461 



AN IMPROVED TECHNIQUE FOR HIGH SPEED PHOTOGRAPHY 



D. A. Senior and C. 0. J. Grove- Palmer 



Naval Construction Research Establishment 

 Rosyth 



January 1950 



Abstract , 



A techniqje is descriDed for high speed cine-photography of the phenomena which take place 

 when small explosive charges are fired under water. At the same time, the explosive pressures 

 are recorded Dy piezo-clectric technique. Pressure records are cstained for a period of 

 60 mini seconds, 3nd high speed photographs are t-aken by means of a 16 mm. Fast ax camera at speeds 

 up to 14,000 frames/ second. At the maximum speed, a cine record in black -nd white or colour is 

 obtained for a period of 80 milliseconds. The liyout of equipment is described, and dct.'.i 1 s of 

 time marking and sequence timing arrangements ar; given. 



Introduction , 



In studying phenomena associated with the detonation of small explosive charges under 

 water, the problem arose of taking motion pictures of an area 3 feet 6 inches square through nine 

 feet of wht-.'r at speeds up to 4,000 fr.Tmcs/second, The illumination of limited areas for this 

 purpose may readily be provided by means of continous light sources, but the power requirements 

 are considerable if the area to b-'' photographed is pxtensivp. (Calculations show;d that in this 



instance, colour photography would require.-ibout l.if k.»'. of pht&fluo3 li.;hting, iMiich was 



cle irly inpricti cabl i) . 



High intensities of illumination may, however, be provided for limited periods by means 

 of photoflash bul6s (1), (2), (3), -nd the technique which is described is based upon this 

 principle. Photogriphs are f-ki^n simultaneously of phenomena ?.bovo ?.nd below the «Qtjr surface. 

 The equipment has been design, d so :s to synchronize the photogr-^Jhy with other methods of 



recording. 



The photographs cover a period of 90 milliseconds at maximum. speed, and therefore give 

 the general history of the explosion phenomena; most information is obtained by projecting the 

 records as a film at normal spEid. From a study of the film it is possible to decide at v/hich 

 points more detail and time resolution are required; the detailed study is then carried out by 

 taking microsecond flash photographs at suitable times from the instant of detonation. 



Vater treatment . 



Experiments have been carried out in a cylindrical tank of diameter 12 feet and depth 

 8 feet around which has been built a laboratory to provide cover over the tank itself and 

 accommodation for control -?.nd recording equipment. 



Writer cl'.rity is m^\intained by c .rcful exclusion of dust 3nd continuous filtration (the 

 filter handles one tank volume per day). An alkalinity of pH.8 is maintained by the addition 

 of sodium carbonate (about 6 g of the anhydrous Salt per day). The addition of 0.6 g potash alum 

 per day then ensures coagulation of suspended m='.tter and its removal by the filter. 



Photo graphi c equipment . 



Camera . 



The western Electric 16 nn. Fastax camera has been used. This camera has a maximum speed 

 of 4,000 frames/ second, the exposure tine then being 80 ni cro i-.-crnds. 



Two modifications hav-- be/n found desirable. The Uns provided by the makers(of focal 

 length 2 inches) does not givr ?. sufficiently large field of view. It has therefore been replaced 



