prism diopter
The prism diopter is the unit of prism power, written with the symbol Δ. A prism of 1Δ deviates a light ray 1 cm at a distance of 1 m, and a prism that deviates a ray 5 cm at 1 m has a power of 5Δ.1 In general, prism power equals 100 times the deviation divided by the distance at which the deviation is measured.1
The prism diopter measures a change in the direction of light, while the diopter of lens power measures a change in vergence. A spherical lens has both kinds of power, because it changes the vergence of incident light and also changes the direction of the incident rays.1
In clinical use the prism diopter also specifies vergences and ocular deviations, because prisms are used to measure them. The displacement is then referred to the center of rotation of the eye.2 As a tangent function, the prism diopter is not linear with angle and grows larger as the deviation increases. For small deviations 1° approximates 1.7Δ, and 1Δ is equivalent to about 0.57°.3,2
More detail
Prisms deflect light toward the base, and the patient sees the light displaced toward the apex. Plastic prisms are held with the rear surface in the frontal plane, close to the position of minimal deviation, and glass prisms are calibrated for the Prentice position, with the back surface perpendicular to the visual axis.3 Prisms cannot be stacked base to base, because the combined strength is much greater than the sum of the individual strengths. Prisms can be stacked with their bases 90° apart, and large deviations are best neutralized by dividing the prism between the two eyes.3
Sources (9)
- Schwartz, Geometrical and Visual Optics, ch. 10
- von Noorden and Campos, Binocular Vision and Ocular Motility, ch. 5
- Yanoff and Duker, Ophthalmology (2023), ch. 11.3
- American Academy of Ophthalmology. 2020. Prisms. Basic and Clinical Science Course, Section 3.
- Grosvenor, T. 2007. Primary Care Optometry. 5th ed. St. Louis: Butterworth-Heinemann Elsevier.
- Irsch, K. 2015. Optical issues in measuring strabismus. Middle East African Journal of Ophthalmology, 22(3), pp.265-270.
- Noorden, G.K. and Campos, E.C. 2002. Binocular Vision and Ocular Motility: Theory and Management of Strabismus. 6th ed. St. Louis: Mosby.
- Rutstein, R. and Daum, K. 1997. Anomalies of Binocular Vision: Diagnosis & Management. St. Louis: Mosby.
- Scheiman, M. and Wick, B. 2014. Clinical Management of Binocular Vision: Heterophoric, Accommodative, and Eye Movement Disorders. 4th ed. Philadelphia: Lippincott Williams & Wilkins.
Reviewed October 2026