Various methods of controlling the amount of radiation provided by X-ray source are known in the prior art. One of the common ways is to arbitrarily set the amount of current through the X-ray tube and terminate the exposure at a predetermined time by means of a timer, as disclosed in U.S. Pat. No. 3,284,631. Another method is to position a phototube adjacent the X-ray sensitive film to provide a measure of the total radiation passing through the object to the film. Current through the phototube which is proportional to the radiation impinging on the tube may be digitized and summed and the exposure terminated when a predetermined total has been reached, as disclosed in U.S. Pat. No. 3,356,847. Other methods have attempted to maintain the voltage and current applied to the X-ray tube constant, to thereby maintain constant X-ray exposure values, as disclosed in U.S. Pat. No. 4,039,811.
Ion chambers of various types and configurations are well known in the art, and may be conventionally connected to provide a signal proportional to the X-ray radiation impinging thereon.