Patent GB-619668-A (GB)
Title: Method and apparatus for portraying intelligence by the phenomenon of luminescence
IPC: H01J29/14
Filed: 1944-06-30
Abstract: 619,668. Cathode-ray tubes ; Luminescent materials. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. June 30, 1944, No. 12543. Convention date, July 22, 1942. [Class 39 (i)] A viewing screen in cathode-ray tube comprises an alkali halide, preferably potassium chloride, or magnesium oxide screen 10, Fig. 3a, which develops colour centres when scanned by an electron beam, and a second screen 12 of material which becomes luminescent when scanned by an electron beam or when irradiated by visible or ultraviolet light, and the emission spectrum of the luminescent screen 12 lies within, coincides with, or overlaps the absorption spectrum of the colour centres. In the form shown in Fig. 3a, the halide screen 10 is scanned by an electron beam, and light from source 3 is projected through screen 10 to excite phosphor screen 12. The colour centres in screen 10 absorb light from source 3 and also light emitted by screen 12 so that the contrast between excited and non-excited areas of screen 10 is increased. The trace is viewed from the halide side or from the phosphor side, Fig. 3c (not shown), and source 3 may be on the halide side or on the phosphor side of the screen. In another form, Fig. 3b (not shown), the screens 10, 12 are some distance apart; the screen 12 may be located outside the envelope with or without intervening lens systems for focusing the light therebetween. In another form, Fig. 3 (not shown), the positions of the screens 10, 12 are interchanged and the phosphor screen is excited by the scanning beam, which penetrates through the phosphor screen 12 and also develops colour centres in the halide screen 10. The phosphor material may have a persistence characteristic longer than the persistance of the human eye so that the phosphor screen can also illuminate the halide screen between successive beam scans. The persistence may be accentuated by using a number of phosphor screens in cascade, as described in Specification 22505/44, as open to inspection under Sect. 91, Fig. 3e (not shown). The phosphor screens may be excited by a light source or by the scanning beam; in the latter case the beam penetrates the halide layer and impinges on the first phosphor screen. The cathode-ray tubes in which the screens are mounted have two windows, Fig. 1 (not shown), and the screens may be supported on one window or independently inside the envelope. The beam is magnetically or electrostatically focused and is deflected by coils to give a circular, radial, or rectangular trace. The light source for exciting the phosphor screen may be a medium pressure mercury vapour lamp or fluorescent lamp. The phosphor material may be alpha willemite, copper activated zinc cadmium sulphides having a molar ratio of zinc sulphide to cadmium sulphide of 86 : 14, manganese activated zinc aluminate, beta zinc silicate, or zinc beryllium silicate with or without tin as a constituent, wherein the molar proportions of zinc oxide to beryllium oxide to silicon dioxide are 4:2:3. In a modification, the colour centres may be produced in " invisible " preferably ultra-violet light, and ultra-violet light is projected through the halide on to the phosphor screen. The images of the colour centres then appear as dark spots on the luminescent screen. Specification 446,254 is referred to.