Several lead salts are used as detection element materials in various infra-red (IR) sensors. PbS (lead sulphide) is one of the oldest and most commonly used ones. PbS and other lead salts are semi-conductors. When used as IR detection element materials they are classified as photon detectors: this means that unlike the other major type, thermal detectors, they react directly to the photons of incoming radiation hitting the material, rather than to the eventual temperature rise in the material. Radiation can then be measured in either of two ways: either by measuring the tiny electric current the photons cause when they hit the PbS material, or by measuring the change in the material's resistance that the photons cause. Measuring the resistance change is the more commonly used method.
In room temperature PbS elements are sensitive to radiation at wavelengths of approximate 1 to 2.5 μm. In practise this means very short wavelength in the IR band, and is within what is called Near IR, a term that refers to the fact that it is close to the wavelengths of visible light. Only very hot objects emit in these wavelengths.
Cooling the PbS elements by for example pressurised or liquid gas or a Peltier system shifts its sensitivity to wavelength of approximately 2 to 4 μm. Objects that emit in these wavelengths still have to be quite hot; several hundred degrees Celsius.
