Developing a robust thermal polarized calibration source

Abstract

Performance fluctuations of LWIR radiometers necessitate frequent calibration which is further complicated for LWIR polarimeters. Existing compact LWIR polarization calibration methods have inherent limitations; notably, combining an unpolarized source with a wire-grid polarizer can lead to polarized reflections which depend upon the thermal environment. Furthermore, microbolometers, which have risen in popularity due to their low size, weight, and power (SWaP) and cost, have intrinsic diattenuation. This research reviews several approaches to generating a linearly polarized signal in the thermal IR waveband for use as a polarimetric calibration target. The impact of environmental reflections from wire-grid polarizers and the intrinsic polarimetric response of a low-cost uncooled microbolometer is first demonstrated. A wire-grid polarizer is place in direct contact with a heating element to analyze the emission behavior of the wire-grid polarizer. Thin wires are also known to exhibit polarized thermal emission. This phenomena is observed through application of an electrical current through nanoscale wires of a wire-grid polarizer as well as through 20[µm] diameter tungsten wires suspended in free-space. The goal of these investigations is a low SWaP polarimetric calibration source for thermal LWIR polarimeters that is robust to the environmental fluctuations.