Patent GB-1002771-A (GB)
Title: Alignment of television camera
IPC: H04N9/09
Filed: 1963-03-15
Abstract: 1,002,771. Photo-electric measurements. RADIO CORPORATION OF AMERICA. March 15, 1963 [March 29,1962], No. 10473/63. Heading G1A. [Also in Division H4] The invention relates to a colour television camera and is concerned with the provision of means to permit the alignment of the camera to obtain optimum performance by arranging the camera cathode-ray monitor tube to display three light bars, the first representing the intensity of the input light to the camera, the second representing a standard light intensity and the third representing the response of a camera tube to the input light. The camera comprises an imageorithicon tube 17 for generating a luminance signal and three vidicon tubes 18,19, 20 for generating component colour signals. The optical system includes a zoom lens 14 incorporating an iris diaphragm 16 and a beam splitter 23 (e.g. halfsilvered mirror) from which part of the inputlight passes to the image-orthicon tube and the remainder passes to the vidicon tubes by way of colour selective dichroic mirrors 26,27,28. The light path to the image-orthicon tube includes a variable neutral density filter 33. To permit the input light intensity to be measured there is provided a photo-diode 43 mounted on a mask 41 pivoted at 42. The mask is introduced into the light path to the vidicons during the alignment procedure (i.e. it is moved about the pivot to the position shown by solid lines) and is of a size such that it obstructs two-thirds of the image formed on the targets of the vidicon tubes whereby the tubes produce an output only during the scanning of the final one-third of each horizontal line. The output from the photo-diode is applied to a chopper 51 which produces a pulse 52 of amplitude determined by the photo-diode output and lense the input light intensity, the pulse being timed to occupy the first one-third of each horizontal line scan. The camera also includes a pulse source 48 which generates a pulse 49 of amplitude representing a standard light intensity during an interval corresponding to the scanning of the second one-third of each horizontal line. Pulses 49 and 52 are combined in adder 53 and when a switch 40 is closed, applied to adders 58-61 through which the individual camera tube signals pass to pre-amplifiers 62-65. A desired one of the pre-amplifiers signals may be selected by switch 36-39 and applied through an amplifier 66 to the camera cathoderay monitor tube 44. During the scanning of the complete vaster the tube produces three vertical bars, the first 46, being due to pulse 52 and representing the intensity of the input light as measured by photo-diode 43, the second, 45, being due to pulse 49 and representing a standard light intensity, and the third 47, being due to the selected camera tube output. In order that the imageorthicon tube 17 should produce an output only during the scanning of the last one-third of each line the target is controlled by blanking pulses 54 applied via a switch 40a. The image-orthicon tube photo-multiplier 68 is controlled by a gain control 67 and each vidicon has means (not shown) to control the amplitude of output signal by variation of voltage on the signal electrode. During the alignment procedure the camera is exposed to white light from a card of specified reflectance under a specified level of illumination and iris diaphragm 16 is adjusted until bar 46 on the monitor tube 44 equals standard bar 45. Switch 36 is then closed to display the image-orthicon output on bar 47 and the iris diaphragm is varied. If no variation in bar intensity occurs the camera tube is exposed above the knee of its characteristic. Adjustment of neutral filter 33 is made until the tube operates on the linear part of its characteristic. Thereafter gain control 67 is adjusted to equalize bar 47 with bar 45 and then switches 37- 39 are closed in turn and the outputs of the vidicons adjusted to likewise equalize bars 47 and 45.