These iPad-based near vision charts are based on the logMAR principle. All charts have ten words per print size: two each of 4-letter, 5-letter, 6-letter, 7-letter and 8-letter words. The words are selected from a list of the 5000 most common words in American English according to www.wordfrequency.info.
Unlike printed tests, these charts require no external lighting and display on the ubiquitous iPad without need for special software or calibration. The test utilizes the high quality iPad screen to produce print as small as 2 point—smaller than a patient can read at a typical reading distance.
The letter sizes are given in both point size and M units. The first page contains 12 lines with the word and line spacing proportional to letter size. The second page contains larger print sizes. The third page is for routine clinical use and contains 8 lines with more space between print sizes. The smallest size is 5 point so patients don’t get frustrated by smaller print not being visible.
The test can be performed at any distance and the lowest line recorded. The Snellen fraction may be specified as test distance/lowest line read, e.g. 0.4/1.25 for 1.25 M print read at 40 cm. This would convert to a Snellen equivalent of 20/63.
The logMAR design of the chart may be leveraged to take more precise measurements. Credit may given for each individual word read correctly (0.01 log units). This may be calculated by recording the logMAR value for the last size read completely and subtracting 0.01 for each subsequent word read. 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. Reading speed can also be estimated: Words/minute = 600/seconds taken for 10 words.
The letters display best on the Retina Display.
Other Ridgevue Vision tests are available from the iTunes store.
For more information visit ridgevue.com