Autorefractometer

S
AUTOREFRACTOMETER
Moderator - Dr. (Prof) ARVIND L. TENAGI
Presenter - Dr. Devanshu Arora
Wednesday, May 13th, 2015 1
Department of Ophthalmology, JNMC,
Belagavi

INTRODUCTION
S Refractometry is the estimation of refractive error
with a machine, called refractometer or optometer.
S Automated Refractometers (AutoRefractors) are
designed to objectively determine the refractive error
& are of various types depending upon the
underlying principle they are based on.
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S Over the last 200 years or so attempts have been
made to automate the process of refraction, but with
little success
S Until recently, when successful autorefractors were
developed, over the last 30 years, which could
objectively determine a patient’s refractive status
with an acceptable level of reliability.
S With the advent of technology these equipments
have become more sophisticated & increasingly
precise. Indeed, there are publications to support the
notion that modern autorefractors are more accurate
and repeatable than retinoscopy
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Why the need?
S The reason for its increasing popularity is primarily
that automated refraction devices offer speed,
reasonable accuracy and repeatability.
S With the increasing load of patients in any
ophthalmology practice, the practitioners are faced
with the challenge of completing all tasks (including
history, thorough examination & refraction being an
important part of it) within a fixed time frame. An
autorefractor will, therefore, increase the speed and
efficiency of the refraction process.
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S The use of these instruments in delivering
repeatable, unbiased data is invaluable in academic
& research studies wherever refractive and
keratometric parameters need to be recorded.
S However, we should not forget that retinoscopy
provides certain information not provided by
conventional autorefractors. For example, it informs
the practitioner about media opacities
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HISTORY & OPTICAL
PRINCIPLES
S The present day autorefractors are based on the
principles used in earlier attempts for automation of
refraction.
S It is therefore important to understand the underlying
principles on which they function as well as the
difficulties which prevented the successful
automation of refraction in the past.
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The Scheiner Principle
S Scheiner discovered in 1619 that the
point at which an eye was focused could
be precisely determined by placing double
pinhole apertures before the pupil.
S Parallel rays of light from a distant object are
reduced to two small bundles of light by the
Scheiner disc.
S 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

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S 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.
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Optometer Principle
S The term ‘optometer’ was first used in 1759 by
Porterfield who described an instrument for
‘measuring the limits of distinct vision, and
determining with great exactness the strength and
weakness of sight’.
S It involved 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.

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S 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.
S 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.
S 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.

S 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.
S A scale can thus be formed which would show the
number of diopters of correction according to the
position of the target.
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Meridional Refractometry
S In the presence of astigmatism, the axes of the
principal meridians must be found and refraction in
both measured.
S However, the need to identify the principal meridians
of astigmatism stood in the way of truly automated
refraction until the principle of meridional
refractometry was discovered in the 1960s.
S Which stated that if the spherical refraction is
measured in at least three arbitrary meridians, the
position of the principal axes and their refractive
powers can be calculated by mathematical
calculation.

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S The mathematical calculation is based on what is
called the sine-squared function.
S The three power measurements at the three
respective meridians provide three points on the
sine-squared function graph. From this, the rest of
the curve can be extrapolated in order to calculate
the maximum and minimum power values, i.e. the
principal focal planes.

EARLY OPTOMETERS
S The earliest instruments were the subjective
optometers in which the patient had to adjust the
instrument to achieve the best subjective alignment
or focus of the target.
S However they proved unsatisfactory because of 3
main limitations:-
1. Alignment Problems
2. Irregular Astigmatism
3. Instrument Accommodation
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S Alignment Problems: As per the requirements of
Scheiner’s Principle, both pinhole apertures must fit
in within the patient’s pupil. Achieving and
maintaining correct alignment of the instrument
required great skill & patience from the examiner &
good cooperation from the patient.
S Irregular Astigmatism: Instruments using the
Scheiner principle measure only the refraction of two
small portions of the pupillary aperture
corresponding to the apertures on the Scheiner’s
disc. In a patient with irregular astigmatism, the best
refraction over the whole pupil may be different in
contrast to the two small pinhole areas of the pupil.

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S Instrument Accommodation:- Inappropriate
accommodation often occurs when a target is
viewed which is known to be within an instrument.
This is called instrument accommodation and leads
to errors in the readings obtained.
S Later, the early objective optometers were
developed, but these required the examiner to focus
or align the image of a target on the patient's retina
& failed to come in main stream use because of
alignment difficulties & instrument accommodation.

MODERN
REFRACTOMETERS
S With the rapid development in electronics and
microcomputers, a number of innovative methods &
instruments for automated clinical refraction have
appeared since 1960.
S In recent years, the automatic infrared optometers
have come to the fore. These are truly objective
instruments as the instrument itself senses the end
point of refraction.
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BASIC DESIGN OF AN
AUTOREFRACTOR
S Autorefractors basically comprise of an infrared
source, a fixation target and a Badal optometer.
S An infrared light source (around 800-900nm) is used
primarily because it is invisible & helps overcome
instrument accommodation to a certain extent.
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S However, at this wavelength, light is reflected back
from the deeper layers of the retina, and this
together with chromatic aberration of the eye, the
refraction of the eye to infrared differs significantly
from its refraction to visible light.
S This difference is of the order of 0.75D – 1.50D more
hypermetropic to infrared. Manufacturers therefore
calibrate the instruments empirically to correlate with
subjective clinical results.
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FIXATION TARGET:-
S A variety of targets have been
used for fixation ranging from
animations to pictures with
peripheral blur to further relax
accommodation.
S Accommodation is most relaxed
when the patient identifies the
scene as one typically seen at a
distance which can be achieved
by using visual fixation targets
composed of photographs or
animations of outdoor scenes.

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S All autorefractors now
use the fogging
technique to relax
accommodation prior to
objective refraction.
S This is the reason why
patients state that the
target is blurred prior to
measurements being
taken – this is the effect
of the fogging lens

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OPTOMETER:-
S Virtually all autorefractors have a Badal optometer
within the measuring head.
S The Badal lens system has main advantage that,
there is a linear relationship between the distance of
the Badal lens to the eye and the ocular refraction
within the meridian being measured.

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TYPES OF
AUTOREFRACTORS
S Fundamentally, there are three types of
autorefractors which derive objective refraction by:
• Image quality analysis
• Scheiner double pin-hole refraction
• Retinoscopy
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Image Quality Analysis
S This method is not used very much in modern-day
autorefractors. It was originally used in the Dioptron
autorefractor developed in the 1970s.
S Here, the optimal position of the optometer lens was
determined by the output signal of the light sensor.
The light sensor matches the intensity profile of the
incoming light from the eye, to the light intensity
pattern from the rotating slit drum.

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How The Image Analyser Determines The Optimal
Position Of The Optometer Lens?
A low intensity profile tells
the autorefractor that the
optometer lens is not in
the correct position to
correct the power. When
the intensity profile
reaches a peak, the
optometer reading is
taken to signify the power
of the meridian being
measured.

Autorefractors based on the
Scheiner Principle
S Most of the latest autorefractors used in practice
today use the Scheiner principle.
S Implementation of this technology in autorefractors is
somewhat different to that used by Scheiner in his
double pin-hole experiment.
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S In general, two LEDs (light emitting diodes) are
imaged to the pupillary plane. These effectively act
as a modified Scheiner pinhole by virtue of the
narrow beams of light produced by the small
aperture pinhole located at the focal point of the
objective lens.
S Ocular refraction leads to doubling of the LEDs if
refractive error is present. After refraction, the retinal
image of the LEDs reflects from the retina back out
of the eye. However, light emanating from the eye is
again reflected by a semi silvered mirror to a dual
photodetector.

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S As the LED system is moved back and forth, the
separation of the two images varies on the
photodetector.
S When the retinal image is single, a single LED
image is centred over both photodetectors. The LED
position corresponds to the refractive error in that
meridian.
S In the case of astigmatism, four LEDs are used and
the power perpendicular to the meridian under test is
measured.

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S It is apparent that alignment of the photodetectors in
such a system is important.
S Basically, it is important that both the patient fixation
and instrument axes are coaxial. If this condition is
not met, then effectively the objective refraction is
conducted from an off-axis point – and this leads to
error. Manufacturers have attempted to reduce these
errors with auto-alignment systems.
S Practitioners who ‘over-ride’ this function, by
continually holding down the joystick button, may
effectively increase the error of measurement due to
the possibility of misalignment.

Autorefractors based on
Retinoscopy Principle
S Some autorefractors (eg. Welch Allen Suresight and
Power Refractor II) use infra-red videorefraction
S A grating, or slit, is produced by a rotating drum.
S Similar principles to retinoscopy are used where the
speed of the reflex is used as an indicator of the
patient’s refraction.
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S It is also referred to as ‘The Knife Edge Principle’.
S Knife-edge refractors use the concept of optical
reciprocity such that radiation from the fundus reflex
is returned to the primary source.
S The slit or ‘knife’ is used to determine the refractive
power of the eye
S The speed and direction of the movement of the
reflex is detected by photodetectors and computed
to derive the meridional power

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S The time difference from the slit reaching each of the
detectors allows the autorefractor to detect the
meridian under investigation.

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S The vertical slit calculates the refraction of the
vertical meridian. The system detects that the
vertical meridian is measured by the way each
detector senses the slit as it passes over the pupil.
S The oblique slit will like wise initiate a different time
dependent response from the detectors, and thus
derive the power within the oblique meridian.

Autorefractors Currently in Use
S Autorefractors are most commonly used to provide
the starting point for refraction to obtain an objective
result before performing subjective refraction.
S Most commercially available Autorefractors available
today come with an inbuilt Automated Keratometer &
are known as Auto Kerato-Refractometer.
S Recently new equipments with addition corneal
topgrahers have been developed in which Corneal
Topography can also be performed.
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Portable Autorefractors
S Since the autorefractor is
stationary, examining light
refraction in children has
remained somewhat challenging.
To address the problem, scientists
developed a portable autorefractor
that is particularly helpful in
examining children as they can
easily adjust themselves
according to different positions of
the patient.
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S 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.
S It is also ideal for vision screenings in community
groups or health fairs.
S 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. Tech-
nicians or doctors can position themselves to make
them work on bedridden patients.

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Intraoperative Refraction
S A new intraoperative approach to IOL power
calculation has been tried which is based solely on
optical refractometry, independent of anatomical and
biometric measurements used in traditional
techniques such as axial length and keratometry.
S Intraoperative aphakic refraction is performed after
cataract extraction using a portable autorefractor &
IOL power calculated using specific formulas for the
same.
S There are certain publications on this technique but
more studies are required before it can become
main stream.

Recent Advances in
Automated Refraction
S Automated Refraction Systems have been
developed in which an autorefractor is connected
with other automated instruments like a phoropter &
an electronic chart and results obtained from the
autorefractor reading are automatically transmitted
to phoropter and various lenses tried, so that both
objective & subjective refraction are performed in an
automated way.
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Automated Refraction System
The standard components of an automated refraction system:
S An Autorefractor
S An electronic and motor driven auto-phoropter, used to
present powered lenses in front of the patient's eyes
S A hardware or software-driven controller that changes the
lenses in the phoropter for the subjective portion of the
testing.
S An Eye Chart to aid in the determination of visual acuity
during the test
S An autolensmeter that measures the powers of the patient's
current pair of glasses or contact lenses.

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Automated Refraction System Video
Disclaimer-
Video incorporated for example purpose only. No commercial
interest intended

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Advantages of automated refraction systems vs. manual
refraction equipment are:
S less manual labour by the practitioner or technician
S more automation of repetitive and iterative tasks in the
refraction
S ability to present former and new values quickly for
validation
S reduced risk of human error
S direct transmission of results to Electronic Medical
Record(EMR) software
S Improved efficiency of practice

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S 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.

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S Additionally, some variations on the traditional
autorefractor have been developed.
S The aberrometer is an advanced form of
autorefractor that examines light refraction from
multiple sites on the eye.
S 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

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S 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.
S Conventional examination techniques &
autorefractors only measure lower-order aberrations
such as myopia, hypermetropia, and astigmatism.
S 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.

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S Wavefront technology, or aberrometry, diagnoses
both lower- and higher-order vision errors
represented by the way the eye refracts or focuses
light.
S Wavefront analysisis 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.

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S Wavefront analysis is approximately 25-50 times
more accurate than the autorefractometer
S 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.

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Basic functioning of Hartmann-Shack Aberrometer

Prescribing directly
from autorefractors
S Although many studies have evaluated the accuracy
and repeatability of autorefractors relative to
subjective refraction, the ability of patients to adapt
and tolerate these prescriptions has not been
addressed.
S Strang-et al conducted an interesting study to
investigate patient tolerance to autorefractor
prescriptions.
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Strang et al Study
S Forty-seven subjects with a mean age of 36.7
(±16.7) and no ocular pathology, were enrolled into
their study. Six autorefractors (Canon RL-10, Hoya
AR-559, Humphrey AR-595, Nidek AR-800, Nikon
NR-5500 and Topcon RM-A7000) were used to
refract the patients in addition to carrying out
subjective refraction. Spectacles were made from
the prescription of one of the six autorefractors
(assigned randomly) and the practitioner.
S Subjects wore each prescription for two weeks. Both
the investigators and the subjects were masked as
to the prescription being worn. After each period,
subjects filled out a questionnaire.

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S The authors’ concluded that prescribing purely from
the autorefractor prescription was unfeasible in
practice.
S Similar studies need to be conducted with modern-
day autorefractors and instruments capable of
automated subjective refraction

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Autorefraction in Irregular eyes
S Corneal shape post refractive surgery is clearly
modified in the majority of procedures.
S Furthermore, specific algorithms are used in lasers
which ablate the cornea to reduce aberrations.
S Most autorefractors (all Scheiner based) perform
refraction through a fixed pupil diameter.
S Therefore, the influence of overall refraction
throughout the pupillary plane will not be addressed.

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S 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.

Conclusion
S Autorefractors are a valuable tool in determining a
starting point for refraction.
S Modern technology has resulted in improvements in
design, size, speed and accuracy.
S There are primarily two principles utilised in current
autorefractors – the Scheiner principle and the
Retinoscopic principle.
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S Improvements in target design ( like auto-fogging
distance targets) attempt to relax accommodation in
patients. The results of autorefraction post refractive
surgery, and in eyes with corneal distortion, should
always be viewed with suspicion.
S Aberrometers may help to provide a better starting
point for refraction in these instances
S Unfortunately, the cost of these systems is
significantly greater than the cost of autorefractors
and is therefore not likely to replace automated
refraction at the present time.

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Thank You For Your Attention

REFERENCES
S Clinical Optics by Andrew R. Elkington 3rd Edition
S Ophthalmology 4th edition- Yanoff & Duker
S Automated refraction Design and applications,
Optometry Today
S Theory & Practice of Refraction by AK Khurana 3rd
Edition
S Internet
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