What is a Bolometer?

A bolometer is a scientific instrument used to detect and measure electromagnetic radiation. This instrument might also be called an actinic balance or a calorimeter depending on the setting, and is typically used in situations where radiation is minute and otherwise difficult to detect. These tools can precisely measure electromagnetic radiation in its various forms, from radio waves to ultraviolet radiation and gamma rays. The operational principle has also been adapted for use in physics and particle detection.

The actual science behind how these instruments work can be somewhat complicated, but the core concept is usually pretty straightforward. All models and setups have some sort of absorber, essentially an element like metal that is able to absorb energy, and a reservoir, which is something that has a constant temperature. The two are connected by some sort of conductor. When energy hits the absorber, the instrument detects any difference between the temperature of the energy and the temperature of the reservoir, which can be an indication of that energy’s overall electromagnetic output.

These sorts of tools are used primarily to measure known radiation outputs, but they can also be used to detect suspected energy fields, particularly those in space. Physicists and astronomers looking for things like black holes, for instance, often use these sorts of tools to detect changes in electromagnetic radiation within given fields in order to get hints and clues about cosmic energy patterns.

It’s widely believed that the American astronomer Samuel Pierpont Langley created the first prototype of this instrument in the late 19th century. The first model was used in conjunction with a telescope to measure infrared radiation on astronomical objects, the moon in particular. The prototype was basic in design. It consisted of two chambers outfitted with platinum strips forming what’s known as a “Wheatstone bridge” structure connected to a galvanometer and battery. Soot covered strips forming the bridge were arranged such that one was left exposed while the other was shielded from radiation exposure. The exposed strip's temperature would increase when it came into contact with the electromagnetic radiation, altering its electrical resistance and essentially creating a temperature sensor.

There are many different variations of the instrument that are used in different settings. A cold electron bolometer (CEB), for example, is a highly sensitive device that detects cosmological radiation. A superconducting-insulator-normal (SIN) metal tunnel junction is what sets the CEB apart from other similar instruments, in large part because its energy loss is used to cool the absorber.

A hot electron bolometer (HEB) works similarly. This is a device used to measure sub-millimeter and far-infrared radiation that cannot be measured by the CEB. It works mainly by detecting energy gain.

A microbolometer is adapted to function as an infrared detector in a thermal camera, commonly known as a Forward Looking Infrared (FLIR) camera. This type of camera works on the same principle as the traditional instrument, and measures infrared radiation with wavelengths between 8 and 13 microns. The electrical resistance recorded by the camera is translated into temperatures, which are used to create an image.

A branch of physics known as particle physics, which studies the basic elements of radiation, frequently uses the term “bolometer” in reference to an instrument known more formally as a particle detector. The particle detector works on the same principle as Langley's original instrument and is used to identify high-energy particles. Scintillation counters and gaseous ionization-type particle detectors are typically used for the purpose of measuring energy associated with radiation and particle characteristics.

As effective as bolometric measuring tools can be, there are some disadvantages to their use, too. In general, these sorts of instruments lack “discriminatory properties,” which means that they do not differentiate between ionized and non-ionized particles. When used as a thermal detector, the instrument does not directly dispel the energy collected by the absorber either, which usually means that it doesn't immediately reset.