2 3/8" Diameter Purple Sphere on Clear Tapered Stem. The radiometer, also known as a light windmill, is an instructive physical object which actually demonstrates a scientific theory, namely, converting light into energy. This theory shows how the radiometer converts light into energy. The radiometer will spin 2000 revolutions per minute in warm light, such as sunlight and well lit rooms. They will not spin using fluorescent lights. These radiometers are truly one of a kind.