objective refraction
TRANSCRIPT
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Company
LOGO Dr Sneha Thapliyal
OBJECTIVE REFRACTION
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CLINICAL REFRACTION
Determines and corrects refractive errors
Objective refraction Subjective refraction
Essential in:Young childrenMental disabilityLanguage difficultyDeaf or mute
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OBJECTIVE METHODS OF REFRACTION
1. Retinoscopy2. Autorefractometry3. Photorefraction4. Electrophysiological method of objective
refraction
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RETINOSCOPY/SKIASCOPY/SHADOW TEST
Objective method of finding out the error of refraction by means of retinoscope utilizing the technique of NEUTRALIZATION
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Far point concept The far point of eye is defined as the point in space that is conjugate with the
fovea when accomodation is relaxed.
Emmetropia: Parallel rays focus on fovea. Retina conjugate with infinity. Far point is at infinity.
Ammetropias: Parallel rays do not focus on retina. Ammetropic eyes require a correcting lens to make retina conjugate with
infinity, i.e., to move far point to infinity
Hyperopia: Deficient refractive power. Parallel rays focus behind retina. Far point is beyond infinity. Plus lens converges rays on to retina and conjugate fovea with
infinity.
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Far point concept Myopia: Excessive refractive power. Parallel rays focus in front of retina. Far point is between infinity and eye. Minus lens diverges rays on to the retina and conjugate
fovea with infinity. Aspherical ammetropias: This indicates different types of astigmatism. This type of errors have two far points. As a set of rays converge at one place and other at different
place due to cornea not having same radius of curvature in all the meridians.
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Principle of retinoscopy:
To bring the far point of patient at the nodal point of the observer at the working distance
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NEUTRALIZATION
1. No movement of red reflex = NEUTRAL POINT
2. Reversal with overcorrection by 0.25 D
3. On alternating the working distance, slight forward, ‘with’ movement and an ‘against’ movement by slight backward movement.
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PREREQUISITES FOR RETINOSCOPY
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PREREQUISITES FOR RETINOSCOPY
1. Dark room: 6 m long2. A trial set: 64 pairs of Spherical lenses(plus and minus) 0.12D 0.25-4.0D 4.0-6.0D 6.0-14D 14-20D
0.25 0.51D2D
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20 pairs of Cylindrical lenses (plus and minus)
0.25-2.00D 2.0-6.0D
10 Prisms: ½ to 6 8,10,12
0.250.5
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Accessories: Plano lenses
Opaque discs Pinhole
Stenopaeic discsMaddox rodsRed and Green glasses
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3. A trail frame Light weight Aluminium alloy- 30 g Adjustable- horizontally and vertically 3 compartments: spherical, cylindrical, prisms Cylindrical compartment : smooth and accurate rotation Side pieces should be joint- tilted while testing for near vision Back lens in trial frames should occupy as nearly as possible the
position of spectacle lens i.e. around 12 mm in front of the cornea.
4. Phoropter or refractor Turn the dial – change the lens before the aperture of the viewing system
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5. Distance-vision chart6. Near vision charts7. Retinoscope:
Invented by Dr Jack Copeland
REFLECTING (MIRROR) RETINOSCOPES
Priestley-Smith’s mirrorPlane mirror
SELF-ILLUMINATED RETINOSCOPES
Spot self-illuminous Streak retinoscope
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HEAD
NECK
HANDLE
SLEEVE UP
PEEPHOLE
SLEEVE DOWN
STREAK RETINOSCOPE
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HEAD
HANDLE
NECK
MIRROR LENSSLEEVEPARASTOP
BULB
BATTERIES
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Contact point b/w neck
and handle
bulb
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Dark mirror
Reflecting mirror
Sleeve
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Observing the optics of retinoscope we find two main systems
- Projection system: Light source Condensing lens Focusing sleeve Current source - Obsevation system: Peep hole
RETINOSCOPE AND IT’S PARTS:
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The retinoscope: how it works?
PROJECTION SYSTEM: illuminates the retina
Light source: a bulb with linear filament that projects a line or streak of light
Condensing lens: focuses rays from bulb onto the mirror
Mirror : placed in the head of instrument at 45 degree angle, it bends the path of light at right angles to the axis of the handle
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Focusing sleeve controls Meridian:Turning the sleeve rotates the streak of light. Vergence:By varying the distance between the lens and
the bulb Bulb: Moved up-Plane mirror effect(Parallel rays) Moved down-Concave mirror effect(converging
rays) Lens: Moved up-Concave mirror effect Moved down-Plane mirror effect
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In Copelands instrument, moving the sleeve up or down moves the bulb. Sleeve up creates plane mirror effect & sleeve down
creates concave mirror effect
Copeland type- fixed lens with moving bulb
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In other Retinoscopes, the lens rather than the bulb moves on moving the
sleeve Sleeve up creates a concave mirror effect & sleeve
down creates plane mirror effect
Fixed bulb type, the lens moves
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OBSERVATON SYTEM: Allows to see the retinal reflex
Light reflected by the illuminated retina enters the retinoscope, passes through an aperture in the mirror & out of the peephole at the rear end of the head.
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Optics in Retinoscopy
Illumination stage: light is directed into the subject’s eye to illuminate the retina
Reflex stage: image of the illuminated retina is formed at subject’s far point
Observation stage: image of the far point is located by moving the illumination across the fundus and noting the behavior of the luminous reflex seen by the observer in the subject’s pupil
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Clinician
So
S1S2
Patient
Optical principles
The light in the pupil is called the “ret reflex”
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The final goal
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Observer Subject
Reflex disappears
Reflex moves down,i.e. with directionof movement of light
Mirror tilts further forwards
Mirror tilts forwards
S2
S2
Far point behindobservers pupil Within pupil
Outside pupil
No S2
S1
Non-neutral point: With movement Far point behind observers
pupil.With movement of reflex, gradual change from red
reflex to no reflex
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With movement
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Observer Subject
Far point in front ofobservers pupil
Reflex disappears
Reflex moves up,i.e. against directionof movement of light
Mirror tilts forwards
Mirror tilts further forwards
S2
S2
Within pupil
Outside pupil
No S2
S1
Non-neutral point: Against movement Far point in front of observers
pupil.Against movement of reflex,
gradual change from red reflex to no reflex
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Against movement
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Reflecting mirror:
Perforated mirror
Central hole: 2.5mm anteriorly 4 mm posteriorly
By a plane mirror, rays are slightly divergent causing less illumination
Small hole corresponding to the central hole reduces illumination in pupillary area by causing central dark patch
Sides of the small hole are blackened to prevent annoying reflexes from entering the eye
Slightly concave mirror with central opening of 4mm and focal length greater than the distance between observer and patient; 150 cm
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LUMINOUS RETINOSCOPE
Streak retinoscope is the most popular Both light source and mirror are incorporated Intensity and type of beam can be controlled.
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Two main techniques of retinoscopy are
Static Retinoscopy: It is the refractive state
determined when patient fixates an object at a distance of 6 m with accomodation relaxed.
Dynamic Retinoscopy: The refractive state is
determined while the subject fixates an object at some closer distance, usually at or near the plane of retinoscope itself with accommodation under action.
RETINOSCOPY TECHNIQUES:
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PROCEDURE OF RETINOSCOPY
Patient is made to sit at a distance of 2/3rd of a meter
Accomodation should be relaxed Use right eye for patient’s right
eye and left eye for left Trial frame is fitted Direct the light from retinoscope
into patient’s pupil Move the retinoscope in
horizontally and observe the red reflex.
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Suitable lens is placed before the eyes to neutralize the band when the pupil will be filled with uniform light
Now move vertically, if still pupil is filled with uniform light, no astigmatism.
Examiner sits just enough to the side to avoid blocking patient’s fixation
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Characteristics of the moving retinal reflex
Speed and brilliance
Low refractive High refractive
Bright FaintMoves rapidly Moves slowly
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2.Width of reflex
Narrow WideHigh degree of ametropia Low degree of ametropia
3. Presence of astigmatism
When the axis does not correspond with the movement of the mirror, the shadow appears to swirl around.
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Examiner should scope both vertical and horizontal meridian.
We correct the astigmatism with cylindrical lens.
Cylindrical lens may be plus or minus, but have power in only one meridian, that which is perpendicular to the axis of the cylinder.
The axis meridian is flat and has no power.
ASTIGMATISM
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CYLINDRICAL AXIS
Break in the alignment between the reflex in the pupil and the band outside it when the streak is off the axis
Rotate the streak until the break disappears
Width of the streak appears narrowest when the streak aligns with the true axis.
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STRADDLING
Confirmation of axis Retinoscope streak- 45
degree off axis in both directions
Correct axis- width of streak equal in both positions
Axis not correct- width is unequal
Narrow reflex is guide towards which cylinder’s axis should be turned
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PROBLEMS IN RETINOSCOPY:
1. Red reflex may not be visible or maybe poor Small pupil Hazy media High degree of refractive error Overcome by causing mydriasis and/or use of
converging light with concave mirror retinoscope2. Changing retinoscopic findings Abnormally active accomodation Overcome by fogging retinoscopy and cycloplegic3. Spherical aberrations Dilated pupils
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4. Conflicting shadows Irregular astigmatism
5. Triangular shadows Conical cornea
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www.phacotube.com
Scissoring shadowsAstigmatismOvercome by: undilated pupil
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Cycloplegics in retinoscopy
When retinoscopy is performed after instilling cycloplegic drugs it is termed as Wet Retinoscopy
1. Atropine 1% ointment Used in children < 7yrs age Dose:1% eye ointment 3times daily for 3days Effect lasts for 10-20days Deduction for cycloplegia with atropine is 1D
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2. Homatropine 2% drops Used in hypermetropes between 7 to 35yrs Dose:1drop instilled every 10mins for 6times &
retinoscopy is performed after 1-2hrs Effect lasts for 48-72hrs Deduction for cycloplegia with homatropine is
0.5D3. Cyclopentolate I% drops
Used in persons between 7 to 35yrs Dose:1drop instilled every 10mins for 3 times &
retinoscopy is performed after 1- 1 1/2hrs Effect lasts for 6 to 18hrs Deduction for cycloplegia with Cyclopentolate is
0.75D
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Only mydriatic [ 10% phenylephrine] may is used in elderly patients to enhance the pupillary reflex when the pupil is narrow or media is slightly hazy
Later it should be counteracted by use of miotic drug[ 2% pilocarpine]
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Clinician
S2
Patient
Working distanceWe now have neutral
We have also introduced negative vergence due to our working distance (WD)
= 1/d (m)Where d = distance in m, measured between your ret and patient’s eye
We have added lenses
To get the right prescriptionwe need to compensateRx = lens power – 1/d
So to get neutral, we needed: lens power = Rx + 1/d
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Working distance compensation
Calculation
For example, if neutrality is achieved with a +3.00DS lens and your working distance is 50cm
Rx = +3.00DS – (1/0.50) = +3.00 – 2.00 =+1.00DS
Working distance lens Before you begin, add a
“working distance lens” A +ve lens to cancel out
the negative vergence As in eg., WD = 50cm WD lens = 1/0.50 =
+2.00DS Neutrality for the same
patient is still +3.00DS WD lens = +2.00DS Lens power = +1.00DS
Rx = lens power - 1/d
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AUTOREFRACTOMETRY
Alternative method of assessing error of refraction by use of an optical equipment called refractometer or optometer
OBJECTIVE SUBJECTIVE
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History Collins(1937) developed the first semi automated electronic
refractionometer.
Safir (1964) automated the retinoscope and this work led to the first commercial autorefractor-the Ophthalmometron.
6600 Autorefractor was the second commercial autorefractor (1969).
Munnerlyn (1978) design using a best contrast principle with moving gratings led to the Dioptron, an automated objective refractor.
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OBJECTIVE SUBJECTIVE
SOURCE OF LIGHTTIME REQUIRED
Infra-red light2-4 min
Visible light4-8 min
INFORMATION PROVIDED Less informative More informative (corrected visual acuity)
PATIENT CO-OPERATION Required less (only has to look straight at target)
More co-operation (patient has to turn knob, focus target, answer questions about appearance of target)
OCULAR FACTORS Better in macular diseases with clear ocular media
Can be done in hazy ocular media
OVER REFRACTION CAPABILITY WITH SPECTACLES, CL, IOL
Difficult No problem
EXPECTED RESULTS Preliminary refractive findings
Gives refined subjective result.
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TOPCON RM 8900/8800
Monitor
Control leverwith measurementswitch
Forehead rest
Chin rest
Examination window
Power switch
1.Print switch 2.Menu switch
3.IOL switch
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SPECIFICATIONS AND PERFORMANCE
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OPTICAL PRINCIPLES
1. The Scheiner principle2. The optometer principle3. Retinoscopic principle4. Knife edge principle5. Ray- deflection princile6. Image size principle
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Basic design
Infrared source (IR-LED)
Fixation target
Badal lens system
Polarized beam splitter
Light sensor
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Autorefractors:
Primary and secondary source of electromagnetic radiation:
Primary- NIR (Near Infrared Radiation): 780-950nm
Efficiently reflected back from the fundusEssentially invisible to the patient
Secondary- back scatter from the fundus Determine sphere power, cylinder power, and
cylinder axis
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Fixation targetAccomodation is most relaxed when target is- Of low spatial frequency Same as typically seen at distance
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Nulling vs open-loop measurement principle
NULLING PRINCIPLE REFRACTOMETERS: Change their optical system until the refractive error
of the eye is neutralized.
OPEN-LOOP PRINCIPLE REFRACTOMETERS: Makes measurements by analysing the
characteristics of the radiation exiting the eye Do not alter their optical system
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ALLOWANCE
•NIR = 800-900nm•The reflectance of the fundus increases towards the red end of the spectrum•Into the infra red, there will be multiple reflections of scattered radiation within the eye which will degrade the image•A 800-900nm will create hypermetropia of 0.75-1.00 DS relative to 550nm
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Basic Principle
Infrared light
Rotatingchopper
Badallens
To PC
Polarisedfilter
Slitimagemore minus 0 more plus
Slitmask
Lightsensor
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BADAL OPTOMETER:
Infrared light is collimated
passes through rectangular masks housed in a
rotating drum
beam splitter
Optometer system
Moves laterally to find the optimal focus of the slit on the retina
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Optimal Focus:
Optimal focus is achieved when a peaksignal is received from the light sensor.
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SCHEINER DOUBLE PIN-HOLE
The original Scheiner double pin-hole was invented in the 16th century.
In a clinical setting, the double pin-hole identifies the level of ametropia in a subject by placing it directly in front of the patient’s pupil
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In a myopic eye, the patient sees crossed diplopic images, whereas in hyperopia, the patient sees uncrossed images.
Crossed and uncrossed doubling can easily be differentiated by asking the patient which image has disappeared, when either top or bottom pin-hole is occluded.
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OPTOMETER PRINCIPLE
Aim: to study the number of dioptres of correction on a scale with fixed spacing when the light from the target on the far side of the lens enter the eye with vergence of different amounts-
0 - + The vergence of the light in the focal plane of the optometer lens is linearly related to the displacement of the target.
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Modern optometers
Limitations of early optometers:1. Alignment problem: both pin-hole apertures
must fit within the pupil.2. Irregular astigmatism: the best refraction over
the whole pupil may be different in contrast to the two small pin hole areas of the pupil.
3. Accommodation: the instrument myopia alters the actual refractive status of the patient.
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Scheiner’s Principle:
Two LEDs (light emitting diodes) are imaged to the pupillary plane.
These effectively act as a modified Scheiner pin-hole by virtue of the narrow pencils of light produced by the small aperture pinhole located at the focal point of the objective lens.
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2 LEDs imaged to the pupillary plane
Ocular refraction l/t doubling of LED if
refractive error is present
Reflection of LEDs from retina back out of the
eye
Reflection by a semi-silvered mirror
Dual photodetectors
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RETINOSCOPIC PRINCIPLE
Autoretinoscopes Source of optical train corresponds to streak
retinoscope. Source of light is drum rotating about a source of
NIR which produces incident rectangular beam. Neutralization is by badal optometer Unwanted reflexes filtered by polarization of
incoming NIR Meridional refractors
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1. Autoretinoscope based on direction of fundus streak motion are nulling refractors.
2. Autoretinoscope based on speed of fundus streak motion are non nulling refractors.
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BEST FOCUS PRINCIPLE:
Aim: to find the best focus of an image on retina through the analysis of maximum contrast that can be captured by autorefractor.
Both meridional and nulling Neutralization is by badal optometer Filters unwanted reflexes by polarization
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Knife-edge principle
Aim: to evaluate the refractive uniformity of the lens. These are nulling autorefracors that are not meridional Based on principle of reciprocity All light returning through the system passes completely
back into the source. Theoretically, no light should escape past the knife-edge.
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Knife edge principle
Foucault Knife edge test for an emmetropic eye. The reflex on the detectormoves over most of the surface
Knife edge test for myopic eye.The motion of the reflex across thedetector provides information on the nature of the refractive error.The speed of the reflex describes the magnitude of refraction
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Ray deflection principle
Aim: to measure Linear deflection of the fundus image in 3 or more
meridia at a fixed distance from the eye Angular deflection of rays Position of far point in those meridia trignometrically
Open loop (non-nulling) meridional refractors Corneal reflexes removed by central aperture in the
plane Coaxial reflexes removed by polarization
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Image size principle
Aim: to measure the size of fundus image in 3 or more different meridia and calculate the full refractive error on the basis of ocular magnification or minification of the image relative to emmetropia.
Non nulling Detection system : fundus camera- CCD camera, video
imaging of the fundus reflex, analysis by sophisticated computer programme
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Conclusion Autorefraction is a valuable tool in
determining a starting point for refraction. Modern technology has resulted in
improvements in design, size, speed and accuracy.
There are primarily two principles utilised in current autorefractors
the Scheiner principle Retinoscopic principle. Improvements in target design (auto-fogging
distance targets and open view autorefractors) attempt to relax accommodation in patients.
Autorefractor should not be used as the final refractive correction without further confirmation.
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