The bolometer, invented in 1878 by Samuel Pierpont Langley, measures the energy of incident electromagnetic radiation. It consists of an absorptive element connected to a heat sink through a thermal link. The absortive element raises its temperature above that of the heat sink when radiation impinges on it. The higher the energy the higher the temperature rises. Old bolometers used metals while newer ones used semiconductors and superconductors as the absorptive elements.

Bolometers can be used to measure radiation energy but there are more sensitive methods, however, for millimeter wavelengths they are among the most sensitive detectors and therefore used for astronomy at those wavelengths. They can also be used to detect non-ionizing particles and even to search for unknown forms of mass or energy. Bolometers are slow to respond and slow to reset.

The term bolometer is also used in particle physics to designate an unconventional particle detector. The first bolometer used for infrared observations consisted of two platinum strips, covered with lampblack, one strip was shielded from the radiation and one exposed to it.

Any electromagnet radiation that fell on the exposed strip would heat it and change its resistance thus acting as a resistance temperature detector. Langley’s bolometer was eventually refined in order to detect thermal radiation from a cow a quarter mile away.