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Angela M Brown, Stevie M Njeru, Mawada Osman; What is the best test distance for the Pelli-Robson Chart?. Invest. Ophthalmol. Vis. Sci. 2019;60(9):3906. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
The Pelli-Robson (PR) chart measures a patient’s contrast sensitivity (CS) by finding the lowest contrast letters he/she can read correctly. Pelli et al (1988) recommended testing at 3m, which places our 4.8 cm letters at 1.0 logMAR, but suggested closer distances for low-vision patients. The manual recommends 1m. Research with sine-wave gratings shows that CS varies with the spatial frequency (SF) of the measuring grating, so test distance for the PR chart probably matters. However, letters are not sine-wave gratings. We studied the effect of test distance on CS using the PR chart.
We tested 48 observers at two distances using the PR chart. The effective spatial frequency of the PR letters was 1.46/distance (Hopkins et al, 2017; see Majaj et al, 2002). The CS of 21 low-vision students (ages 6–18) was tested at 1m and at a closer distance. The closer distance (avg=0.34m) was generally 1 – A (A=Bailey-Lovie logMAR acuity), but students with very poor acuity chose the distance themselves. The CS of 27 elderly adults (ages 66–92) was tested at 1m and 3m. Square-wave acuity (Teller Acuity Cards) and square-wave CS (Ohio Contrast Cards) defined a square-wave contrast sensitivity function (CSF) for each observer (Hopkins et al; see Watson & Ahumada, 2005).
The effect of distance was different for the two groups. For the low-vision students, the effective SF of the 1m letters (1.46 cy/deg) was on the falling limbs of their CSFs. The closer letters (avg=0.5 effective cy/deg) were near their CSF peaks. There was a large (0.43 Log10CS, a factor of 2.7) near/far difference in the CS scores for the low-vision students (t=5.8, p<.0001). Also, the slope of each student’s CSF was correlated with his/her change in CS (Spearman Rank=0.51, p=0.02). By comparison, the effective spatial frequencies of the 1m and 3m letters were both near the elders’ CSF peaks, and there was little difference between the CS scores at the two distances (0.02 Log10CS; 95% conf. int: -0.075–0.023).
For patients who, like our elders, have good visual acuity, 1m vs 3m test distance does not affect the CS measured by the Pelli-Robson chart. However, the recommended test distance of 1m is not close enough for many low-vision patients. Here, moving to an average test distance of 0.34m improved measured CS by a factor of 2.7. At almost three triplets of letters on the Pelli-Robson chart, this improvement is likely to be clinically significant.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
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