Keratometry is a technique used to measure the radius of curvature of the anterior corneal surface using an instrument called a keratometer. The keratometer utilizes the reflective properties of the cornea to measure the size of an image formed by reflection of an object of known size and position, allowing the radius of curvature to be calculated. Commonly, keratometers either use a fixed doubling system with variable mires or a variable doubling system with fixed mires. Measurements from keratometry are used to fit contact lenses and monitor corneal changes from contact lens wear.

1. Keratometry
Presented by,
Loknath Goswami
Long- im Kro
B.Sc Optometry 2nd year

2. Introduction
 Keratometry (Gk. Kerato= cornea, metry =
measurement of)
 It is also known as ophthalmometers
 Keratometers are instruments used to measure the
radius of curvature of the anterior corneal surface
 This measurement is utilize to fit contact lenses and
to monitor corneal changes produced through the
wear of contact lenses

3. A Schematic Diagram

4. Principle
• Keratometers utilize the reflective properties of the
cornea in order to measure its radius of curvature
• By measuring the size of an image, formed by
reflection from the cornea, of an object of known
size and position , a measurement of the radius can
be calculated

5. Principle

6. • The theory of keratometry is depicted in the
figure, where it can be seen that the
magnification of the image is equal to h’/h
where h’ equals the image the size of the
image and h the size of the object. By similar
triangles, it can be seen that
h’/h = f’/x = -r/2x
• From the equation we find that radius of
curvature of the cornea is
• r = 2mx
– Where m = magnification of the image

7. • In theory, the size of the mire image could be
measured by simply placing a measuring
graticule within the microscope.
• However a problem arises in keratometry due
to the continual movement of the patient’s eye.
• Everytime the eye moves, the mire image
moves, which makes it exceedingly difficult to
measure with any degree of accuracy

8. Figure of doubling prism

9. One and two position Keratometers
• Keratometers that require rotation through 90˚ in
order to measure the second principal meridian are
known as two position keratometers
• Keratometers that do not require rotation in order to
measure the second principal meridian are known as
one position keratometers

10. Area of cornea used during
keratometry
• It can be seen from the figure that the light reflected
from the cornea comes not from its centre, but from
two small areas on either side of the instrument axis.

11. • The size of this area is dependent upon the effective
aperture of the keratometer’s objective.
• The principles upon which keratometry is based
assume that a spherical surface exists between these
two areas
• This need not be the case. It is, in fact, well known
that the normal cornea is not spherical, but flattens
off towards its periphery. Because of this and
because different keratometers reflect their mires
from different regions of the cornea, two readings of
the same cornea with two different keratometers
may not give the same radius.

12. COMMONLY FOUND MIRES

13. Eyepiece focusing errors and their
elimination
• The first is due to the mires being positioned closer
to the patient. When this occurs, the size of the mire
image formed by the corneal mirror increases
– Can be eliminated by optically imaging the mires at infinity

14. Eyepiece focusing errors and their
elimination
• The second is produced because the degree of
doubling in an instruments which uses a prism
placed on the eyepiece side of the objective, alters
with the distance from the mire image to the
objective lens. As the mire image is brought closer to
the objective lens, the magnification of the mire
image increases and the original coincidence setting
of the keratometer is upset
– Can be eliminated by using two parallel glass plates in
front of the objective as a variable doubling device, rather
than a moveable prism behind the objective

15. Calibration
• Set the eyepiece
• Mount the test sphere
• Measure the test sphere

16. Types (according to the operation)
• Manual keratometer
 Bausch and Lomb keratometer
 Javal-Schiotz keratometer
 Zeiss (Oberkochen) ophthalmometer
• Automated keratometer
 IOL Master
 Pentacam
 Orbscan
 Corneal Topography
 The Humphrey Auto Keratometer

17. PentacamIOL Master
Orbscan

18. Corneal Topography The Humphrey Auto
Keratometer

19. Types of optical system incorporated in
keratometers
Most of the currently produced
keratometers fall into one of the following three
catogories:
1. Fixed doubling, variable mires
2. Variable doubling, fixed mires
3. Telecentric

20. Javal-Schiotz type keratometer
• It is fixed doubling, variable mire, two position
keratometer

21. Ziess (Oberkochen) keratometer
• It is a variable doubling, two position
keratometer

22. Bausch and Lomb keratometer
• It is one position, variable doubling
keratometer
• Two independently adjustable prisms, situated
behind a special aperture stop, double the
mire image along two mutually perpendicular
meridians

23. Bausch and Lomb keratometer

24. Optical system of Bausch and Lomb
keratometer

25. Optics of Bausch and Lomb
keratometer
• When the instrument is correctly aligned, the
operator sees three images of the instrument’s mires
• The first is produced by light passing through
aperture C and the vertically displacing prism
• The second is produced by light passing through
aperture D and the horizontally displacing prism, and
the third by light passing through aperture A and B
• Back and forth movement of the vertically doubling
prism results in movement of the vertically displaced
image, while movement of the horizontally doubling
prism results in movement of the horizontally
displaced image

26. • The central image formed by the light passing
through A and B is unaffected by movement of either
prism
• The aperture A and B act like a Schiener disc and
double the central image of the mire when the
intermediate image, produced by the objective lens
does not coincide with the focal point of the
eyepiece lens
• This system is designed to assist the operator in
judging when the microscope is out of focus

27. Mires in Bausch and Lomb
keratometer

28. • The mire of the Bausch and Lomb keratometer is
shown in figure. The images of the mire as seen
through the doubling system of the keratometer are
shown in the figure for the conditions where
1. The vertical doubling is correct and the horizontal
doubling is insufficient
2. The vertical doubling is too great and the horizontal
doubling is correct
3. The vertical and horizontal degrees of doubling are
correct
4. The mires are viewed after reflection by an astigmatic
cornea, the axes of which do not coincide with that of the
keratometer

29. Reference
• David B. Henson, Optometric Instrumentation,
Page 91-114
• https://www.ophthalmictechnician.org
• https://en.m.wikipedia.org/wiki/keratometry