Image Intensifier With Channel-type Secondary Emission Multiplier

Abstract

An electronic image intensifier including an electron multiplier. The device comprises a photocathode, a fluorescent screen, and a body between and spaced from the photocathode and screen. The body is provided with a plurality of elongated, longitudinal passageways, the walls of which are secondary emissive. Electrodes are provided on opposite surfaces bounding the channels and an electrically conductive electron-permeable membrane covering the entrance to each passageway.

Description

This invention relates to electron multiplier and image intensifier devices. More particularly the invention relates to "channel intensifier" devices and to electronic tubes employing such devices. Such devices will be defined later but, briefly, they are secondary-emissive electron-multiplier devices comprising a matrix in the form of a plate having a large number of elongated channels passing through its thickness, said plate having a first conductive layer on its input face and a separate second conductive layer on its output face to act respectively as input and output electrodes.

Secondary-emissive intensifier devices of this character are described, for example, in British Pat. Specification Nos. 1,064,073 1,064,074 U.S. Pat. Nos. 3,387,137, 3,327,151 and 3,497,759, while methods of manufacture are described in British Pat. Specification Nos. 1,064,072 and 1,064,075.

In the operation of all these intensifier devices (when incorporated in electronic tubes) a potential difference is applied between the two electrode layers of the matrix so as to set up an electric field to accelerate the electrons, which field establishes a potential gradient created by current flowing through resistive surfaces formed inside the channels or (if such resistive surfaces are absent) through the bulk material of the matrix. Secondary-emissive multiplication takes place in the channels and the output electrons may be acted upon by a further accelerating field which may be set up between the output electrode and a suitable target, for example a luminescent display screen.

As a summary of this art, the devices referred to herein as "channel intensifier" devices (or, more briefly, "channel plates") are devices having a structure as defined in the Patent Specifications referred to above in a definition given in the following terms:

A channel intensifier device is a secondary-emissive electron multiplier device for an electronic tube which device comprises a resistive matrix in the form of a plate the major surfaces of which constitute the input and output faces of the matrix, a conductive layer on the input face of the matrix serving as an input electrode, a separate conductive layer on the output face of the matrix serving as an output electrode, and elongated channels each providing a passageway from one face of the assembly consisting of matrix and input and output electrodes to the other face of said assembly.

With such devices the distribution and cross sections of the channels and the resistivity of the matrix are such that the resolution and electron multiplication characteristic of any one unit area of the device is sufficiently similar to that of any other unit area for any imaging purposes envisaged.

If such a device is used in an imaging tube or system, the latter will be referred to for convenience as an "image intesifier" tube or system rather than as an "image converter" tube or system even in applications where the primary purpose is a change in the wavelength of the radiation of the image.

One of the problems with present channel intensifier devices when used in a tube with a display screen is feedback of positive ions created inside the channels or at the display screen. These ions can be accelerated by the field in the channels and can thus cause secondary electron emission at the photocathode.

A second problem arises owing to the need for a metal backing on the display screen which backing is usually provided to reflect light emitted rearwardly by the phosphor and thus prevent or reduce optical feedback which would otherwise be caused by such light passing backwards through the channel plate and reaching the photocathode. The problem in question is the occurrence of electrical breakdown between channel plate and screen due to the high field which has to be set up between them in order to enable electrons to penetrate the metal backing of the screen and maintain good resolution.

It is an object of the present invention to permit both of these problems to be solved by means of an improved channel plate construction. It is a further object of the invention to provide a solution to the "dark spot" problem as will be explained.

An improvement in or modification of a channel intensifier device as herein defined including an electrically conductive membrane obturating the entrance to each channel which membrane is electron-permeable as defined and is electrically connected to the other membranes.

The term "electron-permeable" is used in the sense that individual electrons can penetrate through a membrane or can produce secondary electrons within the membrane which emerge from the output side thereof.

The membranes may be formed as a continuous layer which is superimposed on the input electrode. In such case the membrane layer may be insulated from the input electrode so as to permit it to be held at a potential different from that of said electrode.

Alternatively, the membranes may be formed as extensions of the input electrode of the device so that said electrode and membranes form, together, a continuous layer.

Preferably the membranes are substantially opaque to visible radiation (and possibly other radiation such as ultraviolet) since this permits the device to perform also the second function of preventing or reducing optical feedback when used in a tube with a display screen. (In this connection the term "substantially opaque" is used to indicate that the membranes must be at least sufficiently opaque to backward radiation from the display screen to prevent cumulative or runaway optical feedback in the tube). In such a tube the screen requires no metal backing and the material of the screen is chosen to emit light of wavelengths to which the members are substantially opaque.

For reasons connected mainly with the electron velocities required to penetrate the membranes, the following description will be based principally on a second aspect of the invention represented by the particular case of an image intensifier tube including the channel intensifier device in operative combination with a display screen on the output side thereof which screen is adapted to emit light of wavelengths to which the membranes are substantially opaque, a photocathode spaced from the input side of said device, and an electron-optical system between said photocathode and said device which system is of the type designed to effect an electron-optical inversion of the image. (A typical example of such an electron-optical system is the so-called electron-optical diode system or equivalents thereof employing more than two electrodes, and the tube may follow a preceding fiber-optically coupled electron-optical image-inverting tube so that the combined system produce an output image which has the same orientation as the input image).

The invention will be described with reference to the accompanying drawing in which:

FIG. 1 is a prior art device shown diagrammatically.

FIG. 2 is a diagrammatic view of one embodiment of the invention.

FIGS. 3, 4 and 5 are diagrammatic views of other embodiments of the invention.

The two problems already discussed will now be explained more fully with reference to FIG. 1 of the accompanying drawing which shows a small part of a known channel intensifier device (with its channels C, input electrode E1, and output electrode E2) and a part of a cooperating display screen S. The screen may be of conventional type laid on a plain glass or fiber-optic window W which may form part of an evacuated envelope. The screen comprises a layer of phosphor S and a metal backing E3 (typically aluminum) which is electron-permeable but substantially opaque to light emitted rearwardly by the phosphor.

One difficulty in operating the device as an image intensifier is achieving adequate resolution. The resolution is limited by the spreading of the electrons after leaving the channel plate (this is indicated by arrows representing electron tracks). One possibility is to reduce the distance d between E2 and E3 and increase the voltage between E2 and E3 (indicated as a source B2) as much as possible. However, there is a limit (about 5 kv./mm.) to the maximum applied field because of the risks of flashover and field emission. Now, unfortunately, 5 kv. over 1 mm. does not have the same effect on the resolution as, say, 10 kv. over 2 mm. As distance d increases, to maintain a given resolution, voltage V must increase as the square of d. Thus at 2 mm. a voltage of 20 kv. is needed, which is more than 5 kv./mm.

What would really be desirable is to reduce distance d to a very small value and then reduce voltage V even more, for example making distance d equal to 0.5 mm. and making the voltage V equal to 1.25 kv., which is only 2.5 kv./mm. However, a minimum of 5 kv. is needed to penetrate the aluminum screen backing E3 which is provided to prevent light feedback. Therefore distance d must be at least 1 mm. which means that there is still a limiting field strength and only just enough resolution.

These problems can be resolved according to the invention by, in effect, transferring the aluminum or like film from the screen to the input side of the channel plate. This is illustrated by the embodiments of the invention which will now be described with reference to FIGS. 2 to 5 of the accompanying drawings.

As shown schematically in FIG. 2 of the drawings, the metal film is effectively constituted by membranes or diaphragms D which are shown obturating the entrances to the channels C. Such membranes are conductive (e.g. aluminum) and may be formed as a continuous layer of film Df which is superimposed on the input electrode E1 of the channel intensifier device as shown. Such membrane layer Df may be in contact with E1 as shown or it may be insulated from the input electrode E1 so as to permit it to be held at a potential different from that of said electrode (see insulating layer Id in FIG. 3.)

Alternatively the membranes D may be formed as extensions of the input electrode E1 of the device so that said electrode and membranes form, together, a continuous layer as shown in FIG. 4.

For image intensifier purposes, an arrangement of a channel plate I with a screen S in accordance with FIG. 2, FIG. 3 or FIG. 4 can be preceded by an electron-optical diode image-inverting stage as indicated in FIG. 5, such stage having, say, several centimeters of axial depth so that a high voltage of, say 5 kv. can be applied (by a source Bo) without risk between its photocathode P and its conical anode A. The latter can, if desired, have a potential different from that of electrode E1, or anode A may be connected to electrode E1 as shown so that the first stage becomes a simple electron-optical diode.

Since 5 kv. can safely be applied, the electrons from the photocathode penetrate easily through membranes D and the latter stop light feedback and also allow the voltage between the channel plate and screen to be reduced with an accompanying reduction of the spacing between channel plate and screen. In spite of this reduced spacing, the screen is still just as bright as, or brighter than, aluminized screen at 5 kv. For example, the E2-S spacing d (FIG. 2) can be made 0.5 mm. and the voltage from B2 can be 1.5 kv. (i.e. 3 kv./mm.). Such a field strength allows the resolution to be slightly better than with 5 v. at 1 mm. and there is virtually no danger of field emission (which was made worse in any case by the presence of the aluminum film in the arrangement of FIG. 1).

In addition, the membranes can stop ion feedback, a function which, of course, could not be performed by the conventional metal backing E3 of FIG. 1.

In addition to the reasons given earlier for the combination of a channel intensifier stage with a preceding electron-optical diode or like stage, there are other reasons which are explained in Pat. Nos. 3,487,258 and 3,491,233 copending application Ser. No. 706,755, filed Feb. 20, 1968 and the features of matrix curvature and tilted channels described in the latter two specifications can be applied to devices and tubes employing the present invention. However, from the point of view of simplifying the formation of the membranes, it is preferable to use flat channel plates for the present invention. As for the tilted channel feature the present invention provides in itself an alternative solution to the "dark spot" problem so that the use of tilted channels is not necessary to avoid the dark spot effect. This is because many of the electrons which pass through the membranes are scattered thereby and issue in random directions and, similarly, secondary electrons enter the channels with relatively low energy and random directions.

Although the invention has particular advantages in relation to tubes in which the channel plate is remote from the photocathode (e.g. the tube of FIG. 5), it can also be applied usefully to some tubes of the "proximity" type, i.e. tubes in which the photocathode is placed near to the channel intensifier device without intermediate electron-optical focusing means. This can arise in cases where the resolution required is sufficiently low to allow the channel plate to be spaced from the photocathode by a distance of a few millimeters thus allowing voltages of several kvs. to be applied between them. A practical example of this is an X-ray image intensifier in which this spacing is 5 mm. and the applied voltage is 4 kv.

The membranes D can be made as separate elements with their edges in electrical contact with the input electrode, but the use of a continuous layer or film Df permits easier manufacture, which can be carried out as follows.

First, a lacquer film is formed by methods well known in the art e.g. flotation water. SAid film is then placed over the input face of a matrix which already has a separate input electrode as in FIGS. 2 or 3. Then aluminum is evaporated on to the film. Finally, the channel plate is baked (in known manner) so as to burn off the lacquer and leave an aluminum film in contact with the electrode E1 (FIG. 2) or a prepared insulating layer formed in E1 (as layer 1d of FIG. 3).

As an alternative applicable to FIG. 4, the film is formed on a substrate (e.g. glass) from which it can be later released in known manner. Aluminum is then evaporated on to the film. The film is then released from the substrate and placed on the channel plate matrix with the aluminum side in contact with the matrix. The matrix is then baked to burn off the lacquer.

If the matrix is of glass and the necessary low degree of conductivity has been, or is to be, obtained by reduction of a metal (e.g. lead) compound in the glass, then the above process requires special measures since the baking is done in air or oxygen. In particular, the reduction can be repeated a second time after the baking, or it can be postponed until after the baking.

As a practical example given by way of illustration a channel plate according to FIG. 2 may have substantially the following dimensions and values: --------------------------------------------------------------------------- TABLE

Channel diameter = 15.mu. Channel pitch (distance between channel centers) = 20.mu. Resistivity of the plate (measured between E1 and E2) = 10.sup.8 ohms/cm..sup.2 Electrodes E1-E2 = Nichrome evaporated at a grazing angle. Film Df = Aluminum 500-1000 angstroms thick __________________________________________________________________________

Claims

I claim:

1. An electronic image intensifier device including a photocathode, a fluorescent screen and a secondary emission multiplier between and spaced from the photocathode and screen said multiplier comprising a body of insulating material having a plurality of parallel, closely adjacent narrow passageways therein opening out on opposite faces thereof the surfaces of said passageways being of resistive, secondary-emissive material, a conductive layer on the face of said body facing said screen constituting an output electrode and a conductive layer on the face remote from said screen constituting an input electrode, an insulating layer on said input electrode, and a continuous electrically-conductive electron-permeable membrane covering said insulating layer and obturating the entrance to each passageway.

2. A multiplier tube as claimed in claim 1 wherein the screen is devoid of metal backing.

3. A device as claimed in claim 1 wherein the membrane is aluminum.

4. A device as claimed in claim 3 wherein the membrane has a thickness between 500 and 1000 angstroms.

5. A device as claimed in claim 1 in which the screen is adapted to emit light of wavelengths to which the membrane is substantially opaque.