Bolometer
Definition and meaning of Bolometer in chemistry.
A bolometer is an instrument that measures the power of incident electromagnetic radiation by detecting the tiny rise in temperature it produces in an absorbing element, which changes that element's electrical resistance.
In more detail
The absorbing element (a blackened metal strip, thermistor, or semiconductor flake) is wired into one arm of a Wheatstone bridge; radiant energy it absorbs raises its temperature, and since resistance depends on temperature, the resulting resistance change unbalances the bridge and produces a signal proportional to the radiant power absorbed. Unlike photon (quantum) detectors, a bolometer is a thermal detector: it responds to the total energy absorbed rather than to individual photon energies, so it works over a very broad, wavelength-independent range, making it useful for infrared and far-infrared measurements where photon detectors lose sensitivity.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Detection principle | Radiation-induced heating changes electrical resistance |
| First built | 1878, by Samuel Langley |
| Common elements | Blackened platinum strip or semiconductor thermistor |
In an early dispersive infrared spectrophotometer, IR radiation transmitted through a sample falls on a thermistor bolometer; the tiny resulting temperature rise changes its resistance, and the bridge-circuit output is recorded as the transmittance spectrum.
Frequently asked questions
How does a bolometer differ from a photodiode detector?
A bolometer is a thermal detector that responds to total absorbed radiant energy regardless of wavelength, while a photodiode is a quantum detector whose response depends on individual photons having enough energy to excite charge carriers across a semiconductor bandgap.
Why were bolometers used in infrared spectroscopy?
Because they respond broadly across the infrared region without needing photon energies above a specific threshold, early IR spectrophotometers used bolometers (and similar thermal detectors like thermocouples) before more sensitive photon detectors became practical.