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PRESENTER: DR. PAVITRA K.PATEL
KERATOMETRY & AUTOREFRACTOMETR
Y
KERATOMETRYDefinitionHistoryPrincipleTypes of keratometerProcedure of keratometryInterpretation of findingsClinical usesLimitationsSources of errorSurgical keratometerAutomated keratometer
CONTENTS
KERATOMETRY
“Kerato”- cornea“metry”-measurement of
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.
YEARS INVENTORS1691 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
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 (1s t Purkinje image) , the radius of curvature of cornea calculated as below:
PRINCIPLE
Greater the curvature of cornea, lesser is the image size.
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
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
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
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
Basically, there are two types of keratometer:
Manual keratometer
Auto keratometer
PRINCIPLE: “Constant object size and variable image size”.
BAUSCH AND LOMB KERATOMETER
PARTS:
OPTICAL SYSTEM OF KERATOMETER
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.
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.
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.
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 90 0 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:oEnables examiner to observe magnified view of the
doubled image.
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
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.
3. Focusing of mire:oMire is focused in the centre of cornea.
Patient’s view of mireFirst 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.
4. Measurement of corneal curvature:o Instrument is correctly focused on corneal image so that
central image is no longer doubled.
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.
OBLIQUE ASTIGMATISM
RECORDING OF THE CORNEAL CURVATURE:
INTERPRETATION OF FINDINGS
Remember that it is the power meridian, NOT the axis, being recorded in keratometry.
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.
RANGE OF KERATOMETER:
Range 36.00 to 52.00 DNormal values 44.00 to 45.00 DTo increase the range Place +1.25 D lens in front of
the aperture to extend range to 61 D. ADD 9 DPlace -1.00 D lens in front of the aperture to extend
range to 30 D. SUBTRACT 6 D
PRINCIPLE: “Variable object size and constant image size”.
JAVAL –SCHIOTZ KERATOMETER
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
oMires divided horizontally through the centre.oThey are illuminated by small lamps.o Image of these mires formed by patient’s cornea (1 s t
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.
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.
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
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
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.
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.
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.
When oblique astigmatism is present
Mires are
horizontal,centra
l 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 astigmatis
m.
Corneal radius or power is then measured in this meridian & also in the meridian 900 to it as usual.
1. Helps in measurement of corneal astigmatic error.
oDifference in power between two principal meridians is the amount of corneal astigmatism.
o In 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
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
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 Keratoglobus4.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.
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
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.
Improper calibration
Faulty positioning of patient
Improper fixation by patient
Accomodat-ive fluctuation by examiner
Localized corneal distortion
Excessive tearing
Abnormal lid position
Improper focusing of corneal image
SOURCES OF ERROR IN KERATOMETRY
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.
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
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 master2. Pentacam3. Orbscan4. Corneal topographer
AUTO-REFRACTOMETRY-CONTENTS
AUTO-REFRACTOMETRYDefinition PrincipleTypes of refractometersPortable autorefractorsAdvantages of automated over manualWavefront technology
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
Scheiner principle(161
9)
Optometer principle(175
9)
OPTICAL PRINCIPLES
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.
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.
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.
o It 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.
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.
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.
Development of optometers grouped as follows:
Early refractomete
rs
Modern autorefractor
s
Both are subdivided into subjective & objective optometers.
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.
LIMITATIONS OF EARLIER OPTOMETERS:1. Alignment problem:oAs per Scheiner’s principle, both pinhole apertures must
fit within the patient’s pupil.o If patient’s fixation wanders, reading is invalid.oThus, considerable patient cooperation required.
2. Irregular astigmatism:o In a patient with irregular astigmatism, best refraction
over whole pupil may be different in contrast to two small pinhole areas of pupil.
3. Accomodation:oOn looking into the instrument, patient tends to
accommodate Instrument Myopia.oAlters actual refractive status of patient.oFactors affecting accomodation:
AttentionFatigueDirection of gazeIlluminationImage detailBlur of retinal imagePsychological factors
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.
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 6/18Do not function properly in presence of hazy ocular media with drop in VA of >6/18
Rough refraction may be obtained.
Features Objective refractometers
Subjective refractometers
Over-refraction capability
Over-refraction in pts using spectacles, contact lenses/IOL difficult
No such problem
Expected results Provides preliminary refractive findings.
Provides refined subjective resultsEg. Vision Analyser
COMMERCIALLY AVAILABLE OBJECTIVE AUTOREFRACTOMETERS:
Based on one or more of the following working principles1. The Scheiner principle2. The optometric principle (retinoscopic principle)3. The best-focus principle4. The knife-edge principle5. The ray-deflection principle6. The image size principle
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)
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
radiationEg.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
SUBJECTIVE AUTOREFRACTORS:1. Vision analyser:oUses innovative optical system & equally innovative
methods for subjective refraction.
2. SR-IV programmed subjective refractor:oUses optometer principle
3. Subjective autorefractor-7:oScreening instrumentoHas spherical optics only
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
Portable autorefractor is particularly helpful in examining children as they can easily adjust themselves according to different positions of the patient.
PORTABLE AUTOREFRACTORS
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.
Advantages of automated refraction systems vs. manual refraction equipment are:less manual labour by the practitioner or technicianmore automation of repetitive and iterative tasks in the
refractionability to present former and new values quickly for
validationreduced risk of human errordirect transmission of results to Electronic Medical
Record(EMR) softwareImproved efficiency of practice
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
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
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.
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.
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.
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
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.
THEORY & PRACTICE OF OPTICS AND REFRACTION-3 rd EDITION-A K KHURANA
OPHTHALMOLOGY 3 rd EDITION-YANOFF DUKERCLINICAL OPTICS-3 rd EDITION-ANDREW R. ELKINGTON INTERNET
REFERENCES
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