United States Patent: 3553458

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( 9973 of 9973 )

United States Patent	3,553,458
Schagen	January 5, 1971

ELECTRICAL NEGATIVE-GLOW DISCHARGE DISPLAY DEVICES

Abstract

An electrical display device employing a two-dimensional array of glow discharge cells formed by an insulating plate having a plurality of apertures therein defining a plurality of hollow cathodes. Each of these hollow cathodes is formed by a cathode pin on which a layer of emissive material is provided in each of the apertures spaced from one surface of the plate. The walls of each aperture are also covered with material sputtered thereon and thus forming a cathode recess, or hollow cathode in the plate. An anode common to all the cathodes and spaced from the edge of each recess to prevent cathode-anode tracking of sputtered cathode material is provided. The cathode recesses are viewable through this anode which may be in the form of a grid or transparent plate spaced from the cathode recesses. Each of the cathodes is connected to a common negative electrode through an element of photoconductive material and allows passage of input radiation to each of those elements to provide controlled feedback to each of the cells.

Inventors:	Schagen; Pieter (Surrey, EN)
Assignee:	U.S. Philips Corporation (New York, NY)
Appl. No.:	04/699,267
Filed:	January 19, 1968

Foreign Application Priority Data


Oct 26, 1967 [GB]			2,702/67

Current U.S. Class: 250/214LS ; 315/169.4; 345/204

Current International Class: H01J 17/49 (20060101); H01l 017/00 ()

Field of Search: 250/213 315/169

References Cited [Referenced By]

U.S. Patent Documents


2837661	June 1958	Orthuber et al.
2972803	February 1961	Koury et al.
3334269	August 1967	L'Heureax

Primary Examiner: Borchelt; Archie R.
Assistant Examiner: Abramson; Martin

Claims

I claim:

1. An electrical display panel device comprising a two-dimensional array of glow-discharge cells, said array further comprising an insulating base plate having a plurality of apertures therein, a conductive pin having an emissive surface within each of said apertures, the emissive surface of said pin being spaced from one surface of said plate and defining therewith a recess in the plate the walls of which are covered with said emissive material and constituting a cathode cell of said array, a common anode for all the cells of the array spaced from the edges of each of the cells and through which the cells can be viewed, an inert gas filling said recesses at a pressure at which with a given potential between the anode and at least one of the cathodes there is a negative glow discharge which fills the recess and a common negative supply electrode for all the cells which electrode is connected to each cathode through an element of photoconductive material and allows passage of input radiation to each of said elements.

2. A display device as claimed in claim 1 in which the negative electrode is a single continuous photoconductive layer arranged to be common to all the cells.

3. A display device as claimed in claim 1 in which the negative electrode comprises a separate photoconductive element for each cell.

4. A display device as claimed in claim 1 in which the negative electrode is positioned so that the photoconductive action is mainly in the direction of thickness of the display device.

5. A display device as claimed in claim 3 in which the elements are positioned so that the photoconductive action of each element is mainly in directions substantially parallel to the image of the display device.

6. A display device as claimed in claim 1 wherein the gas filling is contained between an output window plate and the plate in which the cathode recesses are formed.

7. A display device as claimed in claim 6 wherein the plate between the cathode recesses is opaque.

8. A display device as claimed in claim 6 wherein the plate between the cathode recesses is translucent.

9. A display device as claimed in claim 1 including an electrical resistance in series with each of the cells.

Description

This invention relates to electrical discharge display devices. The invention relates to some extent to relatively simple devices for displaying simple patterns such as diagrams, numerals, words and the like which do not require half-tone capabilities. The invention also relates to devices for displaying more complex images such as radar displays which, however, may also not require half-tones. However, the invention relates principally to devices capable of producing television-type or X-ray type images containing a range of half-tones and, for this reason, the following description is given mainly in terms of devices of this latter type.

During the past few years an increasing amount of effort has been devoted to "flat" display systems of one kind or another many of them being of the solid-state type. Probably the most serious problem with the all solid-state approach to this kind of display is the picture brightness which can be achieved. This problem has two aspects:

A. A sequential address system makes it virtually impossible to obtain a satisfactory brightness from individual picture points unless some storage mechanism is employed which enables each point to emit light, even when the exciting signal is no longer present. Preferably emission should continue until the next "scan" of the element occurs. This makes the associated circuitry and the panel construction very complicated.

B. If the light-emitting phenomenon employed is that of electroluminescence, the present state of the art in this field does not permit a brightness in excess of a few tens of ft.-l. even for constant emission. This is not sufficient for daylight viewing purposes as required in the majority of the possible applications.

For this reason it has been proposed to employ a two-dimensional array of cold-cathode glow-discharge cells as the light sources for the individual picture elements. An early example of such an arrangement is described in U.S. Pat. No. 2,991,394, issued July 4, 1961 and illustrated in FIGS. 10--11 thereof.

A more recent example is the Lear Siegler device reported in an article in Electronic News of July 26, 1965. In this proposal the individual cells consist of positive-column discharge spaces arranged in the form of a two-dimensional matrix of small apertures in a plate of insulating material placed between two electrode systems of parallel wires or semitransparent conductive strips in a "crossbar" arrangement. Any element can be made to emit light by passing a gas discharge through a hole in the insulating matrix at the crossover point of the corresponding cathode strip and anode strip.

Although with this particular arrangement it is possible to obtain a high picture brightness, this cannot be realized in practice unless it is used in a bistable on-off mode, because each element can only be excited to give its correct light output at the instant during which it is scanned, or at most during a full line period. The latter can (although not described in the aforesaid article) be achieved if the incoming picture signal for each line is written into a line storage element during one line scan time, and displayed simultaneously as a complete line during the entire scan time of the picture signal for the next line, which is then being written into a second line store.

Another difficulty with gas discharge devices of this Lear type is the sputtering which takes place from the cathode, due to the bombardment with positive ions. This can severely limit the life of the device and can occur in various ways, for example:

a. Since the cathode strips are thin, it is possible for a substantial part of the cathode material of a cell to be removed from its strip thereby affecting its performance. Serious differences in performance may thus arise between one cell and another due to random removal of cathode material.

b. The sputtered material from a cathode may in time set up partially conductive tracks between adjoining cathodes thus allowing the glow discharges to spread and severely disturbing the operation of adjacent cells.

c. The transparent anode strips may be rendered opaque by deposition of sputtered material thus impeding viewing of the display through the anode plate.

d. If the anode and cathode plates are in direct physical contact with each side of the matrix, the sputtering may in time set up partially conductive tracks from cathodes to anodes thus severely disturbing the operation of individual cells.

Similar sputtering problems occur also in individual cells, and recent work on single and multicell devices by Dr. R. F. Hall has indicated that at least some of these problems can be successfully overcome by the use of an array of recessed cathode glow-discharge cells wherein each cathode is recessed in a slab of insulating material. As described in U.S. Pat. No. 3,465,194 issued Sept. 2, 1969 one such example is shown in FIGS. 1 and 2 of the accompanying drawings, FIG. 1 showing a corner of the cathode array separate from the envelope or cover (containing a semitransparent anode layer) and FIG. 2 showing the two parts assembled together.

It has been found that the cathode recesses can effectively restrict the sputtering action even after the walls of the recesses have been rendered conductive by sputtered cathode material.

With such an arrangement a very high brightness can be obtained, more than adequate for normal display purposes. In addition, it has been shown that the arrangement can reduce the sputtering to such an extent that lifetimes in excess of several thousands of hours are possible without any apparent deterioration in the characteristics of the discharge.

It has also been shown that the characteristic of each individual recessed cathode is in practice substantially independent of its neighbors, is substantially linearly related to the current fed into it, and allows a contrast ratio of at least a few hundred to one. Thus an arrangement of the type shown in FIG. 2 can provide sufficient brightness if many or all of the cells are activated simultaneously to produce an image, but with a scanned or dot-sequential mode of operation the brightness problem remains.

It is an object of the invention to provide an improved recessed-cathode glow-discharge display panel suitable for use by itself as an image intensifier and suitable also for use in combination with driving means adapted to activate individual cells or groups of cells in a predetermined sequence.

The invention provides an electrical display panel device comprising a two-dimensional array of glow-discharge cells, an individual recessed cathode (as herein defined) for each cell, an element of photoconductive material coupled externally to each of said cathodes so as to be in series therewith, a common anode for all the cells through which anode an image composed of glow discharges can be viewed, and a common negative supply electrode for all the cells which electrode is connected to all the cathodes through said elements of photoconductive material and allows passage of input radiation to each of said elements.

The common anode may be a continuous layer which is transparent (to any required degree) to the output radiation provided by the glow discharges of the cells. Alternatively, the anode may be formed as a mesh of opaque conductors between which the glow discharges can be viewed.

In a similar manner, the common negative supply electrode may be a continuous layer which is substantially transparent to the type of radiation which is intended to be used as the input to the panel. Thus, in the particular case of an X-ray image intensifier application, the layer may for example be opaque to visible radiation though semitransparent to the type of X-rays for which the device is designed. As an alternative to a continuous layer, the common negative electrode may be formed as a mesh of opaque conductors between which the input radiation can be admitted to the elements of photoconductive material.

In a somewhat analogous way the device may employ photoconductive elements formed as a single continuous layer arranged to be common to all the cells in such manner that the photoconductive action is in the direction of thickness of said layer.

Alternatively, the device may employ a separate photoconductive element for each cell. In this case the device may be so arranged that the photoconductive action of each element is substantially in directions parallel to the image of the display device.

In any event, input radiation will reach the photoconductive elements so as to actuate them (either simultaneously or sequentially) with differing intensities so as to set up an array of glow discharges providing the desired display. In this process each element of photoconductive material can perform two functions, namely (a) to pass to its cell a discharge current related to the intensity of the input radiation at the point, and (b) to act (in the sequential case) as a storage element so as to increase the duration of the glow discharge and hence the brightness of the display.

For the purposes of this specification a recessed cathode is, broadly, one which provides an exposed emissive surface surrounded by a wall which may be of insulating material or of conductive material. In the latter case the said material may be provided by construction or by sputtering and may or may not be in electrical contact with the emissive surface while being insulated from the walls of neighboring recesses. In the case of cathode-glow operation, the recess (and other parameters of the device) can be designed in known manner so that the glow is contained within it, and this can be done regardless of whether the surrounding wall is conductive or not. Hence the volume of the cathode glow can be held substantially constant in spite of progressive sputtering of the walls of the recess.

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

FIGS. 1 and 2 show a prior art display device;

FIGS. 3 and 4 are elevational views of two embodiments of a display device according to the invention;

FIGS. 5A and 5B are an elevational and plan view respectively of another embodiment; and

FIGS. 6A to 6M show various other embodiments of a device according to the invention.

As a simple preliminary example, the arrangement may be similar to that of FIGS. 1--2 with a layer P of sintered CdSe or other photoconductor applied to the outer side of the glass slab G and metal cathode plugs Kp as shown in FIG. 3 of the accompanying drawings, and a continuous electrode E being applied to the exposed side of the photoconductive layer P.

The gas space may be subdivided into individual cell spaces, but this is not desirable in itself, particularly when using glow discharges which are adequately located by, and contained in, individual recesses. Notional (i.e. nonairtight) subdivision of the gas space into individual cell spaces by a cellular matrix of insulating material may be convenient in that it permits a relatively cheap and simple "sandwich" construction, but even then it is desirable to provide a gap to separate the edge or rim of each recess from the anode so as to meet the aforesaid problem of sputtered conductive tracks from the cathodes to the anode. These two requirements can both be met by appropriate "sandwich" arrangements as will be explained.

In addition to cathode-glow operation, a device according to the invention can be designed to provide also a positive column glow as will also be explained. One arrangement (which will be described) employs an anode recess corresponding to each cathode recess, the anode recess being deeper than the cathode recess containing the positive column part of the discharge.

The material between the cathode recesses may advantageously be opaque for the following two reasons. The first reason is to reduce reflection of light from the display which can thus retain a better contrast under high ambient illumination. Secondly, it can prevent or reduce the feedback of light from the discharges to the photoconductor (this can also be achieved by employing a separate opaque layer between the photoconductor and the plugged slab). However, it may be desirable for the plate to be only partially opaque so as to allow a controlled amount of optical feedback as will be explained.

The energy efficiency of the discharge can be in the order of about 1 lumen per watt for a neon-argon mixture, but may be raised still further by using a gas with whiter light, or alternatively by employing a wavelength conversion from ultraviolet to visible light via an efficient phosphor, analogous to the method employed in fluorescent tubular lamps. The phosphor may be laid on the anode or on the walls containing the cathode glow or (in the case of positive-column glow) both glow zones.

The electrical characteristic of the photoconductor gas-discharge combination may be further improved by adding a certain amount of series resistance. This can be realized in the form of an additional resistive layer, either at the cathode or at the anode side of the sandwich panel. At the cathode side it may be combined with the opaque layer mentioned above by using a material with both the desired resistivity and absorption.

As aforesaid, a controlled amount of feedback from the gas discharge to the photoconductor may be beneficial in certain applications, since it can increase the light gain of the device as well as the afterglow. This may be built into the panel as a permanent amount of light feedback through the plugged slab or the additional "opaque" layer.

Alternatively, it is possible to vary the amount of feedback in the device during operation as required by adjusting the absorption of a suitable material used for the plugged slab or the opaque layer. This may, for instance, be done by flooding the device for a short time with radiation of a suitable wavelength which adjusts the number of absorption centers in the material for the light of the gas discharge.

A device as described above is essentially a flat image converter-intensifier, with the current through each individual gas discharge cell (and therefore its light output) determined by the corresponding photoconductive element. With a photoconductor of the sintered CdSe type and a gas discharge with only 1 lumen/watt efficiency, a lumen gain to visible light of more than 10.sup.3 may be readily obtainable.

A panel of this kind can not only be employed as the "flat" output screen of a scanned display, but can also be used as an image converter panel in its own right. Accordingly more specific embodiments of the invention will now be described by way of example with reference to FIGS. 4 and 5 of the accompanying drawings.

1. FLAT X-RAY IMAGE CONVERTER PANELS

By using a photoconductor layer which absorbs a sufficiently large proportion of the incident X-ray radiation it is possible to construct an X-ray converter panel such as the arrangement shown in FIG. 4 and such a panel may, for instance, be built up in the following way.

A sheet of aluminum provides (with a glass plate WI) the "entrance window" to X-rays and acts at the same time as the negative electrode E. On top of this (and possibly separated from it by a thin film of a suitable different metal to give good electrical contact) is applied a layer of, for instance, 0.5 mm. of sintered CdSe. This is covered with a thin sheet of sintered black glass paste, in which (before the sintering) a large number of holes have been punched. These holes are partly filled up with a suitable metal, or combination of metals K-Kc to an appropriate depth to provide a good electrical contact with the photoconductor layer underneath and a good cathode for the gas discharge above. This can, for instance, be done by electrolytic deposition, using the combined aluminium-photoconductive layer as one electrode. On top of the glassy sheet can be placed the cover glass carrying the anode in the form of a transparent film or (as shown) as a fine wire mesh or grid Am. The appropriate gas filling having been provided, the assembly can then be sealed around the edges to provide the complete panel.

In the form shown in FIG. 4, the anode elements Am are a single set of parallel wires in contact with both the envelope or cover plate Wo and a glass base G. In principle such wires could be replaced by a grid in the form of a perforated metal plate arranged to isolate separate gas chambers for individual picture elements. However, this is not found to be necessary and the wire arrangement of FIG. 4 is convenient in that it permits evacuation and gas filling from the ends of the elongated spaces defined by pairs of wires.

Other methods of construction are, of course, possible and could incorporate wires coated with black glaze and fused together under pressure and high temperature so as to lead to an array of metal plugs in a glass plate similar to the one illustrated in FIG. 2. Such a slab may again have the wires etched back on one side to provide the hollow cathodes, and be covered at its outer side with the photoconductor as in FIG. 3.

2. FLAT IMAGE CONVERTER PANELS

If light is used as the incident radiation, it may be necessary to employ surface photoconductivity, instead of conduction through the thickness of the photoconductive layer. This depends on the absorption coefficient of the material and the diffusion length of the carriers created.

A different kind of construction is that of FIGS. 5A--5B where the negative electrode consists of a mesh Em on the photoconductor, and the metal plugs Kp penetrate into the photoconductive layer P where they are located (FIG. 5B) in the centers of the holes in the cathode mesh. Alternatively the "rigid" construction of the well-known photo conductor electroluminescent display panel could be employed.

As a further alternative, however, the photoconductive material will, where possible, be applied in the same way as in the X-ray panel in the form of a thin continuous layer, and can be chosen for its sensitivity to match the radiation of the application. This can be either ultraviolet, visible, near infrared or even far infrared radiation. The main requirement is that the dark current be sufficiently low compared with the photocurrent to allow a reasonable picture contrast.

3. FLAT DISPLAY PANELS FOR TELEVISION TYPE PICTURES

Possibly the most important application for a panel of the type described in the previous section is for the display of sequential information. In that case it may be activated by a flying light spot modulated with the incoming video signal, the decay time of the photoconductor providing the required storage action in the actual display. Alternatively a line sequential activation can be used.

The following arrangements may be adopted:

a. Activation by a small cathode ray tube

Due to the light intensification of the panel, it is possible to use a very small and simple C.R.T. with a low output and couple this optically via a cheap and low-aperture lens to the panel. The big advantage of the display is that it can now provide a brighter picture than the direct viewing C.R.T. and in addition that it can have a much better performance in high ambient illumination, due to the black background of the glass plate with holes.

b. ACTIVATION BY A SMALL SCANNING LIGHT BEAM

Again, as a result of the light intensification capability of the panel, it is possible to scan the photoconductor with a low-output laser, either of the neon-helium type, or possibly even a solid-state laser, such as GaAs.

c. Activation by a matrix address panel

As mentioned at the beginning, solid-state crossbar matrix displays are in development where the brightness is not sufficiently high. This need not however, be a serious problem if such a panel is used only as the activating element for a gas discharge intensifying panel according to the invention. The two panels can in that case be put in direct contact with one another with the crossbar system providing the low-intensity scanned picture which is then intensified and made continuous by the second panel.

An additional advantage of this approach is that the spectral response of the photoconductor is not necessarily the same as that of the eye. CdSe, for instance seems sufficiently sensitive at about 9000 A. to allow excitation by a matrix of GaAs cells consisting of P-type strips at right angles to n-type strips. Again, it is possible for the activating panel to employ circuitry which allows line storage, as described before, in order to increase the input brightness to the final display panel.

The activation panel may alternatively consist of a crossbar type of panel based on electroluminescent phosphors, either in the dot-sequential mode of operation or with line storage.

As a further alternative the activating panel may be a cross bar device of the kind described in copending application Ser. No. 699,269, filed Jan. 19, 1968.

A variety of individual cell structures will now be described in greater detail with reference to FIGS. 6A to 6M (referred to simply as A to M). In these FIGS. it has been found convenient to separate the glow-discharge stage of the cell from its photoconductive stage since to a large extent the design of the glow-discharge state is independent from that of the photoconductive stage and many combinations are possible.

FIGS. A to G show various glow discharge arrangements and the schematic drawings terminate at a level corresponding to the cathode emissive surfaces K.

In each of these FIGS. the cathode surface K is located in a recess in a cathode plate G (which may be of glass and may be opaque or partially opaque for the reasons stated above). The sidewalls of the recess may or may not be covered by sputtered cathode material or rendered deliberately conductive in some way. In each case the common anode is shown at A as applied to a transparent output window WO. The schematic representation of the anode is intended to include both the case where the anode is a continuous layer which is sufficiently transparent for viewing of the glow discharge image, and also the case in which it is opaque but has regular apertures corresponding to the individual cells or a sufficient number of small apertures which are in a random pattern or in a regular pattern unrelated to the cathode spacing.

FIG. A corresponds to FIG. 3 and does not require further comment as to its operation. The gap between the edge or rim of a recess and the anode is shown at g (as is also done in FIGS. B to D).

In order to permit the arrangement of FIG. A to be assembled as a sandwich construction, it is possible to place the output window WO, during assembly, directly on the cathode plate G, relying on inaccuracies or roughness of the two adjacent surfaces to allow free movement of gas between individual cells and also to prevent sputtering of the cathode material from setting up conductive paths between the cathodes and the anode. Alternatively it is possible to increase the roughness of one or both of the mating surfaces artificially (for example by sandblasting or etching) or to add to either the window plate WO or the cathode plate G a regular pattern of ridges or lugs to act as spacers between the two plates. If such regular spacers are applied to the window WO this may introduce a requirement for registration between the spacers and the cathode recesses. This need for registration can be avoided by forming the regular spacers on the cathode plate instead, e.g. as shown at S in FIG. B. This last alternative is also preferable in that it permits the common anode to be formed as a continuous layer on the plate WO.

FIG. C shows a further alternative arrangement in which the anode of each cell is no longer formed as a surface on the output window opposite the cathode recess. Instead the anode is provided as a metal grid or mesh AM e.g. in the manner described with reference to FIG. 4 so that the anode surfaces of each cell surround or border on the cathode recess instead of being located opposite the recess. Such a grid or mesh is preferably a grid in which each mesh surrounds one cathode recess without, however, forming gastight seals. This calls for a high degree of registration between the meshes of the grid and the cathode recesses (although it has the advantage of providing a sandwich type construction). The need for registration can be obviated by replacing the grid Am by a network of ridges being rendered conductive so as to provide the anode surfaces. An example of this is shown in FIG. D where the conductive anode surfaces are indicated at Ad.

FIG. E shows an alternative construction which is designed to produce a positive column glow in addition to a cathode glow. This is done by making the cathode recess much deeper so as to cause production of a positive column glow in addition to the cathode glow. If there is a single continuous recess as shown, there is some need of a gap (as shown) between the anode A and the output window and the cathode plate GC as in FIG. A, or alternatively a sufficient degree of irregular roughness at one or both of the mating surfaces or a regular anode or spacer pattern as described with reference to FIG. B to D (such a pattern is indicated generically at AS).

As is well known in the art, the provision of a discharge space having this type of geometry, combined with known methods of determining the gas pressure, can give rise to emission of light from a positive column in the region between the cathode glow and the anode.

To avoid the possibility of sputtering gradually affecting the length of the positive column in different degrees for different cells, FIG. F shows an alternative construction in which the deeper discharge space for the positive column glow is provided opposite each cathode recess within a separate matrix structure C, spaced from the cathode plate G by a gap.

In this arrangement the gap or spacers of FIGS. A to D are no longer needed at the anode. As for the gap between cathode plate G and matrix C, it may contain regular or irregular spacer elements formed on either surface as indicated at S.

The arrangement of FIG. F has an anode layer A which may be a continuous transparent layer or a pattern of opaque conductive material as explained with reference to FIG. A. If it is a continuous layer, then the need for registration between the anode and the matrix C is avoided although such registration is still required in this case between the matrix and the cathode recesses.

An alternative arrangement for the anode of FIG. E or FIG. F, which still avoids the need for anode registration, is shown in FIG. G in which a conductive layer is applied to the whole of the output face of the matrix C and extends inwardly to a predetermined extent into each of the discharge spaces. In this case the conductive anode layer A can be opaque and it can be obtained by an evaporation process in which the evaporation is directed at a suitable angle to the face of the matrix so as to ensure the desired depth of penetration into the discharge spaces. A number of possible variants of the photoconductor stage will now be described with reference to FIGS. H to M. Any of the arrangements of FIGS. H to M can be combined with any of the discharge stage arrangements of FIGS. A to G.

FIG. H shows an arrangement in which all the cathode surfaces K are in contact with a continuous layer P of photoconductive material. The contact between each cathode surface and said layer may either be direct or it may be indirectly obtained through a separate coating or plug of "contact" metal such as the layer Kc shown in FIG. 4.

An input window WI is shown adjacent to the layer P but said window is not essential to the structure which may, in certain applications be preceded by an imaging stage of some kind. On the layer P there is shown a negative supply electrode E which may either be semitransparent or may be formed as a pattern of opaque conductive material having apertures such as to pass input radiation Ri from an object to the layer P. Such apertures may be in regular pattern in registration with the cathode surfaces K although random patterns can be used if sufficiently fine.

The simple arrangement of FIG. H corresponds effectively to the arrangements used in FIGS. 3 and 4. In each case the photoconductive action occurs in the form of photocurrents substantially in the direction of the thickness of the layer P as indicated by the arrow X.

A variant of the arrangement of FIG. H is shown in FIG. I where the only change is the subdivision of the photoconductive material into separate plugs each of which is in series with one of the discharge cells. In this case the photoconduction still occurs in the direction of the arrow X.

The arrangement of FIG. J corresponds essentially to that used in FIG. 5. Here the photoconductive material is subdivided into separate islands each of which surrounds and is in contact with a cathode plug Kp . Each photoconductive island is surrounded by conductive material forming a negative supply electrode Em of grid or mesh form. Although this grid is shown as having the same thickness as the photoconductive islands so that it effects the subdivision of the photoconductive material, this is not essential and the grid may in fact be either exposed or, conversely, buried inside the photoconductive material. In particular the problem of registration between this grid and the cathode plugs can be overcome by providing the mesh on the cathode itself. For example, it may be produced as a printed pattern of metal on the cathode plate as indicated in FIG. K (or recessed therein) or it may be provided as a layer on a gridlike network of ridges formed on the cathode plate as shown in FIG. L.

It is not necessary for the cathode plugs Kp to be exposed and, indeed, they may be made flush with the cathode plate G. If the negative supply electrode E.sup.m is also flush with the plate G, then the photoconductive material P may be formed as a continuous layer having constant thickness as shown in FIG. M.

In any event, the photoconductive action of the arrangements of FIGS. J to L is in the form of photocurrents which flow mainly in directions substantially parallel to the plane of the device as indicated by the arrows X.

A set of practical dimensions and values is given below by way of illustration: ##SPC1##

Reverting to the question of providing electrical resistance in series with the cells, a continuous resistive layer may be used, as aforesaid, and may be located between the photoconductor and the cathodes or (if used) cathode plugs. Such layer may also provide a degree of opacity or translucence which may be required for optical purposes as explained more fully below. As an alternative to a continuous layer, individual cathode plugs (such as the plugs Kp of FIGS. 6J to 6M) may be made of resistive material to act as individual resistors.

Reverting now in greater detail to the questions of optical feedback and contrast in high ambient illumination, various specific cases can arise, some of which concern the further possibility of displaying additional information or an additional image by back-projection (the latter term is used to denote systems in which the glow-discharge image has superimposed on it a second image which is in light of a wavelength which substantially does not effect the photoconductor and thus leaves the discharge pattern undisturbed). The following are relevant cases:

I. The material between the cathode recesses is nonreflective and opaque for both glow light and ambient light (this prevents optical feedback and improves contrast by reducing reflection of ambient light).

II. Said material is translucent but not for glow light (this prevents feedback but permits back-projection in a color differing from that of the glow discharges).

III. Said material is translucent to glow light and the photoconductor is sensitive to glow light (this permits a controlled degree of feedback dependent on the degree of translucence.)

IV. Said material is translucent for glow light but the photoconductor is not sensitive thereto (thus feedback will not occur).

V. The translucence is arranged so as to pass light of a third wavelength different from both that of the glow light and that of the light of the intended input image radiation, the photoconductor being insensitive to light of said third wavelength (this permits back-projection in a color different from that of the glow light).

In the above description the term "light" is used to cover also invisible types of light, for example ultraviolet and infrared.

* * * * *

Current U.S. Class:	250/214LS ; 315/169.4; 345/204
Current International Class:	H01J 17/49 (20060101); H01l 017/00 ()
Field of Search:	250/213 315/169