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442 HIGH-SPEED PHOTOGRAPHY November
subject go through its operation once, and closing the shutter. In this way a multiple image is obtained, and knowing the subject distance and focal length of the lens, it is possible to compute rates of movement.
With the increasing use of the single-flash technique it has been necessary to develop shutters which would fire the gas-discharge lamp at the instant the shutter was fully opened. Older techniques required that a slow shutter or the open-flash method be used, but shutters have been developed recently by the major lens manufacturers which are synchronized with the flash and at shutter speeds as high as l/m second. It has been learned, however, that using short focal length lenses may step this figure up to 1/7Bo or Yiooo second.
The rotating-prism motion picture cameras are the most commonly used high-speed cameras today because of their portability. These cameras are light in weight, can run from either alternating or direct current and have lenses available of varying focal length. In the case of the Kodak high-speed camera, lenses of 2J/2 and 4-inch focal length are available with other focal lenses available on special order. With the Western Electric Fastax camera, there are lenses available in focal lengths from 35 mm to 15 inches and development is proceeding on a 30-inch lens.
The Kodak high-speed camera used 16-mm film and operates through the range of 1000 to 3000 pictures per second. Variable speeds lower than 1000 frames a second can be obtained through the use of a continuously adjustable autotransformer such as a Variac. The optical image moves in synchronism with the film through the use of a rotating plane-parallel glass plate of precise thickness which is located between the lens and the film. Two frames are exposed for each rotation of this plate, and a speed of 90,000 revolutions per minute is attained at 3000 pictures a second. The exposure per frame is equal to the reciprocal of 5 times the camera speed. For example, at 1000 frames per second the exposure time per frame is 1/ 16,000 of a second.
A 32-volt universal-type motor operates the camera. To limit acceleration strain, a mechanically coupled rheostat gradually reduces resistance until the preset terminal voltage is attained. At 3000 frames a second, the terminal input to the motor is equal to the full line voltage, usually about 115 volts.