Infrared Polarimetry for Remote Sensing

A more complete understanding of the global distribution of cirrus ice clouds is essential to constraining both long and short-term climate forecasts. Radiometric simulations have shown that joint polarimetric measurements in the sub-mm and long-wave-infrared (LWIR) can significantly increase sensitivity to ice-particle microphysics, however, no existing platforms have polarimetric sensitivity in LWIR. This work describes the design and demonstration of an InfraRed Channeled Spectro-Polarimeter (IRCSP) to address this gap in existing remote sensing technology. The IRCSP utilizes commercially available uncooled microbolometer detectors (UMBs), is less than 10 cm long, has no moving mechanical components, and requires no active thermal control. A robust data-reduction algorithm corrects for both focal plane temperature fluctuations and instrumental polarization error to measure both the AoLP and Degree of Linear Polarization (DoLP) of cold targets. Lab validation studies have demonstrated polarimetric sensitivity to targets with 1.0% DoLP and brightness temperatures of less than 270 K. In 2021, the IRCSP was demonstrated in the near-space environment as a piggyback out of NASA Columbia Scientific Ballooning Facility. Post landing the instrument was retrieved with no damage to the optical payload and collected over 150 minutes of flight data at altitudes above 30 km. These measurements demonstrate the operation of un-cooled microbolometers in the low-pressure environment and are the first known high-altitude observations of a polarized signal from cloud tops in the LWIR. During deployment, the IRCSP reported brightness temperatures between 250-285K with uncertainty of < 1.5K. In addition, the IRCSP detected statistically significant polarization modulation with DoLP between 1 − 20% and preferential AoLP trends. These results support the hypothesis that LWIR polarimetry has the potential to add new sensitivity to existing remote sensing platforms