A multiwavelength scanning radiometer has been used to measure the angular distribution of scattered radiation deep within a cloud layer at discrete wavelengths between 0.5 and 2.3 µm. The relative angular distribution of the intensity field at each wavelength is used to determine the similarity parameter, and hence single scattering albedo, of the cloud at that wavelength using the diffusion domain method. In addition to the spectral similarity parameter, the analysis provides a good estimate of the optical thickness of the cloud beneath the airraft. Results of such analysis are presented for a 50 km section of clean marine stratocumulus clouds off the coast of California on 10 July 1987. These observations were obtained from the University of Washington Convair C-131A aircraft as part of the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment (FIRE). In addition to the radiation measurements, we obtained microphysical and thermodynamic measurements from which the expected similarity parameter spectrum was calculated using accepted values of the refractive index of liquid water and the transmission function of water vapor. The measured absorption for this case of clean maritime clouds is close to, but persistently larger than, theoretical predictions. Finally, measurements are presented of the spectral albedo of the ocean surface under overcast conditions.