The Relation Between Visual Acuity and Illumination

Abstract

1. An apparatus for measuring the visual acuity of the eye at different illuminations is described. The test object is continuously variable in size and is presented at a fixed distance from the eye in the center of a 30° field. Observation of the field is through an artificial pupil. The maximum intensity obtainable is more than enough to cover the complete physiological range for the eye with white light though only 110 watts are consumed by the source. Means for varying the intensity over a range of 1:1010 in small steps are provided. 2. The relation of visual acuity and illumination for two trained observers was measured, using two different types of test object, a broken circle and a grating. The measurements with both test objects show a break at a visual acuity of 0.16, all values below that being mediated by the rods and those above by the cones. The grating gives higher visual acuities at intensities less than about 30 photons and lower visual acuities above that. The maximum visual acuity attainable with the grating under the same conditions is about 30 per cent lower than that with the C. It is shown that the limiting factor in the resolution of the eye for the grating is the diameter of the pupil when it is less than 2.3 mm. and the size of the central cones when the pupil is larger than that. The value of the diameter of the cone derived on that basis from the visual acuity data agrees with that derived from direct cone count in a unit of area. 3. The data for the cones made with both test objects are adequately described by one and the same form of the stationary state equation derived by Hecht for the photoreceptor system. This fact, together with certain considerations about the difference in the nature of the two test objects with regard to the resolvable area, leads to the conclusion that detail perception is a function of a distance rather than an area. All the data for the rods can likewise be described by another variety of the same equation, although the data are too fragmentary to make the choice of the form as certain as might be desired.

Visual Acuity and Illumination in Different Spectral Regions

“…A precise study of this function with white light (Shlaer, 1937) has shown that the data fall into two distinct functions, one at intensities below about 0.1 photon, and the other at intensities above that. This separation is understandable in terms of the duplicity theory.…”

“…The apparatus we used has already been described in detail (Shlaer, 1937). It consists essentially of a device in which the test object is presented at a fixed distance of 1 meter from the eye in the center of a uniform field about 30 ° in diameter, and is viewed through an artificial pupil.…”

Specifying and controlling the optical image on the human retina

“…10). This kind of analysis allowed Shlaer (1937) to calculate the modulation of the retinal image of a 50 cycles/degree square-wave grating, arriving at an estimate of a Michelson contrast of 0.21. Because such a grating could just be resolved, Shlaer was able to draw conclusion about the limits of the retinal detection apparatus for intensity discrimination of adjoining peaks and troughs.…”

“…In two studies, in particular, the optical image played a significant role in the argument. As mentioned above, Shlaer (1937) in an exemplary way attempted to tease out the extent to which optical light spread, receptor spacing and light difference detection were limiting factors in visual acuity, using Hartridge's estimates of the optical spread.…”

“…An artificial pupil when using Maxwellian viewing is thus not an immaterial issue. Shlaer (1937) realized that when the target is a squarewave grating, the diffraction pattern in the plane of the pupil will have the form of its spatial-frequency spectrum, with discrete beams for each spatial frequency entering the pupil in different locations. When converged by the eye's optic on the retina, they will combine, but only to the extent that the higher-order spatial frequencies are in fact admitted into the eye.…”