The PSP, or pyranometer, measures global solar irradiance in the same spectral range as the NIP (280 to 3000 nm). The detector is made up of a circular multijunction thermopile attached to a black mylar disk, which sits horizontally beneath two quartz-glass domes. The inner dome protects the detector from infrared radiation from the outer dome, which may change rapidly with meteorological conditions. The detector is heated by solar energy and the thermopile reacts to the heating by generating a small electrical signal. A calibration factor converts the millivolt signal to an equivalent radiant energy flux in watts per square meter.