Rx glasses define those sets of lenses that are thoroughly prescribed by an ophthalmologist to his patient under strict medical discretion. However, before the prescription of a Rx glasses an examination is conducted to audit the efficacy of vision of the examinee. It aids in the rectification of the eyesight, and when a person wears this pair of glasses, he can see more clearly than the earlier state. The parameters mentioned in the prescription by the ophthalmologist differ, but they prescribe glasses generally due to blurred vision subjected to myopia, hyperopia and presyopia. A Rx eyeglass prescription specifies the power of the lenses in diopters, abbreviated often as D. Rx glasses are normally available in the optical power range of +12D to -12D, with every transition between the powers of the glasses being 0.25D. Using a phoropter the refractive error causing the prescription to eyeglasses is conformed. Photochromic eyeglasses are generally preferred by the doctors.

Defocus aberration within an eye leads to perpetual blurring of image and a phoropter helps to diagnose the extent of refractive error, which in turn helps the optometrist to perceive the type of eyeglasses. An optician will then take the prescription written by an ophthalmologist and order and assemble the frames and lenses to be dispensed and sold to the patient.