bolometers

A bolometer is a sensitive detector that measures electromagnetic radiation by converting absorbed energy into a small rise in temperature in an absorber; the temperature change is then read out by a thermometer. Bolometers are a type of calorimeter and are capable of detecting a wide range of wavelengths, from far-infrared to X-rays, depending on design.

Operation depends on a very small heat capacity at cryogenic temperatures and on a weakly coupled thermal link to a heat sink. Absorbed energy E raises the absorber temperature by ΔT = E/C, where C is the heat capacity. The detector output is proportional to ΔT (or to the thermometer resistance change). The thermal time constant τ = C/G sets the response speed, with low C and moderate G giving fast, sensitive performance.

Common realizations include resistive bolometers using a thermistor, superconducting transition-edge sensors (TES) and kinetic inductance detectors (KID). Magnetic microbolometers and scintillating bolometers are also used in specialized applications. Bolometers are routinely operated at temperatures from tens of millikelvin to a few kelvin to maximize sensitivity.

Readout requires low-noise electronics and, for array instruments, multiplexing. Calibration often depends on known optical loads and careful shielding from temperature fluctuations. The main advantages are broad spectral response, high energy resolution, and large absorbing areas; the main limitations are the need for cryogenics, slower response relative to photon-counting detectors, and sensitivity to microphonic and thermal disturbances.

Applications are concentrated in astronomy and cosmology, notably in infrared, submillimeter, and millimeter wavelengths, including measurements of the cosmic microwave background. X-ray microcalorimeters use bolometric sensors to achieve high energy resolution. Bolometers are also used in terahertz imaging and spectroscopy, material science, and security sensing.