Radiometers are a very basic device invented in 1873 by English physicist Sir Edward Crookes. Basically, it coverts the light into energy and motion. Four square vanes are are inside a partially evacuated glass tube. The vanes are black on one side and silver or white on the other. When light hits the radiometer, the black side rotates away from the source of light. Since the black side gets hotter, the molecules near it have more kinetic energy. This increased kinetic energy causes the molecules to collide with the darker side.

There are many new sophisticated ways to measure the sun's radiation. Most use the basic concept employed by Crookes. Today these devices are meticulously calibrated to avoid error in readings.