A bolometer is a device for measuring the energy of incident electromagnetic radiation. It was invented in 1878 by the American astronomer Samuel Pierpont Langley. 
Bolometers are an instrument used to measure infrared radiation or any other form of radiant energy. They were invented in 1860 by an American scientist called Samuel Pierpont Langley. They are now primarily used in the detection of heat energy transmitted from distant sources. An application of this is in astronomy to measure the heat of the stars. Quanta of infrared light are not sufficiently energetic to be detected photoelectrically. So in the far infra red the usual method of detection depend on the heating effect of radiation on an absorbing surface. The resultant temperature change may be detected by a change of resistance, in a bolometer. It is essential to use a sensitive element with small thermal capacity, well insulated from its surroundings, so that its temperature can respond reasonably rapidly to incident radiation. Working Principal of Bolometers- The operating principle is similar to that of a calorimeter in thermodynamics. However, the approximations, ultra low temperature, and the different purpose of the device make the operational use rather different. In the jargon of high energy physics, these devices are not called calorimeters since this term is already used for a different type of detector (see Calorimeter (particle physics)). Their use as particle detectors is still at the developmental stage. Their use as particle detectors was proposed from the beginning of the 20th century, but the first regular, though pioneering, use was only in the 1980s because of the difficulty associated with having a system at cryogenic temperature. This is where the superconductor comes into the equation. Due to superconductors low temperatures, a slight change in the external temperature will change the superconductor from their superconducting state to a normal state (see History). This gives the superconducting bolometer a key advantage over other forms of the instrument in the fact that it is more sensitive and low noise voltage. To measure the resistance of the superconducting thermometer one has two choices. The first the current biased mode. In this mode the thermometer is biased with a constant current I and the resistance is measured by measuring the voltage. The electrical power in the bolometer will be the current squared times the resistance. The second mode is the voltage-biased mode. The thermometer is biased with a constant voltage V and the resistance is measured by measuring the current. The electrical power in the bolometer will be the voltage squared divided by the resistance. The current-biased mode has the advantage that a voltage can be amplified relatively easily. However, with the arrival of SQUIDS an adequate amplification and detection of a current signal has become possible. The only disadvantage of the superconducting bolometer is that because of its sensitivity it is difficult to use. Types Of Bolometers - Langley's bolometer
The first bolometer used for infrared observatons by Langley had a very basic design: It consisted of two platinum strips, covered with lampblack, one strip was shielded from the radiation and one exposed to it. The strips formed two branches of a wheatstone bridge which was fitted with a sensitive galvanometer and connected to a battery. Electromagnetic radiation falling on the exposed strip would heat it, and change its resistance, the circuit thus effectively operating as a resistance temperature detector. This instrument enabled him to feel his way thermally over the whole spectrum, noting all the chief Fraunhofer lines and bands, which were shown by sharp serrations, or more prolonged depressions of the curve which gave the emissions, and discovered the lines and bands of the invisible infra-red portion.
Microbolometers
Main article: Microbolometer
A microbolometer is a specific type of bolometer used as a detector in a thermal camera. It is a grid of vanadium oxide or amorphous silicon heat sensors atop a corresponding grid of silicon. Infrared radiation from a specific range of wavelengths strikes the vanadium oxide and changes its electrical resistance. This resistance change is measured and processed into temperatures which can be represented graphically. The microbolometer grid is commonly found in two sizes, a 320×240 array or less expensive 160×120 array. Both arrays provide the same resolution with the larger array providing a wider field of view. Larger, 640×480 arrays were announced in 2005.
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