This paper reports experimental results showing the effect of mean retinal illuminance on the modulation transfer function (MTF). The line spread function of the human visual system was computed. The test pattern in which the luminance varied sinusoidally in the horizontal direction was generated on the face of an oscilloscope by modulating the intensity of the beam.
The behavior of the system changes with the mean retinal illuminance. At the highest mean retinal illuminance (1000 td) the MTF shows two peaks which gradually disappear at lower mean retinal illuminances. The line spread function of the entire visual system also exhibits a change in shape dependent on the mean retinal illuminance. In particular, what are deemed inhibitory influences become less prominent at lower mean retinal illuminance. From the line spread function of the optical part up to the retina, the line spread function of the physiological part of the visual system is also evaluated.
A common asymptote appears at high frequencies when the characteristics are plotted in terms of absolute deviation from the mean versus spatial frequency for threshold perception. The asymptote provides an upper bound for the behavior of visual acuity at any retinal illuminance.
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