A polarized scene, which may occur at oblique illumination angles, creates a radiometric signal that varies as a function of viewing angle. One common optical component that is used to minimize such an effect is a polarization scrambler or depolarizer. As part of the CLARREO mission, the SOLARIS instrument project at Goddard Space Flight Center has developed a new class of polarization scramblers using a dual double-wedge pseudo-depolarizer that produces an anamorphic point spread function (PSF).

The SOLARIS instrument uses two Wollaston type scramblers in series, each with a distinct wedge angle, to image a pseudo-depolarized scene that is free of eigenstates. Since each wedge is distinct, the scrambler is able to produce an anamorphic PSF that maintains high spatial resolution in one dimension by sacrificing the spatial resolution in the other dimension. This scrambler geometry is ideal for 1-D imagers, such as pushbroom slit spectrometers, which require high spectral resolution, high spatial resolution, and low sensitivity to polarized light. Moreover, the geometry is applicable to a wide range of scientific instruments that require both high SNR (signal-to-noise ratio) and low sensitivity to polarized scenes.

Classic polarization scramblers are built using birefringent glass that may be either air-spaced or optically contacted together. Examples of birefringent materials include quartz, magnesium fluoride, and calcite. Two popular design forms of polarization scramblers are the Lyot and Wollaston types. The Lyot type uses two plan parallel birefringent plates of different thicknesses to vary the polarization state as a function of wavelength. Integrating the scene across a broad spectrum scrambles the polarized scene. This type reduces the spectral resolution of the scene. The Wollaston type uses two wedged birefringent plates to scramble the polarization state spatially. This type reduces the image quality of the scene. In addition to reducing the image quality of the scene, a single Wollaston scrambler is known to have polarized eigenstates and is ineffective at scrambling certain polarized scenes.

The SOLARIS instrument is designed to measure the solar radiation reflected from the Earth. The design furthers the state of the art in UV to near IR imaging spectroscopy.

This work was done by Peter Hill and Patrick Thompson of Goddard Space Flight Center. GSC-16277-1

NASA Tech Briefs Magazine

This article first appeared in the June, 2012 issue of NASA Tech Briefs Magazine.

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