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Modern eye tests rely less on an optician asking a patient what looks right and more on a machine that measures the required refractive correction and then tells the optician what prescription to try.How does this machine measure what lens strength is required?Chris
Retinoscopy is a technique to obtain an objective measurement of the refractive condition of a patient's eyes. The examiner uses a retinoscope to shine light into the patient's eye and observes the reflection (reflex) off the patient's retina. While moving the streak or spot of light across the pupil the examiner observes the relative movement of the reflex then uses a phoropter or manually places lenses over the eye to "neutralize" the reflex. Retinoscopy is especially useful in prescribing corrective lenses for patients who are unable to undergo a subjective refraction that requires a judgement and response from the patient (such as mentally handicapped or non-verbal people).Retinoscope works on a principle called Foucault's principle. Basically it indicates that the examiner should simulate the infinity to obtain the correct refractive power. Hence a power corresponding to the working distance is subtracted from the gross retinoscope value.