IV. TRANSMISSION OF ENERGY WITHIN 



THE SEA 



8. LIGHT 



J. E. Tyler and R. W. Preisendorfer 



For the past ten years the physical aspects of radiative transfer within the 

 ocean have been under intensive study by both theoreticians and experimental 

 physicists. Probably the most significant advance during this period has been 

 the application of diffusion theory to the interaction of light with the sea and 

 the development of a complete theory of radiative transfer in the ocean. This 

 work has been carried on by Preisendorfer at the Scripps Institution of Oceano- 

 graphy, and to some extent by LeNoble at I'Ecole Superieure de Physique et 

 Chemie. This theory exhaustively treats the phenomena of multiple scattering 

 within a scattering-absorbing medium in which the volume scattering func- 

 tion is not isotropic. The theory is a unique and powerful tool for studying 

 the interaction of electromagnetic radiation with the sea, for predicting flux 

 distributions under water, and for computing the magnitude of important 

 effects such as the deterioration of image contrast along horizontal and inclined 

 paths of sight. The theory has also made possible the development of new 

 instrumentation for studying the optical properties of the sea and has clearly 

 shown the relationships between these various optical properties. The broad 

 physical base which has thus been established for light measurements in the 

 sea has brought about a valuable standardization among various laboratories 

 and has provided a foundation to which special measurements or constructs 

 important to biology or geophysics can be directly related. 



1. Physical Constructs 



A. Radiance 



The most useful construct for the study of underwater light fields is the 

 vector construct, radiance, borrowed from the field of radiometry. Radiance is 

 flux per unit projected area per unit solid angle in a specific direction. It is 

 defined by the equation 



N = PI{A cos dQ) (1) 



and can be measured by means of a simple device illustrated in Fig. 1, or by 

 means of a properly designed optical system. The radiances measured in various 

 directions from a fixed point below the ocean surface can be exhibited in three 

 dimensions by assembling around the point vector arrows whose lengths are 



[MS received June, 1960] 397 



