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 the 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 anterior corneal power using air and corneal stromal refractive indices is higher than clinically useful because it does not take into account the negative contribution of the posterior cornea. Thus, 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. 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. The refractive index of air is 1.000; aqueous and tears, 1.336; and corneal stroma, 1.376. Although the air–tear interface of the cornea is responsible for most of the eye’s refraction, the difference between total corneal power based on stroma alone and with tears is only –0.06 D.
BCSC Section 3, Clinical Optics, covers these topics in greater depth.