Abstract
Measurements of underwater irradiance revealed that the vertical attenuance inupward irradiance for wavelengths above 520nm decreased with increasing depth, while theattenuance in the remaining wavelength region and also the attenuance in the downwardirradiance in the whole wavelength range kept almost constant values. In this paper, it issuggested that the decrease in the attenuance for the upward irradiance above 520nm can beascribed to the Raman scattering of water molecules excited by the intense blue-green lightin the downward irradiance.
The pure water Raman scattering function at a scattering angle of 90° is measured andthe results are used for the theoretical computation of upward irradiance generated by Ramanscattering. Then, the difference between observed upward irradiance and the upward irradianceobtained by extrapolation from that in the shallow layers is computed under the assumptionof constant irradiance attenuance. Since this difference is expected to represent the upwardirradiance generated by Raman scattering, its value is compared with the upward irradiancedue to Raman scattering obtained by theoretical computation. The similarity between thetwo upward irradiances so evaluated supports the view that Raman scattering makes a significantcontribution to upward irradiance in the longer wavelength region.
