An instrument used for the measurement of direct solar radiation. It consists of a silver disk located at the lower end of a tube containing a diaphragm that serves as the radiation receiver for a calorimeter. Radiation falling on the silver disk is periodically intercepted by means of a shutter located in the tube, causing temperature fluctuations of the calorimeter that are proportional to the intensity of the radiation. The instrument is normally used as a secondary instrument and is calibrated against the water-flow pyrheliometer. It was used by the U. S. Weather Bureau as a standard instrument. Currently, this instrument's function has been replaced by an absolute cavity radiometer. See pyrheliometer, cavity radiometer.