\ _ V 



30 

 , deg 



Figure 5. Transmission of light through air-water 

 interface as a function of incidence angle; the 

 dotted line indicates the transmission of light 

 under heavy overcast which diffuses solar insolation. 



performed by Taylor and Smith show that only wavelengths of light in the 480-nm region 

 are transmitted with little attenuation in clear water. Other wavelengths are rapidly filtered 

 out. As a result of this selective filtering process, light with wavelengths outside the 450- to 

 550-nm range (blue-green) has been almost totally filtered out at a depth of 25 m (figure 6). 

 Thus, the selective attenuation of the sunlight spectrum decreases not only the overall intensity 

 of sunlight at a given depth, but also its spectral composition. 



Because of the selective attenuation to which solar radiation is subjected in water, the 

 output of the solar cells drops rapidly with depth. If the typical spectral response of a silicon 

 solar cell (figure 4) is compared with the composition of spectral irradiance at depths in excess 

 of 25 m (figure 6) it becomes obvious that only approximately 25 percent of the potential 

 solar cell power is capable of responding to the available light intensity at that depth. 



Diffused attenuation of sunlight is the major obstacle to light propagation only in 

 waters with exceptional optical properties, i.e., the visual contrast limit depth is in excess of 

 25 m. In seawater with typical optical properties, i.e., the visual contrast limit depth is about 

 5 m, the decrease in underwater irradiance with depth is primarily caused by the suspended 

 animate and inanimate water which either reflects and/or absorbs the downwelling irradiance. 

 Estuaries of rivers, harbors, and bays are the primary locations of water with visibility less 

 than 5 m. 



3. Tyler, J. E., and Smith, R. C, "Measurements of Spectral Irradiance Underwater," Gordon and Breach 

 Science Publishers, New York, NY 1967. 



10 



