A phoropter (or phoroptor) is an instrument commonly used by optometrists and ophthalmologists during an eye examination to measure an individual's refractive error and determine his or her eyeglass prescription. Typically, the patient sits behind the phoropter, and looks through it at an eye chart placed at optical infinity (20 feet or 6 metres), then at near (16 inches or 40 centimetres) for individuals needing reading glasses. The optometrist then changes lenses and other settings, while asking the patient for subjective feedback on which settings gave the best vision. Sometimes a retinoscope or an automated refractor is used to provide initial settings for the phoropter.
The major components of the phoropter are the JCC (Jackson Cross-Cylinder) used for astigmatism correction, Risley prisms to measure phorias and vergences, and the (+), (-), and cylinder lenses.
From the measurements taken, the specialist will write an eyeglass prescription that contains at least 6 numerical specifications (3 for each eye): sphere, cylinder, and axis.
The lenses within a phoropter refract light in order to focus images on the patient's retina. The optical power of these lenses is measured in 0.25 diopter increments. By changing these lenses, the examiner is able to determine the spherical power, cylindrical power, and cylindrical axis necessary to correct a person's refractive error. The presence of cylindrical power indicates the presence of astigmatism which has an axis measured from 0 to 180 degrees away from being aligned horizontally.
Phoropters are made with either plus or minus cylinders. Traditionally, ophthalmologists use plus cylinder phoropters and optometrists use minus cylinder phoropters. One can mathematically convert figures obtained from either type of phoropter to the other.