How do we measure letter chart acuity for low vision patients?
Letter chart acuity represents only one aspect of vision. Yet it is important, (a) because it is so widely used, and (b) because it provides a direct measure of the MAgnification Requirement (MAR = 1/V) of the patient. Traditional letter charts were primarily designed for the normal and near-normal range. At the 20/100 level most have only two letters. Snellen knew this and recommended to use a reduced testing distance for lower visual acuity levels. The ETDRS charts extend measurement to 20/200, but this still does not address the ranges of severe and profound vision loss in Table 3. The Colenbrander 1-meter chart addresses this problem. The chart is designed for use at 1 meter and thus has a six times wider measurement range than charts designed for 6 meters (20 ft). The chart comes with a 1 m cord and occluder attached, so that the 1 m testing distance can be accurately maintained. Letter sizes range from 1 M to 50 M, so that visual acuities from 1/1 (20/20) to 1/50 (20/1000) can be
