The Effect of Instrument Polarization Sensitivity on Sea Surface Remote Sensing with Infrared Spectroradiometers

The sensitivity of Fourier transform infrared spectroradiometers to the polarization state of incident radiance can become significant in radiometric measurements of a partially polarized source such as the sea surface. At off-nadir incidence angles and wavenumbers below about 2750 cm21 (wavelengths longer than 3.6 mm), radiance from the sea surface is partially vertically polarized, because the vertically polarized sea surface emission exceeds the horizontally polarized background reflection. At larger wavenumbers (wavelengths shorter than 3.6 mm), reflected skylight becomes more significant, and the total radiance at off-nadir angles can become horizontally polarized. This paper shows how the inherent polarization sensitivity (25%) of a typical Fourier transform infrared instrument leads to radiometric errors that are small (;0.1 K) but significant for radiometrically demanding sea surface remote sensing applications at large incidence angles. For incidence angles below approximately 458, the polarization-induced error for longwave infrared measurements typically is less than approximately 0.05 K and therefore often can be neglected in sea surface radiometry. However, in polarimetric measurements used, for example, to increase contrast between man-made objects and the background, the instrument polarization sensitivity must be considered always.