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300 BOURNE AND BEESON September
time necessitates an extremely high illumination; for example, even with a very rapid film such as Kodak Super XX, an illumination of some 10,000 ft-c (foot-candles) will be required to expose an average subject with an aperture of //2.8 at 1500 frames/sec. Owing to the slow emulsion of color film, it is even more difficult to provide sufficient illumination for high-speed color cinephotography, and the photographer is continually faced with the problem of obtaining enough light to allow a small enough lens aperture to give adequate depth of focus. Fortunately in many applications of high-speed cinephotography the subject is small in size and an illuminated area of only 6 to 12 in. in diameter is often sufficient. It is fortunate too that at these high speeds long periods of exposure are rarely necessary and useful total exposure times generally lie between 1 and 5 sec. It thus appears that a lamp providing up to 100,000 ft-c over an area of not more than 1 ft in diameter, for a duration up to 5 sec, would satisfy many of the needs of the high-speed cinephotographer. Because many of the applications are in factories, laboratories and industrial organizations, light weight, robustness and portability are other essential requirements for the equipment.
The difficulty of obtaining adequate illumination was stressed in this Society's Symposium3 which included a number of valuable papers dealing with various aspects of the subject. In one paper dealing with lamps for high-speed photography, the requirements of the ideal light source were outlined and the paper then described the various methods which are being used for providing illumination. Each of these methods has certain limitations.
In the past, photographers have generally used standard filmstudio incandescent spotlights for lighting the subject. For example, one M-R 414 Fresnel-lens spotlight with a 5-kw incandescent lamp produces approximately 10,000 ft-c over a 12-in. diameter spot at a distance of 5 ft. It is however, difficult to group enough spotlights closely together to obtain sufficient illumination; again, heating of the subject is extremely severe so that special means of cooling are often necessary. Another interesting but expensive method of lighting used in the United States4 is to produce a short continuous flash by successive firing of a number of aluminum foil flashbulbs mounted on a rotating disc and passing in turn in front of a mirror. Highly loaded, short-life filament lamps intended only for intermittent burning have also been employed successfully.5 The electronic flashtube is another light source which provides an extremely high light intensity but the duration of the flash is only a few microseconds so that, while it is eminently satisfactory for taking