Patent GB-1272448-A (GB)
Title: Improvements in automatic light control system
IPC: G03B7/08
Filed: 1969-08-26
Abstract: 1,272,448. Television. GENERAL ELECTRIC CO. 26 Aug., 1969 [27 Nov., 1968], No. 42499/69. Heading H4F. [Also in Divisions G1 and G2]  The intensity of the light admitted to the surface of a photo-resistive element e.g. a television camera 1, Fig. 1, or photographic plate (la, Fig. 2, not shown) is controlled by means of a variable neutral density filter 2 e.g. two coaxial oppositely rotatable discs whose opacity increase in opposite directions, one such rotatable disc and a fixed neutral density sector, a neutral density film tape loop (Fig. 2, not shown) or a variable aperture iris. To vary the density of filter 2 the discs are counter-rotated via mechanical coupling arrangement 11 by means of a motor 10 which is responsive to the output of an error sensing circuit 12. The system operates in two modes. For small variations in light intensity the system operates in its "normal" mode in which the output of camera 1 (or a photoelectric cell 16, Fig. 2, not shown, a beam splitter being employed) is amplified 13 and fed as one input to a bi-level logic switch 15 wherein it is switched together with a "normal" mode reference potential from source 14 to error sensing circuit 12. Except where the difference between these signals is great (see later) they are compared, and the difference energizes motor 10 causing rotation of the discs forming filter 2. Thus the system operates linearly in the "normal" mode to maintain constant photosurface illumination. An analogue circuit 16 is mechanically coupled to arrangement 11 and produces an "overexposure" mode control signal which varies directly with the position of the discs in filter 2. During "overexposure" mode operation circuit 15 connects the "overexposure" control signal to be compared by circuit 12 with an "overexposure" reference potential supplied by a source 17 so set that any difference therebetween is nulled when filter 2 is in its most dense position. In the "over-exposure" mode, circuit 12 causes motor 10 to increase the density of filter 2, whenever it presents less than maximum density, as quickly as possible rather than to linearly compensate for changes in scene brightness. The system is switched into either its "normal" or "overexposure" mode by operation of a control unit 20 which produces a bi level command signal which is coupled to switching circuit 15. When scene brightness is below a predetermined level and when photosurface illumination is not undesirably high, the command signal is at a first level to operate the system in the "normal" mode. The control unit 20 produces a command signal having a second level in response to scene brightness over the predetermined level or undesirably high photosurface illumination to initiate "over-exposure" mode operation. To provide additional protection of the camera 1 during "overexposure" mode operation, control unit 20 operates a shutter 21, the solenoid 23 which controls its position being energized to open the shutter in response to the first level command signal and de-energized in response to the second level command signal so that the shutter is closed by spring 22. Control unit 20 is made responsive to photosurface illumination during "normal" mode operation by connecting the "normal" mode control signal from amplifier 13 thereto. If scene brightness suddenly increases so that the photosurface illumination reaches the override level (predetermined level) before the filter can compensate, the "normal" mode control signal becomes sufficiently great to operate unit 20 to produce a command signal of the second level. "Overexposure" mode operation is initiated and the discs at filter 2 rotate towards their most dense position. At the same time the shutter 21 is closed. Consequently the "normal" mode control signal goes to its lowest level and the input to control unit 20 is such that "normal" mode operation is resumed. If the photosurface illumination is still high when shutter 21 is reopened, "overexposure" mode initiation is resumed. The system thus cycles between modes until adequate light compensation is provided. In addition the control unit 20 is made responsive to scene brightness by photo-cell 25, so that when scene brightness is above a predetermined level control unit 20 initiates and maintains "overexposure" mode operation. Light from the scene is transmitted to photo-cell 25 by a lens 26 and a neutral density filter 27. If the image sensor 1 comprises a tube having a filamentary cathode rather than a solid state device, additional protection for the sensor 1 during periods of warm-up is provided by coupling control unit 20 to a voltage delay circuit 30 which is connected to the power supply 31 of the sensor 1, so that during warm-up the system is operated in the "overexposure" mode.