GB593764A - A new or improved exposure control system for photographic purposes - Google Patents

Abstract

593,764. Photographic exposure control. GRAVES, A., DAWES, T. J., and ALLTOOLS, Ltd. May 4, 1945, Nos. 11415 and 31497. [Class 40 (iii)] In an apparatus for the control of photographic exposures, e.g. in X-ray photography, a condenser is charged through a photo-electric cell from a H.T. source, and discharged at a predetermined potential by a neon or similar discharge device through a thyratron. In one embodiment, Fig. 1, light from X-ray activated fluorescent screen 1 activates a pilot photoelectric cell 3 rendering the grid of the thyratron 4 sufficiently positive for the tube to fire. The anode current through relay coil 9 opens and closes contacts 7 and 10 respectively. A bank of photo cells 2 is thereby connected between HT3 and earth in series with parallel connected variable condenser 5 and neon tube 8, the comdenser charging rate depending on the resistance of bank 2, which normally varies linearly with the intensity of the light, and on the values of the capacity selected by the arm 6. At a predetermined condenser voltage the neon tube 8 strikes with a consequential rise of thyratron 11 grid voltage, which fires and operates a relay (not shown), to extinguish the X-ray beam. To re-set for the next exposure, the HT+ lead is momentarily broken, deenergizing tubes 4 and 11 causing contacts 7, 10 to revert to normal. Where the time base is too short for the employment of a mechanised relay 9, a gas-discharge type may be substituted. The apparatus may also be used for night-photography, when the thyratron 4 opens and closes a shutter interposed between the light and the cells 2. In a second embodiment, Fig. 2 (not shown), an X-ray beam and the time base circuit are switched on manually, and the change of condenser voltage amplified thermionically before application to the grid of the thyratron operating the X-ray beam extinguishing switch.