Keratometry is used to measure the curvature of the cornea by analyzing the reflection of light off its surface. It works by projecting illuminated circles called mires onto the cornea and measuring the size of the reflected image to calculate the radius of curvature. The main uses of keratometry include measuring corneal astigmatism, estimating contact lens power, and detecting irregularities like keratoconus. Modern instruments automate the process but traditional keratometers require aligning the mires and adjusting knobs until the doubled images come into close alignment. Factors like blinking, eye movements, and irregular corneas can impact the accuracy of measurements.

1. Keratometry
Dr. Karan Bhatia
Senior Resident
AEH, Pondicherry

2. Keratometry (Ophthalmometry)
• Kerato = Cornea
• Metry = Measurement

3. Zones of the Cornea
• Central Zone (apical zone/corneal
cap/central spherical zone) – 4 mm,
radius of curvature does not vary by more
than 1 D or 0.05 mm
• Area where refraction differs by <0.25 D
• Paracentral zone – 4 - 8 mm.
• Peripheral zone – 8 - 11 mm.
• Limbal zone – rim of cornea, 0.55 mm
wide.
Optical
zone

4. What is Keratometry ???
Measurement of the anterior surface of the cornea,
across a fixed chord length, usually 2-3 mm,
which lies within the optical spherical zone of the cornea.

5. Uses of Keratometry
• Measurement of corneal astigmatism.
• Estimate radius of curvature of cornea  helps in contact lens fitting.
• Assess integrity of cornea and/or tear film.
• Detection of irregular astigmatism – keratoconus/pterygium/corneal scarring.
• Assess refractive error in cases with hazy media (Rough estimate, comparison of
two eyes).
• Establish baseline data – should be done in all patients.
• Patient may later want Contact lens or develop an injured/diseased cornea.

6. Uses of Keratometry
• IOL Power calculation (Pre-op Cataract Surgery workup).
• Pre & post surgical astigmatism.
• D/D of axial versus curvatural anisometropia.
• Detect Rigid Gas Permeable lens flexure.
• Progressive myopia.

7. Principle
• Anterior surface of cornea –
CONVEX MIRROR
• ↑ Curvature – ↓ Image Size.
• From Image Size formed by
anterior surface of cornea (1st
Purkinje image) – radius of
curvature of cornea can be
calculated

8. Principle
Therefore ,
B’P/A’B’ = BP/AB
F/i = u/o
r/2 i = u/o ( since F = r/2)
r = 2 ui/o
A
B
Q
S
A’
B’P
u (BP)  constant for any instrument

9. Relationship b/w Radius of curvature and
Diopteric Power of cornea
D = n-1/r
r=1.3375-1/D mm
D – Dioptric power of cornea
n – Index of refraction of cornea (1.3375)
r – Radius of cornea in metres

10. Doubling Principle
Miniature involuntary
eye movements during
fixation of eye
Image formed by
anterior surface of
cornea also moves
(impossible to measure)
DOUBLING
PRINCIPLE

11. Doubling Principle
Biprism
2 images
Move equally as eye moves
Depending on the position of prism – if distance↓, doubling ↑

12. Keratometers
Fixed Doubling
• Bausch and Lomb, Topcon &
Mognon
• Variable – Image size & mire
separation.
• Fixed – Object height & doubling
device distance.
Variable Doubling
• Haag Streit, Javal Schiotz
• Variable – Object height & doubling
device distance.
• Fixed – Image size & mire
separation.

13. Keratometers
One position Keratometer
• Double images are produced side
by side as well as at 90º from each
other.
• The principle meridians are
assumed to be at right angle to each
other.
Two position Keratometer
• Require rotation about the axis to
measure each of the principle
meridians.

14. Keratometers
Ziess Oberkochen Keratometer

15. Keratometers
Humphrey Keratometer Javal Schiotz Keratometer

16. Keratometers
Bausch & Lomb Keratometer

17. OCCLUDER
FOREHEAD REST
ASSEMBLY
CHIN REST
LAMP ASSEMBLY
SIGHTING POST
AXIS ASSEMBLY
EYEPIECE
ROTATING GRIP
ELEVATION ASSEMBLY
BASE ASSEMBLY

18. FOREHEAD
REST
KNOB
FOCUSING
KNOB
LOCKING
KNOB
VERTICAL
MEASURING
DRUM
HORIZONTAL
MEASURING
DRUM
CHINREST
KNOB
ELEVATION
KNOB

31. Caliberation
• Should be done regularly to ensure the accuracy of “K” readings
• Mount a 5/8 inch steel ball bearing at the position close to that normally of
the patient’s eye.
• The steel ball has a known radius of curvature, which upon proper
calibration of the keratometer, can be correctly read.

33. Preparation
• Focus the eyepiece of the keratometer
for the examiner’s eye
• Set the adjustable eyepiece as far
counter-clockwise as possible
• Place a white sheet of paper in front of
the instrument’s objective lens to
retroilluminate the reticle (i.e., cross
hairs)
• Turn the eyepiece clockwise until the
reticle is first seen in sharp focus

35. Preparation – Adjust
instrument for patient
• Adjust height of patient’s chair & instrument to a
comfortable position for both patient & examiner.
• Instruct patient to place chin on chin rest &
forehead against forehead rest & adjust for the
patient.
• Raise or lower chin rest until patient’s outer
canthus is aligned with hash mark on upright
support of instrument.
• From outside instrument, roughly align barrel with
patient’s eye by raising or lowering instrument and
by moving it to left or right until a reflection of
mire is seen on patient’s cornea.

36. Procedure – Instruct patient
• Keep eyes open wide and blink
normally.
• Try not to move the head nor speak.
• Look at the reflection of own eye
in the keratometer barrel.

37. Procedure
• Look into the keratometer and refine the
alignment of the image of the mires (three
circles) on the patient’s cornea.

38. Procedure
• Look into the keratometer and refine the
alignment of the image of the mires (three
circles) on the patient’s cornea.

39. Procedure
• Look into the keratometer and refine the
alignment of the image of the mires (three
circles) on the patient’s cornea.
• Focus the mires and adjust the instrument so
that the reticle is centered in the lower right
hand circle.

40. Procedure
• Look into the keratometer and refine the
alignment of the image of the mires (three
circles) on the patient’s cornea.
• Focus the mires and adjust the instrument so
that the reticle is centered in the lower right
hand circle.
• Adjust the horizontal and the vertical power
wheels until the mires are in close apposition.

41. Procedure
• Look into the keratometer and refine the
alignment of the image of the mires (three
circles) on the patient’s cornea.
• Focus the mires and adjust the instrument so
that the reticle is centered in the lower right
hand circle.
• Adjust the horizontal and the vertical power
wheels until the mires are in close apposition.

42. Procedure
• Look into the keratometer and refine the
alignment of the image of the mires (three
circles) on the patient’s cornea.
• Focus the mires and adjust the instrument so
that the reticle is centered in the lower right
hand circle.
• Adjust the horizontal and the vertical power
wheels until the mires are in close apposition.
• To locate the two principal meridians of the
patient’s cornea, rotate the telescope until the
two horizontal plus signs of the mires are
perfectly continuous with one another.

44. Oblique Astigmatism
2 + signs will not be
aligned
Entire optical instrument
is rotated till the two plus
signs are aligned

45. • A scale associated with it indicates in
degrees, one meridian of oblique
astigmatism.
• Corneal radius of power is then
measured in this meridian and in the
meridian 90 degrees.

46. Astimatism
• Irregular: principal meridians are not perpendicular to each other
• Produce distorted mires
• Regular: principal meridians are perpendicular
• With-the-rule: more power in the vertical meridian (greatest curvature) and horizontal
meridian is flatter
• Example: 45.00 D @ 90º/43.25D @180º
• Against-the-rule: more power in the horizontal meridian and vertical meridian is flatter
• Example: 42.50 D @115º/44.87 D @025º
• Oblique: principal meridians lie between 20° & 70° and 110° & 160°

47. Bausch & Lomb Keratometer
• 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 aperture to extend
range to 61 D
• ADD 9 D
• Place -1.00 D lens in front of aperture to extend range to 30D
• SUBTRACT 6 D

48. Automated Keratometers
• Focuses the reflected corneal image on to an
electronic photosensitive device,which
intantly records the size and computes the
radius of curvature.
• No doubling device is needed.
• Measures angle size in many meridians so it
computes angle as well as power in many
meridians.
• Absence of annoying glare of brightly
illuminated mires.
• Do not calculate clarity of cornea.

49. Surgical Keratometer
• Attached to operating microscope.
• Helpful in monitoring the astigmatism during Corneal
surgery.
• Accuracy limited –
• Difficulty in alligning patients visual axis & Keratometers’s
optical axis.
• Caliberated for a fixed distance from anterior cornea.
• Different microscope objective lenses result in different
focal lengths and therefore different working distance.
• Air in the anterior chamber results in the second target
reflection.
• External pressure on the globe results in a change in a
corneal curvature.

50. ProblemsSolutions
• Unable to locate keratometric mires  instrument and/or patient not aligned
properly.
• Transient Mire clarity  Ask patient to blink & measure quickly/put artificial tears.
• Transient Mire focus  Ensure that patient’s forehead is secured against the
headrest.
• Unsteady Patient gaze  Close other eye.
• H & V mires cannot be measured concurrently  Irregular Astigmatism.
• Only 1 minus sign is visible  Patient’s eyelid drooping .
• Only 1 plus sign is visible  Occluder is in the way.

51. Limitations of Keratometry
• Measures refractive status of a very small central area of cornea (3 mm), ignoring
the peripheral corneal zones.
• Accuracy lost when measuring very flat or very steep cornea.
• Small corneal irregularities would preclude the use of keratometer due to irregular
astigmatism.
• One position instruments assume regular astigmatism.
• Distance to focal point is approximated by distance to the image.
• Autokeratometers do not evaluate the quality of cornea

52. “You can't invent Google, Facebook or the
iPod
unless
you've mastered the basics.”
-- Amy Chua
ThankyouAcknowledgements – Dr. Swapnil Chinche
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