Spreading Of Light Beams In Ocean Water

Abstract Laboratory measurements of the irradiance distribution due to a laser beam and the radiance distribution due to a point source in a hydrosol have been made and the results analyzed. The analysis indicated that after proper accounting for absorption, the distri­ butions may be scaled as a function of the scattering length rather than the more commonly used parameter, the attenuation length. The two distributions have also been shown to be equivalent as required by the optical reciprocity theorem. The measurements have given new insight into understanding the spreading of light underwater.IntroductionIn the study of the characteristics of the natural light field underwater, it is sometimes necessary to look at simple lighting geometries in order to understand some of the basic physical questions which arise. This is especially true in the case of the lighting distribution near the air-water interface or around artificial underwater light sources. In the boundary layer just below the air-water interface and within a couple of optical distances of it, the initial radiance distribution present at the interface (due to multiple scattering processes in the atmosphere) undergoes rapid and complex changes as a function of the inherent optical properties of the water column.These changes, mainly due to the shape of the volume scattering function of the hydrosol, have been extremely difficult to predict analytically due to the characteristics of the equation used to describe the propagation of light underwater. Solutions of this equation, the equation of radiative transfer, while being extensive and using many different approximation techniques, have yet to give adequate insight into the changes in the distri­ bution which occur due to spatial variations of the optical properties of the hydrosol.The same problem also occurs when studies are made of the lighting distribution around artificial light sources underwater. Although the initial lighting or radiance distribu­ tion is usually much less complex and more easily defined, the same analytic difficulties are present and prevent a full understanding of the character of the lighting distribution in the hydrosol.In-order to circumvent these theoretical difficulties experimental measurements were made of the underwater irradiance and radiance distributions produced respectively by a uni-directional point source (or a laser beam) and an omni-directional source. The measurements were made in a laboratory tank where the optical properties of the hydrosol could be varied and controlled.The resulting distributions, essentially the so-called beam spread and point spread, are important since theoretically knowledge of these distributions allows a user to compute the lighting distribution produced by any arbitrary source through use of the super-position or convolution integral1 '*.The hydrosol used in the laboratory experiments was adjusted so that its volume scattering function duplicated as closely as possible the volume scattering function of natural oceanic water. However, there were differences between the two hydrosols, therefore the final results presented should not be looked upon as a precise representation of the lighting distribution occurring in natural waters under similar lighting geometries. The primary objective of the research was to develop an understanding about the character of the lighting distribution underwater due to simple sources and the changes which occur as the optical properties of the hydrosol are varied. The measurements and their analysis has provided this understanding.Inherent optical propertiesIn characterizing the optical properties of a hydrosol, there are four important inherent properties. These are a, the volume attenuation coefficient; s, the total volume scattering coefficient; a, the absorption coefficient; and a, the volume scattering