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Infrared polarisation measurements of targets and backgrounds in a marine environment

Author: Cremer, F. · Schwering, P.B.W. · Jong, W. de · Schutte, K. · Jong, A.N. de
Type:article
Date:2001
Publisher: SPIE
Place: Bellingham, WA.
Institution: TNO Fysisch en Elektronisch Laboratorium
Source:Watkins, W.R.Clement, D.Reynolds, W.R., Targets and Backgrounds VII: Characterization and Representation, 16-17 April 2001, Orlando, FL, USA, 169-180
series:
Proceedings of SPIE
Identifier: 95397
doi: doi:10.1117/12.440074
Keywords: Marine · Image sensors · Infrared radiation · Light reflection · Optical recording · Synchronization · Targets · Light polarization · Marine environment · Targets and backgrounds

Abstract

The infrared (IR) radiation emitted or reflected in an off-normal direction from a smooth surface is partially polarised. This principle can be used for enhanced discrimination of targets from backgrounds in a marine environment. It has been shown that (man-made) targets do not demonstrate a pronounced polarisation effect when observed from near normal direction whereas the sea background radiation has a certain degree of polarisation in slant observation path. A measurement setup has been constructed for collecting polarised IR imagery. This setup contains a rotating polarisation filter that rotates synchronously with the frame sync of the camera. Either a long wave IR (LWIR) or a mid wave IR (MWIR) camera can be mounted behind the rotating polarisation filter. The synchronisation allows a sequence of images to be taken with a predefined constant angle of rotation between the images. Out of this image sequence three independent Stokes images are constructed, containing the normal intensity part, the vertical/horizontal polarisation and the diagonal polarisation. Upto 20 full linearly polarised images can be acquired per second. Measurements are taken at the North Sea coast with this setup. The recorded images are analysed to determine the influence of polarisation on the detection of small targets in such an environment. Furthermore differences between polarisation contrasts in MWIR are analysed