Some Variables Influencing Vernier Acuity

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Abstract
Since there is little experimental evidence on the influence of various stimulus factors on vernier acuity, functions relating vernier acuity to illumination have been obtained for two different exposure times and at three regions of the visible spectrum. An apparatus providing the necessary stimulus controls has been described in detail and the procedure is fully explained.Vernier acuity, as measured by the reciprocal of the standard deviation of “alignment” settings, varies with illumination for both indefinitely long and for short exposures. At low brightness vernier acuity is low, rises rapidly with increase in illumination, and finally levels off at a constant level at high brightnesses.Vernier acuity functions for short exposures are displaced downward and along the illumination axis relative to the function for long exposures. Thus maximum vernier acuity is never so high for short exposures and the maximum is reached at higher illuminations than with long exposures.The relationship between vernier acuity and illumination for both long and short exposures is fitted by Hecht and Mintz’s theoretical equation for visual acuity based on the brightness discrimination of the eye.Variations in the functions with duration of exposure of the acuity object are interpreted in the light of the intensity-time relationship and the findings of Graham and Kemp on the effect of exposure time on brightness discrimination.To investigate the influence of wave-length on vernier acuity, acuity vs. illumination functions have been found for red, yellow, blue, and white light. The experiments were carried out on two different apparatus.The two sets of data obtained on the two apparatus show that all the functions are of the same shape, but that slightly different maximum acuities are reached with the different colored illuminations. Red gives the highest acuity, yellow next, white next, and blue least. When refractive correction is made to allow for sharp focusing in blue illumination, the highest acuities are found with this color.It was concluded that wave-length of illumination is of some significance in vernier acuity, since higher acuities are found with nearly monochromatic light than with white. This improvement is perhaps caused by reduction of blurring of retinal images by restriction of the range of wave-lengths.

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