This iPad-based letter contrast sensitivity test is similar in principle to the well-established Pelli-Robson Chart, the Mars Letter Contrast Sensitivity Test and Rabin’s Small Letter Contrast Sensitivity Test.
Letter contrast sensitivity testing is fast to administer and easy for patients to understand, due to its similarity to visual acuity charts. Letter-based tests have been shown to be more repeatable than printed tests using gratings and are widely used in large clinical trials supported by the US National Institutes of Health.
Unlike other tests, the test requires no external lighting and runs on the iPad without need for special software or calibration. The test utilizes the high quality iPad screen to produce letters with contrasts as low as 0.5%, invisible to the average patient.
The test presents four lines of five letters on each page. The contrast of each successive line decreases in 0.1 log unit steps.
There are six sequences representing three sizes: 7.5 M, 3.8 M and 1.9 M. At 1 meter these are equivalent to 20/150, 20/75 and 20/38, respectively. In terms of spatial frequency, these correspond to 4, 8 and 16 cycles per degree. Users can adjust the test distance to suit their needs. Note that the letters will be 20% smaller on the iPad Mini (6 M, 3 M, and 1.5 M)
Important Notes
The letters have all been calibrated for the Retina Display. Test Procedure The test should be performed at 1 meter with screen brightness set at halfway and Auto-Brightness OFF. Room lights may be dimmed or left on, but care taken to avoid troublesome reflections
Five letters are on each line and the contrast decreases by 0.1 log units per line. The values at the top of the page is log contrast sensitivity (= –log10contrast). Thus 1.0 represents 10% contrast and 1.5 is around 3% contrast.
Subjects read the letters, starting with those of high contrast until no letters on a given page is read correctly. To save time, normally-sighted patients can start on third or fourth page. Patients should be encouraged to look at a page for several seconds. Patients must guess the letter, even if they claim to be unable to see anything.
Scoring
Credit is given for each individual letter read correctly (0.02 log units). This may be calculated by either: Noting the contrast sensitivity on the last line on which the patient read all letters and adding 0.02 for each subsequent letter read, or; Simply recording the total number of letters read correctly and multiplying by 0.02 For more info visit ridgevue.com or email info@ridgevue.com
