Patent GB-987867-A (GB)
Title: Apparatus for amplifying the brightness of an optically formed image
IPC: H04B10/00
Filed: 1961-06-29
Abstract: 987,867. Optical projection systems. GES ZUR FORDERUNG DER FORSCHUNG AN DER EIDGENOSSISCHEN TECHNISCHEN ROCHSCHULE. June 29, 1961 [June 30,1960], No. 23606/61. Heading G2J. [Also in Divisions H1 and H4] Relates to an improvement in apparatus for amplifying the brightness of an image of a kind, such as that disclosed, for example, in Specification 741,154, where the image is employed to cause deformation of a control layer 18 bearing a metallic mirror surface 17 which is located in a Schlieren optical system (not illustrated) for modulating the light from a separate high intensity source. The control layer is subjected to an electrostatic field provided by potential source U1 connected between electrodes 19 and 23 and the deformation is effected by including a photoconductive layer 14a which is illuminated by the image and produces variations in the electrostatic field in correspondence with the resistance variations imparted by the image. Layer 14a is illuminated through an optical raster 12a. In accordance with one form of the invention, electrode 23 is made in the form of spaced apart strips, alternate ones of which are connected together and connected over leads 23a, 23b to potential source U2. The edges of the strips extend orthogonally to the edges of the mirror bars in the Schlieren optical system. In the absence of image light the electrostatic field pattern which is set up causes a uniform wavy deformation of the mirror surface with the crests and valleys extending in the direction of the electrode strips and hence at rightangles to the Schlieren mirror bar. The resulting light diffraction thus takes place along the bars and no light is transmitted. If, however, an image is projected via optical raster 12a a distortion of the uniformity of the electrostatic field pattern is produced which includes components extending orthogonally to strips 23. The resulting deformation of mirror 17 is similarly oriented with the result that light diffraction takes place across the Schlieren mirror bars causing light to be transmitted. It is essential that the strips of optical raster 12a should be aligned at an angle which is substantially different from 0 degrees and 90 degrees with respect to the strip 23; the preferred angle is 45 degrees. The invention has the advantage of permitting photoconductor 14a to be an extremely thin layer, a thickness of 0. 5� is referred to), which may be produced with extreme uniformity by evaporation techniques. The sources U1 and U2 may be A.C. or D.C. sources. Source U1 may be dispensed with. In a second form of the invention, Fig. 7, (not shown), optical raster 12a is in the form of a pattern of spaced circular opaque or transparent area. In a third form of the invention, Figs. 8-11 (not shown), the rasters take the form of two superposed line rasters respectively disposed in alignment with and orthogonally to the strips of electrode 23, one raster being transmissive of only red light and the other being transmissive of only blue light. With this arrangement light is transmitted through the Schlieren system to produce an image only in response to the simultaneous illumination of the photoconductor by a red and blue image. Raster 12a may be formed of a material, such as bismuth sulphide, which is opaque to visible light but transmits infra-red light. By this means the photo-conductive layer may be illuminated uniformly with infra-red light with the object a reducing photo-conductive lag effects. Reference is made to the use in the Schlieren system system of mirror bars which are in the form of concentric annuli. In this case electrode 23 is formed of sector shaped members whose edges extend radially to said annuli. The light source used in the Schlieren system may have a wavelength different from that of the original image, reference being made to the use of the apparatus as an infrared to visible image-converter.