High-accuracy spectropolarimetric imaging using photoelastic modulator-based cameras with low-polarization coatings

Under NASA's Instrument Incubator Program (IIP), we are developing an electro-optical imaging approach to enable multiangle, multispectral, and polarimetric measurements of tropospheric aerosol column abundances and microphysical properties. From low Earth orbit, the measurements would be acquired from the ultraviolet to shortwave infrared at ~1 km spatial resolution over a broad swath. To achieve a degree of linear polarization (DoLP) uncertainty of 0.5% in several spectral bands, we temporally modulate the linear-polarization component of the light at a rapid rate, enabling each detector within a focal-plane array, combined with polarization analyzers, to measure the ratio of the linear Stokes components Q or U to the total intensity. Our system uses tandem photoelastic modulators (PEMs) within a reflective camera. Because the system must measure intensity in certain spectral bands and polarization in others, it is essential that the camera have low diattenuation, particularly in the intensity bands. We report on the status of our PEM-based camera concept, with particular emphasis on experimental and theoretical work to design a set of mirror coatings that minimize diattenuation over the range of spectral bands planned for the camera.