GB2180095A

GB2180095A - Image intensifier faceplates

Info

Publication number: GB2180095A
Application number: GB08624418A
Authority: GB
Inventors: Peter Baldwin Banks; David George Holland
Original assignee: English Electric Valve Co Ltd
Current assignee: Teledyne UK Ltd
Priority date: 1983-10-11
Filing date: 1986-10-11
Publication date: 1987-03-18
Also published as: GB8624418D0; GB2180095B

Abstract

Previously, shielded image intensifier faceplates have comprised a parallel-sided core of clear glass surrounded by an outer shielding opaque layer 23, resulting in vignetting. The invention provides a shielded faceplate in which the inner core 21 has a transverse dimension which decreases along its length, being smallest at the photocathode surface 20, and which therefore does not suffer from vignetting. <IMAGE>

Description

SPECIFICATION Image intensifiers This invention relates to image intensifiers and is particularly concerned with the image intensifierfaceplates and a method of manufacturing them.

A conventional image intensifier is illustrated in Figure 1,which is a longitudinal section. It comprises a glass faceplate 1 on which a photocathode 2 is laid down, a micro-channel plate electron multiplier3 and a phosphor screen 4 on a glass substrate 5. The glass faceplate 1 and substrate 5form 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 multiplier3, 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 ofthe 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 multiplier3 and phosphor screen 4, the glassfaceplatel and glass substrate 5 have central raised portions extending towards the electron muitiplier 3. Thus the electrical connections 6 are spaced much further apart than would otherwise be the case.

In such an image intensifier light incident atthe periphery ofthe glass faceplate 1 may undergo reflection within the faceplate 1, as shown at 7, resulting in spurious signals. This may be reduced by employing a shielded faceplate consisting of a central transparent region surrounded by light absorbing material.

In a previous method of manufacturing such a shielded faceplate 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 byatubular clear glass envelope 10 which is evacuated. The temperature is then raised until the glasses fuse (Figure 3) producing a solid cylindrical block. This is sliced transverselyto its longitudinal axis X. One ofthe slices is shown in Figure 4. The slice is then machined to the required shape shown in Figure 5, to produce a shielded faceplate having a clear glass core 11 surrounded by an outer region of black glass 12.

This method has a number of disadvantages. The machining required is expensive and, in the case of obtaining the initial blackglasstube,isextremelydif- ficult, 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 ofthe clear core 11 causevignetting,orfading oflightenteringthefac- eplate at the periphery ofthe core 11.

According to a first aspect of the invention there is provided a method of manufacturing a shielded fac eplateforuse in an image intensifier comprising a photocathode, including the steps of: arranging in a mould a memberofafirsttype of glass,transparent to lightto which the photocathode is sensitive, adja cent another member of a second type of glasswhich absorbs lighttowhich the photocathode is sensitive, the mould being of a material to which glass does not fuse;; heating the members so thattheyfuse, thereby forming a block having ashape conforming to the internal configuration of the mould and having an inner region ofthe first type ofglass 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 ta ken to include infra-red and u!tra- violet radiation in addition to visible light.

If the photocathode is, for example, sensitive only to nearinfra-red radiation and the red end ofthevis- ible spectrum, the second type of glass could be blue glass. This is advantageous because it may then be visuallyinspectedforflaws, 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 blockformed by the method includesa portionofthefirsttypeofglasswhich 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 ofthefirsttype of glass which is bounded by the encircling portion of the second typeofglass, allowing lightto pass from one side to the other of the faceplate, although the discofthesecondtype 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 oxidation.

According to a feature ofthe invention, a shielded image intensifierfaceplate is manufactured buy 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 faceplate for an image intensifier including a photocathode, comprising an inner core of glass transparent to lightto which the photocathode is sensitive and an outer shielding region of glass which absorbs lighttowhich the photo cathode is sensitive which surrounds and is adjacent to the inner core, the innercore having atransverse dimension which decreases along its length, being smallest at the photocathode surface.Such a construction does not tend to suffer from vignetting to the same extent as a conventional shielded faceplate having a parallel sided innercore. Evenwherethe refractive indices of the glasses forming the inner coreandoutershielding differbyonlyasmall amount, incoming lightstrikingthe boundàrybe- tween 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 likelyto occurwhen the diameter of the inner core increases from its smallest value atthe photocathodeto the front of the faceplate which receives the light.

Thus the shielded faceplate 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 elec tron multiplier; and a glass faceplate which supports the photocathode,the glass faceplate being in accordance with this invention as previously set out.

The invention is nowfurtherdescribed byway of example with reference to Figures 6 to 11 of the accompanying drawings in which: Figure 6shows the initial glass members and Figures 7to 11 illustrate a method in accordance with the invention and are transverse sections.

With reference to Figure 6 a clear glass disc 13 having a diameter of 30 mm and 9 mm thickness and a black glass disc 14 with a diameter of 30 mm and 1.2 mm thick are polished on all surfaces and comprise the start components. They are placed in a graphite mould 15 having a movable member 1 5A (Figure 7) with the clear glass disc 13 above the blackglassdisc 14 as shown. The temperature is then raised to about 800"C when the glasses soften (Figure 8) and the member 1 5A moved inwards so thatthey are forced into contact with the walls of the mould 15.The glasses fuse to form a block 16 having a clearglass region 17 and a region 18 of black glass, as shown in Figures 9 and 10.The heating process takes place in an inert atmosphere to prevent oxidation ofthe mould 15, and initiallyat reduced pressure.

Fusion will commence in a circumferential region, and thus any gas evolved from the glass surface dur ingthefusionwillform a bubble or blister in the enclosed central region. Since at a later stagethis part of the block 16 is usually removed (to permit lightto pass th rough the block 16 to the photocathode) the bubble or blister may not be objectionable. However if there are an undesirablenumberofbubblesthe problem may be reduced by drilling a small hole in the centre ofthe black glass disc 14 prior to the heating process. Then any gases evolved during fusion may escape. The block 16 is then removed from the mould.

The plane surfaces ofthe block 1 6 are then ground down to size (Figure 11), removing a layer of black glass 19 to form a flat surface 20 (to which the photocathode can be applied), and the surfaces polished.

The faceplate thus formed has an inner cylindrical re gion2l,theradiusofwhich increases along its length from the photocathode surface 20 to the front surface 22 of the faceplate, and a surrounding shielding region 23 of blackglass.

Claims

1. A method of manufacturing a shielded faceplate for use in an image intensifier comprising a photocathode, including the steps of: arranging in a mould a memberofafirsttype ofglass,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 material to which glass does not bond; heating the members so that they fuse, therebyforming a block having a shape conforming to the internal configuration of the mould and having an inner region ofthe first type of glass and an outer region ofthe second type of glass; and removing the block from the mould.

2. A method as claimed in claim 1 and wherein the block so formed includes a portion ofthefirst type of glass which is completely encircled by a por tion ofthesecondtypeofglass.

3. A method as claimed in claim 1 or 2 and wherein the first and second types of glass have substantiallythe same refractive index.

4. A shielded image intensifier faceplate substantiallyas illustrated in and described with reference to Figure 11 ofthe accompanying drawings.

4. A method as claimed in claim 1, 2 or 3 and wherein the members are discs which have flat surfaces arranged adjacent each other in the mould.

5. A method as claimed in claim 4 and wherein, afterthe block is formed, some ofthe 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.

6. A method as claimed in any preceding claim and wherein the members are heated in an inert atmosphere.

7. A method as claimed in any preceding claim and wherein the members are initially heated at reduced pressure.

8. A method as claimed in any preceding claim and wherein the material ofthe mould is graphite.

9. A shielded image intensifier faceplate manufactured by a method as claimed in anyofthepreced- ing claims.

10. A shielded image intensifier faceplate as claimed in claim 9 and wherein the outer region surroundsthe inner region which has atransverse dim- ension which decreases along its length, being smal lest at the photocathodesurface.

11. A shielded faceplate for an image intensifier including a photocathode, comprising an inner core of glasstransparentto lightto which the photo- cathode 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.

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 glassfacepiate which supports the photocathode, thefaceplate being as claimed in any of claims 9, 10 or 1.

13. A methodbf manufacturing a shielded image intensifierfaceplate substantially as illustrated in and described with reference to Figures 6 to 11 of the accompanying drawings.

14. A shielded image intensifier faceplate substantially as illustrated in and described with referenceto Figure 11 ofthe accompanying drawings.

Superseded claims 1-4 New or amended claims:- 1-4 CLAIMS

1. Ashielded faceplate for an image intensifier, comprising aninnercoreofglasstransparenttolight of one wavelength distribution and an outer shielding region of g lass, which absorbs l ig ht of the said wavelength distribution, which surrounds and is adjacenttotheinnercore,theinnercorehavinga transverse dimension which decreases a long its length.

2. Afaceplateasclaimed in claim 1 and wherein the glass ofthe innercore andthe glass ofthe outer shielding region have substantially the same refractive index.

3. 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 faceplate which supports the photocathode, the faceplate being as claimed in claim 1 or 2 and the transverse dimension of the inner core being smallest atthe photocathode surface.