Abstract
Intensity variations of light from a discharge lamp, occurring at twice the alternating supply frequency, can lead to a cyclic variation in frame-to-frame exposure if such lamps are used as light sources in motion picture film work. The visibility of picture luminance fluctuations brought about by such exposure variations is discussed, and limits of film exposure variation are recommended. Practical magnitudes of exposure variation are examined. Using these factors, relationships are derived between camera frame frequency and shutter angle, and lamp supply frequency and intensity variation magnitude, such that practical picture luminance variations are not perceptible. An assessment of a 'safe' lamp supply frequency is described, above which the appearance of picture luminance fluctuations may be discounted. The effect of ripple asymmetry (differing magnitudes of successive intensity maxima) is discussed. Experimental techniques for measuring intensity variations under standardized conditions are described, and the characteristics of two currently-available metal-halide discharge lamps are examined.
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