Patent GB-701884-A (GB)
Title: Gamma control apparatus
IPC: H04N5/20
Filed: 1952-04-29
Abstract: 701,884. Copying telegraphy; television. RADIO CORPORATION OF AMERICA. April 29, 1952 [May 1, 1951], No. 10805/52. Class 40 (3). [Also in Group XL(c)] In a copying-telegraphy (i.e. colour printing or television transmitter, the scanning apparatus comprises means for scanning successive areas of the subject which are sufficiently small to resolve a desired amount of detail and further scanning means, synchronized with the first scanning means, which scans the subject without resolving the same amount of detail ; the signals derived from the second scanning means being used to effect "gamma" or contrast control of the signals derived from the first scanning means. The control signals are used to reduce the contrast in large areas of the picture ; however, due to the lack of small-area definition in the control signals, the contrast in small areas of the picture is not reduced. Copying-telegraphy.-Fig. 1 illustrates a colourprinting system employing three colour-separation transparencies 10, 20 and 30 and a further transparency 36 representing brightness. A scanning raster on the screen of a cathode-ray tube 2 is focused on each of the transparencies by means of suitable optical systems including partially silvered mirrors 4, 12 and 28, the caster however being but imperfectly focused on the transparencies 36 so that the signals generated in photo-cell 40 correspond to a lower definition than those generated in photo-cells 16, 26, 34. The control signal generated in the photo-cell 40 may be used in either of two ways ; it may either be applied via a non-linear amplifier 44 to control the responses of the photo-cells 16, 26 and 34, or it may be applied via a second non-linear amplifier 46 to control the brilliance of the raster on the screen of the cathode-ray tube. In a modification, Fig. 6 (not shown), a single colour transparency is used together with one representing brightness, the colour separation being effected by a suitable optical system including dichroic mirrors. Television.-In a film-scanning system employing flying-spot scanning a sharply focused image of the scanning spot is produced in the film gate 104, Fig. 7, and a corresponding signal is developed in the photo-cell 108. An out-offocus image of the scanning spot is produced in the film gate 114 and the corresponding signal developed in the photo-cell 120 is used to effect contrast control. This control may be achieved by applying the output from photo-cell 120 either via non-linear amplifier 124 to control the response of the photo-cell 108 or via non-linear amplifier 126 to control the brilliance of the spot on the screen of the cathode-ray tube. The flying-spot scanner may scan a full raster on the screen of the cathode-ray tube 100, the film being moved intermittently, or it may scan a single line on the tube in which case the film is moved continuously. In a modification, Fig. 8 (not shown), both scanning processes are effected within the same film-gate, an optical system being provided to cause the two rasters to be projected on the film-gate from slightly different directions. In a further modification, Fig. 9, employing two camera tubes, the image produced on camera tube 166 is scanned by a de-focused electron beam and the output from the tube is applied through a non-linear amplifier to a gain control device 162 controlling the amplitude of the output signals from the tube 160. Should the invention be applied to systems employing films or transparencies which have been so treated as to reduce their contrast range, further non-linear amplifiers, as indicated by the dotted rectangles in Figs. 1, 7 and 9, may be provided to compensate for the further reduction in contrast provided by the invention thus restoring the large-area contrast to its value in the film or transparency and increasing the contrast in the detail above its original value.