De pyr pyrós, fuego del sol y metron , measured. Apparatus for measuring solar radiation on the earth or on any other surface. They also call it solarimeter and actinometer, from aktis aktinos, ray of the sun. Similar ones are periheliometer, pyriheliometer and periheliograph which quantifies radiation in a graphic record. The measurement is in kilowatts per square meter. All these instruments are useful in meteorology and photovoltaic technology.