The bolometer is a photon detector with an extremely large sensing range. It is sensitive to all electromagnetic radiation up in to cosmic radiation wavelengths. The basic bolometer consists of a photon absorber, an extremely sensitive thermoelectric sensor and often a cooling system. When a photon hits the absorber, its temperature increases slightly. This is detected by the temperature sensor. The bolometer is very sensitive, but it has a long recovery time, usualy several milliseconds since the last photon hit. If the frequency of photon hits is lower than the recovery time, you will get an accumulative measurement of the energy of all the photons.

If the time between each hit is longer than the recovery time, the output will indicate the energy of the last photon. Since the energy is dependent on the frequency of the electromagnetic radiation (The higher the frequency the higher the energy), You can use a bolometer to measure the EM frequency of the photon. For example if a x-ray photon hits the bolometer the output variation will be higher than if a IR photon hits it. This means that the bolometer can be used in broadband spectroscopy as long as the time between each photon hit is longer than the recovery time.

The temperature sensing component can be a thermistor. In the most sensitive bolometers a superconducting thin film thermistor is used. This thermistor is biased to just the superconducting temperature, and even a small temperature rise changes the current flow through it. This change is measured by a Superconducting Quantum Interference Device.

Recently the technology needed for making an array of these devices has been developed. These arrays can be used in an X-ray telescope, for example.