Retinoscope

A Retinoscope is a diagnostic instrument used by optometrists and ophthalmologists to measure the refractive error of the eye — that is, to determine whether a person is nearsighted (myopic), farsighted (hyperopic), or has astigmatism.
It works by projecting a beam of light into the patient’s eye and observing the movement of the reflected light (reflex) from the retina.