Eye doctors use a wide variety of tests and procedures to examine your eyes. These tests range from simple ones, like having you read an eye chart, to complex tests, such as using a high-powered lens to visualize the tiny structures inside of your eyes. Regardless, when you have an eye exam, it will usually take anywhere from half an hour to an hour or more, depending on the doctor, his or her caseload and the number and complexity of tests required for your eyes.
Here are the basic tests that you are likely to encounter during a routine eye exam (your doctor also has dozens of less routine ones at his or her disposal):
The eye doctor will often perform this test early in the eye exam in order to obtain an approximation of your prescription from which to start.
In retinoscopy, the room lights will be dimmed and you will be given a large target (usually the big "E" on the chart) to fixate on. As you stare at the "E," the eye doctor will shine a light at your eye and flip lenses in a machine in front of your eyes. Based on the way the light reflects from your eye, the doctor is able to "ballpark" your prescription — sometimes right on the money! This test is especially useful for children and non-verbal patients who are unable to accurately answer the doctor's questions.
This is the test that the eye doctor uses to determine your exact prescription. During a refraction, the doctor puts the instrument called a phoropter in front of your eyes and shows you a series of lens choices. He or she will then ask you which of the two lenses in each choice looks clearer. Based on your answers, the doctor will continue to fine-tune your power until reaching a final prescription that is then used in your eyeglasses and contacts.Refraction determines your level of farsightedness, nearsightedness, astigmatism and presbyopia.
Autorefractors and Aberrometers
Your eye doctor also may use an autorefractor or aberrometer to automatically determine your prescription. With both devices, a chin rest stabilizes your head while you typically look at a pinpoint of light or other image.
An autorefractor evaluates the way an image is focused on your retina, where vision processing takes place. While autorefractors are very useful in certain cases such as evaluating young children, some studies show that subjective eye exams tend to produce more accurate results. Subjective eye tests and autorefractors may be used together to produce your prescription.
An aberrometer uses advanced wavefront technology to detect even obscure vision errors based on the way light travels through your eye. Aberrometers are used for custom or wavefront LASIK vision correction procedures.
While there are many ways for your eye doctor to check how your eyes work together, the cover test is the simplest and most common.
During a cover test, the eye doctor will have you focus on a small object at distance and will then cover each of your eyes alternately while you stare at the target. As they do this, eye doctors observe how much each eye has to move when uncovered to pick up the fixation target. This helps them to detect strabismus, or eye turn, which can cause amblyopia, poor depth perception and other binocular vision problems. The test is then repeated up close.
The slit lamp is an instrument that the eye doctor uses to examine the health of your eyes. The slit lamp, also called a biomicroscope, allows the doctor to get a highly magnified image of the structures of the eye in order to thoroughly evaluate them for signs of infection or disease.
During this test the doctor will have you place your chin on the chin rest of the slit lamp and will then shine the lamp's light at your eye. The doctor looks through a set of oculars (much like a microscope in a science lab) and examines each part of the eye in turn. Your doctor will first examine the structures of the front of the eye (lids, cornea, conjunctiva, iris, etc.). Then, with the help of a special high-powered lens, he or she will view the inside of the eye (retina, optic nerve, macula and more). A whole range of eye conditions and diseases can be detected with slit-lamp examination, including cataracts, macular degeneration, corneal ulcers, diabetic eye disease, ... the list goes on and on.
The Glaucoma Test
There are a few variations of the glaucoma test. The purpose of all of them is to measure the pressure inside your eye. The most common glaucoma test is the "puff-of-air" test, technically known as the non-contact tonometer.
For this test, the eye doctor will again start by having you put your chin on the machine's chin rest. While you look at a light inside the machine, the doctor will line it up and puff a small burst of air at your open eye. It is completely painless, and the tonometer does not come in contact with the eye. Based on your eye's resistance to the puff of air, the machine calculates your intraocular pressure.
The other type of glaucoma test is performed with an applanation tonometer. While there are several versions of this instrument, the most common is mounted on the slit lamp. For this test, the doctor will put a yellow eyedrop in your eye that numbs it. Your eyes will feel slightly heavy when the drops start working. This is not a dilating drop — it is simply a numbing agent combined with a yellow dye. Then the doctor will have you stare straight ahead in the slit lamp while he or she touches a bright-blue glowing instrument to the front of each eye and manually measures the intraocular pressure.
Applanation tonometry is also painless. At most, you may feel the tonometer probe tickle your eyelashes. The whole test takes just a few seconds. Since glaucoma is often the result of an increase of pressure inside the eye, these are important tests for ensuring the long-term health of your eyes.
Visual Field Test
In some cases, your eye doctor may want to check for the possible presence of blind spots (scotomas) in your peripheral or "side" vision by performing a visual field test. These types of blind spots can originate from eye diseases such as glaucoma. Analysis of blind spots also may help identify specific areas of brain damage caused by a stroke or tumor.
Dilating drops are drops that the eye doctor puts in your eyes to make the pupils bigger in order to get a better view of the internal structures of the eye. The pupil is like a window to the inside of the eye. Essentially, dilating drops open the window wider to allow the doctor a better view of the eye's internal structures. Dilating drops usually take about 20 to 30 minutes to start working. While the dilating drops are working, you will be sensitive to light (because more light is getting into your eye) and may notice difficulty focusing on objects up close. These effects can last for up to several hours depending on the strength of the drop used.
Once the drops have taken effect, the doctor will use a variety of instruments and light sources to look inside of your eyes. You should bring sunglasses with you, to minimize glare and light sensitivity on the way home; if you've forgotten yours, the staff may give you a temporary pair. Dilation is very important for people with risk factors for eye disease, because it allows for the most thorough evaluation of the health of the inside of your eyes.
These are the most common tests performed during a standard comprehensive eye exam. Contact-lens evaluations, laser-surgery work-ups and pediatric eye exams often require additional, more specialized tests. Your doctor will select the right tests for your eyes and explain each test as it is performed. Regardless of the type of test being performed, it is important that you follow directions closely and be honest in your answers. This will ensure accurate results and the most thorough evaluation of your eyes.