Optical system

	Optical system   |   Optical characteristics   |   The eye muscles   |   Pupil size   |   The retina   |   Visual acuity

Pupil Size
One of the main reasons that higher light levels improve acuity is that the pupil becomes smaller, reducing the effect of the eye's refractive errors. (This is like using a small lens opening on a camera to increase depth of field.) When a point of light is focused on the retina, the actual result is a small, round blur. When the pupil becomes larger or smaller, so does the blur.

Pupil Size and Field Luminance

Pupil size is a function of the weighted average of the luminances (popularly called brightness) within the field of view. Pupil size is influenced much more by the part of the retina associated with central, or foveal, vision than by the outer areas of the retina.

The relationship between field luminance and pupil size (Reeeves, 1920). (The graph can be changed to cd/m² by multiplying millilamberts by 3.183.)

Figure 15 shows the relationship between field luminance and pupil size.

Pupil Size and Accommodation

When the lens of the eye accommodates to view a near object, the pupil will become smaller. For example, if luminance is held constant and the point of fixation is changed from 100 to 33 cm, average pupil diameter would be reduced about 15% (Alpern, Mason and Jardinico, 1961). Actually, the variations in viewing distances at a typical workstation are apt to cause larger changes in pupil size than would differences in the luminances at the workstation.

Logarithmic scales are often used when presenting data from visual studies, which may be misleading to some people. Figure 15 is an example of this. The vertical axis of that figure is linear and the horizontal axis is logarithmic. Each gradation on the vertical axis represents the same amount of change, but each graduation on the horizontal axis represents a value 10 times that of the preceding one.

The same data can be plotted with a linear axis.

The relationship between field luminance and pupil size, plotted with a linear scale of luminance values (Millilamberts may be changed to cd/m² by multiplying mL by 3.183.)

Figure 16 expresses the same data as Figure 15.

The following listing presents some levels of field brightness and associated "typical" conditions

There is a relationship between visual acuity and pupil size, but Figure 15 clearly shows that the relationship between light level and pupil size is of importance only for low light levels. In other words, there is a point beyond which increasing the light levels does not improve visual acuity due to pupil size.

Rate of change of pupil size

Under constant illumination, the opening of the pupil tends to stabilize. There is no reason to assume that it is more stressful to the eye to maintain a two-millimeter pupil than a four-millimeter pupil. Stress may be produced, however, by forcing the pupil size to change constantly. The sphincter and dilator pupilae respond relatively slowly and continue to respond for a length of time after the stimulus to change has stopped. Not only may unusual demands by placed on the iris muscles, but the pattern of stimulations may make the pupil attempt to dilate and contract simultaneously. Pupil dilation and contraction rates are shown in Figure 17.

Change the size of the pupil when going from a dark field to a field of over 100 millilamberts and the reverse (Reeves, 1920).

As long as the stimuli to change pupil size are presented at a rate comparable with the pupillary response time, the pupil does not appear to show fatigue. Campbell and Whiteside (1950) demonstrated this by presenting a small light beam near the edge of the pupil. When the light beam entered the eye, it served as a stimulus to contract the pupil. The contraction then cut off the light beam and the lower retinal illuminance became the stimulus for the pupil to dilate. That arrangement caused the pupil to oscillate continuously at a rate of change determined by the pupillary response mechanism itself. Under those conditions, no fatigue over time was detected in the response mechanism. Fry and King (1975), however, showed that when stimuli of a magnitude sufficient to produce a significant change in pupil size are presented at a slightly higher rate (about 3 Hz) than the pupil can respond to, the response is dampened and discomfort is produced.

It is difficult to set up a situation with a display workstation that challenges the pupillary response system. If, however, a VDT visual task requires fixations back and forth between a display and paper as rapidly as three times a second, users may find it useful to avoid bright task lighting on the paper.

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