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116 Transactions of S.M.P.E., July 1927
posite shadow but in the presence of other hght the composite shadow loses much of its "cubist" tone and becomes merely soft-edged and indistinct.
If the harsh shadows of direct sunlight are desired in portraiture this beam will give the desired results. To illustrate this a "close up" was taken with the sitter at 25 feet from the mirror. The photograph of Fig. 6 shows clear and distinct shadows and there is but little evidence of the multiple character of the beam.
Test Data.
Among the tests that have been made with the polygonal mirror there is one that is of direct interest to the Society of Motion Picture Engineers, and the data of this test will be given in preference to several other tests that were more elaborate and of higher accuracy. This test was made at the Famous Players-Lasky Studio, and certain features of the test therefore are a faithful duplication of studio practice.
A 150 ampere arc was focused (on the axis) to give the designed spread of 30 deg. It w^as found that, due to mechanical limitations, the arc could not be brought closer than 12.75 in. to the mirror, whereas the designed operating focal length was 12.20 in. The effect on the beam was not great because the arc is not sensitive to focal adjustment and to produce a sensible change in beam formation the lamp must be moved through several inches.
The beam was directed against a wall one hundred feet away with the center of the beam 5 feet above floor level. Reading stations were marked off at 5 foot intervals for the width of the beam, and photometer readings were taken at each, making two complete traverses of the beam. Constant current of 150 amperes was held in this and in the test that followed.
The measured beam formation is given by curve B of Fig. 7. This curve shows a central zone 15 degrees wide over which the beam is substantially uniform in intensity and an annular zone 7 degrees wide of gradually decreasing intensity. The width of this outer zone is abnormally wide due to interference by the lamp barrel with the light from the outer zone of the mirrors. With a barrel designed to conform to the beam dimension the beam will have the form of curve A, Fig. 8, which is a computed curve. The central zone of uniform intensity is nearly 20 degrees wide, and the whole width is 32 deg. Curve B of this Figure is the computed beam, taking account of