Another aspect of lighting which bears 

 heavily on viewing range concerns the lights' 

 position on the submersible. Mentioned 

 above were the effects of light scattering by 

 particulate matter (organic and inorganic) in 

 the water column. Surprisingly, even the 

 clearest waters have considerable amounts 

 of such material which scatters and reflects 

 light, thus creating viewing conditions quite 

 similar to those of an automobile's head- 

 lights in a fog. (This suspended material is 

 generally called "snow" by submersible oper- 

 ators, and it is a real-time indicator of 

 whether the submersible is descending or 

 ascending.) The majority of submersibles 

 have their lights attached directly to the 



fairing or in the immediate vicinity of the 

 viewport (Fig. 10.1). To illuminate an object 

 at some distance from the submersible the 

 light must travel a two-way path (out and 

 back) and is subject to scattering in both 

 directions. The result is to limit both the 

 range of viewing and the intensity of light 

 returning to the observer. Another problem 

 is created with this arrangement when pho- 

 tographing through the viewport, because 

 generally, but not always, the lights are 

 aimed to concentrate on one spot forward 

 and downward in front of the submersible; 

 this creates a "hot spot" which overexposes 

 one portion of the photograph and underex- 

 poses others (Fig. 10.7). 



Fig- 10.7 A photograph of the sea floor taken from ALVIN Note the overexposed region or "hoi spot" and underexposed regions at the extremities. Subsequent lighting 



rearrangements provided more uniform exposure. (U.S. Navy) 



479 



