GB792051A - Improvements in or relating to circuit arrangements for automatically terminating x-ray exposures - Google Patents

Abstract

792,051. Photo-electric control of X-ray exposure times. PHILIPS ELECTRICAL INDUSTRIES, Ltd. June 11, 1954 [June 16, 1953], No. 17220/54. Addition to 751,786. Class 40 (3). In a circuit arrangement for automatically terminating an X-ray exposure a radiationsensitive indicator disposed behind the subject comprises a number of radiation cells individually responsive to radiation passing through different parts of the subject. The output current of each cell alters the charge on a capacitor connected in the grid circuit of a corresponding discharge tube and the summated anode currents of the discharge tubes operates to cut off the X-rays when it attains a preset value. The invention resides in so arranging the circuit that when the intensity of X-rays on any of the cells exceeds a predetermined average value the corresponding discharge tube is driven to cut off or saturation before the summated currents attain the preset value. X-rays from tube 3 supplied by the mains 20, fall on film 5 after passage through the subject 4 under examination and then produce a light pattern on fluorescent screen 2 corresponding to the local variations of X-ray intensity on the film. The photo-cells 1 are each responsive to light from different parts of the fluorescent screen 2. Immediately prior to an exposure the normally closed switch 16 is opened to charge capacitors 7 to the voltage of battery 10. When the exposure is started the switch is closed but the triodes are cut off by the high negative bias. The resistances of photo-cells 1 are reduced by the fluorescence from the screen in response to X-rays incident on the film 5, thereby allowing current to flow from battery 6 to reduce the charge on associated capacitors 7. The anode current in the triodes 8 rises until when the summated currents exceed a predetermined level relay 13 operates to open the energizing circuit of the X-ray tube. If the intensity of illumination of the film is markedly uneven certain of the capacitors become discharged well before the others but the effect of this high local illumination in the exposure time is reduced by the fact that grid current limits the current in the associated valve so that it is the illumination of the least illuminated parts of the film which ultimately determines the exposure time. In an alternative arrangement the capacitors are uncharged at the start of an exposure and a relay corresponding to 13 is energized to close the supply circuit. On exposure the capacitors are charged to drive the grids negative so that when the total current falls to a preset level the relay releases to open-circuit the X-ray tube supply. In this case the effect of excessive illumination of individual cells is reduced by their being driven to cut-off. The fluorescent screen may be dispersed and cells giving a response to X-rays used instead of the photo -cells.