To transform radiant energy into kinetic energy in the infrared and visible spectrum. The "Crookes radiometer," also known as a "light mill" or "solar engine," consists of an airtight glass bulb within which a partial vacuum has been made. Inside is a pinwheel mounted on a spindle. The vanes enter rotation when exposed to light, and the speed of rotation is proportional to the intensity of the light source, allowing quantitative estimation of the intensity of electromagnetic radiation.