What is the definition or description of: refraction?

Glasses prescription: Refraction is the determination of a glasses prescription. Usually a phoropter is used, which the patient looks through at an illuminated screen. The examiner tries different lenses to determine the combination that gives the sharpest vision for each eye. Refraction can also be done using an autorefractor, but this is typically used as a starting point to be refined with the manual refraction.