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When the measurable response of a detector to incident radiation is a rise in temperature, the device is called a thermal detector. The receptor in the thermal detectors is typically a blackened surface that efficiently absorbs light at all wavelengths. Thermal detectors include thermocouples and bulk devices that respond to a rise in temperature with a significant change in resistance (e.g. bolometers and Thermistors), pyroelectric detectors that exploit pyroelectric effect and the Golay cell that measures the expansion of a gas induced by radiation incident on the gas enclosure.