Abstract: A highly accurate thermoelectrically controlled radiant energy detector achieving sufficient measurement stability to permit it to monitor the radiant energy output of a calibrated standard instrument, such as a luminance/illuminance source, which is to generate a standard quantity of radiant energy to be used, e.g., in the field testing and calibration of photometric instruments. The radiant energy detector provides a highly stable and drift free photometric measurement using a silicon photovoltaic detector arranged to receive radiant energy upon a sensitive surface thereof and to emit an electrical output signal related to the magnitude of incident radiant energy. A thermally conductive member, such as a mounting block, carries the light detecting element, and temperature sensing means, such as the semiconductor junction of a transistor, is in thermal communication with the conductive member.