GROOVED PHOSPHOR SCREENS FOR ONLINE PORTAL IMAGING

Video-based systems for on-line portal imaging utilize a metal plate c oated with Gd2O2S phosphor at a typical thickness of 500 mg/cm2. A new screen design is proposed wherein the conventional flat phosphor coat ing is replaced by a much thicker phosphor layer (1000-2000 Mg/cm2) pe netrated by either lineal grooves or pyramidal holes comparable to the system pixel size. By increasing the surface area of the phosphor, th e grooves or holes allow light from deep layers of the phosphor to esc ape by a process of internal reflection. In addition, the escaping lig ht is strongly forward peaked, improving optical coupling to the video camera. The processes by which grooved screens intensify light output have been modeled in a simple computer program that gives approximate agreement with experiment. Prototype screens have been constructed th at provide several times the forward light output of flat screens, and that improve DQE(f) in light photon limited systems for spatial frequ encies below 0.4 mm-1.