A high radiation‐flux, measuring calorimeter has been designed for use in imaging furnaces. It consists of a black‐body receiver provided with a cooled aperture and cylindrical housing. It can be exposed to flux densities of the order of 400 cal/cm2sec for extended periods without deterioration. Thermopiles measure the temperatures of the inlet and outlet cooling water. The calorimeter is an absolute measuring device with a response time of one minute. The design virtually eliminates heat losses and permits the measurement of a wide range of heat fluxes. The probable error in the radiation‐flux measurement is ±5%.