LBNL Report Number
An operator of a visual display terminal typically is confronted with two flickering light sources: VDT phosphor with its inherent refresh rate, and the ambient fluorescent illumination reflected from the screen. Due to differences in frequency, the two sources of flicker can produce low-frequency beats in the net light intensity presented to the VDT operator. These low frequencies, on the order of a few hertz, can lead to adaptation of visual sensory mechanisms.
We have constructed an experiment in which a stable, high-amplitude beat is produced, with frequency chosen to enhance the possibility of detecting a visual system response. By adjusting the environmental lighting to produce a sinusoidal flicker at 69 Hz, and employing a VDT with a 61-Hz refresh rate, we produced an 8-Hz beat frequency in the light emanating from the VDT screen.
By measuring temporal contrast sensitivity at 8 Hz as a test of sensory visual system response, we found that two of three subjects showed a reduced sensitivity when exposed to this stimulus, as compared with three sets of control conditions in which the 8-Hz beat was not present. We discuss possible relations between this sensory adaptation and the factors of subjective visual fatigue and reduced levels of task performance. Further work is underway to examine whether adaptation occurs under conventional office lighting and typical mains frequency variations.