The Pelli-Robson (PR) chart has been a popular and successful way to measure clinical contrast sensitivity (CS), but in a recent study, researchers examined whether a shorter test distance—closer than the one meter the PR Chart provides—might be better for patients with low vision.
Contrast sensitivity by PR was measured on two groups: low-vision students 20 years old and younger, and elderly patients age 65 and older. Student PR was measured at one meter and at a closer distance, while elder PR was measured at one and three meters.
The study showed performance on the PR test improved when students with low vision viewed the chart from a closer vantage point. By comparison, little difference was found between the CS performance at one meter and three meters among the elder participants. The average CS over one line on the PR chart was higher at the closer distance than at one meter for the students, but there was no effect of one versus three meters test distance for the elders.
“When using the PR chart, test distance is more important than is generally believed,” the authors concluded in their study. “Although still not as good as the performance revealed by the Ohio Contrast Cards, the performance of school-aged students with low vision (but not the performance of elders who seek primary vision care) can be improved by a factor of 2.5 by simply adjusting the test distance based on the student's letter acuity.”
Njeru SM, Osman M, Brown AM. The effect of test distance on visual contrast sensitivity measured using the Pelli-Robson chart. Transl Vis Sci Technol. 2021 Feb; 10(2):32