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Journal of the Optical Society of America A

Journal of the Optical Society of America A


  • Vol. 19, Iss. 10 — Oct. 1, 2002
  • pp: 2085–2099

First observation of the fourth neutral polarization point in the atmosphere

Gábor Horváth, Balázs Bernáth, Bence Suhai, András Barta, and Rüdiger Wehner  »View Author Affiliations

JOSA A, Vol. 19, Issue 10, pp. 2085-2099 (2002)

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In the clear sky there are three commonly known loci, the Arago, Babinet, and Brewster neutral points, where the skylight is unpolarized. These peculiar celestial points, bearing the names of their discoverers, have been the subject of many ground-based investigations, because their positions are sensitive indicators of the amount and type of atmospheric turbidity. According to theoretical considerations and computer simulations, there should exist an additional neutral point approximately opposite to the Babinet point, which can be observed only at higher altitudes in the air or space. Until now, this anonymous “fourth” neutral point has not been observed during air- or space-borne polarimetric experiments and has been forgotten, in spite of the fact that the neutral points were a basic tool in atmospheric research for a century. Here, we report on the first observation of this fourth neutral point from a hot air balloon. Using 180°-field-of-view imaging polarimetry, we could observe the fourth neutral point at 450, 550, and 650 nm from different altitudes between 900 and 3500 m during and after sunrise at approximately 22°–40° below the anti-solar point along the anti-solar meridian, depending on the wavelength and solar elevation. We show that the fourth neutral point exists at the expected location and has characteristics similar to those of the Arago, Babinet, and Brewster points. We discuss why the fourth neutral point has not been observed in previous air- or space-borne polarimetric experiments.

© 2002 Optical Society of America

OCIS Codes
(010.3920) Atmospheric and oceanic optics : Meteorology
(100.0100) Image processing : Image processing
(120.5410) Instrumentation, measurement, and metrology : Polarimetry
(260.5430) Physical optics : Polarization
(280.0280) Remote sensing and sensors : Remote sensing and sensors
(290.1310) Scattering : Atmospheric scattering

Original Manuscript: February 20, 2002
Revised Manuscript: April 30, 2002
Manuscript Accepted: May 7, 2002
Published: October 1, 2002

Gábor Horváth, Balázs Bernáth, Bence Suhai, András Barta, and Rüdiger Wehner, "First observation of the fourth neutral polarization point in the atmosphere," J. Opt. Soc. Am. A 19, 2085-2099 (2002)

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  29. D. K. Lynch (David.K.Lynch@aero.org) wrote to G. Horvath on 26November2001: “The 4th neutral point has been a favorite topic of mine for some years, yet I have never published the results. I have observed it twice, once in 1983 and again in 1994 from aircraft, but I have not published the results because my calibrations were not too good. The existence, however, is certain.”

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