Observation of non-principal plane neutral points in the in-water upwelling polarized light field

Kenneth J Voss,Arthur C R Gleason,George W Kattawar,Howard R Gordon,Yu You

doi:10.1364/oe.19.005942

Abstract

Neutral points are specific directions in the light field where the three Stokes parameters Q, U, V, and thus the degree of polarization simultaneously go to zero. We have made the first measurement of non-principal-plane neutral points in the upwelling light field in natural waters. These neutral points are located at approximately 40°- 80° nadir angle and between 120° - 160° azimuth to the sun which is well off of the principal plane. Calculations show that the neutral point positions are very sensitive to the balance in the incident light between the partially polarized skylight and the direct solar beam.

Highlights

Neutral points, positions where the light field polarization goes to zero, have been studied since skylight was found to be polarized
Arago discovered that skylight was polarized in 1809 and is credited with observing the Arago neutral point soon after [1]. This point is located above the anti-solar point, and is observable from the surface only at large solar zenith angles, but from elevated positions it can be seen at smaller solar zenith angles [2]
Our data showed that the position of this neutral point depended on the wavelength and solar zenith angle

Summary

Introduction

Positions where the light field polarization goes to zero, have been studied since skylight was found to be polarized. Arago discovered that skylight was polarized in 1809 and is credited with observing the Arago neutral point soon after [1]. This point is located above the anti-solar point, and is observable from the surface only at large solar zenith angles, but from elevated positions it can be seen at smaller solar zenith angles [2]. A second neutral point, the Babinet neutral point, was discovered by Babinet in 1840 [3] and is located near the sun, but above the solar position. A third neutral point, the Brewster point, was discovered by Brewster in 1842 [4] and is located near the sun, but below the solar position. A fourth neutral point, was found by Horvath et al [2], but is located below the horizon and is only visible from high altitudes, such as from a balloon or space

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