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A phoropter is a common name for an ophthalmic testing device similar to the refractor. It is commonly used by eye care professionals during an eye examination, and contains different lenses used for refraction of the eye during sight testing, 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 eye care professional 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 battery of spherical and cylindrical lenses, and 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 and possibly pupillary distance.
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 and orthoptists use plus cylinder phoropters and optometrists use minus cylinder phoropters. One can mathematically convert figures obtained from either type of phoropter to the other.
Trademark and origin of the term
Phoroptor is a registered trademark currently owned by Reichert Technologies, filed Apr 25, 1921 by DeZeng Standard of New Jersey, with the USPTO, serial number 71146698. The word was coined at that time for the newest version of their optometer. DeZeng was purchased in 1925 by American Optical of Massachusetts, who continued to market the product, but the term, often spelled phoropter, has become a genericised trademark for all brands of modern vision testers, especially since AO's main competitor, Bausch and Lomb, stopped making their Greens' Refractor in the 1970's. Reichert bought AO's refracting equipment division in the 1980's, and their current version is named "Ultramatic RX Master Phoroptor".
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- Dictionary.com, Definition of "phoropter", American Heritage Stedman's Medical Dictionary. Retrieved 10-10-10.