Lead sulfide
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Lead sulfide (UK English sulphide) is a chemical compound, most often purified from the mineral galena.
Applications
Lead sulfide and several other lead salts are used as detection element material in various infra-red sensors. Of these, PbS (lead sulfide) is one of the oldest and most commonly used.
PbS and other lead salts are semi-conductors. When used as IR detection element material they are classified as photon detectors: this means that unlike thermal detectors (the other major type), they react directly to the photons of incoming light radiation hitting the material. Thermal detectors react only to the eventual temperature rise in the material caused by the energy from the photos. Because of this, radiation can then be measured in two ways: either by detecting the tiny electrical current the photons cause when they hit the PbS surface, or by measuring the change in its electrical resistance. Measuring the resistance change is the more commonly used method.
At room temperature, PbS elements are sensitive to radiation at wavelengths between approximately 1 and 2.5 μm. This corresponds to the shorter wavelengths in the IR band, and is called near-IR, a term which refers to the fact that it is close to the wavelengths of visible light. Only very hot objects emit radiation in these wavelengths.
Cooling the PbS elements, for example using pressurised or liquified gas or a Peltier system shifts its sensitivity range to between approximately 2 and 4 μm. Objects which emit radiation in these wavelengths still have to be quite hot; several hundred degrees Celsius; but not as hot as those which are detectable by uncooled sensors.
Other compounds used for this purpose include indium antimonide (InSb), which has somewhat better properties for detecting the longer IR wavelengths.
See also
Physical properties
- Molecular weight: 239.28
- Density:
- Dielectric constant: 200
- Energy gap: