22I^ PHOTOGRAPHY OF UNDERWATER EXPLOSIONS 



6.4. Direct Photography 



The applications of photographic techniques which have so far been 

 described involve special optical or mechanical arrangements to obtain 

 detailed information about pressures and velocities. The more con- 

 ventional methods of flash and motion picture photography are also of 

 great importance, a particular advantage being that information is ob- 

 tained over a field of view rather than merely at points in the field. 

 This advantage is particularly evident in studying shock wave fronts, 

 the form and motion of the gas volume, and effects at targets. A num- 

 ber of examples of such "straight" photography and results obtained by 

 its use are given in other sections, but a few representative illustrations 

 are presented here to indicate their application and limitations.'^ 



A. Form of shock fronts. Although shock fronts in water are not 

 self-luminous, large pressure gradients are made visible when suitably 

 illuminated as a result of the accompanying changes in refractive index 

 which have a focusing lens effect. In the case of a spherical shock front, 

 for example, the refraction effect acts to concentrate uniform illumi- 

 nation from behind the spherical front into paths nearer that of the 

 tangential ray grazing the spherical surface than the ones in the absence 

 of the front (see Fig. 6.4). The region near the grazing ray therefore 

 appears more brightly illuminated on a photograph. The apparent 

 radius of the shock front seen in this way is evidently somewhat greater 

 than the true value, because the tangent rays subtend a larger angle at 

 the camera than the secant lines from the great circle on the sphere in 

 the plane normal to the optical axis. This geometrical error is small 

 unless the camera is close to the shock front, but must be taken into 

 account in measurement. 



A number of photographs of shock waves in successive stages of de- 

 velopment are reproduced in Plates I and II. The pictures were 

 taken by light from flash charges behind the main charge and facing the 

 camera. The desired time interval between detonation of the main 

 and flash charges was obtained by use of primacord fuse, as described 

 in section 6.2. The camera shutter was left open through the interval 

 of both explosions, and the outline of the main charge is made visible 

 in the center of the pictures by light from its detonation. Bare charges 

 were intentionally used to permit this double exposure and to reduce the 

 amount of stray material in the field of view to a minimum. 



The development of the shock wave from a spherical charge of 

 pentolite 2^ inches in diameter is shown in the 3 records of Plate I, 

 taken at times of 13, 58, and 104 microseconds after detonation. The 



^ These illustrations are from photographs taken at the Underwater Explosives 

 Research Laboratory, Woods Hole. These and many other similar photographs are 

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



