On occasion, especially when doing an eye exam on a small child the eye doctor will direct a light in the eyes. But why? This test is called a retinoscopy examination, which is a preliminary way to assess the refractive error of your eye. By examining the way light reflects off your retina, the optometrist can assess if 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.
How well your eyes are able to focus during the exam is really what we're looking for. We shine light into your eye because we are looking for what's known as the red reflex. The retinoscope aims light into your eye, and a red or orange light reflects through your pupil and off your retina. This process measures your focal length, or in simpler words, to measure the precise angle at which light refracts off your retina. And this is what lets us know how well your eye focuses. And if we see that you aren't focusing well, we hold a variety of lenses with varying prescriptions in front of your eye to see which one fixes the error. That lens power is the prescription you require to fix your sight with glasses or contact lenses.
All this happens in a darkened room. The patient will usually be asked to look at something ahead, just behind the doctor. Unlike other eye exams, your doctor won't ask you to read letters off charts. This means that a retinoscopy exam is also a really useful tool to determine the prescriptions of the speech-impaired, or young children.