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KERATOMETER
By
Dr . Alshymaa Moustafa
Ophthalmology Specialist
Definition
■ 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.
MANUAL
KERATOMETER
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
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.
■ 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.
■ 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).
■ The Javal–Schiøtz style keratometer employs a fixed-image doubling device, and the mire
separation is variable.
■ The Bausch + Lomb style of keratometer employs a variable image doubling device,
and the mire dimensions are constant.
AUTOMATED KERATOMETER
■ Automatic keratometers use principles similar to those used in automated
lensmeters and refractors, measuring the amount of deflection of the reflected light.
THANKS

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Keratometer

  • 1. KERATOMETER By Dr . Alshymaa Moustafa Ophthalmology Specialist
  • 2. Definition ■ 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 another.
  • 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.
  • 8.
  • 9. ■ The Bausch + Lomb style of keratometer employs a variable image doubling device, and the mire dimensions are constant.
  • 10.
  • 11.
  • 13. ■ Automatic keratometers use principles similar to those used in automated lensmeters and refractors, measuring the amount of deflection of the reflected light.