Concurrent polarimetric measurements of painted metal and illuminating skylight compared with a microfacet model

Abstract

Previous visible-band laboratory measurements have shown that polarization data can be used to determine optical properties of materials such as the index of refraction with controlled illumination sources. For outdoor measurements, the complex illumination formed by the polarized sky for visible wavelengths makes this process considerably more difficult. This paper reports polarization measurements for horizontal painted-metal and PVC plates and the background atmosphere from a quickly changeable dual-field imaging polarimeter which provides polarization of ground-based objects nearly concurrently with full-sky polarization. A microfacet model has been developed which accounts for the polarized sky illumination and solar-reflecting and flat-reflecting microfacets. Data from this model have been used to explain the primary features of the polarization observed when viewing painted metal and PVC plates outdoors with clear skies. Future work will attempt to use this model with polarimeter data to retrieve the index of refraction of the observed plates.