Long wave infrared polarimetric model: theory, measurements and parameters

Material parameters, which include the complex index of refraction, (n,k), and surface roughness, are needed to determine passive long wave infrared (LWIR) polarimetric radiance. A single scatter microfacet bi-direction reflectance distribution function (BRDF) is central to the energy conserving (EC) model which determines emitted and reflected polarized surface radiance. Model predictions are compared to LWIR polarimetric data. An ellipsometry approach is described for finding an effective complex index of refraction or (n,k) averaged over the 8.5?9.5??m wavelength range. The reflected S3/S2 ratios, where S2 and S3 are components of the Stokes (Born and Wolf 1975 Principles of Optics (London: Pergamon) p?30) vector, are used to determine (n,k). An imaging polarimeter with a rotating retarder is utilized to measure the Stokes vector. Effective (n,k) and two EC optical roughness parameters are presented for roughened glass and several unprepared, typical outdoor materials including metals and paints. A two parameter slope distribution function is introduced which is more flexible in modelling the source reflected intensity profiles or BRDF data than one parameter Cauchy or Gaussian distributions (Jordan et al 1996 Appl. Opt. 35 3585?90; Priest and Meier 2002 Opt. Eng. 41 992). The glass results show that the (n,k) needed to model polarimetric emission and scatter differ from that for a smooth surface and that surface roughness reduces the degree of linear polarization.