged to be connected to a 10,000 volt source of either polarity, as needed, and the other half of the wires were always connected at the same time to a 2,000 volt source of the same polarity as the corona source used. These wires then acted as stabilizing electrodes for the higher potential corona wires and provided a uniform corona source over the 8 1/2 inch square frame. The arrangement was then as shown in FIG. 46A. The steps for making a print were carried out as described above, allowing one second of corona discharge to uniformly charge the grid in the step illustrated by FIG. 46A, one second of exposure to an image of 3 foot candles intensity for the step illustrated in FIG. 46B, and 1 1/2 seconds of application of corona discharge to imagewise charge the record sheet in the step illustrated by FIG. 46C. The imagewise charged record sheet was then developed to produce an excellent positive image of the original with a positively charged liquid developer as is well known in the art. As many as 100 prints were made from a single exposure of the photoconductor by quickly replacing the record sheet and applying the corona charging step illustrated in FIG. 46C to charge the new record sheet. It was found that the charging step of FIG. 46C could be carried out in a time as short as 1/8 of a second if the potential to which the photoconductor was charged in the step of FIG. 46A was increased to just below that value which would damage the photoconductor, and if the exposure used in the step of FIG. 46B was increased somewhat. It was found that many useful combinations of the potentials of the electrodes in the different steps of the process and of the times used for each of the steps exist, and that contrast, sensitivity, number of prints which can be made, latitude of exposure, and other important photographic quantities characterizing the process depend strikingly on the proper adjustment of all these parameters. The values given are one set of times and potentials which gave excellent results with a particular grid. Any variation in the quality of the grid or of other factors affecting the process may require different sets of values for all these quantities.

In most xerographic processes it is desirable to charge a fairly thin layer of insulator or photoconductor so that the voltage does not rise to very high values when the required amount of charge has been deposited. In this particular process, however, it is often advantageous to use a relatively thick insulating layer on the record sheet, for the high voltage attained by deposition of a normal amount of charge helps in performing very rapid development of the charge image. The step of depositing charge on the record sheet is in this version quite independent of the voltage formed on the record sheet, within wide limits, and the rapid development made possible by higher voltages is very important in the rapid production of multiple copies.

The version of this process described with respect to FIGS. 46A, 46B, and 46C employs positive charging of a photoconductor to produce positive-to-positive reproduction of prints. By appropriate changes in potentials on the various electrodes in the different steps, negative-to-positive images may be formed using the same photoconductor charged positively in the first step. Analogously, this photoconductor may be charged negatively in the first step of the process, and either positive-to-positive or negative-to-positive images may be made. These changes from positive-to-positive to negative-to-positive images are achieved by appropriate changes in some of the potentials used on the different electrodes during the step of depositing charge on the record sheet. They are, as stated, the results when the charge image on the record sheet is developed with a toner charged oppositely to the polarity of charge on the record sheet. Of course the less desirable method of changing from positive-to-positive to negative-to-positive reproduction by development with toner particles having the same polarity of charge as the charge on the record sheet can also be used, as is well known in the art, but much better results are achieved when development is with a toner bearing a charge opposite to that on the record sheet. Four possible versions of this process are listed below, together with typical values of the potentials used on the various electrodes in each of the steps.

VERSION A

Positive-to-positive process with a photoconductor that stores positive charge:

Step 1

Charge photoconductor

Uncoated grid, grounded

Photoconductor-coated grid, -300 volts

Positive corona

Step 2

Expose photoconductor

No corona

Expose

(Insert record sheet)

Step 3

Charge record sheet

Uncoated grid, -1500 volts

Photoconductor-coated grid, -1750 volts

Negative corona

Negatively charged image

(Develop with positively charged toner)

VERSION B

Positive-to-positive process with photoconductor that stores negative charge:

Step 1

Charge photoconductor

Uncoated grid, grounded

Photoconductor-coated grid, +300 volts

Negative corona

Step 2

Expose photoconductor

No corona

Expose

(Insert record sheet)

Step 3

Charge record sheet

Uncoated grid, +1500 volts

Photoconductor-coated grid, +1750 volts

Positive corona

Positively charged image

(Develop with negatively charged toner)

VERSION C

Negative-to-positive process with photoconductor that stores negative charge:

Step 1

Charge photoconductor

Uncoated grid, grounded

Photoconductor-coated grid, +300 volts

Negative corona

Step 2

Expose photoconductor

No corona

Expose

(Insert record sheet)

Step 3

Charge record sheet

Uncoated grid, -1500 volts

Photoconductor-coated grid, -1220 volts

Negative corona

Negatively charged image

(Develop with positively charged toner)

VERSION D

Negative-to-positive process with photoconductor that stores positive charge:

Step 1

Charge photoconductor

Uncoated grid, grounded

Photoconductor-coated grid, -300 volts

Positive corona

Step 2

Expose photoconductor

No corona

Expose

(Insert record sheet)

Step 3

Charge record sheet

Uncoated grid, +1500 volts

Photoconductor-coated grid, +1220 volts

Positive corona

Positive charge in image

(Develop with negatively charged toner)

All of the above four versions of this process have been operated successfully using the potentials shown. In addition, when using negative corona to charge the photoconductor, both direct and reversal prints have been made using zinc oxide in resin binder as the photoconductive coating on the grid.

The values of potentials given in the listing above for the four versions of this process are correct for an initial charging of the photoconductor to a potential of about 300 volts and a decay by exposure of the surface potential to 250 volts in the strongly illuminated areas and to about 280 in the background, or less strongly illuminated areas. It should be noted that the characteristics of the photoconductors used are such that even a very little stray light in the darker parts of the image causes some photodecay in these areas. The photodecay of charge is, however, always much greater in the more strongly illuminated portions of the image. It is possible to work with either greater or smaller amounts of photodecay of the charge image by suitable changes of electrode potentials and time of charging in the step of charging the record sheet. Using less photodecay leads to greater photographic sensitivity, without any loss of contrast, but sometimes with some limitation on the number of prints which can be made from a single exposure. Using more photodecay, and adjusting the potentials accordingly, can, especially with an increase in the time used to charge the record sheet, lead to an increase in contrast. The sensitivity of the system is also dependent on the initial potential to which the photoconductor is charged. Maximum gain is obtained at potentials just short of irreversible breakdown of the photoconductor. In the case described above, excellent prints were made by charging to 300 volts, but charging the photoconductor to 360 volts destroyed it's photoconductive properties. Tests of the above photoconductor coating indicated that when charged to 300 volts it could be used for 100,000 or more copies without deterioration. It should be noted that this particular system is especially good in this respect. It appears that the conductive grid between the corona charger and the photoconductor coated grid to some extent shields the photoconductor from some of the effects of the corona and gives the photoconductor a much longer life than it has in some of the other processes described.

FIG. 47 shows the exposure step of a single grid structure useful in the general process described above with respect to FIGS. 46A, 46B, and 46C. FIG. 47 shows a conductive core or grid 1298 which is completely covered with a photoconductive layer 1300. One side of the photoconductive layer 1300 is covered with a protective coating 1302 of an insulating or a light absorbing material or preferably a material having both properties. This construction provides a structure particularly suitable for reflex exposure of the photoconductor 1300. FIG. 47 shows a document 1294 in position for copying. A mylar sheet 1296, not always necessary, may be used to protect the photoconductor 1300 from contact with the document 1294 to prevent charging by friction or discharging by contact of the photoconductor 1300. If the mylar sheet 1296 is thin (0.0005 inches is a suitable thickness) it can protect the photoconductor 1300 without causing any loss of image sharpness. In operation the grid structure of FIG. 47 is equivalent to that of FIG. 7, but in many cases the structure of FIG. 47 is easier to manufacture. With both grid structures there is an advantage over normal reflex exposure practice because the incident light does not strike the light sensitive material, in this case the photoconductor 1300. Only after reflection from the document does the exposing light strike the light sensitive material. Thus a much higher contrast is achieved than is normally obtained with a reflex exposure process. The grid structure of FIG. 47 can be used in the process illustrated in FIGS. 46A, 46B, and 46C, with the exposure carried out as shown in FIG. 47 rather than as shown in FIG. 46B.

FIG. 48 schematically illustrates an apparatus using a grid such as is shown in FIG. 7 or FIG. 47 in the general process illustrated in FIGS. 46A, 46B, and 46C. In FIG. 48, a grid 1304 consists of a conductive grid completely covered with a layer of photoconductor, and coated on the upper side with an insulator or light absorbing material to prevent any effect of light incident from above. The grid 1304 is mounted on, and with its conductive core in conductive contact with, conductive drums 1306 and 1308 which are grounded for convenience and safety. In operation drums 1306 and 1308 rotate in a clockwise direction, driven by any suitable means (not shown) to move the grid 1304 from the drum 1306 to the drum 1308. As a section of the grid 1304 travels past a corona charger 1314 mounted in a grounded conductive shield 1316, the grid 1304 is charged as in the step illustrated in FIG. 46A. A conductive grid 1312 connected to a suitable potential source (not shown) determines the potential to which the grid 1304 is charged. As the same section of the grid 1304 continues its translational movement it passes over drum 1310 in firm pressure contact with a thin sheet 1328 of mylar which in turn is in firm pressure contact with a document 1326 to be copied. As this section of the grid 1304 passes over the drum 1310, the photoconductor is imagewise reflex exposed by a light source 1324. This corresponds to the exposure step shown in FIG. 47, and is equivalent to the exposure of the photoconductor in FIG. 46B. As the same section of the grid 1304 continues its translational motion it passes near a record sheet 1330 supported by a backing electrode 1332 and driven synchronously with the grid 1304 by drive rollers 1334. As the grid 1304 passes near the record sheet 1330, a corona discharge is initiated by means of a corona source 1320 mounted in a grounded corona shield 1322. A conductive grid 1318 acts in the same fashion as the conductive grid 1286 of FIG. 46C. The corona source 1320 supplies a source of charges or ions and the deposition of these ions in an imagewise manner onto the record sheet 1330 is controlled by the potentials on the conductive grid 1318 acting with the point-to-point surface potential of the photoconductor on grid 1304. Thus an imagewise charge pattern is deposited on the record sheet 1330. This step is similar to that shown in FIGS. 46C. The imagewise charge pattern on record sheet 1330 may be developed by any known xerographic means (not shown) to provide a positive, rightreading copy of the original document 1326.

The invention has been described in considerable detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

Claims

I claim:

1. An electrographic process for producing a reproducible electrostatic charge image corresponding to an image to be recorded, on a radiation responsive image grid means comprising an electrically conductive core coated on one side with a layer of photoconductive insulating material sensitive to radiant energy and on the other side with an electrically insulating material, said process comprising the steps of:

uniformly charging the photoconductive layer on said image grid means by directing a flow of ions from an ion generating source through an electrically conductive control grid means onto the photoconductive layer while maintaining said control grid means at a predetermined potential relative to said source for limiting the charge level on said photoconductive layer; and

subsequently exposing said uniformly charged photoconductive layer through said control grid means, while maintaining said charge level, to a radiation image for producing a corresponding electrostatic image on said photoconductive layer, the image grid means and control grid means modulating any subsequent flow of ions directed thereto.

2. The electrographic process according to claim 1 wherein said electrically conductive control grid means comprises at least one grid that is positioned adjacent the side of said image grid means facing said ion generating source.

3. The electrographic process according to claim 1 wherein said charging step includes charging said photoconductive layer to a level of potential substantially the same as that at which said control grid means is maintained.

4. An electrographic process for producing an electrostatic charge image corresponding to an image to be recorded on a layer of electrically insulating material adjacent an electrically conductive backing member, comprising the steps of:

directing a first flow of ions from a corona source through an electrically conductive control grid means onto a radiation responsive image grid means comprising an electrically conductive core completely coated with a layer of photoconductive insulating material, while maintaining said control grid means at a predetermined potential relative to said source, to uniformly charge said photoconductive layer;

subsequently exposing said image grid means to a radiation image corresponding to said image to be recorded for producing an electrostatic image on said photoconductive layer; and

thereafter directing a second flow of ions from said source through said control and image grid means and onto said layer of electrically insulating material, while biasing said image and control grid means at predetermined and individually different potentials relative to said backing member, whereby said control grid means and said electrostatic image on said photoconductive layer modulate said second flow of ions to produce an electrostatic image corresponding to that on said photoconductive layer on said layer of insulating material.

5. The electrographic process according to claim 4 wherein said layer of electrically insulating material is positioned in contact with said backing member before the step of directing said first flow of ions.

6. The electrographic process according to claim 4 wherein said layer of electrically insulating material is positioned in contact with said backing member after the step of exposing said image grid means.

7. The electrographic process according to claim 4 wherein said control grid means is maintained at a predetermined level of potential, said image grid means being uniformly charged to a level of potential substantially the same as that at which said control grid means is maintained before said exposing step occurs.

8. The electrographic process according to claim 4 wherein, during said first flow of ions, the potential at which said control grid means is maintained is substantially equal to that of said backing member and said image grid means is maintained at a potential of opposite polarity to the ions in said first flow of ions.

9. The electrographic process according to claim 4 wherein, during said second flow of ions, said image and control grid means are biased by potentials of the same polarity as the ions in said second flow of ions.

10. The electrographic process according to claim 4 wherein the step of directing said second flow of ions is repeated for each reproduction of the electrostatic image on said photoconductive layer that is made on a layer of insulating material.

11. A method of modulating the flow of ions from an ion source toward an electrically conductive printing platen, which method utilizes ion modulating means spaced from and between said ion source and said printing platen comprising electrically conductive grid means and photoconductive grid means, said method comprising:

establishing on said photoconductive grid means an electrostatic charge pattern of charged and discharged portions corresponding to an imagewise pattern, the charged portions being of a first potential and polarity and the discharged portions being of a second potential lower than said first potential,

electrically biasing said conductive grid means to a third potential of magnitude intermediate said first and second potential;

electrically biasing said platen to a potential lower than said third potential; and

directing ions from said source toward said platen;

whereby ion flow from said source is modulated by said electrostatic pattern on said ion modulating means.

12. The method in accordance with claim 11 wherein the electrically conductive grid means is arranged between said ion source and said photoconductive grid means and said photoconductive grid means is arranged between said conductive grid means and said platen.

13. The method in accordance with claim 11 wherein the ions directed from said source toward said platen are of a polarity opposite from said first polarity.

14. The method in accordance with claim 11 wherein the ions directed from said source toward said platen are of said first polarity.

15. A method of modulating the flow of ions from an ion source toward an electrically conductive printing platen, which method utilizes ion modulating means spaced from and between said ion source and said printing platen comprising electrically conductive grid means and photoconductive grid means, said method comprising:

establishing on said photoconductive grid means a uniform electrostatic charge of a first potential and polarity;

exposing said photoconductive grid means to an image-wise pattern of radiation to discharge corresponding portions of said uniform charge to produce an electrostatic pattern of charged and discharged portions corresponding to the radiation pattern, the discharged portions being discharged to a second potential lower than said first potential;

electrically biasing said conductive grid means to a third potential of magnitude intermediate said first and second potential;

electrically biasing said platen to a potential lower than said third potential; and

directing ions from said source toward said platen;

whereby said ion flow from said source is modulated by said electrostatic pattern on said ion modulating means.

16. The method in accordance with claim 15 wherein the electrically conductive grid means is arranged between said ion source and said photoconductive grid means and said photo-conductive grid means is arranged between said conductive grid means and said platen.

17. The method in accordance with claim 15 wherein the ions directed from said source toward said platen are of a polarity opposite from said first polarity.

18. The method in accordance with claim 15 wherein the ions directed from said source toward said platen are of said first polarity.