Field determinations of the remote sensing reflectance signal are necessary to validate ocean color satellite sensors. The measurement of the above-water downwelling irradiance signal E d(0+) is commonly made with a reference plaque of a known reflectance. The radiance reflected by the plaque (L dspec) can be used to determine E d(0+) if the plaque is assumed to be near Lambertian. To test this assumption, basic experiments were conducted on a boat under changing sky conditions (clear, cloudy, covered) and with different configurations for simultaneous measurements of L dspec and E d(0+). For all measurement configurations, results were satisfactory under a clear sky. Under cloudy or covered skies, shadow effects on the plaque induced errors up to 100% in the determination of E d(0+). An appropriate measurement configuration was defined, which enabled E d(0+) to be determined with an accuracy of better than ±15% regardless of the sky conditions.
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