The keratometer is used to approximate the refracting power of the cornea by determining the curvature of the central outer corneal surface. It does this by measuring the image size of a reflected mire in each of the principal meridians, accomplished by lining up prism-doubled images at a distance regulated by sharpness of focus. Note that doubling of the image is performed to avoid problems and inaccuracies from involuntary eye motion. There are 2 basic methods by which the doubled mire images are aligned with one another. For example, in the Javal-Schiøtz-style keratometer (Haag-Streit USA, Mason, OH), the mire separation is adjusted while the image doubling is constant (Fig 8-4A). In the Bausch + Lomb (Bridgewater, NJ) style of keratometer, on the other hand, the mire location is fixed and the image doubling is variable (Fig 8-4B). Corneal refractive power is inferred from the calculated radius of curvature using the formula for surface power D = (n–1)/r. In practice, a correction for the small refractive effect (minus power) of the corneal back surface is incorporated in the value for the refractive index of the cornea.
Figure 8-4 Keratometer principle. The curvature of an annulus of the cornea about 3 mm in diameter is determined by measuring the image size of reflected mires in each of the principal meridians. This is accomplished by the examiner lining up the prism-doubled images. Corneal refractive power is inferred from the obtained radius of curvature using the formula for surface power D = (n – 1)/r, where D is the corneal power in diopters, n is the keratometric refractive index at 1.3375, an empirically derived “standardized” refractive index for the cornea that takes the minus power of the corneal back surface into account, and r is the radius of the corneal curvature (in meters). A, The Javal–Schiøtz style keratometer employs a fixed-image doubling device, and the mire separation is variable. B, The Bausch + Lomb style of keratometer employs a variable image doubling device, and the mire dimensions are constant.
(Reproduced from Guyton DL, et al. Ophthalmic Optics and Clinical Refraction. Baltimore: Prism Press; 1999. Illustration modified by Kristina Irsch, PhD.)
Conventional keratometry is performed at only 1 diameter, approximately 3 mm, and is therefore lacking the detail provided by more elaborate topography.
Automatic keratometers use principles similar to those used in automated lensmeters and refractors, measuring the amount of deflection of the reflected light.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.