The linearly polarized light field in clear, tropical marine waters: spatial and temporal variation of light intensity, degree of polarization and e-vector angle
Thomas W. Cronin1,* and
Nadav Shashar2
1 Department of Biological Sciences, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA and
2 The Interuniversity Institute of Eilat, PO Box 469, Eilat 88103, Israel
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Fig. 1. Analysis of polarization at one direction of view. Data were collected at 09:01h Eastern Daylight Time on 16 August 1999, with the polarimeter looking North at 60° elevation. (A) Averaged, uncorrected spectra of intensity at three orientations of the polarization analyzer mounted in front of the radiance collector. Blue line, analyzer at 0° (horizontal); gray line, analyzer at 45°; red line, analyzer at 90°. (B) Spectra of partial, linear polarization obtained by analyzing the data in A. Blue line, normalized radiance (corrected for spectrometer spectral response; the curve is normalized to 5.24x1011photonscm-2sterad-1nm-1s-1); black line, e-vector angle (0° is horizontal, with angles increasing counterclockwise; right-hand axis); green line, degree of polarization (1.0 is fully polarized light).
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Fig. 2. The polarization light field near midday; data were obtained between 12:16 and 13:22h Eastern Daylight Time on 16 August 1999. Each panel shows the spectra of radiance, degree of polarization and e-vector angle at one orientation of the polarimeter, indicated by the compass directions at the top and elevations (0–90°) in the panels on the left. Blue lines, radiance (all curves are scaled such that 1.0 on the left-hand ordinate is equivalent to 2.92x1012photonscm-2sterad-1nm-1s-1); black lines, e-vector angle (0° is horizontal, with angles increasing counterclockwise; right-hand ordinate); green lines, degree of polarization (left-hand ordinate). Note that data for South are plotted twice, at each end of each row, to provide continuity.
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Fig. 3. Representations of the polarized light field near midday at four wavelengths, obtained between 12:16 and 13:22h Eastern Daylight Time on 16 August 1999 (same data as illustrated in Fig.2). Each part of the figure represents the polarization pattern in the upper hemisphere of view at the wavelength indicated, as seen by an observer looking upwards (note that East and West are reversed from their locations on a standard compass because of this unusual direction of view). The outer circle represents the horizontal, with the inner circles representing elevations of 30° and 60°. The lighter inner region of the figure represents the theoretical location of Snell’s window (the angular extent of the sky above the water’s surface after refraction at the air/water interface), although this was not clearly seen at the measurement depth of 15m. At each location where the spectrum of partial linear polarization was measured and analyzed (as in Fig.1), the e-vector angle is indicated by the tilt of the line relative to a tangent to the circle passing through that point, and the percentage polarization is indicated by the thickness of the line (key provided at lower right). The approximate location of the sun midway through the measurement series is indicated by the sun symbol within Snell’s window.
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Fig. 4. Representations of the polarized light field at mid-morning at four wavelengths, obtained on 15 August 1999 between 09:23 and 11:42h Eastern Daylight Time. Otherwise as in Fig.3.
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Fig. 5. Representations of the polarized light field near sunset at four wavelengths, obtained on 15 August 1999 between 17:51 and 19:21h Eastern Daylight Time. Otherwise as in Fig.3.
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© The Company of Biologists Ltd 2001
