Abstract: A circuit for operating an X-ray tube, such as in a medical diagnostics apparatus, is connected with a high-voltage generator and a switch mechanism for connecting and disconnecting the X-ray tube to the high-voltage generator. A resistor is disposed in a circuit branch which can be connected in parallel across the X-ray tube by a switch controlled by the aforementioned switch mechanism. If the X-ray tube is disconnected from the power source, the switch closes the circuit branch and causes the resistor to be connected across the X-ray tube. As a result, the X-ray tube voltage decreases relatively quickly after the high-voltage is disconnected. The resistor across the X-ray tube may be a variable resistor, such as a high-voltage triode.

Abstract: In an illustrated embodiment, a detector having a plurality of measuring fields is provided with a computer circuit for the formation of a difference signal which corresponds to the difference between the greatest and the smallest output signal of the measuring fields, a computer circuit for the formation of a sum signal which corresponds to the sum of the greatest and the smallest output signal of the measuring fields, and a division circuit is connected to provide an output as a function of the difference signal divided by the sum signal, as a measure of contrast over the selected measurement surface. The contrast signal can be utilized to control the duration of an exposure. Further, the contrast signal can be used for an automatic adjustment of the x-ray tube high voltage during an exposure so as to achieve an optimum picture contrast.