How to use a Snellen Chart
Visual acuity is typically measured monocularly rather than binocularly with the aid of an optotype chart for distant vision, an optotype chart for near vision, and an occluder to cover the eye not being tested. The examiner may also occlude an eye by sliding a tissue behind the patient's eyeglasses, or instructing the patient to use his or her hand. This latter method is typically avoided in professional settings as it may inadvertently allow the patient to peek through his or her fingers, or press the eye and alter the measurement when that eye is evaluated.
1. Place the chart at 20 feet (or 6 meters) and illuminate to 480 lux at that distance.
2. If the patient uses glasses, then the test is performed using them.
3. Place the occluder in front of the eye that is not being evaluated. The first evaluated eye is the one that is believed to see less or the one the patient says that is seeing less.
4. Start first with the big optotypes and proceed to the smaller ones. The patient has to identify every one on the line being presented and communicate it to the physician.
5. If the measurement is reduced (below 20/20) then the test using a pinhole should be done and register the visual acuity using the pinhole. Both measures should be registered, with and without using pinhole.
6. Change the occluder to the other eye and proceed again from the 4th step.
7. After both eyes have been evaluated in distant visual acuity, proceed to evaluate near visual acuity placing a modified snellen chart for near vision (such as the Rosembaum chart) at 14 inches (or 35 centimeters). Then repeat the test from the 2nd step.
In some cases, binocular visual acuity will be measured, because usually binocular visual acuity is slightly better than monocular visual acuity.
Infant vision
The newborn’s visual acuity is approximately 20/400 and 20/20 by age 2 years. [8]
Terms Associated with the Snellen Chart
O.D Ocular dexter (right eye)
O.S Ocular sinster (left eye)
O.U. Oculi unitas (both eye)
Prism diopter
+ Plus or convex lens
C.F. Count fingers
H.M. Hand movement
L.P. Light perception
N.L.P No light perception
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.
