LogMAR chart

A LogMAR chart or Bailey-Lovie chart or ETDRS chart (Early Treatment Diabetic Retinopathy Study) comprises rows of letters and is used by ophthalmologists, optometrists and vision scientists to estimate visual acuity. This chart was developed at the National Vision Research Institute of Australia in 1976, and is designed to enable a more accurate estimate of acuity than do other charts (e.g., the Snellen chart). For this reason, the LogMAR chart is recommended, particularly in a research setting. A LogMAR chart or Bailey-Lovie chart or ETDRS chart (Early Treatment Diabetic Retinopathy Study) comprises rows of letters and is used by ophthalmologists, optometrists and vision scientists to estimate visual acuity. This chart was developed at the National Vision Research Institute of Australia in 1976, and is designed to enable a more accurate estimate of acuity than do other charts (e.g., the Snellen chart). For this reason, the LogMAR chart is recommended, particularly in a research setting. When using the LogMAR chart, visual acuity is scored with reference to the Logarithm of the Minimum Angle of Resolution, as the chart's name suggests. An observer who can resolve details as small as 1 minute of visual angle scores LogMAR 0, since the base-10 logarithm of 1 is 0; an observer who can resolve details as small as 2 minutes of visual angle (i.e., reduced acuity) scores LogMAR 0.3, since the base-10 logarithm of 2 is near-approximately 0.3; and so on. The chart was designed by Ian Bailey and Jan E Lovie-Kitchin at the National Vision Research Institute of Australia. They described their motivation for designing the LogMAR chart as follows: 'We have designed a series of near vision charts in which the typeface, size progression, size range, number of words per row and spacings were chosen in an endeavour to achieve a standardization of the test task.' The Snellen chart, which dates back to 1862, is also commonly used to estimate visual acuity. A Snellen score of 6/6 (20/20), indicating that an observer can resolve details as small as 1 minute of visual angle, corresponds to a LogMAR of 0 (since the base-10 logarithm of 1 is 0); a Snellen score of 6/12 (20/40), indicating an observer can resolve details as small 2 minutes of visual angle, corresponds to a LogMAR of 0.3 (since the base-10 logarithm of 2 is near-approximately 0.3), and so on. Each letter has a score value of 0.02 log units. Since there are 5 letters per line, the total score for a line on the LogMAR chart represents a change of 0.1 log units. The formula used in calculating the score is: