DIFFUSE-REFLECTANCE OF OCEANIC WATERS .3. IMPLICATION OF BIDIRECTIONALITY FOR THE REMOTE-SENSING PROBLEM

The upwelling radiance field beneath the ocean surface and the emergin g radiance field are not generally isotropic. Their bidirectional stru cture depends on the illumination conditions (the Sun's position in pa rticular) and on the optical properties of the water body. In oceanic case 1 waters, these properties can be related, for each wavelength la mbda, to the chlorophyll (Chl) concentration. We aim to quantify syste matically the variations of spectral radiances that emerge from an oce an with varying Chl when we change the geometric conditions, namely, t he zenith-Sun angle, the viewing angle, and the azimuth difference bet ween the solar and observational vertical planes. The consequences of these important variations on the interpretation of marine signals, as detected by a satelliteborne ocean color sensor, are analyzed. In par ticular, the derivation of radiometric quantities, such as R(lambda), the spectral reflectance, or [L omega(lambda)](N), the normalized wate r-leaving radiance that is free from directional effects, is examined, as well as the retrieval of Chl. We propose a practical method that i s based on the use of precomputed lookup tables to provide values of t he f/Q ratio in all the necessary conditions [f relates R(lambda) to t he backscattering and absorption coefficients, whereas Q is the ratio of upwelling irradiance to any upwelling radiance]. The f/Q ratio, bes ides being dependent on the geometric configuration (the three angles mentioned above), also varies with lambda and with the bio-optical sta te, conveniently depicted by Chl. Because Chl is one of the entries fo r the lookup table, it has to be derived at the beginning of the proce ss, before the radiometric quantities R(lambda) or [L omega(lambda)](N ) can be produced. The determination of Chl can be made through an ite rative process, computationally fast, using the information at two wav elengths. In this attempt to remove the bidirectional effect, the comm only accepted view relative to the data-processing strategy is somewha t modified, i.e., reversed, as the Chl index becomes a prerequisite pa rameter that must be identified prior to the derivation of the fundame ntal radiometric quantities at all wavelengths. (C) 1996 Optical Socie ty of America