Calibration and performance validation of optical elements in a photoelastic modulator-based polarimetric camera

As part of NASA's Instrument Incubator Program (IIP), we have been developing enabling technologies for the Multiangle SpectroPolarimetric Imager (MSPI), a candidate instrument for the Aerosol-Cloud-Ecosytem (ACE) mission. ACE is one of several satellite concepts identified in the 2007 National Research Council Earth Sciences Decadal Survey. MSPI is a multiangle, multispectral, high-accuracy polarization imager, and is envisioned to contain multiple cameras pointed at different viewing angles, with intensity imaging in several spectral bands between the ultraviolet and shortwave infrared, and accurate polarimetric imaging in a subset of the bands. To achieve a degree of linear polarization (DOLP) uncertainty of 1%, we temporally modulate the linear-polarization component of the incoming light at a rapid rate, enabling each detector within a focal-plane array combined with polarization analyzers to measure the relative proportions of the linear Stokes components Q or U to the total intensity I. Our system uses tandem photoelastic modulators (PEMs) within a reflective camera design. We report on the status of our prototype camera development, with particular emphasis on theoretical and experimental work on the required and measured performance of optical elements within the system including: the spectropolarimetric filters, quarter wave plates, and tandem PEMs. We also report on the end-to-end measurement and calibration of camera polarization aberrations using a custom polarization state generator (PSG). Careful design and control of scattered light enables the PSG to generate polarization states between DOLP of 0.07% and 40% with an uncertainty of 0.05%, making it a precision tool for polarimetric calibration and validation of MSPI.