Snellen developed his optotypes on a 5 × 5 grid, on which the line thickness is one unit and the letter width and height are five units. This system is followed for the majority of letter and number charts. For children’s tests (such as Allen photos and other test signs), it is frequently abandoned and much thinner lines are used.
Snellen Chart Definition
Precision Vision uses stylized photos, called Patti Pics, for which Snellen’s 5 × 5 principle is followed. They lead to less change in the determined acuity when children finish from photo charts to letter charts. Alternative signs that can be used for illiterate adults or in countries that are not acquainted with the Roman alphabet are the toppling E test and the Landolt C or damaged ring.
Letter charts are not just used to measure visual skill, they are also used as targets for subjective refraction. This is the primary reason range skill is determined more often than near skill. At a far away, accommodation is relaxed, so that the refraction can be more accurate. At a longer test distance, the effect of small modifications in the topic’s position is less important and can be ignored.
As today’s exam lanes typically are smaller sized than 20 feet (6 meters), charts are often developed for much shorter distances. This is no problem for visual acuity measurement, as long as the actual test distance is properly accounted for. For use in refraction, much shorter distances are less desirable. At 4 m (the suggested distance for ETDRS charts) the accommodative demand is 0.25 Diopters and can not be disregarded. In smaller sized spaces, the use of mirrors is advised to increase the test distance.
Letter Size in Snellen Charts
Snellen used a rather verbose definition to show the letter size: “the range in meters (feet) at which the letter subtends 5 min of arc.” Louise Sloan simplified this by defining the M-unit as the size that subtends 5 min of arc at 1 meter. Hence, the verbose declaration “this letter subtends 5 min of arc at ‘x’ meter” can be simplified to “this letter is ‘x’ M-units”. Here the number denoting the M system size is the seeing distance in meters for 20/20 skill.
The majority of Precision Vision eye charts carry the letter size classification in M-units. This makes it simple to calculate the visual acuity if the chart is used at other distance than the one for which it was developed by inserting the new test distance (in meters) and the letter size (in M-units) straight into the above formula.
You can download Snellen Chart in PDF format (for printing) from here.
Interpretation of the Snellen Eye Chart
It is a persistent urban myth that 20/20 would represent normal, typical or even twenty-twenty. This is not so. Snellen intentionally chose his reference requirement (5 minutes of arc) as a size that is “easily recognized by normal eyes.” Therefore, nearly all normal eyes will equate to or go beyond the referral requirement. If 20/20 equaled typical acuity, half of the population would fall short of 20/20, because that is the definition of average.
How to Use Snellen Eye Chart?
Visual skill values are understood best by the list below easy guideline. On a Snellen chart we figure out the line that the individual can simply recognize. If that line is two times as big as the referral standard (20/20), we specify that individual’s MAgnification Requirement (MAR) is 2x. If the MAgnification Requirement is 2x, the visual skill is 1/2 (20/40). Likewise, if the MAgnification Requirement (MAR) is 5x, the visual skill is 1/5 (20/100); if MAR = 10, visual acuity = 1/10 (20/200), and so on.
The MAR and the visual skill scale are opposites. A high MAR value shows low or poor visual acuity; a low MAR value indicates good skill. Their relationship is also true in reverse. A patient with 20/60 (1/3) visual acuity needs 3x magnification to reach the referral standard. This can be achieved with big print that is 3x normal, with a 3x magnifier or with a 3x telescope. Given that normal vision is much better than 20/20, some extra zoom is preferable for comfy and sustainable performance.