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HIGH-SPEED PHOTOGRAPHY 441
The General Electric Company has developed a circuit which is similar to the Edgerton circuit but instead of using the gas tube as described above a high-pressure mercuryvapor lamp namely the H6 is used. With this particular lamp and its associated circuit, flashes of 1 to 2 microseconds' duration are obtained at a very high energy level.
With a continuously moving film camera and a commutator for firing the lamps in synchronism with the film movement the frequencies can be stepped up considerably so that up to 3000 pictures per second can be obtained from a given firing circuit. However, recently, electronic switching equipment has been developed so that six circuits may fire into a given tube to obtain pictures up to 20,000 per second. In the computation of the picture-taking speed and the film speed it is necessary to balance the exposure time or the duration of the flash so that it causes no blur on the film as the film is moving continuously. Therefore, the shorter the flash the better the result in terms of a produced photographic image. If the film is moving at 100 feet a second and a flash occurs in 1 microsecond the film will move 1.2 mils which is within the resolving power of the films used in high-speed motion picture photography. In taking pictures of this type, no shutter is used, hence under ordinary circumstances the room must be darkened sufficiently to prevent the film from being fogged by extraneous room light. This type of photography has been used very extensively in many types of industrial research in providing solutions to problems which have been troublesome to engineers for many years. There has been developed recently a unit which fires the lamp up to rates of 10,000 per second and furnishes sufficient illumination to light a field 12 inches square. The power supply needed for this is 21/z kilowatts and the exposure time is 1 microsecond per flash. The larger units supplied with the General Radio Company circuit will provide sufficient illumination to light a comparatively large field. Higher picture-taking rates than these stated above can be used when. the subject is in silhouette or photographed by the schlieren method. Methods of schlieren photography will be discussed later.
Another application of the gas-discharge-tube technique is to use the flashing lamp as a multiple-exposure device on a stationary piece of film. If the rate of firing is known, it is possible to compute acceleration and deceleration of various subjects under study by opening the shutter, starting the flashing lamp, having the test