Glossary
Visual Acuity Testing Glossary
Plain-language definitions of the visual acuity, optotype, and ophthalmic-chart terms used throughout AcuityMaster — written by Mark S. Brown, MD.
- Visual acuity
- The eye's ability to resolve fine detail, typically measured by the smallest optotype a patient can correctly identify at a set distance. Commonly recorded in Snellen (e.g. 20/20) or logMAR notation.
- Snellen chart
- The traditional eye chart introduced in 1862, with a large letter at top and progressively more, smaller letters per line. Fast for routine screening but with unequal letters-per-line and spacing.
- ETDRS chart
- A standardized chart from the Early Treatment Diabetic Retinopathy Study: 5 letters per row, equal logarithmic size steps, and equal spacing. The gold standard for clinical trials and retinal-disease monitoring.
- LogMAR
- The logarithm of the Minimum Angle of Resolution — the score derived from ETDRS testing. 0.0 logMAR equals 20/20; lower is better. Each 0.1 logMAR equals one line (5 letters).
- Optotype
- A standardized symbol (letter, number, or picture) used to measure visual acuity, designed so its detail subtends a defined visual angle at the test distance.
- Sloan letters
- A set of 10 letters (C D H K N O R S V Z) of equal legibility used as standard optotypes in ETDRS and other clinical charts.
- Landolt C
- A ring with a gap in one of several orientations; the ISO 8596 reference optotype. The patient identifies the gap direction.
- Tumbling E
- An optotype showing the letter E rotated in four directions; the patient indicates which way it points. Useful for non-verbal patients or those who don't share the examiner's alphabet.
- HOTV
- A pediatric matching test using only the letters H, O, T, and V. Suitable for children roughly ages 2.5–5 who can match letters to a card.
- Lea Symbols
- Pediatric optotypes (circle, square, house, apple) calibrated to blur equally at threshold, for preverbal and preliterate children from about age 3.
- Contrast sensitivity
- The ability to distinguish an object from its background at varying levels of contrast. Important for cataract, corneal, and retinal disease where standard high-contrast acuity may appear normal.
- Worth 4-Dot
- A test of binocular fusion and suppression using four illuminated dots viewed through red/green glasses.
- Fixation disparity
- A small misalignment of the eyes during binocular viewing, assessed to evaluate binocular vision and oculomotor balance.
- Astigmatic dial
- A radial line pattern used to detect and estimate the axis of astigmatism during refraction.
- Crowding phenomenon
- The tendency, pronounced in amblyopia, to read isolated optotypes far better than letters presented in a row. Crowding bars or full lines are used to reveal it.
- Amblyopia
- Reduced vision in one or both eyes from abnormal visual development, often detected using crowded optotypes and pediatric charts.
- ANSI Z80.21
- The American National Standard governing visual acuity test devices, including background luminance requirements of 80–320 cd/m².
- ISO 8596
- The international standard defining the Landolt C optotype and the testing of visual acuity.
- Luminance
- The measured brightness of a surface, expressed in candelas per square meter (cd/m²). Acuity charts must fall within a defined luminance range for valid results.
- Decimal acuity
- A notation expressing visual acuity as a decimal fraction (e.g. 1.0 = 20/20, 0.5 = 20/40), common outside the United States.
- Random letter presentation
- Displaying letters in a randomized order between visits so patients cannot memorize the chart — a key advantage of software charts over fixed printed or projector charts.
- Mirror mode
- A display mode that reverses the chart so it can be shown to the patient via a mirror, used in compact exam lanes.
See these tests in your own lane
15-day fully functional trial — every chart and optotype, no credit card.