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Visual Acuity Testing (Snellen Chart)
Visual Acuity Testing (Snellen Chart)
Measures clarity of vision using a standardized letter chart
Visual Acuity Testing (Snellen Chart)
Visual Acuity Testing (Snellen Chart)
Measures clarity of vision using a standardized letter chart
Instructions
Ask the patient to stand 20 feet (6 meters) from the Snellen chart. Test one eye at a time, covering the other, and record the smallest line of letters they can read clearly. Glasses or contact lenses should be worn if normally used. Document the result as a fraction (e.g., 20/20 or 6/6).
Overview
When to use
Why use
Evidences
Interpretation
(Note: This tool is designed as a quick mobile screening aid for visual acuity and is not a substitute for in-office eye examinations. Current evidence on smartphone-based Snellen visual acuity apps is limited, and none have consistently demonstrated accuracy within one line of standard clinical testing, indicating the need for further validation. Differences in screen resolution and text display may also affect test reliability.)
Result | Meaning |
20/20 (6/6) | Normal vision |
Worse than 20/20 (e.g., 20/40) | Reduced visual acuity |
Better than 20/20 (e.g., 20/15) | Above-average vision |
Legal blindness (≤20/200) | Severe visual impairment |
Visual acuity testing with the Snellen chart quantifies distance vision using standardized “optotype” letters sized to subtend five minutes of arc at a fixed test distance; despite widespread use since 1862, Snellen measurements show greater variability than logMAR/ETDRS charts, especially with poorer acuity, which has implications for research and monitoring precision.
https://www.ncbi.nlm.nih.gov/books/NBK558961/
Report acuity in Snellen fraction and, when needed for analysis, convert to logMAR for comparability across studies and for monitoring change over time; conversion improves statistical properties and inter-study comparability. Comparative studies show ETDRS logMAR charts have better test–retest reliability than Snellen, with 95% tolerance limits for change approximately ±0.14 logMAR (ETDRS) versus ±0.18 (Snellen), and reduced practice effects; differences widen with poorer acuity
https://www.nature.com/articles/eye2009147
Snellen acuity is central to primary care and community screening workflows, but pediatric programs often employ age-appropriate optotypes (HOTV, LEA) with crowding or instrument-based screening, per expert panels and U.S. screening recommendations targeting amblyopia and refractive errors in preschoolers.
https://pmc.ncbi.nlm.nih.gov/articles/PMC4274336/
For longitudinal monitoring or research-grade endpoints, prefer ETDRS/logMAR and document refractive status, testing distance, and letter-by-letter scoring to enhance sensitivity to change and reduce measurement error relative to line-based Snellen scoring.
https://pmc.ncbi.nlm.nih.gov/articles/PMC2814576/
Overview
When to use
Why use
Evidences
The Snellen chart is the most widely used tool for assessing visual acuity, which refers to the sharpness or clarity of vision. Visual acuity testing is fundamental in ophthalmology and general practice, serving as a baseline for diagnosing eye conditions, monitoring changes, and evaluating treatment effectiveness.
The Snellen chart consists of rows of letters that decrease in size with each line, representing different visual thresholds. The test is performed at a fixed distance, typically 20 feet (or 6 meters), and the result is expressed as a fraction. For example, 20/20 (or 6/6) indicates normal vision, while 20/40 means the patient sees at 20 feet what someone with normal vision sees at 40 feet. This assessment helps detect refractive errors like myopia, hyperopia, and astigmatism, as well as monitor visual impairment caused by conditions such as cataracts, macular degeneration, or diabetic retinopathy.
Although the Snellen chart does not evaluate peripheral vision, depth perception, or color vision, it remains an essential first step in eye examinations, guiding further testing and management. Its simplicity, reliability, and widespread use make it a cornerstone of both routine vision screening and ophthalmic evaluation.
Overview
When to use
Why use
Evidences
Interpretation
(Note: This tool is designed as a quick mobile screening aid for visual acuity and is not a substitute for in-office eye examinations. Current evidence on smartphone-based Snellen visual acuity apps is limited, and none have consistently demonstrated accuracy within one line of standard clinical testing, indicating the need for further validation. Differences in screen resolution and text display may also affect test reliability.)
Result | Meaning |
20/20 (6/6) | Normal vision |
Worse than 20/20 (e.g., 20/40) | Reduced visual acuity |
Better than 20/20 (e.g., 20/15) | Above-average vision |
Legal blindness (≤20/200) | Severe visual impairment |
Visual acuity testing with the Snellen chart quantifies distance vision using standardized “optotype” letters sized to subtend five minutes of arc at a fixed test distance; despite widespread use since 1862, Snellen measurements show greater variability than logMAR/ETDRS charts, especially with poorer acuity, which has implications for research and monitoring precision.
https://www.ncbi.nlm.nih.gov/books/NBK558961/
Report acuity in Snellen fraction and, when needed for analysis, convert to logMAR for comparability across studies and for monitoring change over time; conversion improves statistical properties and inter-study comparability. Comparative studies show ETDRS logMAR charts have better test–retest reliability than Snellen, with 95% tolerance limits for change approximately ±0.14 logMAR (ETDRS) versus ±0.18 (Snellen), and reduced practice effects; differences widen with poorer acuity
https://www.nature.com/articles/eye2009147
Snellen acuity is central to primary care and community screening workflows, but pediatric programs often employ age-appropriate optotypes (HOTV, LEA) with crowding or instrument-based screening, per expert panels and U.S. screening recommendations targeting amblyopia and refractive errors in preschoolers.
https://pmc.ncbi.nlm.nih.gov/articles/PMC4274336/
For longitudinal monitoring or research-grade endpoints, prefer ETDRS/logMAR and document refractive status, testing distance, and letter-by-letter scoring to enhance sensitivity to change and reduce measurement error relative to line-based Snellen scoring.
https://pmc.ncbi.nlm.nih.gov/articles/PMC2814576/
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