There may be a few assessments that you have experienced at an eye exam and questioned how they work. Having a bright light shined into your eyes may be an example. This is one way eye doctors determine the refractive error of your eye, and it's called a retinoscopy exam. It sounds fascinating, but by examining the way light reflects off your retina, the optometrist can decide whether you are nearsighted, farsighted or have astigmatism. This is how they can also get a pretty good reading on the prescription required to correct your vision.
Essentially, what we are doing during the retinoscopy exam is checking to see how your eye can focus. When light shines into your eye using a retinoscope, a reddish light reflects off your retina, through your pupil. This is known as the red reflex. We use the light to measure your focal length, or in layman's terms, it will calculate the angle of refraction of light off your retina which lets us know how well your eye focuses. And if it's apparent that you can't focus properly, we hold a few lenses with varying prescriptions in front of the eye to see which one fixes the error. This is exactly how we calculate what prescription your glasses or contact lenses need to be.
The retinoscopy exam is usually performed in a dark room. To make your eyes easier to examine, you'll generally be instructed to keep your eyes fixed on an object behind the doctor. Unlike other eye exams, your doctor won't ask you to read any charts. This means that a retinoscopy exam is also a really good way to accurately determine the prescriptions of the speech-impaired, or young children.