title: Validation of an iPad test of letter contrast sensitivity.
abstract: OBJECTIVE
  An iPad-based letter contrast sensitivity test was developed (ridgevue.com) consisting of two letters on each page of an iBook. The contrast decreases from 80% (logCS = 0.1) to 0.5% (logCS = 2.3) by 0.1 log units per page. The test was compared to the Pelli-Robson Test and the Freiburg Acuity and Contrast Test.
  
  
  METHODS
  Twenty normally sighted subjects and 20 low-vision subjects were tested monocularly at 1 m using each test wearing their habitual correction. After a 5-minute break, subjects were retested with each test in reverse order. Two different letter charts were used for both the Pelli-Robson and iPad tests, and the order of testing was varied systematically. For the Freiburg test, the target was a variable contrast Landolt C presented at eight possible orientations and used a 30-trial Best PEST procedure. Repeatability and agreement were assessed by determining the 95% limits of agreement (LoA) ± 1.96 SD of the differences between administrations or tests.
  
  
  RESULTS
  All three tests showed good repeatability in terms of the 95% LoA: iPad = ± 0.19, Pelli-Robson = ± 0.19, and Freiburg = ± 0.15. The iPad test showed good agreement with the Freiburg test with similar mean (± SD) logCS (iPad = 1.98 ± 0.11, Freiburg = 1.96 ± 0.06) and with narrow 95% LoA (± 0.24), but the Pelli-Robson test gave significantly lower values (1.65 ± 0.04). Low-vision subjects had slightly poorer repeatability (iPad = ± 0.24, Pelli-Robson = ± 0.23, Freiburg = ± 0.21). Agreement between the iPad and Freiburg tests was good (iPad = 1.45 ± 0.40, Freiburg = 1.54 ± 0.37), but the Pelli-Robson test gave significantly lower values (1.30 ± 0.30).
  
  
  CONCLUSIONS
  The iPad test showed similar repeatability and may be a rapid and convenient alternative to some existing measures. The Pelli-Robson test gave lower values than the other tests.
authors.affiliation: *OD, PhD, FAAO †OD, MS, FAAO ‡OD, FAAO §MCOptom, PhD, FAAO School of Optometry (PSK, MEJ, EJK), Indiana University, Bloomington, Indiana; and College of Optometry (MAB), University of Houston, Houston, Texas.
authors.forename: Pete S
authors.initials: PS
authors.lastname: Kollbaum
authors.forename: Meredith E
authors.initials: ME
authors.lastname: Jansen
authors.forename: Elli J
authors.initials: EJ
authors.lastname: Kollbaum
authors.forename: Mark A
authors.initials: MA
authors.lastname: Bullimore
mesh_terms: D000328:Adult; D034201:Computers, Handheld; D015350:Contrast Sensitivity / Q000502:physiology*; D003936:Diagnosis, Computer-Assisted / Q000295:instrumentation*; D005260:Female; D006801:Humans; D008297:Male; D015203:Reproducibility of Results; D014787:Vision Tests / Q000295:instrumentation*; D015354:Vision, Low / Q000175:diagnosis* / Q000503:physiopathology; D055815:Young Adult