Snellen chart
A Snellen chart is an eye chart used by eye care professionals and others to measure visual acuity.  Snellen charts are named after the Dutch ophthalmologist Hermann Snellen who developed the chart in 1862.

# Description
The traditional Snellen chart is printed with eleven lines of block letters. The first line consists of one very large letter, which may be one of several letters, for example E, H, N, or A.  Subsequent rows have increasing numbers of letters that decrease in size.  A patient taking the test covers one eye, and reads aloud the letters of each row, beginning at the top.  The smallest row that can be read accurately indicates the patient's visual acuity in that eye.
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 block letters, and are intended to be seen and read as letters. They are not, however, letters from any ordinary typographer's font. They have a particular, simple geometry in which:
- the thickness of the lines equals the thickness of the white spaces between lines and the thickness of the gap in the letter "C"
- the height and width of the optotype (letter) is five times the thickness of the line.
Only the nine letters C, D, E, F, L, O, P, T, Z are used in the traditional Snellen chart.
Wall-mounted Snellen charts are inexpensive and are sometimes used for rough assessment of vision, e.g. in a primary-care physician's office. Whenever acuity must be assessed carefully (as in an eye doctor's examination), or where there is a possibility that the examinee might attempt to deceive the examiner (as in a motor vehicle license office), equipment is used that can present the letters in a variety of randomized patterns.
According to BS 4274:1968 (British Standards Institution) "Specification for test charts for determining distance visual acuity" the minimum illumination for externally illuminated charts should be 480 lx, however this very important parameter is frequently ignored by physicians, making many test results invalid.

# Snellen fraction
Visual acuity = Distance at which test is made / distance at which the smallest optotype identified subtends an angle of 5 arcminutes.

# "20/20" (or "6/6") vision
Snellen defined standard vision as the ability to recognize one of his optotypes when it subtended 5 minutes of arc. Thus the optotype can only be recognized if the person viewing it can discriminate a spatial pattern separated by a visual angle of 1 minute of arc.
In the most familiar acuity test, a Snellen chart is placed at a standard distance, twenty feet in countries where that is the customary unit of measure. At this distance, the symbols on the line representing "normal" acuity subtend an angle of five minutes of arc, and the thickness of the lines and of the spaces between the lines subtends one minute of arc. This line, designated 20/20, is the smallest line that a person with normal acuity can read at a distance of twenty feet.
Three lines above, the letters have twice the dimensions of those on the 20/20 line. The chart is at a distance of twenty feet, but a person with normal acuity could be expected to read these letters at a distance of forty feet. This line is designated by the ratio 20/40. If this is the smallest line a person can read, the person's acuity is "20/40," meaning, in a very rough kind of way, that this person needs to approach to a distance of twenty feet to read letters that a person with normal acuity could read at forty feet. In an even rougher way, this person could be said to have "half" the normal acuity.
In countries using the metric system, the standard chart distance is six meters, normal acuity is designated 6/6, and other acuities are expressed as ratios with a numerator of 6.
Acuity charts are used during many kinds of vision examinations, such as "refracting" the eye to determine the best eyeglass prescription. During such examinations, acuity ratios are never mentioned.
The biggest letter on an eye chart often represents an acuity of 20/200, the value that is considered "legally blind." Many people with refractive errors have the misconception that they have "bad vision" because they "can't even read the E at the top of the chart without my glasses." But in most situations where acuity ratios are mentioned, they refer to best corrected acuity. Many people with moderate myopia "cannot read the E" without glasses, but have no problem reading the 20/20 line or 20/15 line with glasses. A legally blind person is one who cannot read the E even with the best possible glasses.

# Criticism
Snellen charts have been the target of some criticism.  The fact that the number of letters increases while the size decreases introduces two variables, rather than just one.  Some people may simply memorize the Snellen chart before being tested by it, to give the impression that their vision is good. Several studies indicate that the crowding together of letters makes them inherently more difficult to read.  Another issue is that there are fairly large and uneven jumps in acuity level between the rows. To address these concerns, more modern charts have been designed that have the same number of letters on each row and use a geometric progression to determine the size of each row of letters. Also, some letters are harder to distinguish than others, such as P vs F, C vs G, Q vs O, etc.