The Eppley Normal Incidence Pyrheliometer is a World Meteorological Organization First Class Pyrheliometer designed, as its name implies, for the measurement of solar radiation at normal incidence.
The NIP incorporates a wire-wound thermopile at the base of a tube, the aperature of which bears a ratio to its length of 1 to 10, subtending an angle of 5°43'30". The inside of this brass tube is blackened and suitably diaphragmed. The tube is filled with dry air at atmospheric pressure and sealed at the viewing end by an insert carrying a 1 mm thick, Infrasil II window. Two flanges, one at each end of the tube, are provided with a sighting arrangement for aiming the pyrheliometer directly at the sun. A manually rotatable wheel (not shown) which can accommodate three filters, while leaving one aperature free, is provided.
The pyrheliometer is mounted on a power-driven equatorial mount for continuous readings. Please see Solar Trackers.
A calibration certificate traceable to the World Radiation Reference and a temperature compensation curve are included.