What is "pyrheliometer"

Pyrheliometer \Pyr*he`li*om"e*ter\, n. [Gr. ? fire + ? sun + -meter.] (Physics) An instrument for measuring the direct heating effect of the sun's rays.

n. A device that measures the intensity of solar radiation received on the surface of the earth.

A pyrheliometer is an instrument for measurement of direct beam solar irradiance. Sunlight enters the instrument through a window and is directed onto a thermopile which converts heat to an electrical signal that can be recorded. The signal voltage is converted via a formula to measure watts per square metre. It is used with a solar tracking system to keep the instrument aimed at the sun. A pyrheliometer is often used in the same setup with a pyranometer.