ABSTRACT
Polarimetry is radiometry with polarization elements. The algorithms for measuring the Stokes parameters and Mueller matrix elements from a series of radiometric measurements with polarization elements is a linear estimation problem and lends itself to efficient solution using linear algebra, usually with a least-squares estimator, the pseudoinverse, to find the best match to the data. An important consideration is to keep the null space, a set of signal patterns which will not be generated by actual Stokes parameters, from entering into the data analysis. A polarization generator consists of a light source, optical elements, and polarization elements to produce a beam of known polarization state. A polarization generator is specified by the Stokes parameters S of the exiting beam. A polarization analyzer is a configuration of polarization elements, optical elements, and a detector which performs a flux measurement of a particular polarization component in an incident beam. Rotating element polarimetry, oscillating element polarimetry, and phase modulation polarimetry are sequential-in-time methods that acquire a series of measurements over time. Division of amplitude polarimetry and division of wavefront polarimetry can measure all four elements of the Stokes parameters simultaneously. The polarimetric measurement matrix W has rows containing the set of analyzer vectors. The polarimetric data reduction matrix is the pseudoinverse of W.
Stokes polarization images contains information on the orientation of a scattering surface, its refractive index and texture, as well as the polarization state of the light incident on the surface. Color is dependent on quantum mechanical energy levels in the material. So, color and intensity are often poorly correlated with the polarization. Micro-polarizer arrays of linear polarizers matched to focal planes are common for imaging polarimeters. Polariscopes are simple polarimeters which examine samples placed between pairs of polarizers to screen samples for retardance or stress birefringence.
An important variable linear retarder is the photoelastic modulator or PEM formed from shapes with high mechanical resonance Q. Applying force to transparent materials induces retardance through stress-induced birefringence. Due to Q’s of greater than 10,000, large retardances can be generated with low power. PEMs are used as circular retardance modulators in NASA’s remote sensing polarimeters the Airborne Multiangle SpectroPolarimetric Imager (AirMSPI) and Multi-Angle Imager for Aerosols (MAIA, scheduled for launch in 2021). The MAIA research combines space-based aerosol measurements with health and hospital records to develop the relationships between health problems like cardiovascular and respiratory diseases and premature deaths with the multiyear record of aerosols and air pollution.
