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Infrared polarization measurements and modeling applied to surface-laid antipersonel landmines

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Author: Cremer, F. · Jong, W. de · Schutte, K.
Institution: TNO Fysisch en Elektronisch Laboratorium
Source:Optical Engineering, May, 5, 41, 1021-1032
Identifier: 95467
Keywords: Physics · Infrared polarization model · Land mine detection · Thermal infrared radiation · Linear polarization · Thermal imagers


Linear polarization of thermal infrared (TIR) radiation occurs when radiation is reflected or emitted from a smooth surface (such as the top of a landmine) and observed from a grazing angle. The background (soil and vegetation) is generally much rougher and therefore shows less pronounced linearly polarized radiation. This difference in polarization is utilized to enhance detection of landmines using TIR cameras. A setup has been constructed for the acquisition of polarized TIR images. This setup contains a polarization filter that rotates synchronously to the frame sync of the camera. Either a long wave infrared (LWIR) or a mid wave infrared (MWIR) camera can be mounted behind the rotating polarization filter. The synchronization allows a sequence of images to be taken with a predefined constant angle of rotation between the images. Using this image sequence, three independent Stokes images are calculated, consisting of the unpolarized radiance, the difference between vertically and horizontally polarized radiances, and the difference between the two diagonally polarized radiances. A model has been developed that describes the polarization due to reflection of and emission from a smooth surface. This model predicts the linear polarization for a landmine ‘‘illuminated’’ by a source that is either hotter or cooler than the surface of the landmine. The measurement setup has been used to validate the model. The measurements agree well with the model predictions and can be used for estimating the real part of the refractive index of the rubber surface of the landmine. Besides the indoor measurement, outdoor measurements have also been performed. The results of these measurements show that under the given conditions the majority of landmines can be observed in the polarized radiance, whereas they are not clearly visible in the normal (unpolarized) radiance or the visual image.