Prism correction

Eye care professionals use prism correction as a component of some eyeglass prescriptions.

Sometimes an image that one eye sees is not in line with what the one other eye sees, and so creates an impression of two images instead of only one. A lens which includes some amount of prism correction, will displace the viewed image, as seen by that eye, toward a direction so as to bring that image into proper binocular sync with the image that is viewed by the other eye. This prism correction is used to treat muscular imbalance or other conditions (see vergence dysfunction) that cause errors in eye orientation, such as double-vision.

Prism correction is measured in prism dioptres. A prescription that specifies prism correction will also specify the "base", which is the direction of displacement. Whether a patient needs this type of correction or not is determined by a variety of methods, which can include the use of the Polatest (a test for the evaluation of binocular vision) elaborated by Hans-Joachim Haase.[1][2][3]

Prism dioptres

Prism correction is commonly specified in prism dioptres, a unit of angular measurement that is loosely related to the dioptre. 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). The original unit symbol was pdptr (1 pdptr = 1 cm/m).[4][5]

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.

${\displaystyle P=100\,\tan d\!}$

where ${\displaystyle P}$ is the amount of prism correction in prism dioptres, and ${\displaystyle d}$ is the angle of deviation of the light.

For a prism with apex angle ${\displaystyle a}$ and refractive index ${\displaystyle n}$,

${\displaystyle d=(n-1)\,a}$.

Prentice's rule

Prentice's rule, named so after the optician Charles F. Prentice, is a formula used to determine the amount of induced prism in a lens:[6]

${\displaystyle P=cf/10}$

where:

P is the amount of prism correction (in prism dioptres)
c is decentration (the distance between the pupil centre and the lens's optical centre, in millimetres)
f is lens power (in dioptres)

The primary 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.

An additional use of the rule is for determining the amount of unprescribed prism that is introduced if the lens is not correctly centred on the wearer's pupil. This can be used for tolerance control of lenses, for example when glasses must be made with lenses that are too small, so that the optical centre of one or both lenses must be displaced from the pupil position.