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Eye Charts Explained

Snellen Eye Chart explainedEye doctors have used eye charts for years to help determine a persons visual acuity. Popular forms of eye charts include the Snellen chart, the Lea test and the Landtolt C.

No matter the eye chart used, each shows many rows of test symbols, called optotypes, each of a different size. Generally, the chart begins with the larger sized rows and continues down the chart, becoming progressively smaller.

Variations of Eye Charts

Other varieties of eye charts have also been created in order to test young children and illiterate adults. These types of eye charts do not require the patient be capable of recognizing letters. For example, one type of eye chart uses pictures for testing.

Others still use letters, such as the capital “E” but have the letter pointing in various directions. In this way, the patient only has to indicate which way the E is pointing. Another variation of the eye chart is the “broken ring”. This chart has rows of circles with varying segments missing. The patient then describes the location of the broken areas.

The Snellen Eye Chart

Eye doctors have utilized the Snellen eye chart since its development in 1862. Traditionally, this eye chart contains eleven lines of block letters. The first line of the Snellen eye chart simply has the large capital E at the top. Each additional row contains more letters, but the letters are smaller than those found in the previous row.

In order to complete this test, the patient must cover one eye. He or she then reads the letters out loud from each row, starting with the top row. Once the patient reaches the smallest row he is capable of reading, the patients visual acuity is determined for that eye.

Optotypes Used for the Snellen Eye Chart

The Snellen Eye Chart utilizes very specific forms of optotypes. First of all, each must be in the form of a block letter, though the letters are not of standard font. Instead, the thickness of every line used on the chart must be equal to the thickness of the white spaces found between the lines.

The gap in the letter C also must be equal to this gap. In addition, the height and the width of the optotype must be five times as thick as the line. Furthermore, on the letters C, D, E, F, L, O, P, T, and Z are used for the traditional Snellen chart.

Use of the Snellen chart

The least expensive form of Snellen chart is the wall-mounted type. Primary care physicians generally use these in order to form a rough estimate of the patients vision. For more careful measurement of visual acuity, specialized eye charts are used. These are most often used by eye doctors who need to assess a patients visual acuity and by places such as the Department of Motor Vehicles, where a client may be tempted to fake accurate eye sight.

Understanding Visual Acuity

Visual acuity how clearly a patient is able to see objects. With the Snellen eye chart, visual acuity is reported as the distance from where the test was made as compared to the distance at which the patient can read the smallest optotype size.

According to Hermann Snellen, the ophthalmologist who invented the Snellen eye chart, “standard vision” is the ability to recognize the optotype from a specific distance. The most common measurement used with this chart is 20/20. The 20/20 line on the Snellen eye chart represents the smallest line a person with normal vision should be capable of reading from a distance of 20 feet.

The letters located three lines above the 20/20 line are twice the size of those on the 20/20 line. A person with normal vision should be able to read these optotypes from a distance of 40 feet away. This line, therefore, is designated as 20/40.

If the 20/40 line is the smallest line a patient is capable of reading, he or she has 20/40 visual acuity. This means the person must be as close as 20 feet away to see the same thing a person with normal vision can see from 40 feet away. Essentially, this person has half the visual acuity of a person with normal vision.

The top line on the Snellen eye chart is a visual acuity of 20/200, which is considered legally blind. A person capable of reading this chart through the aid of glasses, however, is not considered to be legally blind. To be defined in such a way, the person must be unable to read the top line even with the best-corrected vision.