keratometry

Keratometry is the measurement of the curvature of the anterior surface of the cornea, expressed as diopters of corneal power or as a radius of curvature in millimeters. It provides information about the steepness or flatness of the cornea and is central to assessing corneal astigmatism and estimating refractive power for optical planning.

Most keratometry is performed with a keratometer, an instrument that projects mires (reflections) onto the cornea and analyzes their size and shape as the eye fixates. The readings yield the keratometric powers of the flat and steep meridians, commonly reported as K1 (flat) and K2 (steep). A mean keratometry value is often given as the average of these two meridians. The central cornea is typically modeled as a circle of about 3 millimeters in diameter, and standard calculations assume a fixed index of refraction to convert corneal radius to diopters, using the approximate relationship D = 337.5 / r (where r is the radius in millimeters).

Applications include planning intraocular lens (IOL) power in cataract and refractive surgery, guiding contact lens fitting, and screening for corneal conditions such as keratoconus. Keratometry is also used in conjunction with more comprehensive corneal topography to assess irregular astigmatism and to monitor postoperative changes.

Limitations include its focus on the central, anterior cornea and assumption of a fixed refractive index; it does not measure posterior corneal curvature and can be affected by tear film quality and irregular corneal astigmatism. Modern devices may combine keratometry with broader corneal topography for a fuller surface map.