VIII. RADIATION MEASUREMENTS 



Robert B. Elder 



A. INTRODUCTION 



Radiation studies are made for a number of purposes. The type of 

 instrumentation used for any radiation study depends a great deal upon 

 the purpose of the study. In physical oceanography, radiation studies 

 have a twofold purpose. From these studies the rate can be determined 

 at which radiant energy from the sun and sky is absorbed and converted 

 into heat in the upper layers of the sea. This determination is a princi- 

 pal factor in the computation and prediction of thermal structure changes. 

 Secondly, the extinction coefficients determined by underwater meas- 

 urements give information concerning the optical purity of the water 

 and the amount of suspended matter. 



The parameters to be measured in a heat budget study include the 

 incoming and reflected radiation at the sea surface and the attenua- 

 tion and scattering beneath the surface. These parameters are a func- 

 tion of the angle of the sun, the sea state, the turbidity of the atmosphere 

 and water, and the distribution of energy in the solar spectrum and 

 its differential absorption. 



The physical factors which influence the visual detectability of 

 submerged objects have been an important subject of study for some 

 time. In this case it is important to know the optical principles which 

 govern the transmission of an image through water. Many factors 

 combine to govern the visual detectability of submerged objects to an 

 observer below or above the surface. Detection of objects by an observ- 

 er either below or above the surface requires that the reflectivity or 

 the color of the object differ sufficiently from that of its background, 

 so that the optical signal reaching an observer exceeds his contrast 

 threshold despite attenuation by the intervening water. Other factors 

 which affect the contrast of an object viewed from above the surface 

 include the same parameters that have been indicated for heat budget 

 considerations. However, sea state becomes more important than the 

 others when detection through the surface is attempted. Although the 

 amplitude of the waves is of little importance in optical studies, the 

 slope of the water surface determines the reflective and refractive 

 effects. An observer looking through a wind-roughened surface receives 

 his images from changing angles because of the rapid and random 

 changes in wave slope. 



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