bolometer, instrument for detecting and measuring radiation, e.g., visible light, infrared radiation, and ultraviolet radiation, in amounts as small as one millionth of an erg. The bolometer was invented in 1880 by Samuel P. Langley. Basically it consists of a radiation-sensitive resistance element in one branch of a Wheatstone bridge; changes in radiation cause changes in the electrical resistance of the element. The radiation-sensitive element may be a platinum strip, a semiconductor film, or any other substance whose resistance is altered by slight changes in the amount of radiant energy falling on it.