4. DEFINITION:
Keratometry is measurement of curvature of the anterior
surface of cornea across a fixed chord length, usually 2-
3 mm, which lies within the optical spherical zone of
cornea.
Expressed in Dioptric power.
Keratometer also called as Ophthalmometer.
5. YEARS INVENTORS
1691 Christoph Scheiner –Description of corneal
curvature
-Compared size of the bars in a window-
lens & cornea
1796 Jesse Ramsden- Inventor of 1st model of
keratometer with 3 essential elements
1854 Helmholtz improved Ramsden’s design for
laboratory use
1881 Javal & Schiotz modified Helmholtz’s
instrument for clinical use
1980 Development of autorefractometer
HISTORY
6. 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.
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:
PRINCIPLE
Greater the curvature of cornea, lesser is the image size.
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= radius of curvature, h=height of object, h1=height of the image
n1= refractive index of cornea (1.337),n=refractive index of
medium from which light originates (air=1)
r = 2 x h1/h
D= (n1-n) /r x 1000
8. 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
9. Keratometer is based on 2 concepts:
Fixed object size
with variable image
size
(Variable doubling)
Fixed image size
with variable object
size
(Fixed doubling)
Eg. Bausch and Lomb
keratometer
Eg. Javal- Schiotz
keratometer
10. Doubling principle:
Because of involuntary eye movement image formed on
cornea would be constantly moving.
To overcome this Ramsden devoloped Doubling
technique.
A prism is introduced into the optical system so that 2
images are formed .
The prism is moved until the images touch each other.
Depending on the position of prism, if distance doubling
11. Basically, there are two types of keratometer:
Manual
keratometer
Auto
keratometer
15. OPTICAL SYSTEM AND OTHER PARTS:
1. Object: Circular mire with two plus & two minus
signs.
oLamp illuminates the mire by means
of a diagonally placed mirror.
oLight from the mire strikes the
patient’s cornea & produces a
diminished image behind it.
oThis image becomes the object for the remainder of
optical system.
16. 2. Objective lens:
oFocuses light from the image of the mire (new object)
along the central axis.
3. Diaphragm and doubling prisms:
o4 aperture diaphragm is situated near objective lens.
oBeyond the diaphragm are two doubling prisms, one
with its base up & other with its base out.
oPrisms can be moved independently, parallel to the
central axis of instrument.
17. Light passing through left
aperture of diaphragm is
made to deviate above
the central optical axis by
a base-up prism.
Light passing through
right aperture is deviated
by base –out prism,
placing the second image
to the right of the central
axis.
Light passing through
upper & lower apertures
does not pass through
either prism & an image
is produced on the axis.
18. Total area of upper & = Area of each of
lower apertures the other two apertures
Therefore, brightness of the images is equal.
Upper and lower apertures also act as Scheiner’s disc
doubling the central image, whenever the instrument is
not focused precisely on central mire image.
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.
19. 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:
oEnables examiner to observe magnified view of the
doubled image.
20. PROCEDURE OF KERATOMETRY:
1. 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
Steel ball of known radius of curvature is placed
before keratometer & its value is set on the scale
or dial
21. Mires are focused by clockwise & anticlockwise
movement of eyepiece through trial & error
When mires are in focus, the calibration is
complete.
2. Patient adjustment:
o Seated in front of the instrument.
o Chin on chin rest & head against head rest.
o Eye not being examined is covered with occluder.
o Chin is raised or lowered till patient’s pupil & projective
knob are at the same level.
22. 3. Focusing of mire:
oMire 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.
23. 4. Measurement of corneal curvature:
oInstrument is correctly focused on corneal image so that
central image is no longer doubled.
24. 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.
In presence of oblique
astigmatism, two plus
signs will not be
aligned.Entire instrument
rotated till they are
aligned.
Corneal radius of
Power is then
measured.
28. Spherical cornea
• No difference in power b/w 2
principal meridia
• Mires seen as perfect sphere.
Astigmatism
• Difference in power b/w 2 principal meridia.
• Horizontally oval mires in WTR
astigmatism.
• Vertically oval mires in ATR astigmatism.
• Oblique astimatism principal meridia b/w
300-600 & 120-1500.
Irregular anterior corneal
surface
• Irregular mires.
• Doubling of mires.
Keratoconus
• Pulsating mires(Inclination & jumpimg
of mires on attempt to adjust the
mires).
• Minification of mires in advanced
cases (K >52 D) due to increased
amount of myopia.
• Oval mires due to large astigmatism.
• Irregular,wavy & distorted mires in
advanced keratoconus.
29. RANGE OF 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
the aperture to extend range to 61 D.
ADD 9 D
Place -1.00 D lens in front of the aperture to extend
range to 30 D.
SUBTRACT 6 D
31. OPTICAL SYSTEM AND PARTS:
1.Object:
oConsists of two mires (A & B), mounted on an arc on
which they can be moved synchronously.
oSince the two mires together form the object, the
variable size is attained by their movement.
OPTICAL SYSTEM OF KERATOMETER
One mireStepped,
has green filter
Other mire Rectangular,
has red filter
32. oMires divided horizontally through the centre.
oThey are illuminated by small lamps.
oImage of these mires formed by patient’s cornea (1st
Purkinje image) acts as an object for the rest of the
optical system of the keratometer.
2.Objective lens & doubling prism:
oForms double image of the new object.
oDoubling prism used Wollaston type.
33. oProduces fixed image doubling by birefringent (double
refracting) characteristic of material of which it is
made.
3. Eyepiece lens:
oEnables examiner to observe magnified view of the
doubled image.
34. PROCEDURE OF KERATOMETRY:
1.Instrument adjustment:
oWhite paper held in front of the objective piece & black
line focused on it.
oThen instrument is calibrated to make it ready for use.
2.Patient adjustment
35. 3.Adjustment of mires:
oMires are focused in the centre of patient’s cornea.
Patient’s view of mires Examiner’s view of
doubled mire image
36. 4.Recording of keratometric readings:
oOnly central pair of images is used when measurements
are made.
oWhen two control images just meet, the scales
associated with the mire separation indicate the correct
corneal radius & dioptric power of the cornea.
37. Radius of curvature
first found in one
meridian.
Then entire optical
system rotated 900
about its central axis.
Measurement of radius of
curvature in second
meridian which is
perpendicular to 1st one is
then made in similar way.
38. When corneal astigmatism is
present Overlapping of
mires or they may move
further apart.
Since stepped mire (staircase
pattern) is green &
rectangular mire is red, area
of overlap appears whitish.
Each step of mire 1 D of
corneal power ,thus the
number of steps overlapped
gives approximate degree of
astigmatism.
39. When oblique astigmatism is present
Mires are
horizontal,central
bisecting lines of
images are not
aligned.
Instrument is
rotated until the
control lines are
aligned.
Scale associated
with instrument
rotation
indicates, in
degrees, one
meridian of
oblique
astigmatism.
Corneal radius or power is then measured in this
meridian & also in the meridian 900 to it as usual.
40. 1. Helps in measurement of corneal astigmatic error.
oDifference in power between two principal meridians is
the amount of corneal astigmatism.
oIn Optometry, astigmatism is corrected by minus
cylinder lens.
oFrom K readings, meridian of least refracting power
indicates the position of minus axis of the correcting
cylinder.
CLINICAL USES OF KERATOMETERS
41. Eg 1. OD 42.50D at 180 / 44.50D at 90
Corneal astigmatism = 2.00D
Correcting cylinder = -2.00DC x 180
WTR astigmatism
Eg 2. OD 42.75D at 180 / 42.00D at 90
Corneal astigmatism = 0.75D
Correcting cylinder = -0.75DC x 90
ATR astigmatism
42. 2. Helps to estimate radius of curvature of the anterior
surface of cornea Use in contact lens fitting.
3.Monitors shape of the cornea Keratoconus
Keratoglobus
4.Assess refractive error in cases of hazy media.
5.IOL power calculation.
6.To monitor pre- & post-surgical astigmatism.
7.Used for differential diagnosis of axial versus
curvatural anisometropia.
43. LIMITATIONS 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
refractive
status of small
central cornea
(3-4 mm)
Loses accuracy
when
measuring very
flat or very
steep cornea
44. Small corneal
irregularities
preclude use of
keratometer
due to irregular
astigmatism.
One-position
instruments
assume regular
astigmatism.
Distance to
focal point is
approximated
by distance to
image.
46. SURGICAL/OPERATING KERATOMETER
Attached to operating microscope.
Helpful in monitoring astigmatism
during corneal surgery.
Accuracy limited:
1. Difficulty in aligning patients
visual axis & keratometer’s
optical axis.
2. Calibrated for a fixed distance
from anterior cornea.
3. Different microscope objective
lenses result in different focal
lengths & therefore different
working distance.
4. External pressure on globe
results in change in a corneal
curvature.
47. 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.
ADVANTAGES:
• Compact device
• Very short time consuming
• Comparatively easy to operate
48. Availability of autokeratometer:
oEither available alone or more commonly in association
with autorefractometers as autokeratorefractometers.
Eg: Nidek ARK 2000-S autokeratorefractometer
oAutomated keratometry can be performed using
following instruments:
1. The IOL master
2. Pentacam
3. Orbscan
4. Corneal topographer
50. Refractometry (optometry) is
an alternative method of
finding out the error of
refraction by the use of an
optical equipment called
refractometer or optometer.
AUTOREFRACTOMETRY
52. 1. SCHEINER PRINCIPLE:
oScheiner in 1619 observed that
refractive error of the eye is determined by using double
pinhole apertures before the pupils.
Parallel rays of light from a distant object are reduced
to two small bundles of light by the Scheiner disc.
These form a single focus on the retina if the eye is
emmetropic; but if there is any refractive error two
spots fall on the retina.
53. By adjusting the position of the object (performed optically by the
autorefractor) until one focus of light is seen by the patient, the far
point of the patient’s eye and the refractive error can be determined.
54. 2. OPTOMETER PRINCIPLE:
oPorterfield, in 1759 coined the term optometer to
describe an instrument for measuring the limits of
distinct vision.
oPrinciple permits continuous variation of power in
refracting instruments.
oIt involves a convex lens placed in front of the eye at its
focal length from the eye (or the spectacle plane) and a
movable target is viewed through the lens.
55.
56. Light from the target on the far side of the lens enters
the eye with vergence of different amounts, depending
on the position of the target.
If the target lies at the focal point of the lens, light from
the target will be parallel at the spectacle plane, and
focused on the retina of the Emmetropic eye.
Light from the target when it is within the focal length
of the lens will be divergent in the spectacle plane while
light from a target outside the focal length of the lens
will be convergent.
57. The vergence of the light in the focal plane of the lens
is linearly related to the displacement of the target from
the focal point of the lens.
A scale can thus be formed which would show the
number of diopters of correction according to the
position of the target.
58. Development of optometers grouped as follows:
Early
refractometers
Modern
autorefractors
Both are subdivided into subjective & objective optometers.
59. 1. EARLY SUBJECTIVE
OPTOMETERS:
o Patient is required to adjust
the instrument for best focus.
o Unsuccessful due to
instrument accomodation.
o Examples: 1.Badal
Optometer
2.Young’s Optometer
EARLY REFRACTOMETERS
1. EARLY OBJECTIVE
OPTOMETERS:
o Rely on examiner’s decision on
when the image is clearest.
o Thus, they were objective only
in sense that the patient’s
subjective choice had been
replaced by the choice of an
experienced examiner.
o Based on optometer principle, &
most of them incorporated
Scheiner principle as well.
60. LIMITATIONS OF EARLIER OPTOMETERS:
1. Alignment problem:
oAs per Scheiner’s principle, both pinhole apertures must
fit within the patient’s pupil.
oIf patient’s fixation wanders, reading is invalid.
oThus, considerable patient cooperation required.
2. Irregular astigmatism:
oIn a patient with irregular astigmatism, best refraction
over whole pupil may be different in contrast to two
small pinhole areas of pupil.
61. 3. Accomodation:
oOn looking into the instrument, patient tends to
accommodate Instrument Myopia.
oAlters actual refractive status of patient.
oFactors affecting accomodation:
Attention
Fatigue
Direction of gaze
Illumination
Image detail
Blur of retinal image
Psychological factors
62. General comparison of subjective & objective
instruments:
MODERN REFRACTOMETERS
Features Objective
refractometers
Subjective
refractometers
Source of light Low levels of invisible
infrared light to perform
refraction
Visible light
Time required for
refraction(BE)
2-4 mins 4-8 mins
Information provided Do not provide this
information EXCEPT
Humphrey Automatic
Refractor which
provides VA capability.
Supply more
information & corrected
VA obtained as a part of
refracting procedure.
63. Features Objective
refractometers
Subjective
refractometers
Patient cooperation
factors
Requires less patient
cooperation
(>5 years)
Patient should be able to
turn a knob to focus
various targets or
answer simple questions
about appearance of
target.
(>8 years)
Ocular factors Give better results in
presence of macular
diseases with clear
ocular media.
Less better
Performance is equal in presence of hazy ocular
media with vision upto 6/18
Do not function
properly in presence of
hazy ocular media with
drop in VA of >6/18
Rough refraction may
be obtained.
65. COMMERCIALLY AVAILABLE OBJECTIVE
AUTOREFRACTOMETERS:
Based on one or more of the following working principles
1. The Scheiner principle
2. The optometric principle (retinoscopic principle)
3. The best-focus principle
4. The knife-edge principle
5. The ray-deflection principle
6. The image size principle
66. Autorefractors based on Scheiner principle:
1. Acuity Systems 6600 (NA)
2. Grand Seiko (RH Burton’s BAR 7 in the USA; BAR 8
with AutoK)
3. Nidek (Marco’s AR-800 & 820 in the USA; ARK -900
with AutoK)
4. Takagi (not available in the USA)
5. Topcon (NA)
67. Auto refractors based on retinoscopic principle:
Based on one of the following 2 characteristics of
retinoscopic fundus reflex
Direction of motion of
observed fundus reflex with
respect to direction of motion
of incident radiation
Eg.Baush & Lomb
Speed of motion of the
observed fundus reflex with
respect to speed of motion of
insident radion.
Eg. Nikon NR-5500, Nikon
Retinomax, Tomey TR-
1000,Nidek OPD-Scan
69. Autorefractors are most commonly used to provide the
starting point for refraction to obtain an objective result
before performing subjective refraction.
Most commercially available Autorefractors available
today come with an inbuilt Automated Keratometer &
are known as Auto Kerato-Refractometer.
Recently new equipments with addition corneal
topographers have been developed in which Corneal
Topography can also be performed.
AUTOREFRACTORS CURRENTLY IN USE
70. Portable autorefractor is
particularly helpful in
examining children as they
can easily adjust themselves
according to different
positions of the patient.
PORTABLE AUTOREFRACTORS
71. The portable autorefractor holds great promise in the
future for better eye health, because it can also allow
optometrists to conduct preliminary eye examinations
for those who cannot get to a doctor’s office.
It is also ideal for vision screenings in community
groups or health fairs.
With the advent of handheld autorefractors, it can be
used on patients with certain disabilities, such as those
who cannot hold their head up straight. Technicians or
doctors can position themselves to make them work on
bedridden patients.
72. Advantages of automated refraction systems vs. manual
refraction equipment are:
less manual labour by the practitioner or technician
more automation of repetitive and iterative tasks in the
refraction
ability to present former and new values quickly for
validation
reduced risk of human error
direct transmission of results to Electronic Medical
Record(EMR) software
Improved efficiency of practice
73. Recently, a tool has been developed which works by
combining a simple optical attachment with software on
a smartphone which enables assessment of Refractive
Error.
RECENT ADVANCES IN AUTOMATED
REFRACTION
74. Additionally, some variations on the traditional
autorefractor have been developed.
The aberrometer is an advanced form of autorefractor
that examines light refraction from multiple sites on the
eye.
Aberrometry measures the way a wavefront of light
passes through the cornea & crystalline lens, which are
the refractive components of the eye. Distortions that
occur as light travels through the eye are called
aberrations, representing specific vision errors.
WAVEFRONT TECHNOLOGY IN
REFRACTION
75. Several types of visual imperfections, referred to as
lower and higher-order aberrations, exist within the eye
and can affect both visual acuity and the quality of
vision.
Conventional examination techniques & autorefractors
only measure lower-order aberrations such as myopia,
hypermetropia, and astigmatism.
However, these do not account for all potential vision
imperfections. Higher-order aberrations can also have a
significant impact on quality of vision and are often
linked to glare and halos that may cause night vision
problems.
76. Wavefront technology, or aberrometry, diagnoses both
lower- and higher-order vision errors represented by the
way the eye refracts or focuses light.
Wavefront analysis not "an upgraded" version of corneal
topography or autorefraction but a visual equity
measuring device that takes all elements of the optical
system into consideration i.e. the tear film, the anterior
corneal surface, the corneal stroma, the anterior
crystalline lens surface, the crystalline lens substance,
the posterior crystalline lens surface, the vitreous and
the retina.
77. Wavefront analysis is approximately 25-50 times more
accurate than the autorefractometer.
Now that higher-order aberrations can be accurately
defined by wavefront technology and corrected by new
kinds of spectacles, contact lenses & refractive surgery,
they have become more important factors in eye exams.
78. Corneal shape post refractive surgery is clearly
modified in the majority of procedures.
Furthermore, specific algorithms are used in lasers
which ablate the cornea to reduce aberrations.
Most autorefractors (all Scheiner based) perform
refraction through a fixed pupil diameter.
Therefore, the influence of overall refraction throughout
the pupillary plane will not be addressed.
AUTOREFRACTION IN IRREGULAR EYES
79. In eyes with a normal corneal shape, the results will not
be affected but in pathological eyes such as post graft,
keratoconus and post refractive surgery, the departure of
corneal shape from normality may induce significant
errors compared to subjective refraction.
80. THEORY & PRACTICE OF OPTICS AND REFRACTION-3rd
EDITION-A K KHURANA
OPHTHALMOLOGY 3rd EDITION-YANOFF DUKER
CLINICAL OPTICS-3rd EDITION-ANDREW R. ELKINGTON
INTERNET
REFERENCES
Relationship between radius of curvature & dioptric power of the cornea
4.Assess refractive error in cases of hazy media(rough estimate on the basis that the normal measurement is 43.5 D-comparison of the two eyes in these cases is useful).
5. IOL power calculation-K readings are taken with the help of keratometer & along with axial length, these are utilized to calculate IOL power in SRK formula for IOL power calculation.
1 CT..900,whereas cornea in reality is aspheric
2 CT..ignoring the peripheral corneal zones
Other limitationUse of para-axial optics to calculate surface power.
Iol master-for IOL power calculation
Pentacam-Comprehensive eye scanner which scans both ant. & post corneal surface & corneal thickness
Orbscan(corneal topographer)-Overall architecture of cornea including corneal thickness,corneal surface power & shape of the cornea
The parallel rays of light entering the eye from a distant object are normally focused on a point on the retina in an emmetropic patient.
They are limited to 2 small bundles when double pinhole apertures or a scheiner’s disc is placed in front of the pupil
In a myopic eye, the 2 ray bundles cross each other before reaching the retina, and 2 small spots of light are seen.
In a hypermetropic eye, the ray bundles are intercepted by the retina before they meet & thus again 2 small spots of light are seen.
Far point is the point farthest from the eye at which an object is accurately focused on the retina when the accommodation is completely relaxed
Light from the target when it is within the focal length of the lens will be divergent in the spectacle plane while light from a target outside the focal length of the lens will be convergent.
1st line to speak Efforts have been made to eliminate limitations of old refractors:
Patient ccooperation factors:Children >5 years refracted with objective refractometers while for subjective refractometer use the child should be about 8 years of age.
Regardless of the instrument being used, children should be subjected to cycloplegic refraction.