RAMJI PANDEY

1. KERATOMETRY
bY. RAMji PAndEY
cOnsulTAnT OPTOMETRisT
c l guPTA EYE insTiTuTE
MORAdAbAd

2. What is Keratometry?
• The technique for measuring the curvature of the anterior surface of
the cornea across fixed chord length usually 3 mm which lies within
the optical spherical zone of cornea, with the help of keratometer, is
known as keratometry.
Keratometer is also known as ophthalmometer.

3. History of Keratometer
• In 1691, Scheiner ( description of corneal curvature-compared size of
the bars in a window-lens & cornea).
• In 1796,Ramsden-Inventor of 1st
model of keratometer with three
essential elements.
• In 1854,Helmholts improved Ramsden,s design for laboratory use
• In 1881, Javal & Schiotz modified Helmholtz’s instrument for clinical
use
• In 1980, the development of autokeratometer

4. Types of keratometer
• Manual keratometer
- one position keratometer Eg. Bausch and Lomb keratometer
- two position keratometer Eg. Javal- Schiotz keratometer
• Automated keratometer

5. Parts of a B & L Keratometer

6. Bausch and Lomb Keratometer
Principle
• Keratometry is based on the fact that the anterior surface of the
cornea acts as a convex mirror & the size of the image formed
varies with its curvature
• Greater the curvature of cornea, lesser is the image size.
• Therefore, from the size of the image formed by the anterior
surface of cornea (1st
Purkinje image) , the radius of curvature of
cornea calculated as below

7. • Optical principle involved is the relationship between
the size of an object and size of the image of that object reflected
from surface.
• Radius of curvature is determined by the apparent size of the
image of bright object (mires) viewed by the reflection from
anterior corneal surface which acts as a convex mirror.
r = 2 x h1/h
where r= radius of curvature, h=height of object, h1=height of the
image
Diagnostic procedures in ophthalmology by H.V Nema , Nitin Nema

8. • The corneal radius can be transformed into dioptric power in
the following way
D = (n1-n) /r
Where n1= refractive index of cornea (1.337),n=refractive
index of medium from which light originates (air=1), r= radius
of curvature
Thus equation can be simplified into
D = 337.5/ r
Diagnostic procedures in ophthalmology by H.V Nema , Nitin Nema

9. Principles of Keratometry AB is the object and A' B' is the image. By measuring the
size of the object and image, curvature of the convex surface can be calculated

10. Optics

12. OPTICAL SYSTEM AND OTHER PARTS
1.Object: Circular mire with two plus & two
2. minus signs.
• Lamp illuminates the mire by
means of a diagonally placed
mirror.
• Light from the mire strikes the patient’s
cornea & produces a
diminished image behind it.
This image becomes the object for the remainder of
optical system.

13. 2. Objective lens:
•Focuses light from the image of the mire (new object) along the
central axis.
3. Diaphragm and doubling prisms
•4 aperture diaphragm is situated near objective lens.
•Beyond the diaphragm are two doubling prisms, one with its base
up & other with its base out.
•Prisms can be moved independently, parallel to the central axis of
instrument.

15. • Thus, image-doubling mechanism is unique in Bausch and Lomb
keratometer, in that double images are produced side by side as
well as at 900
from each other
• This allows the measurement of the power of cornea in two
meridia, without rotating the instrument.
Therefore, it is also known as ‘one-position keratometer’.
4. Eyepiece lens:
Enables examiner to observe magnified view of the doubled image.

16. video

17. Procedure
Instrument Adjustment:
• Instrument is calibrated before use.
• White paper held in front of objective lens & a black line is
focused sharply on it.
• Keratometer is then calibrated with steel balls
• of known radius of curvature. Steel ball is placed before
keratometer & its value is set on the scale or dial.
• Mires are focused by clockwise & anticlockwise movement of
eyepiece through trial & error.
Theory and Practice of Optics and Refraction by A.K Khurana

18. Calibration set

19. Patient adjustment:
• Adjust the height of the patient’s chair and the instrument to
a comfortable position for both the patient and the examiner.
• Instruct the patient to place his chin in the chin rest and his
forehead against the forehead rest and adjust for the patient.
• Eye not being examined is covered with occluder.
• Chin is raised or lowered till patient’s pupil & projective knob
are at the same level

20. Focusing of mire:
Mire is focused in the centre of cornea.
Patient’s view of mire
First view seen by the examiner.
Note that the central image is
doubled, indicating that instrument is
not correctly focused on the corneal
image of the mire

21. Instrument is correctly focused on corneal image so that central
image is no longer doubled.

22. Measurement of corneal curvature
To measure curvature in horizontal
meridian, plus signs of central &
left images are superimposed
using horizontal measuring control.
To measure curvature in vertical
meridian, minus signs of central
& upper images are coincided
with the help of vertical
measuring control.

23. • In presence of oblique astigmatism, two plus
signs will not be aligned .Entire instrument rotated till they
are aligned.

24. video

25. Extended Keratometry
• Range  36.00 to 52.00 D
• Normal values  44.00 to 45.00 D
• To increase the range  Place +1.25 D lens in front of the
objective of the telescope to extend range to 61 D.
 ADD 9 D
• Place -1.00 D lens in front of the objective of the telescope to
extend range to 30 D.
 SUBTRACT 6 D

26. • Conversion Formula with + 1.25D Trial lens
Drum K- Reading x 116.59% = Extended k reading
• Conversion Formula with - 1.00D Trial lens
Drum K- Reading x 85.70% = Extended k reading
www.eyedock.com

27. Uses of Keratometer
• Measurement of corneal astigmatism. Difference in
power between two principal meridians is the amount of corneal
astigmatism.
• Estimate radius of curvature of cornea which helps in selection of
base curve in CL fitting.
• Assess integrity of tear film.
• Monitors shape of the cornea- Keratoconus Keratoglobus.
• Assess refractive error in cases of hazy media.
• IOL power calculation.
• To monitor pre- & post-surgical astigmatism.
• Used for differential diagnosis of axial versus curvatural
anisometropia.

28. Limitation of keratometry
• Measurements of keratometer based on false assumption that
cornea is a symmetrical spherical or spherocylindrical structure ,
with 2 principal meridia separated from each other by 900.
• Measures status of small central cornea (3-4 mm)
• Loses accuracy when measuring very flat or very steep cornea.
• Small corneal irregularities preclude use of keratometer due to
irregular astigmatism.

29. Sources of error in keratometry

31. Automated Keratometer
• Focuses reflected corneal image on to an electronic
photosensitive device, which instantly records the size &
computes the radius of curvature.
• Target mires are illuminated with infrared light, & an infrared
photodetector is used.

32. Either available alone or more commonly in association
with autorefractometers as autokeratorefractometers.
Eg: Nidek ARK 2000-S autokeratorefractometer
Automated keratometry can be performed using following
instruments:
1. The IOL master
2. Pentacam
3. Orbscan
4. Corneal topographer

33. Surgical/ operating keratometer
Attached to operating microscope. Helpful in monitoring
astigmatism during corneal surgery with limited accuracy.

34. Care and Maintenance
• Keeping the instrument covered when not in use.
• Cleaning the body & the optics (removal of dust)
• Periodic lubrication of all moving parts.
• Replacement of bulb when required.
• Checking the on-off switch and the power cable , replacing them
when necessary.
• Check the callibration everyday before use.

35. What not to do??
While cleaning do not disturb the optics especially the two
prisms inside used to get the extra images

36. THANK YOU