These iPad-based single-letter visual acuity charts are designed for pediatric use and contain the 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 no crowding or contour bars in this version
There are 14 letter sizes from 100 feet (30 M) to 5 feet (1.5 M). The size progression follows the logMAR scale.
At each letter size the four letters appear in a random order on consecutive pages. The tester can jump to the next larger or smaller size by clicking on the grey words at the bottom of the screen.
The charts can be used for the assessment of severely visually impaired patients. At a test distance of 4 feet (1.2 meters) the largest letters will correspond to 20/800 (6/240).
It is recommended that three of four letters need to be identified correctly for testing to progress to the next smallest size. Alternatively, the more rigorous ATS testing procedure can be used.
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 letters display best on the Retina Display.
Other Ridgevue Vision tests are available from the iTunes store.
For more information visit ridgevue.com