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

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Fig. 3. High-speed shutter used with frame camera. at the objective lens, the beam splitter, and the final relay lenses. This insures even illumination for all the points of the final image and the minimum exposure time for each picture. To obtain a rate of 3,400,000 frames/sec, a series of 85 framing lenses were used for each of the 180° arcs and the rotating mirror operated at 10,000 rps (revolutions per second). The loss of light entailed by the use of a beam-splitting mirror may be objectionable to some users even though a factor of 2 or 3 in exposure usually makes only a minor change in the final print quality. In that case the use of a roof mirror in place of the partial reflecting mirror is advisable. The use of a roof mirror also introduces the need for the objective lens to be of twice the aperture and may result in appreciable distortion in the final image. The objective lens axis is then placed midway between the A and B optical paths. The shutter for the camera is located between the field lens L.2 and the beam splitter, since this part of the system has a small aperture. The shutter must be closed within 1/20,000 sec after the start of exposure or recycling and multiple exposure will occur. Since a mechanical shutter cannot operate in such a short time it was necessary to devise a much faster mechanism. A small block of glass can be rendered sufficiently opaque within a few microseconds by shattering it with a shock wave from a high-explosive detonator. Such an arrangement has been found practical if the glass is enclosed in a suitable steel case. Camera Construction The exterior of the camera is shown in Fig. 2. The tube at the top is the focusing mount for the 24-in. objective lens used. The electrical controls for operation of the camera are mounted on the circular end plate. The rectangular projection on the plate is the highspeed explosive shutter mentioned above. It is easily removed for loading with a glass block and the explosive detonator. The viewfinder is located just above the shutter. The adjustable time-delay and detonator firing circuits are housed within the camera body. The two semicircular relay lens rings and film holders are seen at the rear of the camera. Standard 35mm camera cassettes are used to hold the film. The disassembled explosive shutter is shown in Fig. 3. It is made of steel and withstands the detonator explosion 506 December 1952 Journal of the SMPTE Vol. 59