Journal of the Society of Motion Picture and Television Engineers (1950-1954)

'were taken to record the motion of i Structures near the charge. The thought here was that, while the permanent displacement of the ground beyond the i edge of the crater could be determined by other means, the actual motion of the structure just following the detonation might be much more complicated than static measurements could reveal. In a similar manner the permanent displacement of the ground beyond the edge of the crater could be determined flby measuring the displacement of stakes [driven into it. Such stakes were successfully photographed with the result Hthat unsuspected details of the motion of the ground were revealed. In a few cases we also obtained 16mm Kodachrome pictures of the entire phenomena, ; using the 70-S Bell & Howell cameras I Which were set up at a considerable |distance from the charge. While from jthe analyst's standpoint these pictures i provide information only about the I growth and spread of the dust cloud : originating from the explosion, they are [' of considerable value in that they give Jan overall picture useful in orienting : the observer before looking at the highspeed motion pictures made with the pastax camera. The problem we faced in taking highspeed pictures in the interior of the .structures was that of supplying sufficient i illumination. Suggestions were made [that 60-in. searchlights, or flares of the type employed by the Air Force, could be used. The searchlights, however, I require a larger power source than j could be moved over the rough terrain, I which made them impractical in a field i operation of this type. On the other Jhand, flares are difficult to control, | dangerous to handle, and produce large j quantities of smoke. We constructed an apparatus consisting of a power supply and a motor-driven commutator I connected to set off 40 groups of Sylvania I2A flashbulbs in sequence (see Fig. 5). iTnese bulbs were mounted in reflectors ) constructed of sheet aluminum along the general lines of an overgrown bread tin standing approximately 36 in. high by 12 in. wide. Each reflector contained two rows of twenty sockets, a pair of reflectors providing a total of 40 pairs of bulbs. The connections to the commutator were such that from 1 to 8 bulbs could be connected in parallel and flashed simultaneously, and the speed of the commutator was set so that 40 sets of bulbs were fired in sequence at such a rate that the maximumintensity portion of the curves overlapped to provide essentially continuous illumination. This arrangement operated satisfactorily, although field experience made it apparent that improved operation would result from moistureproofing the electrical equipment and the use of a power source capable of supplying more instantaneous current than the 22^-v radio batteries. For field use this unit had the prime advantage that the energy that supplied the illumination was stored chemically in the flashbulbs and required only triggering energy from a battery, while the motor driving the commutator could be supplied from the same generator that operated the high-speed cameras. With this equipment, it was possible to secure satisfactory pictures when the camera was operated at speeds of approximately 1000 frames/sec and the area to be photographed consisted of 400 sq ft or so. This area was lighted from a distance of approximately 120 ft using 8 flashbulbs to the group; i.e., 320 flashbulbs were used to provide illumination for the two seconds needed to photograph the event. Two cameras were used to photograph the interior of these structures: the Bell & Howell 70-S running at 128 frames/sec and the Fastax set at varying speeds between 132 frames/sec and about 1200 frames/ sec. An observation of incidental interest is that the frame rate on the Bell & Howell camera was apparently sufficiently low for the negative to show no flicker in the light source, while the R. M. Blunt: Photography of Underground Explosions 411