Performance of automatic exposure controls when used with rare-earth intensifying screens

The performance of four phototiming systems was evaluated for use with six different intensifying screens. Phototimer response was studied as a function of patient thickness, kVp, field size, and exposure time. Each system demonstrated a sufficient range of sensitivity to produce acceptable film densities with all screens using an average-sized phantom; three systems produced overexposures of thin phantoms with almost all screens. With each phototimer, the kVp response for most screens was similar, allowing the use of a single kVp-compensating circuit. Exposure times had to be twice the minimum response time to avoid overexposure.