Self-shading effect by radiance meters on upward radiance observed in coastal waters

The ratio between recorded and true upward radiance in coastal waters has been determined from observations at 445, 514, and 546 nm for solar zenith angles in the range 40-60 degrees. The ratio follows an exponential function of KLR, where K-L is the vertical attenuation coefficient of upward radiance and R is the instrument radius, for values of KLR up to 0.50. The observed relative self-shading errors for an instrument radius of 7.5 cm in average optical conditions for the Oslofjord are in the range 0.04-0.18. They seem to differ by less than 0.07 from estimates based on Monte Carlo simulations by Gordon and Ding. The errors decrease with increasing solar zenith angle, but the observed errors are smaller than predicted by the Monte Carlo model. No variation in error with depth has been observed within the upper 2 m.