These iPad-based visual acuity charts are scaled for the iPad Mini. Designed for pediatric use they 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.
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 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 is half the letter size.
It is recommended that 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 severely visually impaired patients. At a test distance of 4 feet (1.2 meters) the largest letters will correspond to 20/200 (6/60).