The majority of<unk> s 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<unk>  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<unk>  calculates the refraction of the eye, sphere, cylinder and axis. Modern<unk> s are based on the idea patented by Antonio Medina Puerta.
Autorefractor
Autoinjector
Lensmeter
Thalamic stimulator
Pupilometer
Focometer
Oculocardiac reflex
Retrobulbar block