Prism correction
Encyclopedia
Eye care professionals use prism correction as a component of some eyeglass prescription
s. A lens
with prism correction displaces the image, which is used to treat muscular imbalance or other conditions (see vergence dysfunction) that cause errors in eye orientation. Prism correction is measured in prism dioptres. A prescription that specifies prism correction will also specify the "base", which is the direction of displacement.
. The prism dioptre of a lens is equal to one hundred times the tangent of the angle by which it displaces an image seen through the lens. Prism dioptre is represented by the Greek symbol delta (Δ).
A prism of power 1Δ would produce 1 unit of displacement for an object held 100 units from the prism. Thus a prism of 1Δ would produce 1 cm visible displacement at 100 cm, 2Δ would produce 2 cm displacement at 100 cm, and so on.
where
is the amount of prism correction in prism dioptres, and 
is the angle of deviation of the light.
For a prism
with apex angle
and refractive index
,
.
where:
The practical use of Prentice's rule is that under certain circumstances, the prescribed prism can be obtained without grinding prism into the lenses, by decentering the lenses as worn by the patient.
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...
s. A lens
Corrective lens
A corrective lens is a lens worn in front of the eye, mainly used to treat myopia, hyperopia, astigmatism, and presbyopia. Glasses or "spectacles" are worn on the face a short distance in front of the eye. Contact lenses are worn directly on the surface of the eye...
with prism correction displaces the image, which is used to treat muscular imbalance or other conditions (see vergence dysfunction) that cause errors in eye orientation. Prism correction is measured in prism dioptres. A prescription that specifies prism correction will also specify the "base", which is the direction of displacement.
Prism dioptres
Prism correction is commonly specified in prism dioptres, a unit of angular measurement that is loosely related to the dioptreDioptre
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...
. The prism dioptre of a lens is equal to one hundred times the tangent of the angle by which it displaces an image seen through the lens. Prism dioptre is represented by the Greek symbol delta (Δ).
A prism of power 1Δ would produce 1 unit of displacement for an object held 100 units from the prism. Thus a prism of 1Δ would produce 1 cm visible displacement at 100 cm, 2Δ would produce 2 cm displacement at 100 cm, and so on.
where
is the amount of prism correction in prism dioptres, and is the angle of deviation of the light.For a prism
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...
with apex angle
and refractive indexRefractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
,.Prentice's rule
Prentice's rule is a formula used when prescribing prism correction:where:
- P is the amount of prism correction (in prism dioptres)
- c is decentration (in centimetres)
- f is lens power (in dioptreDioptreA 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...
s)
The practical use of Prentice's rule is that under certain circumstances, the prescribed prism can be obtained without grinding prism into the lenses, by decentering the lenses as worn by the patient.