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A POLYGONAL FLOODLIGHTING MIRROR
Frank Bexford* and M. W. Palmer**
Synopsis
The reasons that make it desirable for the motion picture studio to employ large floodhghting units are the high levels of illumination required for high speed photography and the pecuHar sensiti\dty characteristic of the photographic film. These two factors indicate the use of a high intensity arc, and there is a national tendency to take the high intensity searchhght just as it has been developed for military service and by refocusing get a beam of wide spread. The defects of this method are illustrated and it is shown that a more suitable optical arrangement is to use a polygonal mirror rather than a paraboloid. The method of computing the dimensions of the polygons is given along with data on a photometric comparison of the two types of reflectors.
Studio Use of High Power Floodlights.
THE intensity of illumination required for the taking of motion pictures is of the general order of one hundred times the illumination under which ^Ye may comfortably work and read in our offices and homes, and the motion picture art requires much special lighting equipment to attain these high intensities. There is a further difference between the every day use of artificial illuminants and studio practice, and that is the obvious fact that in the office or home we are interested in the reaction of the eye, but in the studio we are interested in the reaction of the photographic film. The film centers its reaction on the violet and ultra-violet regions of the spectrum, and as a result the brightness of illumination as measured by the eye with the aid of a photometer is not a reliable measure of the photographic intensity. One of the outstanding results of this difference is the use in studios of several light sources, the high intensity arc among them, that are not commonly considered as illuminants for floodlighting purposes.
The high intensity arc is essentially a high current arc and this fact alone leads to a few large units rather than a number of small ones, and the unique demands here made call for a type of unit that is not essential in any other phase of the illuminating art.
As motion picture sets have become increasingly larger, it has become more and more necessar}^ to use high intensity sources with
* Physicist, Illuminating Engineering Laboratory, General Electric Company. ** Electrical Engineer, Famous Players-Lasky Corporation.
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