What is a Bolometer?

            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 the metal strip the temperature rises. This increase in temperature measured by a bolometer. This is a very sensitive instrument.

Bolometer

            A U.S. scientist, Samuel P. Langely invented this instrument 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 infrared detectors. It is because it does not need 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 changes is produced by the absorption of radiation this causes a change in electrical resistance of the material used in the fabrication of the bolometer. This change in resistance can be used to sense radiation.

            There are several types of bolometer in use today. One type has two 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 0.001 degree centigrade.

            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.

What is a Pyrometer?

            A pyrometer is an instrument used for measuring very high temperatures-especially those that can not be measured through ordinary thermometers. For example, pyrometer is used to measure furnace temperature.

Pyrometer

            There are two main kinds of pyrometers-radiation pyrometer and optical pyrometer.

            In a radiation pyrometer, the radiation form the hot object is focused on to a thermopile, which is a collection of thermocouples. When the thermopile gets heated due to the intercepted radiation, it produces a voltage. The amount of voltage developed depends upon the temperature. Paper calibration permits this electrical voltage to be converted into the temperature of the hot object.

            Sometimes, a bolometer is used instead of a thermopile. A bolometer has two strips of the platinum metal. When the platinum strips heat up, the electrical resistance of the strips changes. The change of resistance can be used to measure the temperature. Such pyrometers are sometimes called resistance pyrometer.

            Optical pyrometers use the color of light of the hot object to measure its temperature. Optical pyrometers, for example, measure the temperature of incandescent bodies by comparing them visually with a calibrated incandescent filament. The calibrated light indicates the temperature for various stages of temperature change. As the object gets heated the color changes, which are compared with the matching color. When the color is matched, the temperature is read off from the display unit.

            The voltage can be changed to maintain a constant color or temperature as required. A part form the main kinds of pyrometers, two other kinds are recognized as important by physicists. They are: (a) Platinum Resistance Thermometer (b) Thermo-Electric Thermometer. The pyrometers are also used to measure temperature form a long distance by making use of the law of radiation.