A radiometer or roentgen meter is a device for measuring the radiant flux (power) of electromagnetic radiation.
Generally, a radiometer is an infrared radiation detector or ultraviolet detector. The name Radiometer is frequently used to refer to a Crookes radiometer (“light-mill”), an early model device wherein a rotor (having vanes which are dark on one side, and light on the other) in a partial vacuum spins when exposed to light.
A common myth (one originally held even by Crookes) is that the momentum of the absorbed light on the black faces makes the radiometer operate. If this were true however, the radiometer would spin away from the non-black faces, since the photons bouncing off those faces impart more momentum than the photons absorbed on the black faces. Photons do exert radiation pressure on the faces, but those forces are dwarfed by other effects. The final explanation depends on having just the right degree of vacuum, and is more related to the transfer of heat than the direct effect of photons.