How It Is Done

Visual acuity testing

Visual acuity tests are used to evaluate eyesight. Several types of visual acuity tests may be used.

• The Snellen test checks your ability to see at distances. It uses a wall chart that has several rows of letters. The letters on the top row are the largest; those on the bottom row are the smallest.
  • You will stand or sit 6 m (20 ft) from the chart and be asked to cover one eye and then read the smallest row of letters you can see on the chart. If you are unable to cover your eye, an eye patch will be placed over your eye.
  • Each eye is tested separately. You may be given a different chart or asked to read a row backward to make sure that you did not memorize the sequence of letters from the previous test.
  • If you wear glasses or contacts, you may be asked to repeat the test on each eye while wearing them.
  • Let your doctor know if you have trouble reading the letters on one side of the row, or if some letters disappear while you are looking at other letters. You may have a visual field problem, and visual field tests may be needed.
• The E chart tests the vision of children and people who cannot read. The E chart is similar to the Snellen chart in that there are several rows, but all of the rows contain only the letter E in different positions. The top row is the largest and the bottom row of Es is the smallest. You will be asked to point in the same direction as the lines of the E. Similar charts use the letter C or pictures. These charts are also available in a hand-held card.
• The Near test uses a small card (Jaeger chart) containing a few short lines or paragraphs of printed text to test your near vision. The size of the print gradually gets smaller. You will be asked to hold the card about 35 cm (14 in.) from your face and read aloud the paragraph containing the smallest print you can comfortably read. Both eyes are tested together, with and without corrective lenses. This test is routinely done after age 40, because near vision tends to decline as you age (presbyopia).

If you cannot read any of the letters or print on these charts because of poor vision, your visual acuity will be tested by other techniques, such as counting fingers, detecting hand movements, or distinguishing the direction or perception of light sources (such as room light or a penlight held up close to the face).

Visual acuity tests usually take about 5 to 10 minutes. They may be performed by a nurse, a medical assistant, an ophthalmologist, an optometrist, a teacher, or some other trained person. Testing may be done at a doctor's office, school, workplace, health fair, or elsewhere.

Refraction

Refraction is a test that measures the eye's need for a corrective lens (refractive error). For this test, you will be asked to describe the effects of looking at an eye chart through various corrective lenses.

Your doctor may use eyedrops to widen (dilate) your pupils before you start this test. The eyedrops take about 15 to 20 minutes to dilate the pupil fully.

The doctor may put a device (called a phoropter) in front of your eyes. The device contains many different lenses. Testing one eye at a time, the doctor will ask you to compare the effects of two lenses (first one lens, then the other). You should state which lens of each pair gives you better vision. The doctor will continue to test your eyes with different lenses until it is determined which lenses correct your vision the best. Your doctor may use a hand-held device (retinoscope) to shine light into your eyes. A series of trial lenses will be placed in front of your eyes and adjusted until the light rays are properly focused on your retina.

Visual field tests

Visual field tests are used to check for gaps in your range of vision. They can help detect eye diseases or nervous system problems that limit your ability to see objects clearly in the entire visual field or in one part of it. Several tests are commonly done to evaluate a person's visual field.

• The confrontation test. Your doctor sits or stands 0.6 m (2 ft) to 1 m (3 ft) in front of you. You cover one eye while fixing your gaze on his or her nose. He or she slowly moves a finger or hand from the outer edge of your visual field toward the centre and from the centre toward the edge through all areas of your visual field. You will focus your eye on your doctor's nose and signal when you first see his or her finger or hand. The test is then repeated for the other eye.
• The Amsler grid test checks for macular degeneration, a disease that causes loss of vision in the centre of your visual field. The test uses a 10 cm (4 in.) square chart with straight lines that form boxes. The grid has a black dot at the centre. The chart is held about 35 cm (14 in.) from your face. You cover one eye while focusing your other eye on the black dot. The test is then repeated on the other eye. Tell your doctor if:
  • You cannot see the black dot.
  • You see a blank or dark spot (other than the centre dot).
  • The lines in the grid look wavy, blurred, or curved instead of straight. You will be asked to point to the specific abnormal area of the grid.
• Perimetry testing uses a machine that flashes lights randomly at various points in the visual field. You look inside a bowl-shaped instrument called a perimeter. While you stare at the centre, lights will flash, and you press a button each time you see a flash. A computer records the location of each flash and whether you pressed the button when the light flashed in that location. At the end of the test, a printout shows any areas of your visual field where you did not see the flashes of light. In an alternative manual perimetry test, your doctor moves a light target and notes your visual field on paper.
• Thetangent screen test uses a black screen with concentric circles and lines leading out from a centre point (like a bull's-eye). Sitting 1 m (3 ft) to 2 m (7 ft) away from the screen, you cover one eye while fixing your gaze on a target point marked on the screen. Test objects of various sizes at the tip of a wand are then moved inward from the outer edge of the screen toward the centre. You will signal when you can see the object, and that point is then marked on the screen. The points on the screen where you see the objects are connected to provide an outline of your visual field. The test is then repeated for the other eye. An alternative manual tangent screen test uses a white object against a black background. If you wear glasses, you will keep them on for this test.

Colour vision tests

Colour vision tests check your ability to distinguish colours. In the most commonly used colour vision test, you look for different coloured numbers or symbols hidden in varying backgrounds of coloured dots.

First, you are shown sample patterns and told what symbols and numbers you can expect to see. You then sit at a table and cover one eye. The doctor holds the colour test patterns about 35 cm (14 in.) away from you. Some patterns are harder to pick out than others. As the doctor holds up a pattern, you will identify the number or symbol you see and trace it using a pointer. Some patterns may not have a number or symbol. The test is then repeated with the other eye.