Moving the incident beam along a certain pupillary meridian (indicated by arrows in the figure) will result in a reflected beam that goes in the same or opposite direction as the incident beam. However, in a hyperopic eye the reflection is directed to the opposite side of the pupil. Because the retina can be considered a spherically concave mirror (reflecting about 4% of incident light), the beam is reflected back in more or less the original direction in a myopic eye. It uses focal shift in a different way, starting with the observation that the retinal image of a light beam coming from a superior direction is located below the optical axis in a myopic eye and above the optical axis in a hyperopic eye. The automatic retinoscope is an automated version of the handheld retinoscope and is implemented in an objective, serial, and double-pass fashion. The uncomplicated nature of the technique makes it robust for extreme aberrations. The x-y scanner, comprising 2 separate scanners for the x- and y-directions, repeatedly moves the beam to a new entry position until homogeneously spread measurements are available for the whole pupil area. Using a beam splitter and lens, the retinal image is captured on a linear array of photodetectors and is made available for further processing. Once in the eye, local aberrations in the beam's entry position cause a focal shift of the retinal image with respect to a certain reference position. Ray tracing uses a narrow laser beam that is directed into the eye parallel to the eye's line of sight by means of an x-y scanner. The ray tracing principle is a serial, double-pass method using forward projection that can be implemented in both an objective and subjective manner.
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