1. An apparatus for measuring the heating effects of the sun.
2. An instrument that measures the intensity of direct solar radiation.
3. A general term for the class of actinometers (name for any instrument used to measure the intensity of radiant energy, particularly that of the sun) that measure the intensity of direct solar radiation.
4. An instrument that measures the total intensity of direct solar radiation received by the earth.

Actinometers may be classified, accordingly to the quantities that they measure, in the following manner:

• Pyrheliometer, which measures the intensity of direct solar radiation.
• Pyranometer, which measures global radiation (the combined intensity of direct solar radiation and diffuse sky radiation)
• Pyrgeometer, which measures the effective terrestrial radiation.