bolometer

A bolometer is a sensitive detector that measures electromagnetic radiation by detecting the heating effect it produces in an absorber. Absorbed radiation raises the absorber’s temperature slightly, and a thermometer placed in contact with the absorber converts this temperature rise into an electrical signal. The signal is proportional to the incident power integrated over time, subject to the device’s response characteristics.

Bolometers rely on a high temperature coefficient of resistance in the thermometer and a low heat capacity in the absorber. To achieve high sensitivity, the device is often cooled to cryogenic temperatures, from a few kelvin to tens of kelvin, to reduce thermal noise and heat capacity. A weak thermal link to a bath ensures a measurable temperature change for small absorbed powers.

Performance is described by the noise-equivalent power (NEP), the minimum detectable power, and by the response time or time constant τ = C/G, where C is heat capacity and G is thermal conductance. Readout typically uses a constant bias while measuring changes in resistance or voltage; the setup is optimized to minimize parasitic heat loads and electronic noise.

Bolometers come in several forms. Semiconductor bolometers use thermistors on a micromachined membrane; infrared microbolometers are widely used in uncooled thermal cameras. Superconducting bolometers, including transition-edge sensors (TES) and other microcalorimeter designs, offer very high sensitivity for astronomy and submillimeter work. Materials include doped amorphous silicon, vanadium oxide, and various superconductors.

Applications include astronomy (cosmic microwave background and submillimeter astronomy), infrared spectroscopy, and security or industrial monitoring. The bolometer was invented in the late 19th century by Samuel P. Langley.