1. A signal processing system for radiation imaging apparatus comprising: (a) detector means capable of producing an output signal representing apparent orthogonal spatial coordinates and the associated energy level of a radiation event; (b) means for determining the energy level distribution of radiation events occurring within predetermined elements in response to an image of known energy level, each of said elements defining a predetermined portion of the apparent image field; and (c) means for causing each element to accept substantially the same number of events in response to said image.