A precision optical instrument used to measure the refractive power of lenses or to determine the optical correction needed for vision.
From Greek 'diopter' (optical power) + '-meter' (measuring instrument), literally 'a device that measures optical power.'
Before digital eye exams, optometrists used dioptometers as their primary tool to measure exactly how much lens power you needed—it's a very precise instrument that directly measures light refraction.
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