Nomenclature

American Society of Heating, Refrigerating and Air-Conditioning Engineers

Power from the sun expressed as the radiative flux over an area (W/m^2)

Instrument to measure solar irradiance.  There are two main types: (1) thermopile and (2) Silicon photodiode.  A thermopile pyranometer measures a flat spectrum covering the 300 to 50,000 nanometer range while a Silicon photodiode only measures o the absorption spectrum of silicon.

Instrument to measure the direct component of irradiance. When placed on a 2-axis tracker to point at the sun, it measured direct normal irradiance (DNI).