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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: = 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)
A dioptre (British spelling) or diopter (American spelling), symbol dpt, is a unit of measurement with dimension of reciprocal length, equivalent to one reciprocal metre, 1 dpt = 1 m −1. It is normally used to express the optical power of a lens or curved mirror , which is a physical quantity equal to the reciprocal of the focal length ...
The prism value is measured in prism diopters, and Base refers to the direction of displacement. PL is an abbreviation for plano or plain, meaning no prescription PD or IPD Pupillary Distance or Interpupillary Distance, respectively.
The Four Prism Dioptre Reflex Test (also known as the 4 PRT, or 4 Prism Dioptre Base-out Test) is an objective, non-dissociative test used to prove the alignment of both eyes (i.e. the presence of binocular single vision) by assessing motor fusion.
Vertex distance is important when converting between contact lens and glasses prescriptions and becomes significant if the glasses prescription is beyond ±4.00 diopters (often abbreviated D).
t = center thickness (in metres); n = refractive index; P = front base curve (in 1/metres); h = vertex distance (in metres); F = back vertex power (in 1/metres), (essentially, the prescription for the lens, quoted in diopters ). If the difference between the eyes is up to 3 diopters, iseikonic lenses can compensate.