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Thermal Detectors

A thermal detector absorbs radiation and changes temperature. Because the power in absorbed radiation is typically rather small (<10^-7 W), the detector itself should be small so that it has a low heat capacity.

A thermocouple is the joining of two dissimilar-metal or metal alloy wires or films. When this occurs, a potential difference is formed between the other ends of the metals. Since potential differences are temperature-dependent (called the Seebeck effect), temperature values or changes in temperatures can be determined by calibration.

A bolometer is a semiconductor or thin metal strip whose resistance decreases with temperature. They are small, and typically painted black to better absorb radiation.

A Golay detector is a small pneumatic chamber filled with gas and covered with a thin membrane. When radiation strikes the detector, the gas warms, increasing the internal pressure and deforming the membrane. Deflection of the membrane can be measured mechanically or optically.

A pyroelectric detector uses a crystal of a pyroelectric material, which has a strong temperature-dependent electric polarization. The change in electric polarization causes a measurable current, which changes fast enough to respond to the output of an interferometer. The most common material used is deuterated triglycine sulfate, (dTGS). Lithium tantalate (LiTaO₃) and lead zinc titanate (PZT) are also used.