MEASURING CONTRAST SENSITIVITY

An important adjunct to visual acuity testing. While visual acuity tests measure how big an object must be in order to be seen, contrast sensitivity measures how much contrast (luminance variation) a pattern must have to be seen.

The Pelli-Robson chart has large letters of equal size that can be seen by most low vision patients. The letters decrease in contrast down the chart, with 3 letters shown for each level of contrast sensitivity. It is a large wall-mounted chart with a recommended viewing distance of 1 meter.

The Mars Letter Contrast Sensitivity Test is similar to the Pelli-Robson, but has greater accuracy due to its finer contrast decrements and scoring procedure. It is handheld, with a recommended viewing distance of 50 cm.

The SKILL chart has a regular letter chart on one side and a low contrast, low luminance (black on dark gray) chart on the other. The difference in the number of letters that can be read on each side is compared. An eye with normal contrast sensitivity will read four lines less on the dark side of the chart: a larger difference indicates a loss of contrast sensitivity at low luminance.