Autorefractor

thumb|A United States Navy optometrist technician using an autorefractor during a humanitarian assistance project in [[Nicaragua in 2008]] An autorefractor or automated refractor is a computer-controlled machine used during an eye examination to provide an objective measurement of a person's refractive error and prescription for glasses or contact lenses. This is achieved by measuring how light is changed as it enters a person's eye.

==Technique== thumb|Shin Nippon Nvision K-5001 Refkeratometer thumb|The optics inside a Shin Nippon Nvision K-5001 Refkeratometer The majority of autorefractors calculate the vision correction a patient needs (refraction) by using sensors that detect the reflections from a cone of infrared light. These reflections are used to determine the size and shape of a ring in the retina which is located in the posterior part of the eye. By measuring this zone, the autorefractor can determine when a patient's eye properly focuses an image. The instrument changes its magnification until the image comes into focus. The process is repeated in at least three meridians of the eye and the autorefractor calculates the refraction of the eye, sphere, cylinder and axis. Modern autorefractors are based on the idea patented by Antonio Medina Puerta.