PHOTOGRAPHY OF UNDERWATER EXPLOSIONS 213 



A very simple and effective measure of water transparency is ob- 

 tained by using the Secchi disk. This is a circular disk, eight inches in 

 diameter and painted white, which is lowered in the water until it be- 

 comes invisible from above the surface. The depth of water at which 

 this happens is called the Secchi disk reading, and a rough but useful 

 empirical rule is that reasonably good pictures can be taken up to half 

 the Secchi disk reading. This reading in sea water varies greatly with 

 nearby sources of contamination, nature of the bottom, etc., but gen- 

 erally is very much less in coastal waters than offshore. For example, 

 harbor waters on the New England coast may give readings of the order 

 of 20 feet, while readings of 120 feet are obtained 100 or 200 miles off- 

 shore. Tropical waters are much preferable, for this and other reasons, 

 readings of 130-140 feet having been obtained within a mile of islands 

 in the Bahamas for example. The readings quoted here are not in- 

 tended as source data, but merely to give an indication of what order of 

 distances can be used. In tests in Woods Hole harbor on the Massa- 

 chusetts coast, object-camera distances of more than 5 feet are fre- 

 quently impossible, but in tests off the Bahamas motion pictures of good 

 quality have been taken at a distance of 60 feet. 



A further inherent characteristic of water is of course the lower 

 speed of hght corresponding to an index of refraction of the order 1.34, 

 the exact value depending upon the temperature, density, and salinity. 

 The effective object distances and field of view for a camera with air 

 between the lens and film are therefore reduced by this factor, as com- 

 pared to air. The difference in optical path merely requires a changed 

 lens setting for proper focus, but the greater working distance required 

 for a given field, as compared to air, is unfortunate because of the scat- 

 tering effects already discussed. The index of refraction is, however, a 

 useful experimental tool in investigation of pressures and flow following 

 explosions, as discussed in section 6.3. 



B. Explosive light sources. An explosive is an appropriate hght 

 source for study of explosions, and some of the most successful sources 

 for still photographs with extremely short exposures have employed 

 explosive "flash charges." Similar charges have also been proved most 

 satisfactory for fundamental studies of small scale explosions, and a 

 description of their preparation is therefore merited. 



The basic design, developed at the NDRC Explosives Research 

 Laboratory in Bruceton, Pa. (34), consists of a centrally detonated 

 spherical charge of cast 50/50 pentolite (an equal mixture by weight of 

 PETN and TNT, see section 3.1). This charge is surrounded by a 

 concentric layer of argon gas at atmospheric pressure. The spherical 



(33) ; a number of techniques and applications to underwater explosions are described 

 in reports by Eldridge, Fye, and Spitzer (31), and by Swift et al (105). 



