Sometimes, particularly when doing an eye exam on a small child the optometrist will direct a beam of light in the eyes. But why? This test is a retinoscopy examination, which is a preliminary way to determine the refractive error of your eye. Whether you're near or farsighted, or you have astigmatism, examining the way light reflects off your retina is one test your eye doctor can use to see whether you need vision correction.
How well your eyes are able to focus during the retinoscopy exam is the most important thing we look for. When we use the retinoscope to shine light into your eye, a reddish light reflects off your retina, through your pupil. This is known as the red reflex. The degree at which the light reflects off your retina, which is what eye care professionals call your focal length, is exactly what tells us how well your eye can focus. And if we notice that you aren't focusing properly, that's where the lenses come in. We hold up different prescription lenses in front of your eye to see which one fixes the error. And that is precisely how we calculate what prescription your glasses or contact lenses need to be.
The optometrist will run your exam in a dark room. To make your eyes easier to examine, you'll generally be told to keep your eyes fixed on an object behind the doctor. Because a retinoscopy exam doesn't require you to read eye charts, it's also a particularly useful way to determine an accurate prescription for children or patients who have difficulty with speech.