Visual acuity is usually measured with a standard chart (such as the Snellen chart) at a distance of 20 feet. In the Emergency Department and other areas patients may be in a supine position and cannot be tested in the standard way. Placing a visual acuity chart above the patient can allow testing to be done. The size of the acuity chart needs to be adjusted in size to take into account the change in distance between the patient's eyes and the chart.
size of letter for reduced chart in inches =
= (size of letter in standard chart in inches) * (new eye-to-chart distance in feet) / (20 feet)
percentage reduction of original Snellen chart =
= 100% * (1 – ((size of letter for reduced chart) / (size of letter in normal chart)))
Advantages of this approach:
(1) the chart can be mounted in place so that it is always available
(2) handheld acuity charts are held at a distance that requires accomodation
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