Computed-tomography imaging spectropolarimeter (CTISP): instrument concept, calibration and results

Researchers at the U.S. Army Engineer Research and Development Center (USAERDC), in collaboration with researchers at the University of Arizona's Optical Sciences Center, have designed, constructed and developed a complete Stokes imaging spectropolarimeter. CTISP (patent pending) is a polarimetric extension to CTIS (Computed-Tomographic Imaging Spectrometer), developed by the authors associated with the Optical Sciences Center. Currently, CTISP characterizes an object's spectropolarimetric radiance over the 440 to 740 nn range using 20 nm spectral bins and subdividing the FOV with a 32 x 32 resolution for a total of 16x32x32=16384 voxels. The output of CTISP is an estimate of the Stokes vector for each voxel. This paper focuses on the concept and practice of polarimetric calibration of each voxel in CTISP's spatial and spectral response. Voxel polarization calibration is performed using a fully computer-automated spectropolarimetric calibration facility to generate purely polarized linear and circular quasimonochromatic light. The facility includes optics and procedures for optically verifying the polarization state. During calibration CTISP's response to each generated polarized voxel is recorded and used to calculate a polarization characteristic matrix (PCM) for each voxel. CTISP uses four polarization analyzers in an automated rotating filter wheel configuration to acquire four images of the object. The results from the image reconstructions behind four analyzers are utilized with the PCMs to estimate the Stokes vector for each voxel in the object cube. Both the polarization accuracy of the calibration facility and CTISP will be addressed, and results will be presented that indicate CTISP is capable of reconstructing object cubes containing complex spectral, spatial and polarization content.