The refractometer (10) includes a reference projector (14) for projecting a reference pattern on a reference pattern position (42) on a detector during a reference interval, a site-selector (22) for selecting a measurement site (40) on an optical system, and a measurement projector (32) for projecting a measurement pattern through this selected measurement site and onto a measurement pattern position (44) on the detector during a measurement interval. The reference projector, the site selector, and the measurement projector are all aligned along the same optical axis. In the case in which the optical system is a human eye (45), the designated site in typically a site on the cornea and the detector is the retina (43) of the eye. The refractometer provides an aligner for aligning the reference pattern position with a measurement pattern position. The amount by which these two positions are moved in order to achieve alignment provides a measure of the optimal wavefront corresponding to the measurement site.
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