Prism correction

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

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).

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.

$P = 100 \tan d\!$

where P is the amount of prism correction in prism dioptres, and d is the angle of deviation of the light.

For a prism with apex angle a and refractive index n,

d = (n − 1)a.

Prentice's rule

Prentice's rule is a formula used when prescribing prism correction:

P = cf

where:

P is the amount of prism correction (in prism dioptres)

c is decentration (in centimetres)

f is lens power (in dioptres)

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.