Dr Herman Snellen, a Dutch ophthalmologist proposed his
optotypes in 1862 which is still widely used to measure the visual
acuity during clinical refraction. Snellen’s letters were
so constructed that their constituent parts, i.e. the limbs and
spaces between them, each subtends an angle of 1 minute of arc
at a specified distance.
The linear size of the Snellen’s 6 meters
letters is given by the following notation:
tan 1 minute of arc =h/6000 mm
(where “h” is the size or height of the letter)
Taking the value from log table
0.000292 = h/6000 mm
Or, 0.000292 × 6000 = h
Or, h = 1.75mm
Therefore, 5 minutes of arc 5 × 1.75 = 8.75mm
On Snellen’s test chart, 6/6 or 20/20 letter size subtends an
angle of 5 minutes of arc which has a letter size of 8.75mm.
Snellen’s fraction says that visual acuity depends on two factors:
1. Size of the letter seen
2. Distance at which it is presented.
The numerator in the fraction denotes test distance, i.e. the
distance at which the letter subtends 5 minutes of arc and the
denominator indicates the size of the letter presented.
Visual acuity = (Viewing distance)/(Distance at which the letter size subtends
5 minutes of arc)
A visual acuity score of 20/200 implies that the test is d͢͢one at
20 feet and denominator presents the size of the smallest letter
that is being read would subtend an angle of 5 minutes of arc
when at a distance of 200 feet. The angular size of such letter
at 20 feet is 50 minutes of arc and the linear size is 87.50mm as
derived by above equation. Similarly visual acuity score of 20/20
implies that the test is done at 20 feet and the size of the smallest
letter that is being read would subtend an angle of 5 minutes of
arc when at a distance of 20 feet. The angular size of such letter
at 20 feet is 5 minutes of arc and the linear size is 8.75mm as
derived by above equation.
20/20 is being chosen as normal or standard on the basis for
being “easy to recognize”. In fact healthy eye always exceeds the
standard. In population samples the average acuity does not
drop to 20/20 level until the age of 50 or 60 years. In fact a normal
young adults with healthy eyes can easily read over a range which
may extend beyond 20/20 to 20/15 or so. 20/20 is not the level
where the average visual acuity is truncated. Snellen’s fraction
may also be read as:
Visual acuity = (Test distance)/
(Distance from which a normal eye can
see the letter on the chart)
20/200 in the above fraction means that test was conducted at 20
feet and the patient is able to read the size of the letter which a normal
person with healthy eyes can read from a distance of 200 feet.
In USA the distance is expressed in feet and clinician invariably
uses the Snellen’s fraction with 20 feet as the numerator. In most
other countries metric unit is more popular with 6 meter being
the common test distance. Snellen’s fraction is internationally
recognized both in meters and feet. The notations recorded to
show various levels of visual acuity are as under:
Snellens Chart is standardized for size and contrast. Care must
be taken while measuring the acuity to encourage the patient tokeep reading. You may ask the patient to relax and blink regularly.
Avoid making comments while the patient is reading the chart
such as “You got that letter wrong” or “Are you sure?”. They affect
the patient’s behavior and can influence the outcome. Be aware
that frequent presenter may memorize the chart. In such case
you may use different charts, if available or you may ask them to
read backward. In addition to Snellen’s numeric notations, there
are several abbreviations used to record visual acuity in patient’s
prescription and attention should be given to the appropriate
use of these abbreviations:
VA — Visual acuity
VAcc — Visual acuity with correction
BCVA — Best corrected visual acuity
UCVA — Uncorrected visual acuity
VAsc — Visual acuity without correction
Visual acuity charts may be prepared as:
1. Printed panels
2. Projection charts
3. Video display generated.
The charts are often viewed directly. However, when the room
dimensions do not permit the desired test distance, mirrors
may be used to lengthen the optical path from the chart to the
patient’s eyes. High contrast between the black letters and white
background is the basis behind the Snellen’s test chart which is
possible mostly on printed panels. Projector charts and the video
generated charts lack the contrast. However, Projector charts
allows adjustment of optical path length from the patient to the
screen. Computer generated display provides the means to select
different optotypes to change the letter sequence and to vary the
stimulus parameters such as contrast, spacing arrangements
and presentation times. However, pixilated letters, brightness
and contrast of the screen display may alter the acuity results in
critical cases. While placing the test charts one should take care
to avoid glare sources within the patient’s field of view.
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