Exposure control appts. for X=ray generator - has three separate photomultipliers for image amplification and compensated charging capacitor in signal monitoring circuit

摘要

The arrangement consists of an X-ray generator and screen, and an image intensifier which transmits luminous flux through optical fibres to three photo-multipliers. Each photomultiplier covers a different sector of the image field. One of the control functions available at the central console is the selection of photo multipliers to which high voltage and current-controlled supplies are connected. Signals generated in the photomultipliers are passed to an integrator unit which consists of a capacitor charged by the photocurrent and a FET operational amplifier comparator circuit which compares the capacitor terminal voltage with a voltage reference in order to vary the generator 'on' time in accordance with the luminous flux incident on the photomultiplier. The comparator incorporates a current threshold sensing circuit which regulates the high tension voltage to a safe level. A resistor-diode network acts as a non-linear compensator in parallel with the capacitor and responds to a threshold voltage set by the diodes.