GB2081965A

GB2081965A - Image intensifier tubes

Info

Publication number: GB2081965A
Application number: GB7907739A
Authority: GB
Original assignee: English Electric Valve Co Ltd
Current assignee: Teledyne UK Ltd
Priority date: 1979-03-05
Filing date: 1979-03-05
Publication date: 1982-02-24
Also published as: GB2081965B; NL8001283A

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 collar- like resistive element 22a placed around the rim surface of one of the tube's windows and connected so as to act as a high resistance between the relevant photo-layer 12 and its window mounting flange 3 (and hence the H.T. power source). This causes both a temporary reduction in down-tube potential and - when, as is preferred, the photo- layer is the cathode - 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 voltagror high tension (HTFis 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, until 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 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 shell - can 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 100 MQ to 1 G 5Z, and that, where the image intensifier is a multi-staged 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 intensifier 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 collar of resistive material positioned around the peripheral surface of the window bearing the photolayer and forming the sole electrical connection between the photo-layer and its electrode mounting flange.

The flash protection effect of the intensifier of the invention works in a manner somewhat similar to that of the intensifiers of our aforementioned Application No: 23,753/78. Thus, the primary effect of the resistor between photo-layer and electrode body housing is to absorb part of the energy stored in the interelectrode and stray capacitances of the intensifier. However, there is also a secondary effect, which is to cut-off the electron flow by 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 maximurn 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, nowwhere 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 1000tuA) 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 correspondingly 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 photo-layer and its electrode mounting flange, this resistive element taking the form of a collar around the peripheral surface of the photo-layer-bearing window, this collar being the sole electrical connection between the photo-layer and its electrode mounting flange. This collar-shaped resistive element may be so constructed and connected that there are two electrical pathways (around the collar in opposite senses) between the connection points, and thus it constitutes two resistances in parallel, but preferably it is so constructed and connected that there is only one electrical pathway between the connection points (such a construction is described in more detail hereinafter with reference to the drawings).

While the collar may constitute a resistance of fixed value, it is preferred that it effectively be a resistance of variable value (so that there may be chosen the most suitable resistance value for the particular intensifier concerned) -and such an effect may be obtained by "splitting" the resistive parts of the collar into sections (of appropriate resistance) joined by conductive portions constituting tapping points. Then, by selecting two such tapping points as the resistive element connection points, the resistive portions between them adding up to an appropriate resistance (say, 10, 15, 20 or 25 M52), there may easily be selected the most desirable value for the effective resistance of the resistive element.

The need fdr selecting the correct resistance value to be employed is explained as follows.

The resistive collar 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 inter-electrode and stray capacitance. However, the correct de gree 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 S2 to 100 M , 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 25 mm three stage intensifier assembly is typically between 1 OM and 25my, and the exact value can only be selected after a consideration of the factors outlined above.

Though the collar-shaped resistive element could simply be made of material having a suitable resistivity, most conveniently it is constructed of an insulating former having a resistive surface layer. The insulating former is preferably made of a ceramic-like material, for example alumina, while the resistive surface layer is advantageously made of a noble metal oxide, for example palladium oxide (the layer is conveniently applied as a paint or ink fused into place by a subsequent firing step). A typical such noble metal oxide preparation is that available commercially under the name DuPont Resistor Composition No: 9479.

In a preferred embodiment of the invention the electrical connection between the photolayer and the collar-shaped resistive element is made via a conductive ring (as 1 3 in Fig. 1, for example) contiguous with the periphery of the photo-layer, and if such a system is employed then most conveniently the connection between the conductive ring and the resistive element is made by wires extending from the latter to one or two connector tabs extending from the former. In our actual embodiment these connector tabs will extend alongside and in very close proximity to the internal edge of the electrode mounting flange, and accordingly most desirably that internal edge is cut away in the vicinity of the tabs to reduce the possibility of charge leakage or "tracking".

Moreover, also to reduce the possibility of charge leakage, but this time over the window surface between the conductive ring (if used) and the electrode mounting flange's internal edge, that surface is conveniently coated with a layer of resistive material, for example green chromium oxide.

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 collar-like resistive element of the invention should be interposed between the photocathode and the cathode mounting flange; it appears to be easier to manipulate electron flow at low electron velocities - thus, as electrons leave the cathode rather than as they approach the anode.

The collar-like resistive element 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 aformentioned 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 cutaway top plane view of part of the tube part shown in Fig. 2.

The intensifier tube part shown in Figs. 2 and 3 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 1 2 electrically connected to the mounting flange 3 via a conductive ring 1 3. However, this connection via the conductive ring 1 3 is not direct (the ring 1 3 is separated from the flange 3 by a ring (19) of resistive green chromium oxide), but instead is by way of a connector tab (20) contiguous with the ring 1 3 and joined to a connector wire (21) itself joined to a tapping point (30) (see Fig. 3) on a collar-like resistive element (22) mounted (by means not shown) around the peripheral surface (23) of the window 1. The collar-like element 22 is itself connected to the cathode mounting flange 3 by a second connector wire (24) (which is shown in Fig. 2 even though, strictly speaking, it is not visible in this view).

As can be seen more clearly from Fig. 3, the tab 20, which passes through the ring 1 9 of resistive green chromium oxide separating the conductive ring 1 3 from the flange 3, has at its outer end lateral projections (as 24) extending for a short distance along the peripheral surface 23 of the window 1. Moreover, the collar-like resistive element 22 is a ceramic ring having a surface coating of resistive material all around it except on the X-ed portion (25); the "conductive" part of the element 22 is thus that part extending from the tapping point 30 around the element in an anti-clockwise direction (as viewed).

The particular embodiment shown in Fig. 3 has two connector tabs 20 leading off the ring 13 coupled with a number of tapping points (as 30) spread along the conductive part of the collar-like resistive element 22. By judiciously selecting which tab 20 is connected (by wire 21) to which tapping point 30, and then which other tapping point 30 is connected (by wire 24) to the cathode mounting flange 3, so there may be attained the desired value for the effective resistance of the element 22.

Claims

1. An image intensifier 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 collar of resistive material positioned around the peripheral surface of the window bearing the photo-layer and forming the sole electrical connection between the photo-layer and its electrode mounting flange.

2. An intensifier as claimed in claim 1, wherein the collar-shaped resistive element is so constructed and connected that there is only one electrical pathway between the connection points.

3. An intensifier as claimed in either of the preceding claims, wherein the collar-like resistive element constitutes a resistance of variable value.

4. An intensifier as claimed in claim 3, wherein the resistive parts of the collar are "split" into sections (of appropriate resistance) joined by conductive portions constituting tapping points.

5. An intensifier as claimed in any of the preceding claims, wherein the collar-shaped resistive element is constructed of an insulating former having a resistive surface layer.

6. An intensifier as claimed in claim 5, wherein the insulating former is made of a ceramic-like material, while the resistive surface layer is made of a noble metal oxide.

7. An intensifier as claimed in claim 6, wherein the ceramic-like material is alumina, and the noble metal oxide is palladium oxide.

8. An intensifier as claimed in any of the preceding claims, wherein the electrical con nection between the photo-layer and the col lar-shaped resistive element is made via a conductive ring contiguous with the periphery of the photo-layer.

9. An intensifier as claimed in claim 8, wherein the connection between the conductive ring and the resistive element is made by wires extending from the latter to one or two connector tabs extending from the former.

10. An intensifier as claimed in claim 9, wherein the connector tabs extend alongside and in very close proximity to the internal edge of the electrode mounting flange, and accordingly that internal edge is cut away in the vicinity of the tabs to reduce the possibility of charge leakage or "tracking".

11. An intensifier as claimed in any of the preceding claims, wherein, in order to reduce the possibility of charge leakage, the window surface between the conductive ring (if used) and the electrode mounting flange's internal edge is coated with a layer of resistive material.

1 2. An intensifier as claimed in claim 11, wherein the layer of resistive material is one of green chromium oxide.

1 3. An intensifier as claimed in any of the preceding claims, wherein the collar-like resistive element is interposed between the photocathode and the cathode mounting flange.

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

CLAIMS (27 Oct 1981)

1. An image intensifier 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 collar of resistive material positioned around but separate from the rim surface of the window bearing the photo-layer and forming the sole electrical connection between the photo-layer and its window mounting flange.