In the practical application of cineradiography, the radiologist needs a reliable and speedy determination of proper film exposure. Failure to achieve this greatly hampers the full realization of the clinical potential of this method of studying dynamic motion and restricts the extension of its usefulness to other areas of diagnostic roentgenology. As with spot-film radiography, some type of phototiming would be a desirable means of assuring correct film density for any combination of emulsion speed, framing rate, object density, and radiation exposure factors. Unfortunately, phototiming as we know it in conventional radiography cannot be readily achieved without a radical change in commercially available cine cameras. The compact design of these cameras does not provide sufficient space for the insertion of even a small conventional photocell into the area of the filming aperture. It is conceivable that a tiny selenium (cadmium sulfite) type photosensitive crystal could be placed in the camera, but this ...