PHOTOGRAPHY OF UNDERWATER EXPLOSIONS 215 



ably uniform and reproducible shock wave pressures, and hence are 

 very useful in fundamental investigations. 



The spherical intensity distribution from the design of Fig. 6.1(a) 

 is to a large extent wasted in many applications. This waste is accom- 

 panied by the obvious disadvantages of using unnecessarily large quan- 

 tities of high explosives near equipment or gear. Reflectors can of 

 course be used to concentrate more of the total light output in a given 

 region, but a better solution in many cases is simply to use only a suit- 

 able section of the sphere in the form of a conical charge, as shown in 

 Fig. 6.1(b). With an angular opening of 60° this charge has approx- 

 imately V7 the weight of a spherical charge giving the same intensity 

 over this angle of field without reflectors. The charges described are 

 not usable without modification at depths of more than about 20 feet 

 below the surface, but protective cases of various types and other 

 charges permit flash photography at depths of 600 feet or more. For 

 details of these and other designs of flash charges and their preparation 

 the original reports should be consulted. 



Explosive flash charges have the advantages of very high intensity 

 with extremely short duration and are thus best suited to single flash 

 photographs of rapidly changing phenomena, especially underwater 

 shock waves. Although two or more charges have been fired at preset 

 intervals, they are not useful for sequence records of slowly changing 

 phenomena. The use of flash charges has the further disadvantage of 

 involving high explosives, which of course cause underwater effects in 

 addition to those intended to be made visible, and experimental ar- 

 rangements must be planned to prevent interference by these disturb- 

 ances. The use of high explosives also involves hazards. These are 

 already present in photography of an explosion, and it may well be that 

 alternative sources, such as high voltage discharges, result in compa- 

 rable hazards, as well as presenting difficulties in field operations. 



C. Other flash sources. Electrically operated light sources can also 

 be used in underwater flash photography. The General Radio micro- 

 flash unit, Avhich comprises a gas-filled lamp through which a condenser 

 previously charged to 7,000 volts is discharged after ionization of the 

 gas by a triggering impulse, is one electrical counterpart of an explosive 

 flash charge, and the General Electric electronic photohght, employing 

 a high voltage discharge through a capillary arc, is another.^ Both 

 these units give intense flashes with durations of the order of two to four 

 microseconds, and can therefore essentially stop underwater shock 

 waves. A number of investigations at the Taylor Model Basin on 

 shock waves and intersecting shock waves from small charges have 



3 The General Electric "Electronic Photolight" is an example of the capillary arc; 

 underwater flares for photographic purposes are described in a report by E. L. 

 Patterson (81). 



