keratometer is an instrument that is
used to measure the front surface (approx
2 - 4 mm radius) curvature of the cornea.
Information that provides are:
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-therule -- a key factor in contact lens fitting.
degree of corneal astigmatism.
The presence of any corneal distortion.
Keratometry is essential in the fitting of
contact lenses, evaluating and following
patients with keratoconus, and in the
determination of appropriate intraocular
lens implant power for patients undergoing
is a convex refracting surface
In order to find the refracting power of the
cornea, we need to reflect an object of a
known size at a known distance off the
corneal surface. Then determine the size
of the reflecting image with measuring
telescope and calculate the refractive
power of the cornea based on the
refractive index of n= 1.3375
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.
Surface power formula: D = .........
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
Primary Care Optometry-Grosvenor page 224
Adjustments and Parts
A. Focusing Knob
B. Rotating Grip for Locating
C. Chin Rest
D. Locking Knob
E. Elevating Knob
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
Bausch & Lomb Keratometer
position keratometer: it measures two
meridians at the same time
contains two prisms
Eyepiece Adjustment : turn the
eyepiece (EP) counterclockwise fully.
Place a white sheet of paper in front of the
telescope part. Turn the eye piece
clockwise until the specified target in the
instrument becomes in sharp focus.
Adjust the height of the instrument and
align the outer canthus of the patient with
the leveling sight sign (item K in Figure 2)
the non-tested eye (with H).
the patient to look at the center of
the instrument. If you are aligned, you can
see the circles in the eye. The patient can
see the picture of their eye in the
the focusing knob( item A in Fig
2) such that you get a single image of the
central circle.(A,B in Fig 3)
Align the side and upper circles with the
central circle by rotating the body of the
instrument(using item B in Fig 2) .(C in
7) Use the horizontal alignment knob(M) to bring
the crosses on each other.(E in Fig 3)
8) Use the vertical alignment knob(N) to bring
the crosses on each other.
9) Read the values from the scales (M & N) and
the angle from (S).
measures one meridians at one time.
Follow steps 1-4 of the above instrument
5) Adjust the instrument to see a clear image of
the rectangle and staircase targets
Rotate the instrument to align the line in
rectangle with the line in the staircase
Turn the measuring knob until staircase
and rectangle just overlap.
Read the values from the scales.
K flattest meridian@angle
-3.00x180 Mires Clear & Regular
Also determine if the mires are clear and regular.