Background
Why Measure Polarization?
Light coming from the sun has no orientation. After interacting with our atmosphere (or atmospheres of other celestial bodies) it will change polarization based upon the makeup of the atmosphere. A polarimeter measures intensity of light based upon its polarization, and can tell many things about the type of medium that the light has passed through.
By putting a polarization stage in front of a spectrograph, the proposed system will build on polarimetry research in place at Montana State University. By increasing the spectral range of the measurement, the short-wave and mid-wave infrared regions will be able to characterize naturally occurring polarization in the atmosphere.
This research could be used to determine how polarization changes with aerosols, clouds, and reflectance. This would be very helpful in polarization-based navigation or object identification. It could also be used to characterize aerosols and particle size.
Polarimetry at Montana State University
Previous research of polarimetry at Montana State University has been done using an all-sky polarization imager developed by Dr. Joseph Shaw and Dr. Nathan Pust. The instrument has been in use since 2009, and expanded upon the previous capabilities of ASI imagers by allowing the study of sky polarization in partially cloudy skies. The all-sky imager operates in five 10nm bands defined by interference filters centered at 452, 491, 532, 632, and 701nm. This design uses digital images instead of film, providing real time data. Implementation of liquid crystal variable retarders instead of rotating polarization elements makes the system much faster in capturing images. The information collected is used to calculate the degree of linear polarization as well as the angle of polarization.
The data collected from the API system can be used to better understand variations of polarization relating to clouds, surface reflection, and the effect of aerosols in the atmosphere. The research was also done with the goal of validating the polarized radiative transfer code, MODTRAN-P, which was developed at the Air Force Research Laboratory. A sun occulter is used to prevent the device from viewing the sun directly. This prevents overexposure as well as blooming and flaring. However, it also blocks a small portion of the sky.