IMAGE INTENSIFIER FACEPLATE

Abstract: Previously shielded image intensifier faceplates have been manufactured by a method which has a number of disadvantages, requiring difficult and expensive machining, and introducing distortions. These may be overcome by employing the invention, where a clear glass disc l3 and a black glass disc l4 are placed in a graphite mould l5 and located so that they fuse and form a block having a shape conforming to the internal; configuration of the mould. Then the block need only be ground down to size and unwanted parts of the black glass removed.

Description: This invention relates to image intensifiers and is particularly concerned with the image intensifier faceplates and a method of manufacturing them. A conventional image intensifier is illustrated in Figure 1, which is a longitudinal section. It comprises a glass faceplates 1 on which a photocathode 2 is laid down, a micro-channel plate electron multiplier 3 and a phosphor screen 4 on a glass substrate 5. The glass faceplates 1 and substrate 5 form the end windows of a vacuum envelope in which the other elements are contained. In operation an optical image is focussed on the photocathode 2 by an external lens, causing electrons to be emitted. The electrons are accelerated to the electron multiplier 3 where they are increased in number by secondary emission at the channel walls. The secondary electrons are then accelerated towards the phosphor screen 4 by a potential difference set up between it and the electron multiplier 3, producing an optical image corresponding to the image focussed on the photocathode 2 but of very much greater intensity. By increasing the potential difference, a typical value being several thousand volts, the intensification produced is increased. The flat surfaces of the photocathode 2, electron multiplier 3, and phosphor screen 4 must be closely spaced and parallel to ensure good definition in the final image. To achieve this without discharge breakdown occurring between electrical connections 6 to the photocathode 2, electron multiplier 3 and phosphor screen 4, the glass faceplates 1 and glass substrate 5 have central raised portions extending towards the electron multiplier 3. Thus the electrical connections 6 are spaced much further apart than would otherwise be the case. In such an image intensifier light incident at the periphery of the glass faceplates l may undergo reflection within the faceplates l, as shown at 7, resulting in spurious signals. This may be reduced by employing a shielded faceplates consisting of a central transparent region surrounded by light absorbing material. In a previous method of manufacturing such a shielded faceplates a cylindrical clear glass rod 8 is inserted into a black glass tube 9, as shown in Figure 2. The rod 8 and tube 9 are surrounded by a tubular clear glass envelope l0 which is evacuated. The temperature is then raised until the glasses fuse (Figure 3) producing a solid cylindrical block. This is sliced transversely to its longitudinal axis X. One of the slices is shown in Figure 4. The slice is then machined to the required shape shown in Figure 5, to produce a shielded faceplates having a clear glass core ll surrounded by an outer region of black glass l2. This method has a number of disadvantages. The machining required is expensive and, in the case of obtaining the initial black glass tube, is extremely difficult, since its inner surface must be polished to a high quality. In addition, distortions are introduced during the heating process, resulting in a loss of concentricity. Also the parallel sides of the clear core ll cause vignetting, or fading of light entering the faceplates at the periphery of the core ll. According to a first aspect of the invention there is provided a method of manufacturing a shielded faceplates for use in an image intensifier comprising a photocathode, including the steps of: arranging in a mould a member of a first type of glass, transparent to light to which the photocathode is sensitive, adjacent another member of a second type of glass which absorbs light to which the photocathode is sensitive, the mould being of a material to which glass does not fuse; heating the members so that they fuse, thereby forming a block having a shape conforming to the internal configuration of the mould and having an inner region of the first type of glass and an outer region of the second type of glass, and removing the block from the mould. The term 'light' in this Specification should be taken to include infra-red and ultra-violet radiation in addition to visible light. If the photocathode is, for example, sensitive only to near infra-red radiation and the red end of the visible spectrum, the second type of glass could be blue glass. This is advantageous because it may then be visually inspected for flaws, unlike black glass. Since the glasses are shaped by the mould the difficulty and expense of machining incurred in employing the previous method, described above is avoided. Also the method according to the invention is much more flexible than the previous method, allowing different shielding configurations to be easily manufactured, and the distortions which occur with the previous method are reduced. It is preferred that the block formed by the method includes a portion of the first type of glass which is completely encircled by a portion of the second type of glass. Preferably, the first and second types of glass have substantially the same refractive index. This reduces reflections at the boundary between the two and hence the possibility of spurious signals being generated. It is preferred that the members are discs which have flat surfaces arranged adjacent each other in the mould. Preferably some of the second type of glass is removed to expose a flat surface of the first type of glass which is bounded by the encircling portion of the second type of glass, allowing light to pass from one side to the other of the faceplates, although the disc of the second type of glass may be annular, in which case such further processing may not be necessary Preferably, the members are heated in an inert atmosphere, preventing an oxidation. According to a feature of the invention, a shielded image intensifier faceplates is manufactured by a method according to the invention, and preferably the outer region surrounds the inner region which has a transverse dimension which decreases along its length, being smallest at the photocathode surface. According to a second aspect of this invention there is provided a shielded faceplates for an image intensifier including a photocathode, comprising an inner core of glass transparent to light to which the photocathode is sensitive and an outer shielding region of glass which absorbs light to which the photocathode is sensitive which surrounds and is adjacent to the inner core, the inner core having a transverse dimension which decreases along its length, being smallest at the photocathode surface. Such as construction does not tend to suffer from vignetting to the same extent as a conventional shielded faceplates having a parallel sided inner core. Even where the refractive indices of the glasses forming the inner core and outer shielding differ by only a small amount, incoming light striking the boundary between them at a glancing angle (i.e. at a large angle of incidence) will result in a reflected beam of large amplitude. This condition is less likely to occur when the diameter of the inner core increases from its smallest value at the photocathode to the front of the faceplates which receives the light. Thus the shielded faceplates permits a much improved operation of an image intensifier; and according to a third aspect of this invention, an image intensifier includes an electron multiplier; a phosphor screen upon which electrons from said multiplier are incident; a photocathode adapted to receive illumination and to emit electrons to the electron multiplier; and a glass faceplates which supports the photocathode, the glass faceplates being in accordance with this invention as previously set out. The invention is now further described by way of example with reference to Figures 6 to ll of the accompanying drawings in which: Figure 6 shows the initial glass members Figures 7 to ll illustrate a method in accordance with the invention and are transverse sections; and Figure l2 illustrates an image intensifier in accordance with the invention. With reference to Figure 6 a clear glass-disc l3 having a diameter of 30 mm and 9 mm thickness and a black glass disc l4 with a diameter of 30 mm and l.2 mm thick are polished on all surfaces and comprise the start components. They are placed in a graphite mould l5 having a movable member l5A (Figure 7) with the clear glass disc l3 above the black glass disc l4 as shown. The temperature is then raised to about 8000°C when the glasses soften (Figure 8) and the member l5A moved inwards so that they are forced into contact with the walls of the mould l5. The glasses fuse to form a block l6 having a clear glass region l7 and a region l8 of black glass, as shown in Figures 9 and l0. The heating process takes place in an inert atmosphere to prevent oxidation of the mould l5, and initially at reduced pressure. Fusion will commence in a circumferential region, and thus any gas evolved from the glass surface during the fusion will form a bubble or blister in the enclosed central region. Since at a later stage this part of the block l6 is usually removed (to permit light to pass through the block l6 to the photocathode) the bubble or blister may not be objectionable. However, if there are undesirable number of bubbles the problem may be reduced by drilling a small hole in the centre of the black glass disc l4 prior to the heating process. Then any gases evolved during fusion may escape. The block l6 is then removed from the mould. The plane surfaces of the block l6 are then ground down to size (Figure ll), removing a layer of black glass l9 to form a flat surface 20 (to which the photocathode can be applied), and the surfaces polished. The faceplates thus formed has an inner cylindrical region 2l, the radius of which increases along its length from the photocathode surface 20 to the front surface 22 of the faceplates, and a surrounding shielding region 23 of black glass. The faceplates manufactured as described above with reference to Figure 6 to ll may then be included in an image intensifier as shown in Figure l2.

Claims: 1. A method of manufacturing a shielded faceplates for use in an image intensifier comprising a photocathode, including the steps of: arranging in a mould (l5, l5A) a member (l3) of a first type of glass, transparent to light to which the photocathode is sensitive, adjacent another member (l4) of a second type of glass which absorbs light to which the photocathode is sensitive, the mould (l5, l5A) being of material to which glass does not bond; heating the members (l3, l4) so that they fuse, thereby forming a block (l6) having a shape conforming to the internal configuration of the mould (l5, l5A) and having an inner region (2l) of the first type of glass; and removing the block (l6) from the mould (l5, l5A). 2. A method as claimed in claim l and wherein the block (l6) so formed includes a portion (2l) of the first type of glass which is completely encircled by a portion (23) of the second type of glass. 3. A method as claimed in claim l or 2 and wherein the first and second types of glass have substantially the same refractive index. 4. A method as claimed in claim l, 2 or 3 and wherein the members (l3, l4) are discs which have flat surfaces arranged adjacent each other in the mould (l5, l5A). 5. A method as claimed in claim 4 and wherein, after the block (l6) is formed, some of the second type of glass is removed to expose a flat surface (20) of the first type of glass which is bounded by the encircling portion of the second type of glass. 6. A method as claimed in any preceding claim and wherein the members (l3, l4) are heated in an inert atmosphere. 7. A method as claimed in any preceding claim and wherein the members (l3, l4) are initially heated at reduced pressure. 8. A method as claimed in any preceding claim and wherein the material of the mould (l5, l5A) is graphite. 9. A shielded image intensifier faceplates manufactured by a method as claimed in any of the preceding claims. l0. A shielded image intensifier faceplates as claimed in claim 9 and wherein the inner region (2l has a transverse dimension which decreases along its length, being smallest at the photocathode surface (20), and is surrounded by the outer region (23). 11. A shielded faceplates for an image intensifier including a photocathode, comprising an inner core (2l) of glass transparent to light to which the photocathode is sensitive and an outer shielding region (23) of glass which surrounds and is adjacent to the inner core (2l) the inner core (2l) having a transverse dimension which decreases along its length, being smallest at the photocathode surface (20). 12. An image intensifier including an electron multiplier; a phosphor screen upon which electrons from said multiplier are incident; a photocathode adapted to receive illumination and to emit electrons to the electron multiplier; and a glass faceplates being as claimed in any of claims 9, l0 or ll.

Inventors: BANKS, PETER BALDWIN, HOLLAND, DAVID GEORGE
Applicants: ENGLISH ELECTRIC VALVE COMPANY LIMITED
Application year: 1984
Main classification: H01J43
IPC codes: H01J9, H01J29, C03B23