From Snellen to ETDRS
Dr. Hermann Snellen’s chart standardized vision testing and established a reliable process of diagnosis in its time. However, just as all original technology does, there came a point when the Snellen chart needed reform.
In 1976, Dr. Ian Bailey and Dr. Jan Lovie noted significant deficiencies in the Snellen chart. Here are just a few:
· The poor visual acuity lines at the top included 1 to 2 letters, while the good visual acuity lines at the bottom included 8 letters. It is speculated this
design’s purpose was to create equal lengths in each row, but uneven rows promote error in visual acuity measurement.
· Letters were not always the same level of legibility. The letters A, J, and L were noticeably more difficult to discern than C, D, E, G, and O.
· Distances between lines were not consistent. As each row grew smaller, the amount of space between each row decreased. The cluster of optotypes as row space decreased would produce the crowding phenomenon, which has been shown to cause inaccurate acuity measurements.
Bailey and Lovie created their own modified version of the Snellen chart to reduce some of its inaccuracies. Their revised chart included the same amount of letters on each line, a standardization of legibility, and equidistant rows, to name just a few adjustments. The “Bailey-Lovie chart” was also designed to be scored in logMAR units, making it easier to convert results based upon various viewing distances. In 1982, the chart was renamed the “ETDRS chart” as it was recommended to be used in the Early Treatment Diabetic Retinopathy Study (note: the terms “ETDRS chart” and “logMAR chart” are not interchangeable – logMAR is not a type of chart, but rather a term to describe visual acuity notation. This is a common misconception).
The ETDRS chart is fully accessible on modern computerized vision testing systems, thanks to Dr. Bailey and Dr. Lovie’s work to create a more accurate chart!
Source: US National Library of Medicine
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