Applied optical principles: keratometry.

The optical correction of the aphakic eye requires the addition of a significant amount of vergence power to the corneal interface by one of three modalities: spectacle, contact lens, or intraocular lens. No matter which modality is chosen, based upon Gullstrand's parameters, the corneal interface provides approximately 74% of the effective vergence power of the eye; therefore, it is essential to understand the principles underlying the clinical assessment of the refracting power of the cornea. This presentation briefly reviews selected principles underlying the determination of corneal curvature and toricity by keratometry, as well as their relationship to the correcting spectacle lenses, and emphasizes the necessity of surveying the topography of the cornea for the greatest accuracy in these measurements.