Bolometer, ( from the Greek bole — a throw; a ray, and metreo — I measure), a thermal non-selective detector of optical radiation, based on the change in the electrical resistance of a heat-sensitive element when it is heated by absorbing the radiation flux being measured. A bolometer serves to measure the power of integral (total) radiation, and together with a spectrometer — to measure the spectral composition of the radiation.
The relative change of the resistance ΔR/R when its temperature changes by an amount ΔT is described by the approximate relation ΔR/R=βΔT, where β is the temperature coefficient of resistance; for metals β=0.5% per 1 K, for semiconductors β≈4.2% per 1 K. A bolometer is connected in a bridge circuit, two arms of which carry two identical heat-sensitive elements. The radiation is directed onto one element, while the other serves to compensate for changes in the temperature of the surroundings and for radiation interference.
The heat-sensitive element of a bolometer is usually a thin (0.1 — 1 μm) layer of a metal (Ni, Au, Bi and others) whose surface is covered with a layer of black coating having a large absorption coefficient over a wide range of wavelengths, or a semiconductor with a large temperature coefficient of resistance. Semiconductor bolometers are made of Ge and Sb, and also of the oxides of Mn, Ni and Co. Superconducting bolometers, which work under deep cooling (3—15 K), are based on the sharp change in the electrical resistance of a metal on its transition from the normal state to the superconducting one. In the transition range, which amounts to fractions of a K, the temperature coefficient becomes very large (≈5000% per K), which leads to an increase in the sensitivity of the bolometer. Sn, Ta, Nb and others are used as materials for superconducting bolometers. In immersion bolometers the thermoelement is in optical contact with a lens made of a material with a high refractive index. This makes it possible to focus the radiation efficiently onto a receiving area as small as 0.01 mm2 and, by reducing the area of the heat-sensitive element, to lower the sensitivity threshold of the bolometer. Modern bolometers in the spectral range up to 50 μm and with a sensitive element of area 1 —10 mm2 have a sensitivity threshold of 10-11—10-10 W/Hz1/2 with a time constant of 10-3—10-1 s, and superconducting bolometers — respectively 10-12 W/Hz1/2 and 10-4 s. (On the parameters of bolometers (see Optical radiation detectors). Bolometers are used in measuring technology as detectors of infrared radiation.