Journal of the Society of Motion Picture Engineers (1930-1949)

luminosity of the shock wave generated in argon gas by a small amount of a brisant explosive.1 Shock-wave flashes of about 4 to 20/xsec (4 to 20 X 10~6 sec) duration and 300-600 million cp intensity were produced by detonating 0.4-5.0 cm3 of a brisant explosive (tetranitromethane plus toluene) in the end of a small, grooved brass cylinder, above which was a cellophane tube filled with the argon. The cellophane tube and the brass cylinder had the same diameters, namely 8 mm or more. The vertical cellophane cylinder of argon traversed by the luminous shock wave had a height of about 8 cm.2*3 Apparently the selected materials and dimensions favored the production of brief, intense luminosity. Later investigators who employed the argon flashlamp in photographing various explosive phenomena used modified forms of the lamp. Shepherd reported use of a lamp consisting of approximately ^ oz of pressed tetryl inside a cardboard cylinder 2 in. in diameter. The end of the cylinder had a cellophane window.4 The duration of luminosity of the flashlamp was estimated to be 2-4 /*sec. The pictures, which were taken by either front or back lighting (silhouette), illustrated that the light from the argon flashlamp, even though spread over the expanse of the subject at different selected stages in the explosion process, was much more brilliant than the hot gas from the permissible-type explosives. U.S. military investigations during World War II included photographic studies of underwater explosions, which required the development of lamps suitable for (1) the illumination of gas bubbles from explosions at different depths and (2) the illumination of relatively small, highvelocity, demolition explosives detonating relatively near the camera. In order to obtain flashes of high intensity and short duration, attention was given to both the surface area of the charge generating the shock wave at a given instant and to the thickness of the argon layer traversed by the shock wave. Both spherical and conical cast pentolite charges were employed.6 It was found that both the duration and the intensity of the illumination increased with the thickness and the area of the argon gas layer, but details about the methods and results have not been published in the open literature. The present paper includes microsecond photographs and oscillograph reproductions which show the characteristics of the brief flash of intense light developed by a conical explosive charge in an experimental type of flashlamp used at Eastern Laboratory. Experimental Photographs of the argon flashlamp were made with a one-microsecond Rapatronic shutter.6 The shutter is triggered by the light from the explosion by means of a photoelectric cell (RCA 929) and an adjustable time-delay circuit. The Rapatronic shutter consists of crossed polarizers between which is a slug of extra-dense flint glass as shown in Fig. 3. The shutter is opened by causing the plane of polarization to rotate in the glass (Faraday effect) by an axial magnetic field. The 1-jusec exposure is produced when a 24-kv, 0.125-/xf capacitor is discharged through a triggered air gap into a nineturn coil around a slug of extra-dense flint glass 1 in. in diameter. The plane of polarization of the light passing through the glass is rotated by the magnetic field. A doublet camera lens of about 6-in. focal length was used in front of the Rapatronic shutter. Visual focusing was accomplished by rotating one of the polarizers that normally are crossed on opposite ends of the flint glass. The light-time oscillographic trace was displayed on a Du Mont Type 256A ranging oscillograph, and a photograph was made to record the transient. Light from the argon explosion was allowed to fall on the cathode of an RCA Type 929 Winning and Edgerton: Explosive Argon Flashlamp 179