Dual-field imaging polarimeter for studying the effect of clouds on sky and target polarization

Abstract

Polarization signatures of ground-based objects depend upon both the physical characteristics of the object and the polarization of the incident light. Models of polarization can accurately represent the polarization of incident light for clear skies, but the effect of cloudy skies upon object signatures is more difficult to predict. In response to the need for measurements to characterize the effect of variable sky polarization on target polarimetry, a multispectral visible band polarimeter (440 - 710 nm) has been developed. This polarimeter quickly changes to measure both full sky polarization and narrow field-of-view target signatures. Sky polarization is measured with a fisheye lens. Target polarization is measured with a 300 mm telephoto lens. A Liquid Crystal Variable Retarder (LCVR) design has been selected that uses two Meadowlark LCVR-300 variable retarders at set rotation angles. Current calibration work involves the removal of the fisheye effects on the Stokes vector. Future measurements will be used for validation of Polarized MODTRAN.