Bolometer is an instrument that is used to measure radiation temperature. It is a kind of Resistance Bridge. Its one arm consists of a blackened metal strip. When the radiations fall on this metal strip the temperature rises. This increase in temperature is measured by a bolometer. This is an extremely sensitive instrument.
This instrument was invented by a U.S. scientist, Samuel P. Langley in 1880. In this first bolometer a Wheatstone bridge was used along with a galvanometer that produced a deflection proportional to the intensity of the radiation. Since then several other thermal devices have been developed but bolometer remains one of the most effective and convenient infra-red detectors. It is because it does not require cooling and can be operated with great ease.
Now the question arises, what is the principle of working of this instrument? The principle on the basis of which a bolometer operates is that a temperature change is produced by the absorption of radiation and this causes a change in electrical resistance of the material used to fabricate the bolometer. This change in resistance can be used to sense radiation.
There are several types of bolometers in use today. One type has two strips of platinum. When the platinum strips bridge the circuit, their electrical resistance changes. The change of resistance can be used to measure the temperature. This instrument is so sensitive that it can detect changes in temperature as small as 0.0001°c.
Another type of bolometer known as spectrum bolometer consists of a single strip of metal. It is used for examining the distribution of intensity of radiation in a spectrum.
