Lingrey reports that tests with divers and underwater television 

 systems have demonstrated the superiority of the latter from the standpoint 

 of target image interpretation.^^ The tests were conducted in shallow water 

 under ambient lighting, and comparisons were made on the basis of contrast, 

 resolution, and tone response. The general conclusion was that the subjective 

 sightings of a television monitor technician ran about 30% better than the 

 sightings of the diver (Reference 18, p. 57). 



Accurate color rendition with underwater lighting is difficult except 

 at close distances using quartz iodide lamps. Tests with 250-watt mercury 

 vapor, thallium iodide and quartz iodide lamps demonstrated that practically 

 all color rendition was lost at a distance of 3 meters from the light sources.^ ■^ 

 Generally, yellow is the easiest color to distinguish underwater, followed by 

 blue and green. Red should be avoided in underwater applications. 



Underwater Television 



Television offers some improvement over the human eye in both 

 range and image contrast. In turbid waters, however, backscatter can limit 

 usable television range to a distance of a few feet; in extreme cases to a 

 distance of several inches. Backscattering is usually minimized by employing 

 oblique lighting — placing the light source to one side and forward of the 

 camera objective lens. Pulsed range gating using a laser light source offers 

 some improvement in range over conventional vidicon systems. 



Underwater Lasers 



Terrestial laser surveying and alignment systems have been used with 

 great success. As a result, their use in similar applications under water has 

 been suggested. Recent tests at NCEL with a diver operated laser transit 

 indicate a usable range of about 1 50 feet. Tests were conducted in reasonably 

 clear water. Laser range can best be extended by using pulse gated systems, 

 and research in this direction is currently underway. 



Guidelines 



Advanced methods of subsea drilling have utilized guidelines for 

 positioning wellhead systems on the ocean floor. Figure 16 is a drawing 

 showing a longitudinal cross section of a typical drilling ship. The landing 

 base and wellhead are lowered via the pipe string, thereby establishing the 

 guideline system. The blowout preventer and various control systems can 

 then be placed into position via the pipe string, guided by the guidelines. 

 Usually, four guidelines are used. 



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