YAMOUT OPTICAL CENTER

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Keratometry

Keratometer Mechanical Adjustments and Parts

A. Focusing Knob
B. Rotating Grip for Locating the Axis
C. Chin Rest
D. Locking Knob
E. Elevating Knob
EP. Eyepiece
G. Head Rest Adjusting Knob
H. Occluding Shield
HR. Head Rest
J. Chin Rest Adjusting Knob
K. Leveling Sight
M. Horizontal Measuring Drum
N. Vertical Measuring Drum
S. Axis Scale

The keratometer is an instrument that is used to measure the front surface (approx 2 - 4 mm radius) curvature of the cornea. Keratometer readings provide the following information to assist in the overall assessment of the cornea:

· The radii of curvature of the cornea.  

· The directions of the principal meridians of the eye, determining whether the astigmatism is with-the-rule or against-the-rule -- a key factor in contact lens fitting.  

· The degree of corneal astigmatism.  

· The presence of any corneal distortion.

The keratometer usually shows the corneal curvature in diopters of power or in millimeters and diopters.  If a millimeter scale is not given, it can be found from tables or by using the surface power formula using the refractive index 1.3375.
                                       n - 1
Surface power formula: D = .........
                                                       R

D = the dioptric power of the cornea
n = the refractive index of the keratometer used (1.3375)
R = the radius of curvature of the cornea in meters  

If the drum scale only gives diopters, you should be able to calculate the equivalent millimetres radius of curvature

Example Application of Above Formula

What is the radius of curvature of the cornea if the keratometer drum reads 43.50 D ?

D = 43.50
n = 1.3375
R = ?
              n - 1      1.3375 - 1
    R =  .........  =  ................  = 0.007758m
               D            43.50

 Radius of curvature (R) is 7.76 mm  

There are various models of keratometers available, each having its own range and calibration based on a specific refractive index--review manufacturer's manual for specific calibration methods and refractive index numbers.