212 PHOTOGRAPHY OF UNDERWATER EXPLOSIONS 



sion below it, these phenomena are to a considerable extent byproducts 

 of the major physical events. Many of these events can thus be in- 

 ferred only indirectly unless the photography is carried out beneath the 

 surface. The substitution of water as the optical medium and the fact 

 that the phenomena of interest are capable of destructive effects both 

 present difficulties in securing a sharp, adequately illuminated image at 

 the photographic plate, and methods of satisfying this basic requirement 

 are discussed in this section. 



A. Transparency of the water. Water is at best an imperfect medium 

 for transmission of light waves, owing to a greater decrease in intensity 

 with distance from a source than predicted by geometrical considera- 

 tions, such as the inverse square law for a point source. This attenu- 

 ation is largely the result of scattering by impurities, and varies tre- 

 mendously with the amount and nature of such impurities. 



If the laws of geometrical image formation hold, all the light which 

 reaches the camera from a given point on an object to be photographed 

 combines at a corresponding image point on the plane of the film. If 

 scattering and absorption occur in the intervening medium, however, 

 part of this light does not reach the camera and, what is worse, light 

 scattered from other geometrical paths does arrive, thus producing a 

 decrease in definition and contrast of the image. 



This reduction in visibility is inherent in any given medium, and 

 becomes rapidly more serious as the distance increases, as the attenu- 

 ation and scattering increase exponentially with the path traversed. 



Loss of intensity by absorption and scattering could of course be 

 compensated by using brighter illumination, but the failure of the light 

 to form a good image cannot. An upper limit to possible working dis- 

 tances is therefore set by the clarity of the water, and this limiting factor 

 is an important one for measurements on any scale. In tests on a lab- 

 oratory scale, such as might be performed in a tank several feet on a 

 side with detonator caps, reasonably clean tap water is often satis- 

 factory, but must be changed when contaminated by dirt, solid carbon 

 explosion products, or growths. With larger charges, transparency is 

 a more difficult problem, because larger working distances are necessary 

 which in turn require larger volumes of clearer water. If an artificial 

 basin is used, the water must be changed or cleaned periodically, which 

 becomes a considerable expense for work with even a few pounds of ex- 

 plosive. For charges of more than a few pounds, work must almost 

 always be carried out in natural bodies of water, and the clarity of this 

 water is frequently the Hmiting factor which makes satisfactory pic- 

 tures difficult or impossible to obtain.^ 



^ Problems of underwater photography in general have been investigated by the 

 Woods Hole Oceanographic Institution, as reported by Ewing, Vine, and Worzel 



