Sometimes, especially when performing an eye exam on a small child the eye doctor will direct a beam of light in the eye. But what does this do? Firstly, this test is a retinoscopy examination, and if you have issues with accurate vision, this is one way the optometrist might find out. By merely looking at the reflection of light off your retina, the eye doctor can determine whether you are nearsighted, farsighted or have astigmatism. This is how they may also get a pretty good reading on the prescription required to correct your vision.
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 refracts off your retina, which is what eye care professionals call your focal length, is exactly what lets us know how well your eye can focus. If it becomes obvious that you are not focusing well, that's where the lenses come in. We hold up several prescription lenses in front of the eye to see which one corrects your vision. This is precisely how we find out what prescription your glasses or contact lenses need to be.
All this happens in a dark room. The patient will usually be asked to look at something behind the doctor. Because a retinoscopy exam doesn't involve any eye charts, it means that it's also a really great way to determine an accurate prescription for children or patients who have difficulty with speech.