context: Here's a brief guide to the special tests your eye doctor may perform during an eye exam. This test measures the amount of pressure it takes to flatten a portion of your cornea. Pressure readings help your doctor diagnose and keep track of glaucoma. He'll give you drops to numb your eye, then press lightly on it with a tool called a tonometer. This computerized test maps the curve of your cornea. It can show problems with your eye's surface, like swelling or scarring, or conditions such as astigmatism. You might have it before you have surgery, a cornea transplant, or a contact lens fitting. This lets the doctor see how well blood moves in your retina. It helps diagnose diabetic retinopathy, retina detachment, and macular degeneration. The doctor will inject a special dye, called fluorescein, into a vein in your arm. It travels quickly to blood vessels inside your eye. Once it gets there, the doctor uses a camera with special filters to highlight the dye. He takes pictures of the dye as it goes though the blood vessels in the back of your eye. This helps him spot circulation problems, swelling, leaking, or abnormal blood vessels. The doctor uses special drops to expand your eye's pupil (he'll call this dilate). That lets him check your retina for signs of disease. This is what the doctor uses to get your eyeglasses prescription. You look at a chart, usually 20 feet away, or in a mirror that makes things look like they're 20 feet away. You'll look through a tool called a phoropter. It lets the doctor move lenses of different strengths in front of your eyes. You can tell him if things look clear or blurry. Your answers give him your prescription for your glasses or contact lenses. The test will also help him spot presbyopia, hyperopia, myopia, and astigmatism. The doctor uses this microscope to shine a beam of light shaped like a small slit on your eye. He may also dilate your pupils during the test. It can help diagnose cataracts, glaucoma, detached retina, macular degeneration, cornea injuries, and dry eye disease. This test helps diagnose glaucoma. The doctor will use a tool called a tonometer that blows a tiny puff of air, measuring eye pressure indirectly by the eye's resistance to the puff. Applanation instruments can also measure pressure. They are the most accurate, but you'll need local anesthetic. This computerized test can give a remarkably detailed image of the retina and all of its layers. You might get it if you have a serious retina condition, like age-related macular degeneration or retinal detachment. This test uses sound waves to make picture of the inside of your eye. It helps your doctor diagnose and treat tumors, cataracts, or bleeding in your eye. You might also get it before cataract surgery. This measures how well you see at near and far distances. If your child can't yet read, the doctor will use a special test. Your child will look at a letter " E" then tell the doctor the way the legs point with her fingers. You can practice this at home before the test. This measures your peripheral (side) vision. You'll stare at on object in the center of your line of vision (like the doctor's eyes or a computer screen). As you look at the target, you'll note when you see an object moving into your field of vision or, depending on the test, when the lighted spot appears . This test lets the doctor know if conditions like stroke or glaucoma have hurt your vision.