Principle of Snellen chart

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Here is a full explanation of the principle of the Snellen chart, drawn from the textbooks:

Principle of the Snellen Chart

Fig. 1.1 - Snellen visual acuity chart (Kanski's Clinical Ophthalmology)

Core Optical Principle: Minimum Angle of Resolution (MAR)

Distance visual acuity (VA) is directly related to the minimum angle of separation (subtended at the nodal point of the eye) between two objects that allow them to be perceived as distinct. A normal eye can resolve two points that subtend 1 minute of arc at the nodal point of the eye. This is the fundamental basis of the Snellen chart.

Kanski's Clinical Ophthalmology, 10th ed.

Design of the Chart

The chart uses black letters or symbols (optotypes) of progressively decreasing sizes set against a white background.
The top letter on the chart is designed to subtend 5 minutes of arc at a distance of 200 feet (approximately 60 m).
Each subsequent row contains letters that subtend 5 minutes of arc at progressively shorter distances (200 ft, 100 ft, 70 ft, 50 ft, 40 ft, 30 ft, 20 ft, 15 ft, etc.).
Each individual stroke or gap within a letter subtends 1 minute of arc at the designed distance. The letter as a whole fits within a 5 x 5 minute-of-arc grid.
Adams and Victor's Principles of Neurology, 12th ed.

How VA is Expressed (Snellen Fraction)

VA is recorded as a non-mathematical fraction:

VA = Test distance / Distance at which the letter subtends 5' of arc

Notation	Meaning
20/20 (or 6/6)	Normal vision - at 20 ft, patient reads what a normal eye reads at 20 ft
20/200 (or 6/60)	At 20 ft, patient reads only what a normal eye reads at 200 ft
20/15	Better than normal - reads at 20 ft what most read at 15 ft

The standard testing distance is 20 feet (6 metres).
Normal corrected VA in young adults is often better than 6/6.
20/200 (6/60) is the legal threshold for blindness.

Testing Protocol

Each eye is tested separately, with the fellow eye carefully occluded.
The eye with worse vision is tested first.
The patient wears their distance correction (glasses/contacts) for the primary test. An unaided (uncorrected) VA is also recorded for completeness.
The patient should be pushed to read every letter possible.
Best-corrected VA (BCVA) = acuity achieved with optimal refractive correction.

Pinhole Test

If VA is less than 6/6, the test is repeated using a pinhole aperture (an opaque occluder with one or more holes of ~1 mm diameter):

The pinhole allows only a narrow shaft of light to fall on the fovea, eliminating the blur caused by defocused peripheral rays.
If VA improves with pinhole: the reduced vision is due to a refractive error or optical media defect (cornea, lens, aqueous, vitreous) - not a neurological cause.
If VA does not improve (or worsens): suggests macular disease, posterior lens opacities, or a neural/retinal problem.
Kanski's Clinical Ophthalmology, 10th ed.; Adams and Victor's Principles of Neurology, 12th ed.

When Chart Reading is Not Possible

For patients who cannot read chart letters, qualitative testing is used (in descending order of function):

Level	Abbreviation	Definition
Counting Fingers	CF	Can count fingers held at a specified distance (usually 1 m)
Hand Movements	HM	Can detect whether the examiner's hand is moving
Perception of Light	PL	Can only detect the presence/direction of a light source
No Perception of Light	NPL	Completely blind

Variants of the Snellen Chart

Chart	Use
Standard Snellen	Literate adults and children (age 4-6+)
Tumbling E chart	Pre-literate children or non-English speakers (patient indicates direction of letter E)
Allen picture chart	Young children who cannot read
Rosenbaum card	Near vision testing at 14 inches (bedside)
Jaeger card	Near VA bedside test; J1 = 20/20, J7 = 20/50, J13 = 20/100, J16 = 20/200
Teller acuity cards	Infants - uses preferential looking at striped patterns

Summary of Key Numbers

Parameter	Value
Testing distance	20 ft (6 m)
Normal VA	20/20 (6/6)
Each letter subtends	5 minutes of arc
Each stroke/gap subtends	1 minute of arc (at the designated distance)
Top letter readable by normal eye at	200 ft (60 m)
Legal blindness threshold	20/200 (6/60)
Nodal point of the eye	~7 mm posterior to the cornea

Kanski's Clinical Ophthalmology, 10th ed.; Adams and Victor's Principles of Neurology, 12th ed.; Rosen's Emergency Medicine

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