These iPad-based visual acuity charts are designed for pediatric and preschool near vision testing 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. The chart employs a logMAR design with each line 80% the size of the previous.
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 40 cm.
The test should be performed with the screen brightness set at halfway and Auto-Brightness OFF. Room lights may be dimmed or left on, but care taken to avoid troublesome reflections.
There are 13 pages that each have a line of 5 letters. Each line is surrounded on all sides by a border, serving as a crowding bar. The distance between the letters and the surrounding border is around half the letter size.
It is recommended that at least three of five letters need to be identified correctly for testing to progress to the next smallest size.
The charts are not ideal for the assessment of patients with severe visually impairment.
The test can be performed at any distance and the lowest line read recorded. The letter sizes are given in meters (M units) along with the Snellen equivalent if testing is performed at 40 cm. The Snellen fraction can specified as test distance/lowest line read, e.g. 0.4/2.5 and transformed to 20/x or 6/x by simple math. For example a visual acuity of 0.4/2.5 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 with 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