Shape, Curvature, and Power
Three topographic properties of the cornea are important to its optical function: the underlying shape, which determines its curvature and, hence, its refractive power. Shape and curvature are geometric properties of the cornea, whereas power is a functional property. Historically, power was the first parameter of the cornea to be described, and a unit representing the refractive power of the central cornea, the diopter, was accepted as its basic unit of measurement. However, with the advent of contact lenses and refractive surgery, knowing the overall shape and the related property of curvature has become increasingly important.
The refractive power of the cornea is determined by Snell’s law, the law of refraction. Snell’s law is based on the difference between 2 refractive indices (in this case, of the cornea and of air), divided by the radius of curvature. The refractive index of air is 1.000; aqueous and tears, 1.336; and corneal stroma, 1.376. Although the air–tear film interface constitutes the primary refractive element of the eye, the difference between total corneal power calculated based on stroma alone and corneal power calculated with both stroma and tears is only –0.060, a clinically insignificant difference.
Calculating the anterior corneal power using the refractive indices overestimates the true power because it does not take into account the negative contribution of the posterior cornea. Average refractive power of the central cornea is about +43 D, which is the sum of the refractive power at the air–stroma interface of +49 D minus the endothelium–aqueous power of 6 D. For most clinical purposes, a derived corneal refractive index of 1.3375 is used in calculating central corneal power. This value was chosen to allow +45 D to equate to a 7.5-mm radius of curvature. BCSC Section 3, Clinical Optics, covers these topics in greater depth.
Dawson DG, Ubels JL, Edelhauser HF. Cornea and sclera. In: Alm A, Kaufman PL, eds. Adler’s Physiology of the Eye. 11th ed. New York: Elsevier; 2011:71–130.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.