Use of a spectralon panel to measure the downwelling irradiance signal: case studies and recommendations

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.sub.d](0+) is commonly made with a reference plaque of a known reflectance. The radiance reflected by the plaque ([L.sub.dspec]) can be used to determine [E.sub.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.sub.dspec] and [E.sub.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.sub.d](0+). An appropriate measurement configuration was defined, which enabled [E.sub.d](0+) to be determined with an accuracy of better than [+ or -] 15% regardless of the sky conditions. [c] 2004 Optical Society of America OCIS codes: 000.2170, 010.4450, 120.4800.