The pyrheliometer is a broadband instrument that measures the direct (or beam) component of solar radiation at normal incidence, i. e., it always is aimed directly at the sun. It is sensitive to wavelengths in the band from 280 to 3000 nm. To achieve normal incidence to the sun, the NIP is mounted on an Epply equatorial solar "smart" tracker that continuously trains the instrument on the sun. After sunset the tracker resets to the sunrise position. The aperture of the NIP is small enough to minimize the registration of circumsolar radiation but large enough to allow for the performance tolerance of the tracker. Solar irradiance enters the instrument through a crystal-quartz window that is sealed to the aperture. Within the instrument, the irradiance is directed onto a 3/8" square black mylar receiver on which a set of thermocouples (a thermopile) is cemented. The radiation warms the detector and excites the thermopile, which produces an electrical signal. A calibration factor is applied to convert the millivolt signal to an equivalent radiant energy flux in watts per square meter.