For investigation of flicker produced by phasic variations of the brightness level, an electronic flicker apparatus was developed for continuous variation of brightness, area, light-dark ratio, ambient light ratio, and monocular or binocular exposure. The Sylvania glow modulator tube is excited by a steady current and simultaneously by rectangular impulses, thus furnishing an ambient and a flickering light source which can be mixed at any ratio within the total light emission capacity. At the same total light emission (Talbot level), the following ratios of ambient light, in percent of the total emission were investigated with monocular presentation: 0, 25, 50, 75, 85, and 95, at two levels of total brightness. Visual field (1.5°) and time interval ratio 50:50 were kept constant. There was no difference in the subjective sensation of flicker at any ambient light ratio. There was a continuous drop of the fusion frequency of flicker with increasing ratio of ambient light in three subjects. The extreme ratios (zero and 95% ambient light) were tested in 173 subjects. The drop of the fusion frequency of flicker at 95% ambient light ratio was statistically highly significant at both brightness levels, but was more pronounced at the higher brightness. A hypothesis for this phenomenon is presented.
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