The preliminary test that we will discuss this time is the autorefractor. During this test, you are instructed to look at a target, a red barn or a balloon. The instrument bounces light through the refractive system causing the image to move in and out of focus. This process provides an estimate of your glasses prescription and measures the curvature of the cornea.
The refractive system includes the cornea at the front of the eye and the intraocular lens. When glasses are not needed, light moves through the refractive system and focuses on the retina at the back of the eye.
If the refractive system is too strong, then light is focused in front of the retina which is referred to as nearsightedness. If the refractive system is too weak, light is focused behind the retina resulting in farsightedness.
The numbers provided by the autorefractor are used as a starting point in the exam room for the subjective refraction. This is the part of the examination where you as the patient make the decision on which image looks better between 1 and 2.
There are a small handful of conditions that can result in progressive changes in the cornea that disrupts the refractive system reducing visual acuity with or without glasses.
Corneal astigmatism occurs when there is a change in curvature values over the cornea. As mentioned previously, there is more to the annual exam then getting new glasses. Although this instrument provides that starting point for getting new glasses, it also monitors the health of the cornea on the front of the eyes.