Patent GB-532106-A (GB)
Title: Improvements in and relating to methods of producing optical effects
IPC: H01J31/50
Filed: 1939-05-24
Abstract: 532,106. Light valves and relays ; optical apparatus; cathode-ray tubes. BRITISH THOMSONHOUSTON CO., Ltd., and GABOR, D. May 24, 1939, Nos. 15451 and 19720. [Classes 39 (i) and 40 (iii)] [Also in Group XX] A light valve or relay comprises a rigid base member and a thin film separated from it by a distance not greater than one wavelength of visible light and is such that a change in this separation caused by a force or radiation causes a corresponding change in the intensity of light transmitted by the device. To the hypotenuse surface of a right prism 1, Fig. 1, a thin film 2 is clamped by an apertured plate 3. The film 2 is preferably of the same refractive index as the prism and is normally in optical contact with it over the whole of its surface. When radiation i falls on the upper surface of the film it is heated and expands and by buckling outwards is no longer in optical contact with the face of the prism. Under these conditions auxiliary light I incident on that face at an angle greater than that of total reflection is reflected out through the second lower face. When the two surfaces are in contact, in the absence of any primary radiation, the auxiliary light is not totally reflected but passes into the film and is diffused or absorbed at its upper surface. The change of reflected light from total reflection to zero takes place within a separation equal to one wavelength of the auxiliary light, Fig.. 2 (not shown). The film may. on expansion, bulge away from or towards the base plate, Figs. 3, 4 (not shown). in the latter case the base plate being suitably recessed. By this method also the effect seen by reflected light is positive since an increase in primary radiation causes an increase in reflected light. As applied to a screen for viewing picture images as in a cathode ray tube, the film 2, Fig. 7, may be pressed into optical contact with base plate 4 by a fine gauge gauze 5 or the base plate may be formed with closely spaced ridges, Fig. 8 (not shown). The surface of the film 2, Fig. 11, may also be coated with a fluorescent material 6 to assist in focusing an electron beam and to serve to diffuse light transmitted through the film from the auxiliary source so that the image may be viewed by transmitted instead of reflected light ; the large prism forming the base plate may be replaced by a thin plate 12 having a serrated lower surface. As applied to cathode ray tubes for television receivers, such screens may form the outer end of the tube, Fig. 10 (not shown), the image then being focused on a projection screen by using reflected light or a screen having a serrated rear surface may be placed inside the tube, Fig. 12 (not shown) in which case transmitted light is used to form the external image. The principle may also be used to view distant objects by infra-red light. Infra-red rays are focused by suitable lenses 25, 26, Fig. 14, and a plane mirror 27 on the film side of a relay screen 4. Auxiliary light from a lamp 15 is made to fall on the rear face 12 of the screen. The visible image formed by transmitted light is viewed through a lens system 28, 29, 33. In order to compensate for the non-uniform illumination of the screen produced by the light passed through the points of contact of the film and its base plate, a photographic plate 31 previously exposed to the screen m the presence of auxiliary light but in the absence of any primary radiation and subsequently developed is placed in the identical position in which it was exposed. The dark areas in the photographic plate then remove the effect of the light areas in the relay screen. A method for overcoming non-uniform response caused by the initial separation of the film and base not being uniform over the whole surface consists in allowing light from an auxiliary lamp to fall on the film side of the screen after passing through a suitably graded photographic plate. Fig. 15. This light causes heating of the film in a manner similar to the primary radiation but by being correctly apportioned by the photographic plate it causes the separation to be uniform over the whole surface in the absence of any primary radiation. A simple apparatus for viewing objects by infra-red rays and utilizing a reflector in place of lenses is described with reference to Fig. 13 (not shown).