<title>Computed-tomography imaging spectropolarimeter (CTISP): instrument design, operation, and results</title>

A complete Stokes imaging spectropolarimeter has been designed, constructed and tested by 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. CTISP is a polarimetric extension to CTIS, developed by the authors associated with the Optical Sciences Center. Currently, CTISP characterizes an object's spectropolarimetric radiance over the 440 to 740 nm range using 20 nm spectral bins and subdividing the FOV with a 32 X 32 resolution. CTISP's output is a four-fold increase in object cube information when compared to spectral radiance alone as CTISP effectively extracts the polarization information from the radiance of each of the 'N' voxels to form 'N' 4 element Stokes vectors, where N equals (# horizontal resolution elements in FOV)*(# vertical resolution elements in FOV)*(# of wavelength bands). Voxel polarization calibration is performed using a fully computer automated spectropolarimetric calibration facility. The facility generates an object whose spatial and spectral dimensions define a voxel and whose radiance is purely polarized. CTISP's response to each generated polarized voxel is recorded and used to calculate a polarization characteristic matrix (PCM) for each voxel. CTISP utilizes 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. CTISP utilizes a host of software tools to control the calibration facility, perform image acquisition and perform reconstruction and Stokes vector calculation. The order of use and inter-relation of these tools is described. Results will be presented and indicate that CTISP is capable of reconstructing objects containing complex spectral, spatial and polarization content. A spectral comparison is made to a reference spectrometer using a reflectance standard for normalization.