Absolute linearity measurements on a PbS detector in the infrared

The nonlinearity characteristics of a commercially available thin-film photoconductive PbS detector were experimentally investigated in the infrared using the National Physical Laboratory detector linearity characterization facility. The deviation from linearity of this detector was shown to be significant even for relatively low values of radiant power incident on the active area of the detector. For example, the linearity factor was approximately 0.8 when 0.6 (mu)W of radiant power at a wavelength of 2.2 (mu)m was illuminating a spot of 1 mm in diameter on the active area of the PbS detector. These figures demonstrate the poor linearity characteristics of this detector and provide a warning to other users of PbS detection systems. The deviation from linearity was shown to be a function of the size of the spot being illuminated on the detector active area, as well as the wavelength of the incident radiation. The deviation from linearity was shown to be a function of irradiance illuminating the detector for irradiance values lower than 1 (mu)W mm^-2.