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Thermal Detectors

Excerpt from Field Guide to Spectroscopy

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.

Thermocouple Types

Type JFe vs. Cu-Ni
Type KNi-Cr vs. Ni-Al
Type ENi-Cr vs. Cu-Ni
Type TCu vs. Cu-Ni
Type S, RPt-Rh vs. Pt
Type NNi-Cr-Si vs. Ni-Si-Mg

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.


D. W. Ball, Field Guide to Spectroscopy, SPIE Press, Bellingham, WA (2006).

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