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). It is also used to evaluate accommodative ability of the eye and detect latent hyperopia.
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.
Static retinoscopy is performed when the patient has relaxed accommodative status viewing a distance target; dynamic retinoscopy is performed when the patient has active accommodation from viewing a near target.