Snellen’s Primer: What is it? Description, History, Interpretation, Logarithmic Progression and Visual Ability

It was developed by the Dutch ophthalmologist Herman Snellen in 1862 and was adopted by medical professionals in many countries who have used it for more than 100 years.

The Snellen chart, also called the Snellen eye, is a chart used to measure visual acuity by determining the level of visual detail a person can have.

When you visit the eye doctor for a checkup, you may be asked to read an eye chart. Snellen’s chart is a familiar sight in doctors ‘and optometrists’ offices.

It typically displays 11 rows of capital letters, also known as “optotypes,” which are constructed according to strict geometric rules, and each row thereafter has a decreasing number of smaller letters on each bottom line of the graph.

In the traditional table, the first line traditionally consists of the single letter E, and only nine letters are used: C, D, E, F, L, O, P, T, and Z. You cover one eye and read the line over. small number of letters you can see. The test is done on each eye.

From a distance of 20 feet (6 meters), subjects read each line in the table, using only one eye, until they can no longer decipher the shapes of the letters.

Each row of letters is assigned a ratio that indicates the visual acuity required to read it, and the ratio of the lowest line that a person can read represents the individual’s visual acuity for that eye.

In the United States, normal vision is defined as 20/20; in countries that use the metric system, it is 6/6.

A ratio less than 1 (eg 6/10) indicates worse than normal vision; a ratio greater than 1 (eg 6/5) indicates better than normal vision.

In some offices, the table is viewed using a mirror so that testing can be done with less than 20 feet of space.

If you have 20/20 vision, you are considered to have normal visual acuity. Many ophthalmologists and vision scientists now use an improved chart known as the LogMAR chart.

Snellen’s primer has been criticized. One of those criticisms is that the number of letters in each line differs, so the difficulty of differentiating the letters due to size is confused with the difficulties due to visual crowding caused by the proximity of other letters.

It has been established that letters are easier to read when presented on their own.

Another is that row spacing, as well as letter spacing, varies in Snellen’s chart, introducing a third factor that further confuses the measurements.

Yet another criticism is that the progression of the relationships between the letter lines is irregular and somewhat arbitrary, with particularly large gaps at the lower end of the acuity scale.

Finally, the repeatability of measurements taken with the Snellen chart is poor, complicating any effort to measure changes in vision over time.

Alternatives to Snellen’s primer include those developed by:

Edmund Landolt (Landolt C), Sergei Solovin (using Cyrillic letters), Louise Sloan, Ian Bailey and Jan Lovie, Lea Hyvärinen (the Lea table, for preschool children) and Hugh Taylor (the Tumbling E table, for those who are not familiar with the Latin alphabet).

Description of Snellen’s primer

The normal Snellen chart is printed with eleven lines of capital letters. Three lines up, the letters are twice the height of the letters on the 6/6 line (or 20/20 in the US).

Snellen charts published in 1862 used alphanumeric capital letters on the 5 × 5 grid. The original table shows A, C, E, G, L, N, P, R, T, 5, V, Z, B, D, 4, F, H, K, O, S, 3, U, Y, A, C, E, G, L, 2.

Subsequent rows have an increasing number of letters that decrease in size. The symbols on an acuity chart are formally known as “optotypes.”

In the case of the traditional Snellen chart, the optotypes have the appearance of capital letters and are intended to look and read like letters. They are not, however, any ordinary typesetter’s font.

Some clinics do not have 6m eye rails available, and a half-size board that subtends the same angles to 3 meters (9.8 feet), or an inverted board projected and viewed through a mirror, is used to achieve the lettering of the correct size.

At exactly 6 meters away from the patient, the letters on the 6/6 line shall subtitle 5 minutes of arc (angular unit of measure) so that the individual ends of the letters subvert 1 minute of arc.

This means that the table must be sized such that these letters are 8.73 mm high and the highest “E” (6/60) must be 87.3 mm high.

History

The Dutch eye doctor Hermann Snellen developed the Snellen chart in the 1860s.

Dr. Snellen also created a chart called the “Tumbling E” chart, which can be used by people who cannot read, or by young children who do not know the alphabet.

Instead of using different letters, the “Tumbling E” eye chart uses a capital letter E that faces different directions.

The ophthalmologist asks the person being examined to use their fingers to show the direction in which the “fingers” of the E.

Before Dr. Snellen created the standardized eye chart, every ophthalmologist or eye doctor had a chart that they preferred.

Snellen’s primer allowed a person to go from any eye care provider to any eyeglass manufacturer and get the same results.

Eye charts do not help your eye doctor determine if you have an eye disease such as glaucoma or a problem with your retina.

Interpretation of Snellen Eye Test Charts

Snellen designed his optotypes on a 5 × 5 grid, in which the thickness of the line is one unit and the width and height of the letter are five units. This system is followed for most letter and number graphics.

For children’s tests (such as Allen pictures and other test symbols), much thinner lines are often abandoned and used. Precision viewing offers stylized images, called Patti Pics, for which Snellen’s 5 × 5 principle is followed.

They produce fewer changes in acuity measured as children graduate from chart charts to letter charts.

Alternative symbols that can be used for illiterate adults or in countries unfamiliar with the Roman alphabet are the E test and the Landolt C or the broken ring.

Letter charts are not only used to measure visual acuity, they are also used as targets for subjective refraction. This is the main reason that distance acuity is measured more often than near acuity.

At a long distance, the accommodation is relaxed, so the refraction can be more accurate. At a longer test distance, the effect of small changes in the subject’s position is less important and can be ignored.

Since today’s exam lanes are often shorter than 20 feet (6 meters), charts are often designed for shorter distances.

This is not a problem for visual acuity measurement, as long as the actual test distance is accurately accounted for.

For use in refraction, shorter distances are less desirable. At 4 m (the recommended distance for early treatment diabetic retinopathy study charts), the accommodative demand is 0.25 diopters and cannot be ignored.

In smaller rooms, the use of mirrors is recommended to increase the test distance.

Letter size

Snellen used a fairly detailed definition to indicate font size: “the distance in meters (feet) that the letter subtend 5 min of arc.”

Louise Sloan simplified this by defining unit M as the size that subtend 5 min of arc to 1 meter. Therefore, the detailed statement “this letter subtend 5 arc min in ‘x’ meter” can be simplified to “this letter is ‘x’ M units”.

Here, the number indicating the size of the unit M is the viewing distance in meters for an acuity of 20/20. Most precision vision eye charts carry the letter size designation in M ​​units.

This makes it easy to calculate visual acuity if the table is used at any distance other than the one it was designed for by inserting the new test distance (in meters) and letter size (in M units) directly at the point above formula.

Interpretation of Snellen’s primer

It is a persistent urban legend that 20/20 would represent normal, average, or even perfect vision. This is not so, Snellen deliberately chose his reference standard (5 min of arc) as a size that is “easily recognizable by normal eyes.”

Therefore, almost all normal eyes will meet or exceed the reference standard. If 20/20 equals average acuity, half the population would not reach 20/20, since that is the definition of average.

Visual acuity values are best understood by the following simple rule. In a Snellen chart, we determine the line that the person can recognize. If that line is twice as large as the reference standard (20/20), we declare that person’s magnification requirement to be 2x.

If the magnification requirement is 2x, visual acuity is 1/2 (20/40). Similarly, if the magnification requirement (MAR) is 5x, the visual acuity is 1/5 (20/100); if MAR = 10, visual acuity = 1/10 (20/200), and so on.

The magnification requirement and the visual acuity scale are opposite. A high magnification requirement value indicates low or poor visual acuity; a low magnification requirement value indicates good acuity. Their relationship is also true in reverse.

A patient with visual acuity of 20/60 (1/3) needs a 3x magnification to reach the reference standard. This can be achieved with large print which is 3 times normal, with a 3x magnifying glass or with a 3x telescope.

Since normal vision is better than 20/20, additional magnification is desirable for comfortable and sustainable performance.

LogMAR notation

The magnification requirement value is best known by its logarithm: logMAR. Regarding the amplification requirement, a higher logMAR value indicates poorer vision.

Although the logMAR notation is often presented as a visual acuity notation, it is actually a vision loss notation. Zero logMAR indicates standard vision; zero visual acuity indicates blindness.

The logMAR notation is most convenient when used with a graph where the letter sizes follow a logarithmic progression, such as in the Early Treatment Diabetic Retinopathy Study Charts.

Because of this, such charts are often called logMAR charts. In these graphs, each 0.1 unit increase on the logMAR scale indicates a loss of one line on the visual acuity table.

The logMAR scale is widely used in scientific publications where visual acuity values ​​must be plotted to analyze trends or must be averaged across population groups.

Visual ability

People with a visual acuity of 20/40 (0.5) need a 2x magnification. This does not mean, however, that they have lost 50% of their visual skills.

Doctors have long used the expression “won lines” or “lost lines” to indicate changes in visual ability.

The visual acuity score (VAS) provides a convenient scale for estimating visual abilities.

On this scale, 20/20 is rated 100; in tables with a logarithmic progression, each line is worth 5 points and each letter read is 1 point. 50 visual acuity points are 20/200; it is reasonable to estimate that at this level the average person has lost 50% of visual ability.

Zero is reached by 20/2000, a level at which it is difficult to talk about visual acuity, as 100x magnification is needed for detail recognition.

In the AMA Guidelines for the Assessment of Permanent Impairment (5th and 6th Edition), the visual acuity scale is used in the calculation of visual impairment .

A study has shown that AMA calculations provide better estimates of capacity than previously used scales.

Logarithmic progression

The above discussion already indicated why a logarithmic progression of font sizes is desirable and leads to more consistent measurements.

Although the name “Snellen’s primer” should be applied to any letter- or symbol-based graph, some authors use the term Snellen’s primer only for graphs with a non-logarithmic progression.

Practitioners and researchers around the world turn to precision vision for “Early Treatment Diabetic Retinopathy Study” charts and other high-quality vision testing tools.