Phoropter
Encyclopedia
Eye care professional
An eye care professional is an individual who provides a service related to the eyes or vision. It is a general term that can refer to any healthcare worker involved in eye care, from one with a small amount of post-secondary training to practitioners with a doctoral level of education.-Current...
s during an eye examination
Eye examination
An eye examination is a battery of tests performed by an ophthalmologist, optometrist, or orthoptist assessing vision and ability to focus on and discern objects, as well as other tests and examinations pertaining to the eyes....
, containing different lenses used for refraction of the eye during sight testing, to measure an individual's refractive error
Refractive error
A refractive error, or refraction error, is an error in the focusing of light by the eye and a frequent reason for reduced visual acuity.-Classification:...
and determine his or her eyeglass prescription
Eyeglass prescription
An eyeglass prescription is an order written by an eyewear prescriber, such as an optometrist or ophthalmologist, that specifies the value of all parameters the prescriber has deemed necessary to construct and/or dispense corrective lenses appropriate for a patient.If an examination indicates that...
.
Typically, the patient sits behind the phoropter, and looks through it at an eye chart
Eye chart
An eye chart is a chart used to measure visual acuity. Types of eye charts include the logMAR chart, Snellen chart, Landolt C, and the Lea test.-Procedure:Charts usually display several rows of optotypes , each row in a different size...
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
Lens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...
and other settings, while asking the patient for subjective feedback on which settings gave the best vision
Visual perception
Visual perception is the ability to interpret information and surroundings from the effects of visible light reaching the eye. The resulting perception is also known as eyesight, sight, or vision...
. Sometimes a retinoscope or an automated refractor
Automated refractor
An autorefractor or automated refractor is a computer-controlled machine used during an eye examination to provide an objective measurement of a person's refractive error and prescription for glasses or contact lenses. This is achieved by measuring how light is changed as it enters a person's...
is used to provide initial settings for the phoropter.
Phoropters can also measure phoria
Phoria
Phoria is a 5-piece band from Brighton, with influences ranging from Radiohead and Elbow, to Sigur Rós and Cinematic Orchestra.In August 2010, Phoria released the anticipated E.P. 'Yourself Still' as a pay-what-you-want download . The success of this release has accelerated the production of the...
s (natural resting position of the eyes), accommodative amplitudes, accommodative leads/lags, accommodative posture, horizontal and vertical vergence
Vergence
A vergence is the simultaneous movement of both eyes in opposite directions to obtain or maintain single binocular vision..When a creature with binocular vision looks at an object, the eyes must rotate around a vertical axis so that the projection of the image is in the centre of the retina in both...
s, and more.
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
Eyeglass prescription
An eyeglass prescription is an order written by an eyewear prescriber, such as an optometrist or ophthalmologist, that specifies the value of all parameters the prescriber has deemed necessary to construct and/or dispense corrective lenses appropriate for a patient.If an examination indicates that...
that contains at least 6 numerical specifications (3 for each eye): sphere, cylinder, and axis and possibly pupillary distance
Pupillary distance
Pupillary Distance or interpupillary distance is the distance between the centers of the pupils in each eye. This measurement is used when preparing to make prescription eyeglasses...
.
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
. The optical power
Optical power
Optical power is the degree to which a lens, mirror, or other optical system converges or diverges light. It is equal to the reciprocal of the focal length of the device. The dioptre is the most common unit of measurement of optical power...
of these lenses is measured in 0.25 diopter
Dioptre
A dioptre, or diopter, is a unit of measurement of the optical power of a lens or curved mirror, which is equal to the reciprocal of the focal length measured in metres . It is thus a unit of reciprocal length. For example, a 3-dioptre lens brings parallel rays of light to focus at metre...
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
Astigmatism (eye)
Astigmatism is an optical defect in which vision is blurred due to the inability of the optics of the eye to focus a point object into a sharp focused image on the retina. This may be due to an irregular or toric curvature of the cornea or lens. There are two types of astigmatism: regular and...
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.
The phoropters also include prismatic
Prism (optics)
In optics, a prism is a transparent optical element with flat, polished surfaces that refract light. The exact angles between the surfaces depend on the application. The traditional geometrical shape is that of a triangular prism with a triangular base and rectangular sides, and in colloquial use...
lenses which are used to analyze binocular vision and treat orthoptic problems.