GB2081966A - Image intensifier tubes - Google Patents

Abstract

Battlefield image intensifier tubes are protected against damage caused by the relatively enormous heat and electron energy developed if the intensifier should by accident be pointed at a bright light source (such as a shell flash) by a slotted annular resistive element 20 placed around but touching the photo-emissive cathode layer 12 (preferably) to act as a high resistance between the cathode and the cathode-window mounting flange (and hence the H.T. power source). This causes both a temporary reduction in down-tube potential and a pinching- out effect of the electron flow from any point-like source on the photocathode when the tube is subjected to highlight overload conditions. <IMAGE>

Description

SPECIFICATION Image intensifier devices This invention relates to image intensifier devices, and concerns more particularly image intensifier tubes protected against the deleterious effects of flashes of bright light.

Image intensifiers are used to provide a bright, easily visible image of very faint light sources, and specifically of objects illuminated by such faint sources. On the battlefield, for example, present-day image intensifiers can provide a "daylight" image of an object for which the sole source of illumination is starlight.One particular type of intensifier is essentially a vacuum tube sealed at one end by an entrance window internally coated with a photoemissive material (one that emits electrons when illuminated) and sealed at the other end with an exit window internally coated with a phosphor material (one which emits light when struck by electrons); a very large voltage---or high tension (HT+is put across the tube between the entrance window photoemissive layer (the cathode) and the exit window phosphor layer (the anode) so that in operation electrons emitted by the photoemissive layer cathode upon illumination are attracted (suitably focussed) to the phosphor layer anode where they cause light to be emitted, and by appropriately adjusting the various parameters the light emitted by the device is much brighter than that faint light which illuminated it. If the light source is very faint, then several intensifiers may be joined together in a cascade, the output of one being the input of the next, untill the final output is bright enough.

One embodiment of an image intensifier of this general type is shown, in part exploded sectional view, in Fig. 1 of the accompanying drawings. The intensifier tube shown is of the single stage variety. It has a transparent input window (1), of the fibre optic type, sealed by means of a glass frit seal (2) to a cathode input window mounting flange (3). The mounting flange 3 is carried by a cathode body housing (4), to which is electrically connected a getter shield (5). A ceramic body insulator (6) separates the cathode body housing 4 from an anode body housing (7) which supports an anode focusing cone electrode (8). Mounted on an anode output window mounting flange (9) is a transparent output window (10) (also of the fibre optic type), which is sealed to the mounting flange 9 by another glass frit seal (11).

At the input end of the tube, carried by the input window 1, is a photoemissive cathode cathode layer (12) having a peripheral photocathode metal contact layer (1 3) which makes electrical contact with the mounting flange 3, while at the output end of the device, carried by the output window 10, is a luminescent (phosphor) screen (14) having an aluminium backing layer (15) electrically united with the mounting flange 9. In use, an operating potential difference is created between the cathode and anode housings 4 and 7 by means of a d.c. source (represented at 16).

Unfortunately, this type of intensifier suffers from a serious drawback; because it is so sensitive, the effect on it of being illuminated with a bright light-say, the flash of an exploding shellcan be disastrous. Indeed, the resulting flow of electrons within the tube can cause the delicate photo-electron sensitive layers to be destroyed by the heat that is generated by the device as a result of the shell flash, and which the device has to dissipate.

In the Complete Specifications of our Applications for Letters Patent Nos. 23,753/78 (Serial No ; 1/6190/V and 1/6147/V) and 23,754/78 (Serial No ; 1/6191/V) we have described and claimed various ways of protecting an image intensifier tube against flash. In Application No. 23,753/78 we have described and claimed an image intensifier (of at least one stage) which has its luminescent screen or its photocathode decoupled with respect to the internal capacitances of the HT power supply source, and we have stated that conveniently it is the luminescent screen which is decoupled, the decoupling being resistive decoupling effected by a resistor connected between the luminescent screen and the output point of the HT power supply, the value of the resistor being such as to provide a time constant with the capacitance at that power supply output point which is many times greater than the duration of a typical high energy flash likely to occur in operation (though the value of the resistor should not be so large as to cause serious loss of output potential with the order of photo currents used during normal operation).We have gone on to say that the value of the resistor will typically lie in the range of 100 MQ to 1 G, and that, where the image intensifier is a multistaged device, the value of a decoupling resistor utilised towards the input end of the intensifier will be larger than the value of a decoupling resistor utilised nearer the output end.

The present invention concerns a modification of the invention described and claimed in our aforementioned Application No: 23,753/78.

In one aspect, therefore, this invention provides an image intensitier of the kind described wherein at least one of its two photolayers (the luminescent screen and the photocathode) is resistively decoupled with respect to the H.T. power supply connected (in operation) across the anode and cathode body housings, the resistive decoupling being effected by a resistor connected between the photo-layer and the appropriate electrode body housing, this resistor comprising a slotted annular layer of resistive material posi tioned around the periphery of the photo-layer and separating it from its electrode mounting flange, the slots in the annular resistive layer extending in a radial sense and being filled with a less resistive material contiguous with and electrically connecting the photo-layer and its electrode mounting flange.

The flash protection effect of the intensifier of the invention works in a manner somewhat to that of the intensifiers of our aforementioned Application No: 23,753/78. Thus, the primary effect of the resistor between photolayer and electrode body housing is to absorb part of the energy stored in the inter-electrode and stray capacitances of the intensifier. However, there is also a secondary effect, which is to cut-off the electron flow by localised distortion of the electrostatic field within the intensifier.

As regards the first effect, the resistive nature of the electrical connection between the photo-layer and its electrode housing imposes a significant limit on the rate at which electric charge can flow between the two (and hence between the photo-layer and H.T.

power source). Though under very low light levels (and thus very low through-tube electron flow currents) the power source maintains at a maximum the potential drop from cathode to anode, as soon as the light level increases dramatically the initial electron flow in the tube immediately exceeds the value that can discharge through the photo-layer/ electrode housing connection with the power source, and so immediately the potential drop along the tube practically disappears, leaving the following electrons with no, or effectively no, accelerating force to drive them from cathode to anode. Accordingly, the electron beam energy drops dramatically as the pertinent capacitance progressively discharges even though the photoemissive layer may still be emitting vast quantities of electrons, and so the danger of damaging the phosphor layer is removed.

To put it at its simplest, if the resistor is between photocathode and cathode housing then the cathode becomes very much less negative with respect to the anode (as electrons leave it but are not replaced), while if the resistor is between phosphor layer anode and anode housing then the anode becomes very much less positive with respect to the cathode (as electrons reach it but cannot leak away); in either case the potential drop down the tube is significantly reduced, and succeeding electrons have, literally, nowhere to go! As regards the second effect (observed when the resistor is between the photoemissive cathode and the cathode mounting flange), the electric field distortion occurs because, at the photocurrents involved (which can exceed 1000,uA) substantial potentials are developed across the surface of the photoemissive cathode itself (between its edge and the localised emission point).During a high intensity localised flash of light the photocurrent of electrons flowing inwards from the cathode mounting flange to the emission point somewhere on the photoemissive cathode surface becomes increasingly concentrated nearer the emission point, and produces a corresponding rapidly-increasing positive surface potential caused by the finite cathode-sheet resistance. If the resultant electric field parallel to the surface is comparable to the normally undisturbed field perpendicular to the surface (viz, of the order 100 volts/mm), then it is possible for the electron current to be electro-optically limited by an auto bias grid action, as in a conventional thermionic triode tube.This occurs because the local rise in potential on the cathode surface, which is itself caused by the current flow and is proportional to the current flowing, cannot rise beyond the value at which the sign of the electric field in a direction perpendicular to the surface reverses.

The intensifier of the invention provides some or all of the desired decoupling (flashprotection) effect by inserting a resistive element between the photolayer and its electrode mounting flange, this resistive element taking the form of an annular layer having therein radially-extending slots filled with less-resistive material. The terms "radially-extending" and "in a radial sense" mean, in this context, that the slots extend from a point at or near the internal boundary of the annulus to a point at or near the external boundary, and do not constitute part of the periphery of some concentric circle having a radius intermediate the inner and outer radius of the annulus. Thus, "radial" means both truly radial, as extending along a radius, and angled thereto, as extending along a spiral (or similar) line.

The purpose of the slots in the annular resistive layer is to allow the correct resistor value to be attained; this is explained as follows. The resistive layer is present to decouple the photo-layer from its electrode mounting flange (and thus from the H.T.

power source and the major part of the tube interelectrode and stray capacitance). However, the correct degree of decoupling is required to obtain a suitable time constant, and so the actual value of the resistance is an important factor. Moreover, this actual value will depend upon a number of other factors determined by the intensifier design and the materials from which it is constructed, for contributions to the resistive decoupling of photo-layer and H.T. power supply are made by, for example, the inherent resistances of the photo-layer itself and the contacts between various components.The effective total resistance is required to be in the range 1 M to 100 M52, but the exact value depends upon the size of the intensifier, the number of cascade-coupled devices, the type of highlight overload expected, the type of photoemissive cathode, and the electron optics of the intensifier. By way of example, the total effective resistance required for a 25rum three stage intensifier assembly is typically between 10 M and 25 ME2, and the exact value can only be selected after a consideration of the factors outlined above.

The photo-cathode itself is resistive, but its effective sheet resistivity is of a fairly low order (usually about 1 MS3/square) such that it is not itself enough. The photoanode itself is a conductor (the photo layer is an insulator, but the aluminium overlay is, of course, highly conductive), and so it, too, is quite insufficient by itself. Accordingly, the invention places between the photo-layer and its electrode flange an annulus of a much higher sheet resistivity (within the range 103 to 106 MP/square), and such an annulus might have an actual resistance (between its internal and external boundaries) of about 103 MS2. Materials suitable for this layer are the transition metal oxides, used in finely divided form for example, titanium, vanadium and (especially) green chromium oxide.Layers of the latter can have a sheet resistivity of about 106 ME2/square, and are capable of sustaining several kilovolts without electrical breakdown.

The resistance of the annul us must, however, be adjusted within quite fine limits say, to an accuracy of as little as 2 M=and to do so in the obvious manner, by suitably altering the width of the annulus, causes a problem as significant as that with which it deals, for in practice it is difficult to reconcile the requirements of high voltage operation and relatively low resistance. The width tends to have to be too narrow for good control and high-voltagesustaining capability-for example, as small as a fraction of a millimetre. Instead, therefore, the invention provides slots in a relatively wide annulus, the slots being filled with the less-resistive material.Of course, the width of the slots needs careful consideration, as does the length of any slots as are not truly radial, but the level of accuracy and precision required to adjust these to obtain the desired resistance is low, and well within the capabilities of useable production techniques. By way of example, a resistance change of 2 M might require a slot width change, or a slot length change, of as much as 0.5 mm.

The invention thus uses a wide annulus (at least one millimetre wide), which provides a resistance much higher than required, with stripts cut out of it and filled with less-resistive material. One or more (at least three are preferred) such strips can be cut with a ratio of length to width such that when connected in parallel, and subsequently filled with the less-resistive material, their combined resistance is in the required range, typically 1 OMS3. In order to achieve the desired lengthto-width ratio the strips may be cut truely radially or partly tangentially, and straight or curved.

The less-resistive material filling the slots provides, of course, the conductive (relatively speaking) connection between the photo-layer and the electrode mounting flange. As explained, by adjusting the ratio between conductive connections (less-resistive material) and resistive connections (resistive layer), so is adjusted the actual resistance between the photo-layer and its electrode mounting flange (and the H.T. power source).

The less resistive material may, in general, be any material of the right sheet resistivety (and chemical and physical stability). Examples of such materials are noble metal oxides (applied in paint form and fired into place).

However, the photo-emissive cathode materials conventionally employed in intensifier tubes are themselves of a suitable sheet resistivity and stability, therefore can very conveniently be used as the less-resistive material where the slotted resistive element is positioned between the photo-emissive cathode and the cathode mounting flange.

In such a case, the layer of less-resistive material is most conveniently laid down in place while the photo-cathode itself is being formed. Thus, using standard deposition techniques, first there is laid down on the intensifier window a complete annulus of resistive material, then there are cut into the annulus slots of the required width and length, and lastly there is laid down the photo-layer material both to form the photo-layer itself and to fill in the slots in the resistive annulus.

Conveniently, the electrode-mounting-flange ends of the slots are filled with a drop of a "good" conductor-a blob of silver, for exam ple-to ensure a good electrical contact with the flange. It is also advantageous, to ensure uniformity of photo-layer potential distributions, to use a narrow annular metal contact ring to connect the photo-layer to all the slots in parallel. In practice such a ring will be deposited onto the relevant window, the resistive annulus and the photo-layer then being deposited on top.

The further construction of the intensifier may be quite conventional, and so need not be described here.

It is preferred that, of the two photo-layers (the photocathode and the luminescent-screen anode), the slotted annular layer of the invention should be interpcJed between the photocathode and the cathode mounting flange; not only is it most convenient to ruse, as the lessresistive material, the photo-cathode material, but it appears to be easier to manipulate electron flow at low electron velocities-thus, as the electrons leave the cathode rather than as they approach the anode.

The annular resistive layer decoupling resistance of the present invention may, with considerable advantage, be employed together with one or both of the flash-protecting inventions the subject of our aforementioned Applications Nos: 23,753/78 and 23,754/78.

The invention is now described, though by way of illustration only, with further reference to the accompanying drawings, in which: Figure 2 shows diagrammatically a crosssectional view, in exploded form, of part of an intensifier tube of the invention; and Figure 3 shows diagrammatically a plan view of the annular resistive layer used in the embodiment of Fig. 2.

The intensifier tube part shown in Fig. 2 is the photocathode end, and is very similar to the corresponding part of the prior art tube shown in Fig. 1 (indeed, where possible the same reference numerals have been used).

Thus, there is an entrance window 1 sealed by a frit 2 to a cathode mounting flange 3 joined to a cathode body housing 4, and on the internal surface of the window 1 is a photoemissive layer 12 electrically connected to the mounting flange 3. However, this connection is not via a conductive ring (as 1 3 in Fig. 1) but rather via a slotted annular resistive layer (20) electrically connected by a conductive ring (21) to the photoemissive layer 12.

As can be seen more clearly from Fig. 3, the annular layer 20 has slots (30, 31 and 32) cut into it, these slots extending from the inside to the outside of the annulus. Slot 30 is truly radial, slot 31 is straight but angled to a true radius, and slot 32 is curved and angled to a true radius (and follows, in fact, a spiral line). In practice, each slot--and the central area enclosed by the annulus is filled with a layer of photo-emissive material (not shown), and at the other end of each slot is a blob (33) of silver enabling a good electrical connection with the cathode mounting flange (the position of the inner rim of which is indicated by the dashed line 34).

In a particular case, using an S-25 type photo-emissive material (a caesium-treated sodium/potassium/antimony alloy with the approximate empirical formula Na2KSb (Cs)) deposited in a circular layer 25 mm in diameter, and using as the resistive material green chromium oxide deposited in an annular layer about 0.1 mm thick and 2 mm wide, a desired annulus resistance of 5 MS2 could be obtained by cutting three slots (as 30 or 31 or 32) of the following approximate dimensions: 30) 0.6 mm wide (and 2.0 mm long); 31) 1.0 mm wide and 3.0 mm long; and 32) 1.25 mm wide and 4.0 mm long.

Claims (7)

1. An image intensifier of the kind described hereinbefore wherein at least one of its two photo-layers (the luminescent screen and the photocathode) is resistively decoupled with respect to the H.T. power supply connected (in operation) across the anode and cathode body housings, the resistive decoupling being effected by a resistor connected between the photo-layer and the appropriate electrode body housing, this resistor comprising a slotted annular layer of resistive material positioned around the periphery of the photolayer and separating it from its electrode mounting flange, the slots in the annular resistive layer extending in a radial sense and being filled with a less-resistive material contiguous with and electrically connecting the photo-layer and its electrode mounting flange.

2. An intensifier as claimed in claim 1, wherein the sheet resistivity of the material forming the slotted annular layer is from 103 to 106 MSl/square, and the annulus itself has an actual resistance (between its internal and external boundaries) of about 103 M.

3. An intensifier as claimed in either of the preceding claims, wherein the material of which the slotted annular layer is formed is a transition metal oxide, used in finely divided form.

4. An intensifier as claimed in claim 3, wherein the material is green chromium oxide.

5. An intensifier as claimed in any of the preceding claims, wherein the slotted annular layer is at least one millimetre wide.

6. An intensifier as claimed in any of the preceding claims, wherein the actual resistance of the slotted annular layer with its slots filled with the less-resistive material is about 1 OMQ.

7. An intensifier as claimed in any of the preceding claims, wherein the less-resistive material is a noble metal oxide applied in paint form and fired into place.

7. An intensifier is claimed in any of the preceding claims, wherein the less-resistive material is a noble metal oxide applied in paint form and fired into place.

8. An intensifier as claimed in any of claims 1 to 6 and where the slotted resistive element is positioned between the photo-emissive cathode and the cathode mounting flange, wherein the photo-emissive cathode material is used as the less-resistive material.

9. An intensifier as claimed in any of the preceding claims, wherein the electrodemounting-flange ends of the slots in the slotted annular layer are filled with a drop of a "good" conductor to ensure a good electrical contact with the flange.

10. An intensifier as claimed in any of the preceding claims, wherein, in order to ensure uniformity of photo-layer potential distribution, a narrow annular metal contact ring is used to connect the photo-layer to all the annular layer slots in parallel.

11. An intensifier as claimed in any of the preceding claims, wherein the slotted annular layer is interposed between the photocathode and the cathode mounting flange.

1 2. An intensifier as claimed in any of the preceding claims and substantially as hereinbefore described with reference to the accompanying drawings.

CLAIMS (27 Oct 1981)

1. An image intensifier of the kind de scribed hereinbefore wherein at least one of its two photo-layers (the luminescent screen and the photocathode) is resistively decoupled with respect to the H.T. power supply connected (in operation) across the anode and cathode body housings, the resistive decoupling being effected by a resistor connected between the photo-layer and the appropriate electrode body housing, this resistor comprising a slotted annular layer of resistive material positioned around the periphery of the photolayer and separating it from its window mounting flange, the slots in the annular resistive layer extending in a radial sense and being filled with a less-resistive material contiguous with and electrically connecting the photo-layer and its window mounting flange.