FILM LIGHTING USING METAL-HALIDE LAMPS

When a discharge lamp is run from an alternating supply, the intensity of light from it varies between maximum and minimum values at twice the supply frequency. This so-called intensity ″ripple″ can, if such lamps are used as light sources in motion-picture film work, lead to a cyclic variation in frame-to-frame exposure. This in turn can cause the overall luminance of the displayed picture to fluctuate. In this paper, an investigation of the visibility of such luminance fluctuations over the frequency range 0. 1-12. 5 Hz is described. Limits of film exposure variation are recommended, below which these picture luminance fluctuations should not be visible, taking into account a nonlinear transfer characteristic. The magnitude of film exposure variation which can occur in practice is discussed, based on the assumption that the variation in lamp intensity is sinusoidal in character, as is the frequency of the luminance fluctuations which occur when the film is replayed. Using these factors, theoretical relationships are derived between camera frame frequency and shutter angle, and lamp supply frequency and intensity ripple magnitude, such that picture luminance fluctuations due to the presence of the ripple component are not perceptible.