Abstract

The cosine error of in situ seven-channel radiometers designed to measure the in-air downward irradiance for ocean color applications was investigated in the 412683  nm spectral range with a sample of three instruments. The interchannel variability of cosine errors showed values generally lower than ±3% below 50 degrees incidence angle with extreme values of approximately 4–20% (absolute) at 50–80 degrees for the channels at 412 and 443  nm. The intrachannel variability, estimated from the standard deviation of the cosine errors of different sensors for each center wavelength, displayed values generally lower than 2% for incidence angles up to 50 degrees and occasionally increasing up to 6% at 80 degrees. Simulations of total downward irradiance measurements, accounting for average angular responses of the investigated radiometers, were made with an accurate radiative transfer code. The estimated errors showed a significant dependence on wavelength, sun zenith, and aerosol optical thickness. For a clear sky maritime atmosphere, these errors displayed values spectrally varying and generally within ±3%, with extreme values of approximately 4–10% (absolute) at 40–80 degrees sun zenith for the channels at 412 and 443  nm. Schemes for minimizing the cosine errors have also been proposed and discussed.

© 2007 Optical Society of America

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