The Effect of Test Distance on Visual Contrast Sensitivity Measured Using the Pelli-Robson Chart

The Pelli-Robson (PR) chart is widely used to measure clinical contrast sensitivity (CS). It is generally believed that PR testing distance is not critical. Here, we examine whether a closer test distance than the usual 1 meter might be better for patients with low vision.

PR CS was measured on two groups: low-vision students (<20 years old) and elder patients (>65 years old). Student PR was measured at 1 meter and at a closer distance d = visual acuity in log10cy/deg (d = 1.5-logMAR). Elder PR was measured at 1 and 3 meters. Grating CS was also measured using the Ohio Contrast Cards (OCCs).

Average CS was 0.398 log10 units (over one line on the PR chart) higher at the closer distance than at 1 meter for the students, but there was no effect of 1 vs. 3 meters test distance for the elders. The equivalent spatial frequencies of the PR letters at 1 meter were near the acuity limits of students with low vision, but were near the peak of the elders' CS functions. Especially for students with low vision, PR CS was below OCC CS, even when PR was tested at a closer distance.

PR CS should be measured at a distance in meters that is equal to the patient's letter acuity in cy/deg, or 1.5-logMAR.

Contrast sensitivity is highly associated with quality of life, and it is important to measure it accurately. Using a closer distance, or measuring grating CS, can reveal visual abilities missed when patients with low vision are tested using PR at 1 meter.