The ultimate performance which can theoretically be obtained with a fluoroscopic imaging device is determined by the quantum nature of the x-rays themselves. In medical fluoroscopy, the available x-ray intensity is set by the permissible patient dose, so that the information carried by the x-rays is quite definitely limited. This fact was first pointed out by R. C. Mason (1) in 1944. Since then, experimental work and calculations have been carried out by several others. The limitation has usually been expressed in terms of the minimum brightness gain of an ideal intensifier in which the perception is limited by the scintillation effect rather than by the observer's eye. Mason estimated that the scintillations would be noticeable at a brightness gain of about 10, and expressed serious doubt as to whether very large brightness gains (i.e. 10,000) would be of interest. An experimental test of the effect was carried out for a particular case by Coltman and Ebbighausen (2, 3). Under conditions equivalent to a ...