CHANNEL PLATE FOR AN IMAGE INTENSIFIER TUBE, AND PROCESS FOR PRODUCING A CHANNEL PLATE, AND IMAGE INTENSIFIER TUBE PROVIDED WITH A CHANNEL PLATE

Abstract: Channel plates for image intensifier tubes have an end-spoiling that continues into the channels over at least a distance greater than twice the channel diameter. In the channels the thicknesses of the end-spoiling decrease with increasing distances from the channel outputs. Thereby a considerable increase in resolution at the anode of the image intensifier tube is obtained.

Description: Channel plate for an image intensifier tube, and process for producing a channel plate, and image intensifier tube provided with a channel plate. The invention relates to a channel plate for an image intensifier tube. Image intensifier tubes comprise a cathode which under the influence of incident radiation, such as light or x-rays, emits electrons which under the influence of an electric field move towards an anode. The electrons striking the anode form a perceptible initial image. In image intensifier tubes of the proximity focus type an essentially uniform electric field prevails between the cathode and the anode, and there are no focusing elec¬ trodes to focus the electrons emitted by the cathode on the anode. In modern image intensifier tubes of the proximity focus type a channel plate, mostly a multi-channel plate or MCP for short, is placed between the cathode and the anode to increase the image intensification. Such a channel plate comprises a stack of hollow tubes, e.g. hollow glass fibres, extending between an input face and an output face. The input face when mounted faces the cathode or carries the cathode, and the output face faces; the arode. Such a potential difference prevails between input face and output face that an electron entering a channel at the input face moves in the direction of the output face, in which process the number of electrons is increased by secondary emission effects. After leaving the channels the electrons are accelerated in the usual manner in the direction of the anode. The input face and the output face of the channel plate are- each provided with a conducting layer, which is capable of bringing the ^ input face and the output face to a suitable uniform potential. A suitable layer for the output face can be, e.g. a n'ickel-chromium layer, which is applied by vacuum deposition or another suitable technique. A layer applied to the output face always penetrates slightly into the channels. The conductance of the wall of the end of the channels thus obtained is known as "end spoiling". The image resolution obtained from use of a channel plate is directly related to the diameter of the chan- nelε. In practical image intensifier tubes it has been found that with a channel plate whose channel diameter d is 10 rn and centre distance is 12 yum it is possible to achieve a separation capability of approx. 34 line pairs per millimetre. The separation capability can be in- creased by reducing the channel diameter. The object of the invention is to provide a channel plate which produces a greater image resolution with the channel diameter remaining the same. For this purpose, according to the invention a channel plate which has an output face provided with a conducting layer is charac¬ terized in that the conducting layer continues over a relatively great distance from the output face into the channels. The invention will be explained in greater detail below with reference to the appended drawing. Fig. 1 shows schematically an image intensifier tube of the proximity focus type, provided with a channel plate; Fig. 2 shows schematically, on an enlarged scale, a part of an example of an embodiment of a channel plate according to the invention; Fig. 3 shows schematically, on an enlarged scale, a part of a second example of an embodiment of a channel plate according to the invention; Fig. 4 shows a graph clarifying an embodiment of the invention; and Fig. 5 shows schematically a third example of an embodiment of a channel plate according to the invention; Fig. 1 shows schematically, in cress section, an example of an image intensifier tube of the proximity focus type. The tube comprises a tubular housing 1 having an input window or cathode window 2 and an output or anode window 3. The housing can be made of glass, as can the cathode window and the anode window. The anode window is, however, also often an optical fibre plate. The housing can also be made of metal, in the event of the cathode and possibly the anode being arranged in an insulated manner in the housing, for example by using a separate carrier, as described in Dutch Patent Appli¬ cation 79.00878. If the image intensifier is designed for receiving x-rays, the cathode window can be made of thin metal. The anode window must, however, be light-trans¬ mitting. The cathode can also be provided directly on the input face of the channel plate. All such variants are known per se and are therefore not shown in greater detail. In the example shown the actual cathode 4 is on the inside of the cathode window and emits electrons under the influence of incident light or x-rays . The emitted electrons are propelled in a known manner under the influence of an electric field (not shown) in the direc¬ tion of an anode 5 disposed on the inside of the anode window. A channel plate 6 extending approximately parallel to cathode and anode is placed between cathode and anode. A large number of tubular channels, which can have a diameter, e.g., of the order of 8 - 12 /u , extend between the input face 7 of the channel plate facing the cathode and the output face 8 of the channel plate facing the anode. Fig. 2 shows, on an enlarged scale, a part of the channel plate 6 with two tubular channels 10, extending between the input face 7 and the output face 8. The diameter of the channels is indicated by "d" . The output face 8 is provided with a conducting layer 11 for bring¬ ing the output face to a suitable potential . The input face can also be provided with such a conducting layer, as indicated by 12. According to the invention the separation capability of an image intensifier tube equipped with a channel plate can be increased by making the layer 11 continue into the channels over a distance 'a' which is greater than approx. 2d. All this is as shown at 12 in Fig. 2. In a practical image intensifier tube where a channel plate with a conducting layer applied to the output face (end spoiling) was used and continued over a distance of 3d to 4d into the channels, an improvement of the separation capability of approx. 15 to 25% was achieved. According to a preferred embodiment, the thickness of the layer 12 decreases gradually towards the input face from the output face 8 or from an imaginary face some distance from the output face. All this is shown in the graph of Fig. 4. In the graph of Fig. 4 a solid line 40 shows the correlation between the conductance G of the layer 12 in the channels and the distance from the output face 8. It can be seen that the conductivity of the layer 12 decreases gradually from a distance a = approx. 2d. As stated, this effect can, be achieved by allowing the thickness of the layer to decrease. Such an effect can be obtained in another manner, as will be explained in further detail below. It is pointed out that it was found in research that even in the conventional channel plates the conducting layer formed by vacuum deposition also continues at the output side over some distance into the channels. This distance is, however, relatively short (as shown in Fig. 2 by way of example for the input face) and is between 1 1/2 and 2 1/2 d. In Fig. 4 a dotted line 41 shows the variation of the conductance as it can occur in a known channel plate. A conducting layer 12 continuing over a relatively great distance into the channels can be obtained by applying the layer by means of the so-called sputtering technique. It is also possible to form such a layer by vacuum deposition at a relatively high pressure, with the free path length of the molecules not being greater than the channel diameter d. A,suitable pressure value can be of the order of 10 "z mbar. A gradually or stepwise decreasing thickness of the layer 12 can be obtained by, for example, carrying out successive vacuum depositions at different pressures. Fig. 3 illustrates a conducting layer obtained in such a manner in two stages. A first layer 20 is applied to the output face 8 of a channel plate 6 by vacuum 'deposition at a relatively high pressure of, e.g., 10 "2 mbar. A second layer 21 is then applied at a much lower pressure of, e.g., 10 "5 to 10 "6 mbar, but the sequence can also be the other way round. It is pointed out that the layers 20 and 21 can be formed of different material if desired. The conducting layer can also be built up in more than two stages from a number of partial layers which are each vacuum deposited at a different pressure, or by a continuous vacuum deposition process at a gradually decreasing or increasing pressure. It is also possible, for example, to apply first a layer to the desired depth using a sputtering technique and then to apply one or more layers, of the same or different material, by vacuum deposition. As an alternative, the conducting layer can be applied by vacuum deposition -in stages by beginning at low pressure and increasing it in stages. The successive layers then extend to an increasing depth into the channels . In this case also different materials can be used for the different partial layers. Fig. 5 shows schematically, on an enlarged scale, an end spoiling built up in this way from three partial layers 24, 25, 26. Such an effect can be obtained by gradually increasing the pressure during the vacuum deposition. It can be seen from the above that the technique described can be used both for a channel plate not yet provided with end spoiling and for a channel plate already provided with end spoiling applied in the usual manner.

Claims: CLAIMS 1. Channel plate for an image Intensifier tube, said channel plate having an input face and an output face, at least the output face being provided with a conducting layer, characterized in that the conducting layer con- tinues over a relatively great distance from the output face into .the channels. 2. Channel plate according to Claim 1, characterized in that the conducting layer continues over at least a distance a > 2d into the channels, where d is the channel diameter. 3. Channel plate according to Claim 1 or 2, charac¬ terized in that, in the channels the conductivity of the conducting layer decreases gradually from a certain distance from the output face with increasing distance from the output face. 4. Channel plate according to Claim 1, 2 or 3, characterized in that the conducting layer comprises at least two partial layers applied on top of each other. 5. Channel plate according to Claim 4, characterized in that the at least two partial layers extend over different distances into the channels. 6. Channel plate according to Claim 4 or 5, charac¬ terized in that at least two partial layers are of different materials. 7. Channel plate according to one of the preceding claims, characterized in that the thickness of the conducting layer decreases with increasing distance starting at a first distance from output face. 8. Channel plate according to Claim 7, characterized in that the decrease in the thickness is uniform. 9. Channel plate according to Claim 7, characterized in that the decrease in the thickness is in stages. 10. Image intensifier tube of the proximity focus tYP Q r comprising a cathode and an anode, characterized by a channel plate according to one of Claims 1 to 8. 11. Process for producing a channel plate with a conducting layer applied to at least the output face, characterized in that the conducting layer is applied at least partially by vacuum deposition at a relatively high pressure. 12. Process for producing a channel plate with a conducting layer applied to at least the output face, characterized in that the conducting layer is applied at least partially by sputtering to a relatively great depth in the channels opening into the output face. 13. Process according to Claim 11 or 12, charac¬ terized in that the conducting layer is built up in stages from partial layers. 14. Process according to Claim 11 or 12, charac¬ terized in that a first partial layer of a first type of material and at least a further pariial layer of a second type of material are applied. 15. Process according to Claim 11, characterized in that during the vacuum deposition of at least part of the conducting layer the pressure is increased gradually or in stages from a relatively low value. 16. Process according to Claim 15, characterized in that during the vacuum deposition of at least part of the conducting layer the pressure is reduced gradually or in stages from a relatively high value. 17. Process according to Claim 15 or IS, charac¬ terized in that the pressure is varied between approx. 10 "2 mbar and 10 "5 to 10 "B mbar.

Inventors: BOSKMA LIEUWE W, VAN NISSELROY LEONARD G E J, BOSKMA, LIEUWE, W., VAN NISSELROY, LEONARD, G., E., J.
Applicants: OPTISCHE IND DE OUDE DELFT NV, B.V. OPTISCHE INDUSTRIE "DE OUDE DELFT"
Application year: 1989
Main classification: H01J43
IPC codes: H01J9, H01J31, H01J43