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508 JONES AND EYLES November
II. NEW LIGHT SOURCES
New light sources have been described for use with the conventional types of mechanical high-speed cameras. The older method of using high-efficiency tungsten-filament lamps is being replaced steadily by the use of specially modified discharge tubes of various types. Beeson6 has published information on an overrun high-pressure mercury-cadmium vapor lamp. This lamp can be run at a relatively low current for prolonged periods, when it gives a light output equivalent to about 2 kilowatts of tungsten-filament illumination. It can be overrun for short periods, the duration of which depends upon the degree of overload, and it then produces an extremely high intensity of illumination. For example, when run for one second at 120 amperes, it provides sufficient light to illuminate a subject 3 feet square and give adequate exposure on 16-mm Kodachrome film at //2.7, using a Kodak high-speed camera, Type III, running at the rate of 3000 pictures per second. The color rendering, though not perfect, is sufficiently good for most practical purposes. Beeson6 has also produced special illumination equipment consisting of 48 flashbulbs mounted on a rotating disk and triggered by means of a commutator so that each bulb is fired in turn as it passes through the reflector. The bulbs are fired at such a rate that substantially uniform illumination of the subject is obtained over the one second or so required to pass 100 feet of 16-mm film through the Kodak high-speed camera running at the rate of 3000 pictures per second. This equipment has been successfully used with the Kodak high-speed camera in several cases where an extremely high intensity of lighting is needed without undue heating of the subject.
Aldington7 has published information on the gas arc, which consists of a continuously operated high current-density arc discharge in a rare gas, usually xenon. This lamp gives very high illumination for extended periods, but it has to be water-cooled.
III. TIME BASES
While both conventional high-speed cameras and specially designed cameras such as those described above have been used extensively for direct photography, there is a growing feeling that straightforward cinematography is not adequate for the examination of many problems. In the first place, more accurate time bases have had to be introduced than had previously been used. The inclusion of a specially