A phototimer for x-rays or gamma rays controls the exposure of a radiological imaging device. The phototimer sensor comprises a film of a radiosensitive dielectric material, such as thallium bromide, sandwiched between upper and lower metal layers. This film can be about ten microns thick or less, so as to be thin enough to be radiolucent, but still produce sufficient charge carriers under exposure to x-ray or gamma ray radiation. A phototimer circuit arrangement coupled to the metal layers can be configured as a current amplifier or as a voltage amplifier. A cooling arrangement can be incorporated to maintain the sensor at a low temperature to reduce or eliminate thermally activated dark current. The phototimer can be incorporated into a large area radiological imager.