Heat detectors are often classified into four types: pneumatic detectors (Gaolai tubes), thermocouples or thermopiles, thermistors, pyroelectric detectors.
1. Pneumatic detector (Gaolai tube)
Using the principle that the gas-filled vessel receives heat radiation and the temperature increases after the gas expands, the change in the vessel wall is measured to determine the intensity of the infrared radiation. This is an old-fashioned detector, but in 1947, after Gaulle improved the pneumatic detector, using a photocell to measure the small changes in the container wall, so that the sensitivity greatly improved, so this pneumatic detector, also known as Gao Lai components .
Thermistor resistance changes with its own temperature. Its temperature depends on the absorption of radiation, Joule heat generated by the current applied during operation, ambient temperature, and heat dissipation. Thermistors are basically made of semiconducting materials, with NTC (negative resistance temperature coefficient) and PTC (positive resistance temperature coefficient).
Thermistors are usually two-terminal devices, but they are also available in three and four terminals. Two-terminal devices or three-terminal devices are directly heated, and four-terminal devices are indirectly heated. Thermistors are usually made smaller, beaded, ring-shaped, and flake-shaped. A thermistor bolometer made of a negative temperature coefficient oxide semiconductor (usually a mixture of oxides of manganese, nickel, and cobalt) is often composed of two components: one is a main component, facing the window, receiving infrared radiation. The other is a compensating element. The performance is the same as that of the main element. They are independent of each other and are packaged in a single tube. They do not receive infrared radiation and only provide temperature compensation.
3. Thermocouples and thermopiles
Thermocouples are one of the oldest heat detectors and are still widely used. Thermocouples work based on thermoelectric effects. The thermoelectric emf provided by a single thermocouple is relatively small and cannot meet the requirements of certain applications. Therefore, several thermocouples or several thermocouples are often connected in series to form a thermopile. Thermopiles can provide greater thermoelectric emf than thermocouples. Newer thermopiles are made using thin-film technology. Therefore, they are called thin film thermopiles.
4. Pyroelectric detector
Pyroelectric detectors are late development of a heat detector. Today, not only unit pyroelectric detectors have matured, but also multi-element arrays have been successfully applied. Pyroelectric detector detection rate than the photon detector detection rate is low, but its spectral response is wide, at room temperature, has been in infrared thermal imaging, infrared camera tube, non-contact temperature measurement, intrusion alarm, infrared spectrometer, Laser measurement and submillimeter wave measurement have been applied, so it has become an important infrared detector.
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