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A phoroptor is an instrument used by eye care professionals to measure a patient's refractive error in order to determine an eyeglass prescription. Phoroptors can also be used to make other measurements, such as the phoria, or natural resting position of the eye, and the amplitude of accommodation (AA), the eye's ability to focus at near distances. An eye care professional uses a phoroptor to measure a patient's refractive error by letting the patient view an eye chart through the phoroptor, adjusting the phroptor's settings, and asking the patient which settings provide the best vision.
Refractive error is an error made by the eyes in focusing light. A phoroptor is equipped with a series of lenses that refract light and focus images on the patient's retina. As the lenses are adjusted, the degree of correction necessary to address a patient's refractive error can be determined. The optical power of the lenses in the phoroptor, or the degree to which they converge light, is measured in very small increments, 0.25 diopters, in order to determine an accurate eyeglass prescription.
There are three measurements per eye that go into an eyeglass prescription: spherical power, cylindrical power, and cylindrical axis. All three can be measured by a phoroptor. Each eye is different, even in a single patient, so separate measurements must be made for each eye. Therefore, a typical eyeglass prescription consists of six different measurements.
Spherical power refers to the ability of the eye to focus light on the retina. If the spherical power is too high for the length of the eyeball, the patient will be myopic or nearsighted. If the spherical power is too low for the length of the eyeball, the patient will have hyperopia or farsightedness. Cylindirical power and axis are used to measure astigmatism, blurry vision due to the inability of the eye to focus images sharply on the retina.
A phoroptor can also be used to analyze binocular vision, or the way in which the two eyes work together to focus images. Phoroptors are usually fitted with prisms in order to observes and analyze phorias and binocular vision. A phoroptor can measure vertical and horizontal vergences, the simultaneous movement of the two eyes in opposite directions -- towards each other or away from each other -- in order to focus on closer or further objects. Therefore, phoroptors can also be used to treat orthoptic problems, or defects in eye movement.