Index
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 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 familiar in doctors’ and optometrists’ offices.
It typically displays 11 rows of capital letters, also known as “optotypes,” constructed according to strict geometric rules. Each row 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. A small number of letters 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 could 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 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 (e.g., 6/10) indicates worse than normal vision; a ratio greater than 1 (e.g., 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 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 independently.
Another is that row spacing and letter spacing varies in Snellen’s chart, introducing a third factor that further confuses the measurements.
However, another criticism is that the progression of the relationships between the letter lines is irregular and somewhat arbitrary, with huge 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 standard 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 five × five 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 appear 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 exact 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 precisely 6 meters 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 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 five × five 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.
Thinner lines are often abandoned and used for children’s tests (such as Allen pictures and other test symbols). Precision viewing offers stylized images, called Patti Pics, for which Snellen’s five × five principles are 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 used to measure visual acuity and as targets for subjective refraction. This is why distance acuity is measured more often than near acuity.
At a long distance, the accommodation is relaxed so that the refraction can be more accurate. At a longer test distance, the effect of small changes in the subject’s position is less critical 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, mirrors are recommended to increase the test distance.
Letter-size
Snellen used a reasonably detailed definition to indicate font size: “the distance in meters (feet) that the letter subtends 5 min of arc.”
Louise Sloan simplified this by defining unit M as the size that subtends 5 min of arc to 1 meter. Therefore, the detailed statement “this letter subtends five 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 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 the 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 will not reach 20/20 since that is the definition of average.
The following simple rule best understands visual acuity values. A Snellen chart determines the line that the person can recognize. If that line is twice as extensive 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 genuine 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 a large print three 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 a vision loss notation. Zero logMAR indicates normal 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 averaged across population groups.
Visual ability
People with a visual acuity of 20/40 (0.5) need a 2x magnification. However, this does not mean 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. Fifty visual acuity points are 20/200; it is reasonable to estimate that the average person has lost 50% of visual ability at this level.
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.
The AMA Guidelines for the Assessment of Permanent Impairment (5th and 6th Edition) uses the visual acuity scale to calculate visual impairment.
A study has shown that AMA calculations provide better capacity estimates 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 Snellen’s primer only for graphs with a non-logarithmic progression.
