Low Contrast Visual Acuity Charts
Testing is identical to the measurement of visual acuity at high contrast level, i.e., we measure the smallest size of the optotypes that the person can recognize. The threshold is defined as the line on which at least 3 out of the 5 optotypes are correctly recognized. The 2.5% test is the most practical test in clinical use. The resulting threshold point on the curve is far enough from the high contrast value so that the declination of the slope of the curve can be defined. If the person has severely impaired vision, the test must be quite close, which may require use of reading lenses.
Figure 1. Translucent Low Contrast Visual Acuity charts for the small light box. The optotypes are printed at 25%, 10%, 5%, 2.5% and 1.25% contrast. The most often used chart is the 2.5% chart because at 2.5% contrast visual acuity values are close to one half of the visual acuity at full contrast. Thus by measuring at full contrat and at 2.5% the tester finds whether the angle of the slope is the usual one or steeper or lower than normal. Low contrast charts are available also for the ETDRS cabinet.
You use the same technique as you used in measurement of visual acuity at full contrast. Move quickly down the chart and ask the person to identify the first or the last symbol on each line. When the person hesitates or makes an error, move to one line higher and ask the person to read the entire line. If on that line at least 3 optotypes are read correctly, the person may have focused more carefully on the chart and may be able to read one more line. To record the results carefully, record the number of optotypes read correctly, i.e., if on the 2.5% chart one of the symbols was read incorrectly on line 0.3 (6/18, 20/63) record the visual acuity value as 0.3 (-1) at 2.5%.
Figure 2.Test results are marked on the recording sheet at the contrast levels used, e.g. in this picture 2.5% and 1.2%, going along that level toward the right until the visual acuity value, measured at that contrast (A' at 1.2%, B' at 2.5%), is reached. If the person's visual acuity was 1.0 (20/20, 6/6), the line connecting these three points, A', B' and X, depicts the slope of the contrast sensitivity curve of this person.
Record the results: 0.4 (6/15, 20/50) at 2.5% and 0.2 (6/30, 20/100) at 1.2% contrast.
After having used the test for a while, you will not need the recording form any more, except for reporting your results to somebody who is not accustomed to using the test. You will have a mental image about where the threshold is located on the form. You will mark down the name of the test, the contrast level and the visual acuity value that you measured, for example: at 2.5% contrast 0.4 (6/15, 20/50). These two numbers carry the necessary information for follow-up.
When both optotype and grating measurements are made, it is interesting to mark them on the recording form to see the relationship between the different threshold values. The luminance of the tests needs to be kept closely equal, otherwise the results are not comparable. Again, it is important to record the names of the tests and the luminance level used.
Note that in the recording form, the visual acuity and grating acuity values are marked as if the acuity values followed the physical structure of the tests, visual acuity 1.0 (20/20, 6/6) being equal to 30 cpd grating acuity (based on the comparison of Snellen E, a mini grating, with the usual test gratings). You will see that this is seldom true even in the normally sighted persons; in the visually impaired persons the variation can be both ways but usually grating acuity is higher than visual acuity measured with optotypes.
Grating acuity can be very much higher than the optotype acuity. The largest difference might have been measured in a boy who had only a narrow functioning rim of his lower visual field (Hyvärinen and Jacob 2011). His grating acuity value was 4 cpd, which, if it had been converted to a recognition acuity, would have been 0.12 (6/50, 20/160); however, his optotype acuity was 0.004 (6/1500, 20/5000). Thus there would have been a 30 times error in calculation if the grating acuity value had been converted to optotype acuity value. In the Introduction to Low Contrast Tests, in Figure 7.C you learned that the grating acuity value was 2 cpd, which, if converted to optotype acuity, would be 0.06 (20/320, 6/95), yet the measured optotype acuity value was 0.4, nearly 7 times higher.
In the original recording form visual acuity values were given in 5 nominations. This was found confusing by users who were accustomed to one type of writing the visual acuity values. Now there are three different prints to make the recording easier.