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I. INTRODUCTION 
Interesting photographs of underwater explosions in model tanks have been obtained by 
many investigators. Their experiments suffered almost universally from the drawback that 
it was impossible to shoot charges larger than a gram or even a tenth of a gram. A technique 
has been developed at UERL of shooting in the open ocean so that the charge size is limited 
essentially only by the transparency of the water rather thar by the strength of tank. 
This report discusses in detail the methods and apparatus used in obtaining the pictures 
and the results of some two years of experimentation. It indicates the usefulness of photo- 
graphic techniques for studying underwater explosions. 
TI. EXPERIMENTAL METHODS AND EQUIPMENT 
Two main types of picture have been taken; the flash photograph, in which a single 
picture of approximately one microseconc exposure was obtained, and the motion picture, which 
usually gave three or four to several hundred pictures in a single experiment. These latter 
pictures were taken at speeds up to 2500 per second and corresponding exposures down to 100 
microseconds. The two methods wili be taken up in detail. 
1. Methods of flash photography 
(a) General information. -- In taking short exposures of rapidly changing phenomena, 
the shutter of the still camera was opened, the explosion initiated, a short 
duration flash of light emitted, and the shutter closed. During the winter months, 
it was found possible to work in daylight by actuating the instantaneous shutter 
mechamism, set at 1/100 second, with a solenoid and synchronizing the explosion 
electrically. When the sun was brighter, however, this method caused a general 
fogging even at 1/200 second, and it was found necessary to work at night. This 
simplified the problem cf synchronization inasmuch as it was possible to set the 
shutter on bulb, open it with the solenoid, fire the charge, and close the shutter. 
In experiments in which it wss necessary to "stop" shock waves, the light source 
was an explosive flash charge (Section II, 1, (d)). The delay between the time of 
detonation of the subject charge and the flash charge was obtained by the proper 
length of Ensign-Bickford Primacord whose detonation velocity is 0.248 incher per 
microsecond. 
It is possible to obtain multiple exposures on a single plate by firing several 
flash charges in sequence, using primacord timing. 
When extreme speed was not necessary, the flash charge was replaced by a photo- 
flash bulb or Eastman Kodatron Speedlamp. 
(b) Vater transparency. -- One of the important variables in underwater photography 
in the open sea (near land at least) is the transparency of the water. Not only 
does the total amount of transmitted light from a constant light source decrease 
at lower transparency, but, since the lower transmission is due to turbidity, image 
sharpness decreases even if the proper exposure is made. To take this into account, 
a crude transparency measure is made by dropping a white disk eight inches in 
diameter through the water until it disappears from view. The depth of this dis- 
appearance is recorded as the "Secchi disk reading." YY ssa rough rule of thumb, 
it may be stated that fairly good pictures can be taken with object-to-camera 
distances up to half the Secchi disk reading. 
Se Se a 
aly Progress Report on Underwater Photography, by D.E. Kirkpatrick, J W E 
NDRC Section C-4 (Division 6.1 » May 4, 1342 = ie os tee 
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