A phoroptor in use

A phoroptor (sometimes misspelled phoropter) is an instrument commonly used by eye care professionals during an eye examination, containing 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.^[1] 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.

Phoroptors can also measure phorias (natural resting position of the eyes), accommodative amplitudes, accommodative leads/lags, accommodative posture, horizontal and vertical vergences, and more.

The major components of the phoroptor 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 and possibly pupillary distance.

A simple trial frame is used in conjunction with a retinoscope to determine refractive errors. Lens of varying focal lengths may be placed in the trial frame. Trial frames, such as in this picture, are not to be mistaken for phoroptors

The lenses within a phoroptor 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.

Phoroptors are made with either plus or minus cylinders. Traditionally, Ophthalmologists and Orthoptists use plus cylinder phoroptors and Optometrists use minus cylinder phoroptors. One can mathematically convert figures obtained from either type of phoroptor to the other.

The phoroptors also include prismatic lenses which are used to analyse binocular vision and treat orthoptic problems.

[edit] References

[edit] See also

• Ophthalmic lens
• Ophthalmologist
• Optometrist
• Orthoptist