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. Vision scientists now use a variation of this chart, designed
by Ian Bailey and Jan Lovie at the National Vision Research Institute of Australia..
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, or N. Subsequent rows have increasing numbers
of letters that decrease in size. A person taking the test covers one eye from 20 ft.
away, and reads aloud the letters of each row, beginning at the top. The smallest row
that can be read accurately indicates the 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 is five
times the thickness of the line. Only the ten Sloan letters C, D, E, F, L,
N, O, P, T, Z are used in the traditional Snellen chart. The perception of five out
of six letters is judged to be the Snellen fraction.
Wall-mounted Snellen charts are inexpensive and are sometimes used for approximate assessment
of vision, e.g. in a primary-care physician's office. Whenever acuity must be assessed carefully,
or where there is a possibility that the examinee might attempt to deceive the examiner, equipment
is used that can present the letters in a variety of randomized patterns.
BS 4274-1:1968(British Standards Institution) "Specification for test charts for determining
distance visual acuity" was replaced by BS 4274-1:2003 "Test charts for clinical determination
of distance visual acuity — Specification". It states that "the luminance of the presentation
shall be uniform and not less than 120 cd/m2. Any variation across the test chart shall
not exceed 20 %." According to BS 4274-1:2003 only the letters
C, D, E, F, H, K, N, P, R, U, V, and Z should be used for the testing of vision based upon
equal legibility of the letters. 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"(ft)) 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: 20 ft in the US, or 6 metres in the rest of the
world. 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 or 6/6, is the smallest line
that a person with normal acuity can read at a distance of 20 ft.
Three lines above, the letters have twice the dimensions of those on the 20/20 line.
The chart is at a distance of 20 ft, but a person with normal acuity could be expected
to read these letters at a distance of 40 ft. 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 20 ft
to read letters that a person with normal acuity could read at 40 ft. In an even more
approximate manner, this person could be said to have "half" the normal acuity.
At 20 ft, the letters on the 20/20 line should subtend 5 minutes of arc, which means that
the chart should be sized such that these letters are 8.86 mm tall and the topmost
"E" should be 88.6 mm tall. Putting it another way, the eye should be at a distance 68.75
times the height of the top letter. Outside of the US, the standard chart distance
is 6 m, normal acuity is designated 6/6, and other acuities are expressed as ratios with
a numerator of 6. Many rooms do not have 6 m available, and either a half-size chart
subtending the same angles at 3 m, or a reversed chart projected and viewed by a mirror is
used. 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 largest letter on an eye chart often represents an acuity of 20/200, the value that is considered
"legally blind." Some individuals with moderate myopia may not be able to read the large E
without glasses, but have no problem reading the 20/20 line or 20/15 line with glasses.
By contrast, legally blind individuals have a visual acuity of 20/200 or less when using
the best corrective lens. See also
E Chart Landolt C
Lea test References
External links Snellen Chart