Snellen chart

The Snellen chart is the classic tool for testing distance vision (visual acuity) and remains a cornerstone of eye examinations worldwide.

Dutch ophthalmologist Herman Snellen (1834–1908) introduced optotypes, purpose-designed letters drawn on a 5×5 grid to maintain consistent proportions. Each letter is sized so that its height subtends an angle of five minutes of arc at the test distance, with individual strokes subtending one minute of arc. This approach allowed visual acuity to be expressed as a simple fraction (e.g., 6/6), and brought uniformity to what had previously been an inconsistent and subjective practice.

Snellen introduced a formula to express visual acuity the Snellen fraction (d/D):

The formula for the acuteness of vision (v) is expressed by the utmost distance at which the types are recognized (d), divided by the distance at which they are seen at our standard angle of five minutes (D). The distances are taken by french metrical measure.

Snellen 1862

• v = acuteness of vision
• d = distance at which the test is performed (usually 6 m or 20 ft)
• D = distance at which a normal eye can read the letter

The test distance is usually 20 feet (6 meters), because at this distance light rays entering the eye are nearly parallel, minimizing the need for accommodation. A normal eye should distinguish a gap or stroke that forms an angle of 1 minute of arc at the retina (the minimum angle of resolution).

Before Snellen’s innovation, visual acuity assessment lacked uniformity; clinicians used printed text or informal objects such as mustard seeds or stars. Snellen’s success lay in merging scientific optics with clinical practicality, based on the theoretical foundations of his mentor Franciscus Cornelis Donders, who defined accommodation and refractive anomalies.

Today, while logMAR charts (Bailey-Lovie, ETDRS) offer superior accuracy for research, Snellen charts persist worldwide for routine screening due to simplicity and familiarity.

• 20/20 (6/6) vision: You can read at 20 ft (6m) what a normal eye reads at 20 ft (6m) → normal vision.
• 20/40 (6/12) vision: You must be at 20 ft (6m) to read what a normal eye can see at 40 ft (12m) → reduced acuity.
• 20/200 (6/60) vision: You can read at 20 ft (6m) what a normal eye reads at 200 ft (60m) → severe vision impairment.

For context: the largest letter on the Snellen chart (20/200 or 6/60 line) is about 3.5 in (8.9 cm) high at 20 ft (6 m), while the 20/20 (6/6) letters are only 0.35 in (8.9 mm) tall. The heights are designed so that the strokes subtend one minute of arc and the entire letter five minutes of arc at the specified distance

---

History of visual acuity testing

1674 – Robert Hooke (1635–1703) discussed the eye’s inability to distinguish parts when their apparent size falls below a certain angle, introducing the idea that vision depends on angular separation and estimated the limit of normal visual resolution as one minute of arc.

…such is the make of the sensible part of the Eye, viz. of the Tunica Retina, that it cannot distinguish in most Eyes an Angle less than a Minute (though there be some that can to the third of a Minute, but these are very few)…

Hooke, 1680

1754 – Tobias Mayer (1723–1762) published Experimenta circa visus aciem including Experimenta de visus acie, objectis luce diurna illustratis. Mayer systematically studied visual acuity using dots, lines, and grids on white paper to determine the limits of visual acuity (terminus visionis). He expressed acuity in arcseconds and distinguished between “primary” acuity (isolated objects) and “secondary” acuity (crowded objects), a concept anticipating modern research on visual crowding

1843 – Heinrich Küchler (1811–1873) was the first to publish printed vision-test charts in 1843, introducing the Schriftnummerprobe für Gesichtsleidende (printed number and text samples for those with eye disease). In 1844, he followed with a landmark paper, Ueber den Zweck und die beste Einrichtung von Gesichtsproben (On the Purpose and Best Arrangement of Sight Tests) in Medicinische Annalen. Küchler emphasized global standardization and defined essential qualities of test objects as:

1. Universal comprehension (independent of literacy).
2. Differ only by size and visual angle, not by shape or ornamentation.
3. Ease of reproduction for consistency across clinics.

1854 – Eduard Jaeger Ritter von Jaxtthal (1818–1884) produced the Jaeger Test-Types (Schrift-Scalen) for near vision, promoting the principle that reading text was the most functional measure of vision.

1861 – Franciscus Cornelis Donders (1818–1889) formalized the concept of visual acuity as the minimum angle of resolution and advocated standardized testing methods.

1862 – Herman Snellen (1834–1908) published Letterproeven tot Bepaling der Gezichtsscherpte (Test Letters for Determining Visual Acuity), introducing optotypes and the Snellen fraction (d/D).

1874 – Snellen and Edmond Landolt (1846–1926) collaborated on methods for measuring visual acuity in the Graefe–Saemisch Handbuch.

1888 – Edmond Landolt (1846–1926) introduced the Landolt C (broken ring), later adopted as the ISO reference standard for research.

1909 – International Congress of Ophthalmology made the first formal attempt to standardize vision testing, though widespread clinical uptake was slow.

1959 – Louise Sloan (1908–1983) developed Sloan optotypes and introduced the M-unit system for low-vision assessment.

1976 – Ian Bailey and Jan Lovie proposed the logMAR chart, providing uniform letter spacing and logarithmic progression for better accuracy.

1982 – ETDRS chart standardized logMAR principles for clinical trials in diabetic retinopathy, now the research gold standard.

---

Associated Persons

• Person (DOB–DOD)

---

Alternative names

• Name
• Name

---

Controversies

Did they first describe or popularise or plagiarise?

---

References

Historical references

• Hooke R. Lectures in light In: The posthumous works of Robert Hooke. 1705
• Mayer T. Experimenta circa visus aciem. Commentarii Societatis Regiae Scientiarum Gottingensis. 1754; 4: 97-110
• Stampfer S. Ueber die Genauigkeit des Visirens bei Winkelmessungen. Jahrbücher des kaiserlichen königlichen polytechnischen Institutes in Wien. 1834; 18: 211–36.
• Jaeger E. Prüfung des Sehvermögens bei Gesunden wie Kranken In: Ueber Staar und Staaroperationen nebst anderen Beobachtungen und Erfahrungen. Wien: Seidel; 1854: 110-128.
• Küchler H. Schriftnummerprobe für Gesichtsleidende. Darmstadt: Diehl; 1843
• Küchler H. Ueber den Zweck und die beste Einrichtung von Gesichtsproben. Medicinische Annalen 1844; 10: 408-418.
• Snellen H. Probebuchstaben zur Bestimmung der Sehschärfe. 1862 [German]
• Snellen H. Test-types for the determination of the acuteness of vision. 1862 [English]
• Snellen H. Optotypi ad visum determinandum. 1862
• Snellen H, Landolt E. Ophthalmometrologie. Die Functionsprüfungen des Auges. In: Handbuch der gesammten Augenheilkunde [Eds Graefe A, Saemisch T. ] 1874

Eponymous term review

• list

---

---