OpTeC Colloquium

OpTeC sponsors a colloquium throughout the academic year. This is a forum where a wide range of optical science and engineering topics are discussed at a level that can be understood by a broad spectrum of optics students, staff, and faculty. The primary purposes are to train students to give effective scientific talks and to promote cross-disciplinary interactions of students, staff, faculty, and local industry employees. Speakers include MSU students, staff, and faculty, and visitors from external universities, research labs, and companies.

February 26, 2026
4:10 pm
Norm Asbjornson Hall
Room 149

on the Montana State University Campus

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Tracking the Sky’s Neutral Point Positions for Atmospheric Turbidity

Abstract

Monitoring the polarization signature of the sky is valuable for navigation, meteorology, and remote sensing, yet the location of the polarization neutral points remain underutilized as atmospheric diagnostics. Unlike direct line-of-sight measurements, neutral point locations are influenced by scattering throughout the sky dome, providing an inherently integrated, scattering geometry-based probe of atmospheric turbidity. Although neutral points are defined by a balance of polarized radiance components and are therefore randomly polarized in orientation, their locations are not fixed and respond systematically to aerosol loading, microphysical properties, wavelength, and solar geometry.

In this talk, I will present the development of the Ultraviolet Linear Stokes Imaging Polarimeter (ULTRASIP), a ground-based imager operating with 7.2 arcsec/pixel resolution, designed for high-precision, diurnal tracking of sky neutral points. I will relate these observations to recent radiative-transfer sensitivity studies that quantify how aerosol optical depth, fine–coarse mode partitioning, absorption, wavelength, vertical height distribution, and solar zenith angle all influence Babinet neutral point displacement. These simulations reveal that increasing aerosol loading generally produces sunward shifts relative to a molecular-scattering reference, but that the sign and monotonicity of the response depend strongly on aerosol regime and wavelength, with non-monotonic behavior emerging under certain fine-mode–dominated conditions.

Finally, I will discuss how high-precision neutral point tracking with ULTRASIP enables meaningful correlation with AERONET aerosol retrievals and event-scale atmospheric perturbations, establishing polarization neutral points as physically interpretable diagnostics of aerosol loading and microphysical regime. This framework identifies both the promise and the limitations of neutral-point–based aerosol sensing and informs the design and observational strategy of future ground-based sky-polarization instruments.

Biographical Sketch

Meredith Kupinski joined the Wyant College of Optical Sciences at the University of Arizona (UA) in 2008 where she is now a full Professor developing polarimetric instrumentation, polarized light scattering models, and polarization-aware computer vision and graphics capabilities. Prof. Kupinski was the recipient of a postdoctoral NSF fellowship to study polarimetry for aerosol science. In 2016, she was awarded a Jean d’Alembert scholarship at École Polytechnic in France to work on Mueller polarimetry for cervical cancer detection. She received an NSF CAREER award to develop polarimetric importance sampling for computer graphics. She is the PI of a NASA Channeled Infrared Polarimeter for cloud ice observations. She is also the PI of an ONR project on manifold optimization to use quadratic statistics in imaging detection problems. She earned a BS with Highest Honors in Imaging Technologies from the Rochester Institute of Technology and an MS and PhD in Optical Sciences from UA.

Past Speakers