Appendix E: Vision Tests

Participants viewed a standard Snellen-type acuity chart with black letters on a white background. Following procedures suggested for low vision patients by Colenbrander and Fletcher (1992), the viewing distance for this test was reduced from twenty feet to four feet. This was done to provide a more accurate assessment of visual acuity in the low vision range. At the four-foot distance the eleven lines on the chart corresponded to visual acuities ranging between 20/50 and 20/1000. Participants were asked to read as many lines as possible, beginning with the top line and continuing down rows as the letters decreased in size. The experimenter recorded the lowest (smallest) line read correctly.

Participants were then asked to read letters on a Pelli-Robson contrast sensitivity chart. This chart was illuminated as uniformly as possible by a combination of tungsten and fluorescent light sources. The luminance of the white areas of the chart was measured with a Minolta LS-100 luminance meter in nine separate locations and ranged from 65 cd/m2 to 119 cd/m2, which is within the recommended acceptable limits. All letters on this chart were the same size and were presented on a white background. The first three letters on the chart were black, but each consecutive set of three letters provided less contrast than the previous set, until the letters eventually faded to white. Participants read this chart from a distance of 1 m (39 in). The experimenter recorded the last set of three letters from which the participant correctly identified at least two.

The recently revised (Fourth Edition) of the H.R.R. Pseudoisochromatic Plates test (Richmond Products, Boca Raton, FL) was used to screen participants for red-green and blue-yellow abnormalities in color vision. The book of test plates was viewed on a dedicated easel under simulated daylight illumination as recommended by the manufacturer. Although participants were allowed to bring their eyes within approximately six inches of the plate, only a few were able to read any of the color-coded symbols. If participants could not see the symbols embedded in the relatively high contrast demonstration plates, the test was aborted. Because most participants in this study were not able to read any symbols on the H.R.R. Pseudoisochromatic Plates test, no results for this test are reported.