Method Of Forming Electrostatic Latent Images

Abstract

In a method of forming an electrostatic latent image by the steps of applying a first electric field across a photosensitive element including a photosensitive layer and a highly insulative layer so as to deposit a first uniform change of one polarity upon the surface of the highly insulative layer and to establish a uniform charge distribution in the photosensitive layer, applying a second electric field across the photosensitive element to deposit a second charge of the opposite polarity and projecting a light image upon the photosensitive element concurrently with the application of the second field, the second field is applied in two spaced-apart periods and the light image is projected between the periods.

Description

BACKGROUND OF THE INVENTION

This invention relates to a method of forming electrostatic latent images in an electrophotography utilizing corona discharge electrodes in the form of fine wires and more particularly to an improved method of eliminating the effect of corona discharge electrodes upon the projected light image.

According to a recently developed method of electrophotography described more fully, for example, in U.S. Pat. No. 3,457,070, issued July 22, 1969, and in copending U.S. Application Ser. No. 481,365 filed Aug. 20, 1965, hereinafter referred to as the KTA process, an electrostatic latent image is formed by the steps of applying a first electric field across a photosensitive element including a thin and transparent highly insulative layer and a photosensitive layer integrally bonded to the highly insulative layer to deposit a charge of a first polarity upon the surface thereof, applying a second electric field across the photosensitive element to deposit a charge of opposite polarity, and projecting a light image upon the photosensitive element concurrently with the application of the second field whereby to form an electrostatic latent image on the surface of the highly insulative layer corresponding to the projected light image.

Where a corona discharge electrode in the form of fine metal wires is used to establish the first and second fields, as the light image is projected through the metal wires while the second field is being applied across the photosensitive element the shadow of these metal wires affects the quality of the electrostatic latent image or a visible image reproduced therefrom. In order to eliminate this problem it has been tried to oscillate or shift the corona discharge device. However, this approach was not satisfactory because the interval during which a photographic flash lamp flashes is very short.

SUMMARY OF THE INVENTION

It is therefore the principal object of this invention to provide a novel method of forming electrostatic latent images which are free from the shadow of corona discharge metal wires.

Briefly stated, according to one embodiment of this invention, the method of forming electrostatic latent images comprises applying a first electric field across a photosensitive element including a photosensitive layer manifesting persistent internal polarization and a highly insulative layer integrally bonded to the photosensitive layer so as to deposit a first uniform charge upon the surface of the highly insulative layer and to establish a uniform charge polarization in the photosensitive layer; applying a second field across the photosensitive element in the dark to deposit on the surface of the highly insulative layer a second charge of the opposite polarity; projecting a light image upon the photosensitive element; depositing a charge of the opposite polarity on the surface of the highly insulative layer in the dark whereby to form electrostatic latent image on the surface of the highly insulative layer; and exposing the surface of the photosensitive element to uniform light.

The latent images produced in this manner can be developed and transfer printed in the well-known manner. Further, the photosensitive element can be used repeatedly by erasing the latent image and cleaning the surface of the element. The novel process can assure extremely high photosensitivity comparable with that of the KTA process.

The novel method is not only suitable for forming latent images by projecting the light image through the corona discharge electrode unit but also can form more stable latent images than the KTA process by the following reason. Thus, in the second step, a large field is applied across the photosensitive layer in the dark and then the light image is projected in the third step so that the density of charge carriers flowing through the photosensitive layer in response to light increases greatly thus assuring high photosensitivity.

Where the light image is projected upon the photosensitive element concurrently with the application of the second field of the opposite polarity by means of a corona discharge electrode unit as in the KTA process, since the rate of charging the surface of the photosensitive element by the action of corona discharge is not high, the field applied across the photosensitive layer does not increase rapidly. Thus, during such interval wherein the field intensity is still low, it is impossible to expect high photosensitivity. For this reason, with the KTA process, it is impossible to obtain high photosensitivity unless the time of light image projection is carefully preselected. The novel method can completely solve this problem.

The photosensitive layer to be used in this invention is desirable to have a high dark resistance. However, it is more desirable to use a photosensitive layer which includes a plurality of trap levels in the layer at portions near the highly insulative layer integrally bonded to the photosensitive layer, or a photosensitive layer that can manifest persistent internal polarization (PIP) effect. With such a photosensitive layer it is possible to increase the interval between the second and fourth steps without decreasing the intensity of the electrostatic latent image.

BRIEF DESCRIPTION OF THE DRAWING

This invention can be more fully understood from the following detailed description taken in connection with the accompanying drawing in which a single FIGURE diagrammatically shows an electrophotographic apparatus utilized to carry out this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

EXAMPLE 1.

Vapor of monomer of acrylic resin was introduced into an evacuated vessel and a high AC voltage was impressed across two spaced-apart electrode plates disposed in the evacuated vessel to deposit films of the polymer of the acrylic resin on the surfaces of the electrode plates to a thickness of about 3 microns. The coated electrode plate was utilized as a substrate and an alloy of SeTe containing 25 percent of Te was vapor deposited on the polymer film to a thickness of about 34 microns. Near the end of the vapor deposition process, Se was also vapor deposited together with the SeTe alloy and thereafter a thin layer of SeTe along was deposited to obtain a photosensitive layer. A layer of polycarbonate was then applied on the photosensitive layer to a thickness of about 15 microns to complete a photosensitive element.

A photosensitive element 10 was wrapped around a metal or glass cylinder 11 with its highly insulative layer 12 faced outwardly. The photosensitive element 10 shown in the drawing further comprised a photosensitive layer 13 prepared in the above-described manner and a backing electrode 14 which is grounded as shown. In some applications the backing electrode 14 is transparent. The cylinder was rotated in the direction indicated by an arrow.

A first corona discharge electrode unit 15 was placed close to the periphery of the photosensitive element to deposit a charge of -800 volts on the surface of highly insulative layer 12. Then, in the dark, a positive charge was deposited by means of a second corona discharge electrode unit 16 until the potential of the surface of the highly insulative layer increased to substantially zero volt. 0.5 second after termination of this positive corona discharge a suitable light image was projected upon the photosensitive element by means of a flash light optical system 17, and 0.5 second thereafter, the surface of the photosensitive element was charged positively by means of a third positive corona discharge electrode 18. Then uniform light, which may be room light, was projected upon the photosensitive element as schematically shown by arrow A.

Under these conditions, the surface potential was measured and obtained a charge potential of +200 volts at portions of the highly insulating layer corresponding to bright portions of the light image. The electrostatic latent image was developed by applying a powder of charged developer by means of a magnetic brush 19 which are commonly used in the electrophotography to obtain an intense visible image. The developed power image was then transfer printed onto a paper 20 in the conventional manner. The developer powder remaining on the photosensitive element after transfer printing was removed by a cleaning brush 21. The above-described cycle of operation was repeated many times without any trouble.

EXAMPLE 2.

A photosensitive element was prepared comprising an electrode, a photosensitive layer including a photoconductive layer and a charge trap layer, and a highly insulative layer which were bonded together into an integral structure. The electrode may be made of a metal plate, low resistance paper, low resistance synthetic resin, Nesa (trade mark) glass or any other low resistance material. The photoconductive layer may be sintered CdS or CdSe, or vapor deposited CdS, CdSe or Se or a thin layer of a powder of CdS, CdSe or ZnO bonded by a binder of a very low proportion or a thin layer of polyvinyl carbazole whereas the charge trap layer may be composed of ZnS or ZnCd activated with Cu, Ag or Pb, or anthracene, anthraguinone, S, PbO or the like having a large number of impurity levels. The highly insulative layer may be made of any material provided that it can transmit light rays and has high insulating strength. Where Nesa glass is utilized as the electrode, the highly insulative layer may be opaque.

The method of this invention described in connection with example 1 was carried out with various photosensitive elements described above and obtained electrostatic latent images similar to that of example 1.

EXAMPLE 3.

The trap layer was formed by diffusing an impurity into the surface of a photoconductive layer at a high density, thus forming a charge trap layer in the surface portion of the photoconductive layer. Thus, for example, a charge trap layer was formed in the surface layer portion of a sintered photoconductive layer by diffusing an impurity of high concentration into a surface layer portion at a relatively low temperature for a short interval. Other layers were prepared in the same manner as in example 2.

It was found that this photosensitive element also showed satisfactory results.

Stated in another way, this invention comprises a modification of the KTA process wherein the step of applying the second field of the KTA process is divided into two spaced-apart independent periods and between these periods is interposed the projection of the light image independently of the corona discharge, thus eliminating the deleterious effect of the shadow of the corona discharge electrode upon the latent image.

Where photosensitive materials having high PIP effect or high dark resistance are utilized, the method of this invention may be modified by applying such extremely high potential across the photosensitive element during the second step that is not suitable for satisfactory development of the latent image and increases the density of drifting charge carriers contributing to image formation at the time of light image projection and by readjusting the surface potential to a value suitable for developing during the fourth step. This modified method can greatly improve the photosensitivity. The field applied in the fourth step may be a DC or an AC field.

Claims

I claim:

1. In a method of forming an electrostatic latent image by the steps of applying a first electric field across a photosensitive element including a photosensitive layer manifesting persistent internal polarization and a highly insulative layer integrally bonded to the photosensitive layer so as to deposit a first uniform charge of one polarity upon the surface of the highly insulative layer and to establish a uniform charge polarization in the photosensitive layer, applying a second electric field across the photosensitive element to deposit on the surface of the highly insulative layer a second charge of the opposite polarity, and projecting a light image upon the photosensitive element, the improvement which comprises applying said second field in two spaced-apart periods, and projecting said light image between said periods.

2. The method of forming an electrostatic latent image according to claim 1 wherein said first and second fields are applied by corona discharge electrode units.

3. The method of forming an electrostatic latent image according to claim 1 wherein said electrostatic latent image formed on the surface of said highly insulated layer is subjected to light irradiation and developed under ambient light.

4. An electrophotographic apparatus comprising a photosensitive element including a photosensitive layer manifesting persistent internal polarization and a highly insulative layer integrally bonded to said photosensitive layer, a first corona discharge electrode unit to deposit a charge of first polarity on the surface of said highly insulative layer, spaced-apart second and third corona discharge electrode units to deposit a charge of the opposite polarity on the surface of said highly insulative layer, means interposed between said second and third corona discharge electrode units to project a light image on said photosensitive element, whereby to form an electrostatic latent image on the surface of said highly insulative layer corresponding to said projected light image, means to develop said latent image as a powder image, means to transfer print said powder image onto a receptive medium, means to clean the surface of said photosensitive element and means for moving said photosensitive element relative to other elements of the apparatus.

5. The electrophotographic apparatus according to claim 4 wherein said apparatus further comprises means interposed between said third corona discharge electrode unit and said developing means to irradiate said photosensitive element with uniform light.

6. The electrophotographic apparatus according to claim 4 wherein said photosensitive element is wrapped around a rotary cylinder with said highly insulative layer faced outwardly, and said first to third corona discharge electrode units, light image projecting means, said developing means, transfer means and cleaning means are disposed around the periphery of said photosensitive element.