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A Snellen chart is an eye chart that can be used to measure visual acuity. Snellen charts are named after the Dutch ophthalmologist Herman Snellen who developed the chart in 1862 as a measurement tool for the acuity formula developed by his professor Franciscus Cornelius Donders .
Near visual acuity or near vision is a measure of how clearly a person can see nearby small objects or letters.Visual acuity in general usually refers clarity of distance vision, and is measured using eye charts like Snellen chart, LogMAR chart etc. Near vision is usually measured and recorded using a printed hand-held card containing different sized paragraphs, words, letters or symbols.
For 6/6 = 1.0 acuity, the size of a letter on the Snellen chart or Landolt C chart is a visual angle of 5 arc minutes (1 arc min = 1/60 of a degree), which is a 43 point font at 20 feet. [10] By the design of a typical optotype (like a Snellen E or a Landolt C), the critical gap that needs to be resolved is 1/5 this value, i.e., 1 arc min.
Herman Snellen (Dutch pronunciation: [ˈɦɛrmɑn ˈsnɛlə(n)], February 19, 1834 – January 18, 1908) was a Dutch ophthalmologist who in 1862 introduced the Snellen chart to study visual acuity. He took over directorship of the Netherlands Hospital for Eye Patients (Nederlandsch Gasthuis voor Ooglijders), after Franciscus Donders .
An example of the Landolt C eye chart (also known as the Japanese eye chart.). Numerous types of eye charts exist and are used in various situations. For example, the Snellen chart is designed for use at 6 meters or 20 feet, and is thus appropriate for testing distance vision, while the ETDRS chart is designed for use at 4 meters. [16]
Sloan letters, designed by Louise Sloan in 1959, are a set of optotypes used to test visual acuity generally used in Snellen charts and logMAR charts. This set of optotypes consists of ten specially formed "letters", C, D, H, K, N, O, R, S, V and Z. [ 1 ] These letters, unlike the ones used in older Snellen charts, are designed to give acuity ...
The Snellen chart is an eye chart used by eye care professionals and others to measure visual acuity. Snellen charts are named after Dutch ophthalmologist Hermann Snellen who developed the chart in 1862. Vision scientists now use a variation of this chart, designed by Ian Bailey and Jan Lovie.
Often, children who are suspected of having amblyopia are too young to be able to verbally recognize letters on the Snellen chart, making the eye examination challenging. [ 13 ] It is critical to identify eye conditions early in children, as early detection and intervention can save vision and lives.