PHOTOGRAPHY OF UNDERWATER EXPLOSIONS 211 



tures taken at normal rates of 16-64 frames/second show the phenome- 

 non on one or at most only a few frames before it leaves the field of view. 

 For charges of a few hundred pounds or less, frame rates of 1,000 per 

 second, such as are possible with Eastman high-speed or Fastax cameras, 

 are needed. 



A somewhat slower phenomenon, but one which still requires high 

 film speeds, is the rise of the dome of spray thrown up from the surface 

 by reflection of the incident shock wave. The initial velocity of the 

 upper surface of this dome is simply related to the pressure in the shock 

 Avave producing it, and can therefore be used to compute shock wave 

 pressures. The development and contour of the dome is easily seen to 

 be affected by the depth of the explosion, and the phenomenon has been 

 made the basis of several methods for measuring depths of explosion. 



The gaseous products of explosion rise upward after their formation 

 and, after a time determined by the depth and explosive, break through 

 the surface, projecting the familiar plumes of spray if the charge is suffi- 

 ciently close to the surface. The time of the process and the nature and 

 extent of the upward water motion are clearly closely related to the 

 motion of the buoyant gas volume, and can provide valuable evidence 

 as to the nature of the motion when the breakthrough occurs. 



The phenomena above the surface, as sketched briefly in the fore- 

 going, are examined in more detail in Chapter 10, but the usefulness of 

 photographic records suitable for quantitative measurement should be 

 apparent. If such records are to be obtained, two general types of re- 

 quirement must be met. The first of these is the obvious necessity that 

 the photographic quality must be good. Proper exposure, good contrast, 

 and in the case of motion pictures proper frame speed, must all be real- 

 ized for good measurements. These necessities are both straightforward 

 and obvious, but they are mentioned because of the large amount of 

 otherwise useful data which have been lost as a result of such things as 

 surface glare on the water and unsuitable or faulty equipment. 



A second type of requirement is that of proper scales of length and 

 time, incorporated directly in the record if possible. Although it is 

 sometimes possible to make useful measurements without either type of 

 scale, any record is more valuable for them and many measurements are 

 virtually impossible otherwise. An example of the latter is photo- 

 graphic determination of underwater shock wave velocities and pres- 

 sures, which can hardly be accomplished without accurate knowledge of 

 a distance scale and precise indications of film speed in terms of a known 

 standard. 



6.2. Optical Considerations in Underwater Photography 



Although, as suggested in the preceding section, phenomena visible 

 above the water surface can provide much information about an explo- 



