Patent GB-653075-A (GB)
Title: Improvements in or relating to photo-electric apparatus for electronic translation of optical images
IPC: H01J29/02
Filed: 1948-04-15
Abstract: 653,075. Cathode-ray tubes. EDWARDS, B. J., JOHNSTONE, D. M., and PYE, Ltd. April 15, 1948, No. 10535. [Class 39 (i)] In a photo-electric tube a stage of gain is inserted before the conversion of an electron image into an optical image in an image converter or before the scanning process in an image analyser. Such image multiplication or gain is obtained by the use of a normally insulating substance, such as diamond dust, as a current amplifier by bombarding it with electrons. An image converter tube, Fig. 1, comprises an envelope 10 containing a transparent photo-cathode 11, a first (image intensifying) target 12, and a second target comprising a fluorescent screen 13. The target 12 comprises a substance which is normally insulating but becomes conducting when bombarded by electrons. This is in the form of a layer or plate 14 which is coated on the side facing the cathode 11 with a thin electron-permeable conducting metallic film 15 and on the opposite side with a transversely non-conducting electron-emissive film 16. This may be photo-sensitive and caused to emit photo-electrons by illumination from a light source 17 arranged inside or outside the envelope 10. A shield 18 prevents background illumination of the fluorescent screen 13 by the source 17. The photo-cathode 11 is shielded from the source 17 by the target 12. Alternatively the surface 16 may be secondary emissive and a subsidiary electron source may be arranged for bombarding the surface 16 with electrons. A fine mesh screen electrode 19 is arranged close to the emissive surface 16. The envelope 10 is surrounded by a coaxial focus coil 20 for forming an electron image of the emission from the cathode 11 on the target 12 and also of the emission from the side 16 of the target 12 on the fluorescent screen 13. The electrode 15 is earthed and the electrode 19 has a potential of about 30 volts positive. An optical image is projected by means of an optical system 21, on to the photo-cathode 11, the potential of which is 1000 volts negative. The corresponding electron image is projected on to the target 12 so that the electrons have sufficient velocity to penetrate the film 15 and bombard the layer 14. This layer is rendered conducting to an extent at different points thereof depending on the incident electron intensity at those points. The conduction current through the layer 14 at any point, and consequently the emission current leaving the emitting surface 16, is much greater than the incident electron image current. The fluorescent screen 13 is at a potential in the range of 2000-10,000 volts positive. The electrons emitted from the surface 16 are drawn through the mesh electrode 19 to form an electron image on the fluorescent screen 13, which converts it into a visible optical image. In an image analyser tube the emission from the surface 16 of the target 12 is projected on one side of a secondary emitting target and forms a charge image. The other side of this target is scanned by a low velocity beam. In a modification, Fig. 3, the target 212 corresponds to the target 12, Fig. 1, and has a core consisting of a fine metal mesh 215 corresponding to the metallic electrode film 15, Fig. 1. In the interstices of the mesh 215 are secured particles 214 of a substance such as diamond dust corresponding to the layer 14, Fig. 1. On one side of the target structure 215 is formed a photoemissive film 216, corresponding to the film 16, Fig. 1, which may comprise a very thin transparent film of silver or antimony subsequently photo-sensitized with caesium.