Several different tests may be performed during the eye exam. The tests are designed to check your vision and to examine the appearance and function of all parts of your eyes.
Eye muscle test
This test examines the muscles that control eye movement, looking for weakness or poor control. Your eye doctor looks at your eyes as you move them in six specific directions and as you visually track a moving object, such as a pen or a light.
Visual acuity test
This test measures how clearly you can see from a distance. Your doctor will ask you to identify different letters of the alphabet printed on a chart (Snellen chart) positioned usually 20 feet away. The lines of type get smaller as you move down the chart. You cover one eye and read aloud, then cover the other eye and read aloud.
- Refraction refers to how lightwaves are bent as they pass through your cornea and lens. A refraction assessment helps your doctor determine a corrective lens prescription that will give you the sharpest vision. If you don't need corrective lenses, you won't have a refraction assessment.
- Your doctor may use a computerized refractor to measure your eyes and estimate the prescription you need to correct a refractive error. Or he or she may use a technique called retinoscopy. In this procedure the doctor shines a light into your eye and measures the refractive error by evaluating the movement of the light reflected by your retina.
- Your eye doctor fine-tunes this refraction assessment by having you look through a Phoroptor, a mask-like device that contains wheels of different lenses, and judge which combination gives you the sharpest vision. By repeating this step several times, your doctor finds the lenses that give you the greatest possible acuity.
Visual field test (perimetry)
Your visual field is the area in front of you that you can see without moving your eyes. The visual field test determines whether you have difficulty seeing in any areas of your peripheral vision — the areas on the side of your visual field. There are a few different types of visual field tests:
Confrontation visual field exam. Your eye doctor sits directly in front of you and asks you to cover one eye. You look directly at your eye doctor while he or she moves his or her hand in and out of your visual field. You tell your doctor when you can see his or her hand or fingers.
Tangent screen exam. You sit a short distance from a screen and stare at a target at its center. You tell your doctor when you can see an object move into your peripheral vision.
Automated perimetry. Your eye doctor uses a computer program that flashes small lights as you look into a special instrument. You press a button when you see the lights.
Using your responses to one or more of these tests, your eye doctor determines the fullness of your peripheral vision. If you aren't able to see in certain areas, noting the pattern of your visual field loss may help your eye doctor diagnose your eye condition.
Color vision testing
You could have poor color vision and not even realize it. If you have difficulty distinguishing certain colors, your eye doctor may screen your vision for a color deficiency. To do this, your doctor shows you several multicolored dot-pattern tests. If you have no color deficiency, you'll be able to pick out numbers and shapes from within the dot patterns. However, if you do have a color deficiency, you'll either find it difficult to see anything among the dots, or you won't see anything at all within the dot pattern.
A slit lamp is a microscope that enlarges and illuminates the front of your eye with an intense line of light. Your doctor uses this light to examine the cornea, iris, lens and anterior chamber of your eye.
When examining your cornea, your doctor may use eyedrops containing fluorescein (flooh-RES-ene) dye. The orange dye spreads across your eyes to help your eye doctor detect tiny cuts, scrapes, tears, foreign objects or infections on your cornea. Your eyes' tears eventually wash the dye away.
A retinal examination — sometimes called ophthalmoscopy or fundoscopy — examines the back of your eye, including your retina, optic disk and the underlying layer of blood vessels that nourish the retina (choroid). Usually before your doctor can see these structures, your pupils must be dilated with special eyedrops. The eyedrops may sting briefly and might cause a medicinal taste in your mouth as the medication eventually drains from your tear ducts into your throat.
After administering eyedrops, your eye doctor may use one or more of these techniques to view the back of your eye:
Direct examination. Your eye doctor shines a beam of light through your pupil and uses an ophthalmoscope to see the back of your eye. Sometimes eyedrops aren't necessary to dilate your eyes before this exam. You might see afterimages when your eye doctor stops shining the light in your eyes. This is normal and will go away within a minute or two.
Indirect examination. For this exam you might lie down or recline in a chair. Your eye doctor will hold each eye open and examine it with a bright light mounted on his or her forehead — a bit like a miner's lamp. This exam lets your eye doctor see the inside of your eye in great detail and in three dimensions. Since this light is brighter than that in a direct examination, you're more likely to see afterimages, but they disappear quickly.
Slit-lamp exam. In this exam your doctor uses the slit lamp along with an accessory condensing lens or a contact lens. The slit lamp reveals more-detailed views of the back of your eye than do direct or indirect examinations.
The retinal examination takes only five to 10 minutes, but if you're given eyedrops, their effects may not wear off for several hours. Your vision will likely be blurry, and you may have trouble focusing your eyes. If you're particularly sensitive to light, you may need to wear dark glasses (or sunglasses) for a short time. You may not be able to drive, so make sure you have another way back to work or home. Depending on your job, you might not be able to work until the effects of the eyedrops wear off.
A glaucoma test (tonometry) measures your intraocular pressure — the pressure inside your eyes. It helps your eye doctor detect glaucoma, a disease that causes pressure to build up inside your eyes and can lead to blindness. Glaucoma can be treated most effectively if it's caught early.
Methods your eye doctor may use to test your eyes for glaucoma include:
Applanation tonometry. This test measures the amount of force needed to temporarily flatten a part of your cornea. Fluorescein, the same orange dye used in a regular slit-lamp exam, is usually put in your eye to make your cornea easier to see. You'll also receive eyedrops containing an anesthetic. Using the slit lamp, your doctor moves the tonometer to touch your cornea. It doesn't hurt, and the anesthetic wears off within about 20 minutes.
Noncontact tonometry. This method uses a puff of air to test the pressure in your eye. No instruments will touch your eye, so you won't need an anesthetic. You'll feel mild pressure on your eye, which can be uncomfortable, but it lasts only seconds.
Pachymetry. This test measures the thickness of your cornea — an important factor in evaluating your intraocular pressure measurement. After applying numbing eyedrops, your eye doctor uses an instrument that emits ultrasound waves to measure your corneal thickness.
Besides these basic evaluations, you may need more specialized tests, depending on your age, medical history and risk of developing eye disease.