There are some assessments that you have seen during an eye exam and asked yourself what they measure. Having beams of light shined into your eyes may be one of them. Firstly, this test is known as a retinoscopy examination, which is a preliminary way to assess the refractive error of your eye. Whether you're near or farsighted, or you have astigmatism, examining the reflection of light off your retina is one way your eye doctor can determine if you need eyeglasses.
How well your eyes are able to focus under the circumstance we create during the exam is really what we're looking for. We shine light into your eye because we are looking for what we call your red reflex. The retinoscope sends a beam of light into your eye, and a red or orange light reflects through your pupil and off your retina. The angle at which the light reflects off your retina, which is what eye care professionals call your focal length, is the thing that lets us know how well your eye can focus. If it becomes obvious that you can't focus well, that's when we use a set of lenses. We hold different lenses with varying prescriptions in front of your eye to see which one will correct the refractive error. That lens power is the prescription you will need to correct your vision with glasses or contact lenses.
The eye doctor will run your exam in a darkened room. The patient will usually be asked to look at something behind the doctor. Unlike eye examinations you may have had, your doctor won't ask you to read any charts. This means that a retinoscopy exam is also a very good way to accurately determine the prescriptions of those who may struggle with speech, like young children and the elderly.