A refraction test is a type of eye examination that enables an ophthalmologist to determine the correct prescription for a person's eyeglasses or contact lenses, explains Healthline. The test also allows an eye doctor to diagnose different eye conditions, such as astigmatism or hyperopia.
In a refraction test, an optometrist examines the bending of light as it travels through the cornea and the eye lens, states Healthline. She observes the movement of light by using a computerized refractor or focusing light into the eyes. In a digital exam, a machine measures how much light the retina reflects. In a manual test, an eye doctor shines a light into a person’s eyes and observes how much light bounces off the retina to measure the individual’s refractive score.
Next, a person sits in front of a device called a phoroptor and stares at a chart of letters that is projected at a distance of 20 feet, says Healthline. Eye doctors typically use charts with pictures of easily identifiable items for children who are not yet familiar with letters.
An optometrist performs the refraction test separately for each eye, notes Healthline. She uses different lenses of varying strengths and asks the viewer which lens provides the clearest vision. Once the procedure is done, she determines the combination that gives the person 20/20 vision.