These iPad-based visual acuity charts are designed for pediatric use and contain four letters: H, O, T and V. Tests using these letters have been employed extensively in the Amblyopia Treatment Study and Vision in Preschoolers Study.
Unlike printed tests, these charts require no external lighting and runs on the iPad without need for special software or calibration. The high quality iPad screen displays letters small enough to measure visual acuity at test distances as short as 6 feet or 2 m.
There are 11 pages each with a line of 5 letters. Each line is surrounded on all sides by a border, serving as a crowding contour. The distance between the letters and the surrounding border half the letter size.
It is recommended that three of four letters need to be identified correctly for testing to progress to the next smallest size.
The charts are not ideal for the assessment of severely visually impaired patients. At a test distance of 4 feet (1.2 meters) the largest letters will correspond to 20/200 (6/60).
The test can be performed at any distance and the lowest line read recorded. The Snellen fraction is then specified as test distance/lowest line read, e.g. 10/25 or 3/7.5. The letter sizes are given in both feet and meters (M units). Visual acuities taken at any distance can be transformed to 20/x or 6/x by simple math. For example a visual acuity of 4/25 corresponds to 20/125.
The logMAR design of the chart may be leveraged to take more precise measurements. Credit may given for each individual letter read correctly (0.02 log units). This may be calculated by recording the logMAR value for the last line read completely and subtracting 0.02 for each subsequent letter read. Of course this requires that you know the logMAR value. On the last page, there is a table giving the logMAR values for each line for a number of test distances. A spreadsheet that will calculate logMAR based on the patient’s responses for any test distance can be requested at ridgevue.com
The letters display best on the Retina Display.
Other Ridgevue Vision tests are available from the iTunes store.
For more information visit ridgevue.com
