■ Optical instrument which is used to approximate the refracting power of the cornea
by determining the curvature of the central outer corneal surface.
■ Conventional keratometry is performed at only 1 diameter, approximately 3 mm, and
is therefore lacking the detail provided by more elaborate topography.
■ Keratometer may be manual or automated.
4. Optical principal
■ 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 prob lems and inaccuracies
from involuntary eye motion.
■ There are 2 basic methods by which the doubled mire images are aligned with one
5. ■ For example, in the Javal- Schiøtz- style keratometer (Haag- Streit USA, Mason, OH),
the mire separation is adjusted while the image doubling is constant.
■ In the Bausch + Lomb (Bridgewater, NJ) style of keratometer, on the other hand, the
mire location is fixed and the image doubling is variable.
■ 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.
6. ■ 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).
7. ■ The Javal–Schiøtz style keratometer employs a fixed-image doubling device, and the mire
separation is variable.
9. ■ The Bausch + Lomb style of keratometer employs a variable image doubling device,
and the mire dimensions are constant.