Eye Charts, Snellen Eye Charts, Standard Vision Charts

What is an Eye Chart?

As a familiar sight in doctor’s offices, most of us are acquainted with eye charts and have probably been tested with this simple procedure several times throughout our lives. Eye charts help ophthalmologists, optometrists, opticians and other healthcare professionals to actuate an individual’s visual acuity. More specifically, eye charts help to determine distance vision, or distance visual acuity.

Eye charts are regularly utilized as an integral part of a more complete eye exam. They are also often used in schools to help catch vision problems in children as early as possible for intervention.

Available in various formats, most eye charts display several repeating rows of letters or symbols, with the top row in the largest font size, and subsequent rows in gradually smaller sizes. These test symbols are also referred to as optotypes and can be geometric characters or specially shaped numbers or letters.

Standing or sitting a specific distance away, test subjects are then asked to identify these optotypes, usually reading the top row on down until they are unable to reliably determine the characters.

The visual acuity determination is based on the Snellen fraction. Named for Herman Snellen, a Dutch eye doctor in the mid-19th century who originated the Snellen eye chart, this fraction is actually very well known, even if most of us don’t really understand what it means. “Visual acuity is equal to the distance at which the test is made divided by the distance at which the smallest optotype identified by the patient subtends an angle of 5 arcminutes”.

Because the standard distance for eye chart tests in the USA is 20 feet, and 6 meters most everywhere else in the world, this particular distance translates to the line symbols representative of “normal” acuity subtending an angle of 5 minutes of arc, with the spaces between the lines and the thickness of the lines subtending 1 minute of arc. Designated as 20/20 in the USA and 6/6 in most other countries, this line is apportioned as the smallest line an individual can identify at a distance of 20 feet (or 6 meters).

Types of Eye Charts

There are a myriad of eye chart formats to choose from, with the Snellen chart and the Tumbling E chart being two of the most widely used.

Snellen Eye Chart
While there are several variations of the Snellen eye chart originally developed in 1862, the most basic format generally features 11 rows of specific capital letters that are chosen to offer the most equal legibility. The letters are specifically formulated and sized using standard geometric rules for eye chart configuration. Snellen eye charts are usually printed on matte-finished and non-reflecting materials to ensure more accurate testing results.

Tumbling E Eye Chart
Highlighting the same scale as the Snellen eye chart and just as accurate, the Tumbling E eye chart is perfect for use with children or adults who cannot read letters. All of the optotypes on this chart are in the shape of the capital letter, E, in various rotations, facing different directions. These E shapes also get smaller with each descending line. Instead of asking the test subject to speak and identify the different optotypes, they are instead asked to use either hand to indicate which direction the prongs of the E are pointing.

Another variation of the Tumbling E chart is a chart that displays familiar shapes to identify, rather than letters or numbers. Tumbling E’s and moons, stars, hearts and other well-known symbols may be more appropriate for children, those who are illiterate, or those who experience difficulty with speech or reading comprehension.

Rehabmart is pleased to offer a selection of professional eye charts from Graham Field.

Hulet Smith, OT
Rehabmart Co-Founder & CEO

ck