LogMAR charts are used to measure an individual's visual acuity (VA). The name LogMAR is derived from the Logarithm of the Minimum Angle of Resolution. In contrast to other visual acuity charts, such as the Snellen chart, the sizes of the letters progress systematically in geometric progression. The letter size of each line is designated as the logarithm to the base 10 of decimal visual acuity, so the 6/6 (or 20/20) line is LogMAR 0.00 and the 6/60 (20/200) line is LogMAR 1.0. The space between lines and letters also change in proportion[disambiguation needed] keeping the effect of contour interaction constant. Another feature of the LogMAR chart is a fixed five letters on each line, allowing for a more consistent sampling of visual acuity between lines. For this reason it has been recommended that whenever visual acuity is measured, particularly in a research setting, the LogMAR chart is the instrument of choice.
The chart was designed by Ian Bailey and Jan Lovie in 1980 at the National Vision Research Institute of Australia. They described their motivation for designing the LogMAR chart: "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."
Relation to the Snellen chart
In contrast to the LogMAR chart, the Snellen chart which is also used in assessing visual acuity has the following differences:
- Inconsistent numbers of letters on a line
- Unequal graduations from one line to the other
Recording visual acuity using the LogMAR chart
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:
- LogMAR VA = 0.1 + LogMAR value of the best line read - 0.02 X (number of letters read)
Advantages of LogMAR over other charts
- Equal number of letters per line
- Regular spacing between lines and letters
- Uniform progression in letter size
- Final score based precisely on the total of all letters read
- Finer grading scale allows for greater accuracy and improved test/retest reliability
- Grosvenor, Theodore (2007). Primary care Optometry. St. Louis, Missouri: ELSEVIER. pp. 174–175. ISBN 9780750675758.
- Bailey IL, Lovie JE. I (1976.) New design principles for visual acuity letter charts. Am J Optom Physiol Opt. Nov;53(11):740-5.
- Carlson, Kurts, Nancy, Daniel (2004). Clinical Procedures of Ocular Examination. U.S.A: McGraw HIll. p. 10. ISBN 0-07-137078-1.