An Auto Refractor is an instrument used to determine an individual’s prescription by measuring how light reflects through the eye. Auto refractors provide an excellent baseline to determine the correct eyeglass or contact lens prescription. An auto refractor works by shining light into the eye and then measuring how it changes as it bounces off the back of the eye (the ocular fundus). The image moves in and out of focus, taking several measurements of these reflections to determine when the eye is properly focused.