Patent GB-1047659-A (GB)
Title: Improvements in or relating to infra-red viewing systems
IPC: H01J31/48
Filed: 1964-06-26
Abstract: 1,047,659. Image converter tubes. INSTITUT FUR PLASMAPHYSIK G.m.b.H. June 26, 1964 [June 27, 1963], No. 26442/64. Heading H1D. A system for producing a visible image of a source of infra-red radiation includes : means for focusing the infra-red radiation to form an image upon a photocathode in an evacuated vessel; a source of supplementary radiation arranged to be incident, in a controllable manner, upon the photocathode; and means for cooling the photocathode to a temperature at which the photo-electric yield varies rapidly with temperature, whereby the photoelectrons - the emission of which at a given point of the photocathode is determined by the local temperature to which the cathode is raised by the incident infra-red radiation-may be employed to produce a corresponding image. The infrared radiation from an object 3, Fig. 1, is focused through the germanium or indium antimonide window 5 upon the photo-emissive surface of a photo cathode 2 which is sensitive to the emission radiation of a phosphor layer coated upon its rear surface and scanned by an electron beam 8, the photo-electrons being collected by anode 10 and employed to modulate the intensity of a cathode-ray tube 12, the scan of which is synchronized with that of the beam 8. Photocathode 2 (Figs. 2 and 4, not shown) may consist of a photo-emissive layer on a carrier transparent to the supplementary radiation-e.g. mica-on the opposite face of which is coated one or more phosphor layers, or a mixture of two or more phosphors, and may be kept at, for example, - 150 C. by a cooling system 14. To reduce thermal conduction in the plane of the photo-emissive material, the latter may be in mosaic form (Fig. 4, not shown), an electrically conductive, thermally insulating tin chloride layer being interposed between the carrier and the mosaic. The photocathode may also have the photo-emissive material coated directly on either the phosphor layer, which is itself coated on an electron transparent aluminium foil, or on a layer of short-lifetime semi-conductor of suitable energy gap, such that the recombination radiation of the carriers excited by the incident electron beam 8 provides the supplementary radiation. In other embodiments, the photo-emissive material may be coated upon one end wall of the envelope, and the supplementary radiation may be provided by the scanned screen of a cathode-ray tube, with a fibre optical system interposed (Fig. 6, not shown), or by the radiation from a discharge scanned across the cathode by successive reflection from two multifaced mirrors rotating about their axes which are at right angles to one another (Fig. 7, not shown). In a further embodiment (Fig. 8, not shown) the photocathode 17 is uniformly irradiated by a spark-gap or xenon flash tube 36 mounted in the same envelope, the photoelectrons being focused by an axial magnetic field to produce a visible image on the luminescent screen 34; the pulse supplying source 36 may be synchronized with the frame transport frequency of camera 35. Reference is also made to the use of an electron multiplier in conjunction with anode 10, and to the use of X-rays as the supplementary radiation.