Bolometry

Bolometry is a method for measuring radiant energy by converting incident electromagnetic radiation into heat and detecting the resulting temperature rise. Bolometers are broadband power detectors, meaning they integrate energy across a spectral band rather than resolving individual wavelengths.

The basic operation involves an absorber that captures photons and heats up. The temperature change delta T is related to the absorbed energy Q by delta T = Q/C, where C is the heat capacity of the absorber and thermometer. The absorber is thermally linked to a heat sink by a weak conductance G, so the device has a characteristic response time tau ≈ C/G. A thermometer—often a resistive element, a superconducting transition-edge sensor, or a thermistor—converts the temperature change into an electrical signal. Noise sources such as phonon noise, Johnson-Nyquist noise in the sensor, and photon noise limit sensitivity.

Bolometer designs vary, but many use cryogenic operation to minimize heat capacity and achieve high sensitivity. Membrane-supported absorbers and thin-film thermometers reduce C and G, improving responsivity. Modern astronomical bolometers frequently employ superconducting devices such as transition-edge sensors (TES) or microbolometers, and array technologies enable imaging over large fields of view. While some bolometers are used at cryogenic temperatures, uncooled microbolometers are common in infrared camera applications.

Applications span infrared and submillimeter astronomy, cosmic microwave background experiments, terahertz imaging, spectroscopy, and security or industrial monitoring. Performance is characterized by the noise-equivalent power (NEP), time constant, and, for imaging, the noise-equivalent temperature difference (NETD). Bolometry remains a foundational technique for broad-band, sensitive detection of radiant energy across wide spectral ranges.