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Proceedings Paper

Spectral irradiance, radiance, and polarization in blue western Mediterranean waters
Author(s): Niels Kristian Hoejerslev; Eyvind Aas
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Paper Abstract

Spectral downward and upward irradiance, radiance and polarization have been measured in the visible part of the spectrum down to 275 m in blue Western Mediterranean waters. The irradiance reflectance varied by a factor of 10 in the spectral mode and around a factor of 4 with depth. Spectral radiance and polarization were measured at discrete depths where the radiance meter was rotated around the vertical cable for selected zenith angles in order to give the azimuthal radiance distribution, which can be represented by a simple function of elliptic shape. Irradiance calculated from the radiance data sets compares favorably with directly measured irradiance, except for the near-surface measurements where ship shadows and sun-glints are problematic. Inspection of the radiance data indicates that self-shading effects from the instrument are small for the upward radiance. Various distribution functions have been calculated. The average cosine varies around 0.8, the average cosine for the upward radiance around 0.4, and the ratio of the upward irradiance and the nadir radiance is about 4. The latter parameter displays the highest variation in the present data set. The upward radiances in the zenith angle interval 150-180 degrees in the anti-sun direction were about the same at all depths and wavelengths. The vertical attenuation coefficients for these upward radiances approach the asymptotic k-value. This implies that a flat sea surface acts like a Lambert diffusor for water-leaving radiances in the nadir angle interval 0-40 degrees in the anti-sun direction. The degree of polarization varied between 0 and 50 percent, which is somewhat lower than earlier reported in the literature. An analytic expression for the angular distribution of upward radiance is proposed.

Paper Details

Date Published: 6 February 1997
PDF: 10 pages
Proc. SPIE 2963, Ocean Optics XIII, (6 February 1997); doi: 10.1117/12.266433
Show Author Affiliations
Niels Kristian Hoejerslev, Niels Bohr Institute of Astronomy (Denmark)
Eyvind Aas, University of Oslo (Norway)

Published in SPIE Proceedings Vol. 2963:
Ocean Optics XIII
Steven G. Ackleson; Robert J. Frouin, Editor(s)

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