Autorefractometer

AUTOREFRACTOMETER

AUTOREFRACTOMETERModerator - Dr. (Prof) ARVIND L. TENAGI

Presenter - Dr. Devanshu AroraWednesday, May 13th, 20151Department of Ophthalmology, JNMC, Belagavi

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INTRODUCTIONRefractometry is the estimation of refractive error with a machine, called refractometer or optometer.Automated Refractometers (AutoRefractors) are designed to objectively determine the refractive error & are of various types depending upon the underlying principle they are based on.Wednesday, May 13th, 20152Department of Ophthalmology, JNMC, Belagavi

Over the last 200 years or so attempts have been made to automate the process of refraction, but with little successUntil recently, when successful autorefractors were developed, over the last 30 years, which could objectively determine a patients refractive status with an acceptable level of reliability. 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 retinoscopyWednesday, May 13th, 20153Department of Ophthalmology, JNMC, Belagavi

Why the need?The reason for its increasing popularity is primarily that automated refraction devices offer speed, reasonable accuracy and repeatability.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.Wednesday, May 13th, 20154Department of Ophthalmology, JNMC, Belagavi

Since most present day autorefractors have an inbulit automatic keratometer as well. 4

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.However, we should not forget that retinoscopy provides certain information not provided by conventional autorefractors. For example, it informs the practitioner about media opacitiesWednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi5

HISTORY & OPTICAL PRINCIPLESThe present day autorefractors are based on the principles used in earlier attempts for automation of refraction.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. Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi6

Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi7The Scheiner Principle

Scheiner discovered in 1619 that the point at which an eye was focused couldbe precisely determined by placing double pinhole apertures before the pupil.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

Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi8

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 scheiners disc is placed in front of the pupilIn 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.8

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 patients eye and the refractive error can be determined.Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi9

Far point is the point farthest from the eye at which an object is accurately focused on the retina when the accommodation is completely relaxed9

Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi10Optometer Principle

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.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.


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.

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Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi12Light 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.Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi13

Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi14Meridional Refractometry

In the presence of astigmatism, the axes of the principal meridians must be found and refraction in both measured.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.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.

Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi15The mathematical calculation is based on what is called the sine-squared function.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 OPTOMETERSThe 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.However they proved unsatisfactory because of 3 main limitations:-Alignment ProblemsIrregular AstigmatismInstrument AccommodationWednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi16


Alignment Problems: As per the requirements of Scheiners Principle, both pinhole apertures must fit in within the patients pupil. Achieving and maintaining correct alignment of the instrument required great skill & patience from the examiner & good cooperation from the patient. 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 Scheiners 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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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.

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 REFRACTOMETERSWith the rapid development in electronics and microcomputers, a number of innovative methods & instruments for automated clinical refraction have appeared since 1960.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. Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi19

BASIC DESIGN OF AN AUTOREFRACTORAutorefractors basically comprise of an infrared source, a fixation target and a Badal optometer.An infrared light source (around 800-900nm) is used primarily because it is invisible & helps overcome instrument accommodation to a certain extent.Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi20

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.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.Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi21

Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi22FIXATION TARGET:-A variety of targets have been used for fixation ranging from animations to pictures with peripheral blur to further relax accommodation.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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Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi24All autorefractors now use the fogging technique to relax accommodation prior to objective refraction.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


Certain autorefractors use an open view to allow patients an unrestricted binocular view of a distance target25

Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi26OPTOMETER:-Virtually all autorefractors have a Badal optometer within the measuring head.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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Infrared light is collimated & passes through rectangular masks present in a rotating drum. The light passes through a beam splitter to the optometer system & is projected on the retina & a slit image is formed. The polarising beam splitter effectively removes reflected light from the cornea whereas the slit image from the retina passes through the polarised beam splitter and falls on the light sensor. The optometer lens system moves laterally to find the optimal focus of the slit on the retina. Optimal focus is achieved when a peak signal is received from the light sensor.27

TYPES OF AUTOREFRACTORSFundamentally, there are three types of autorefractors which derive objective refraction by: Image quality analysis Scheiner double pin-hole refraction RetinoscopyWednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi28

Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi29Image Quality AnalysisThis method is not used very much in modern-day autorefractors. It was originally used in the Dioptron autorefractor developed in the 1970s.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.


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.

Sub-optimal position position of optometer lens- results in low detector output

this is performed for three meridians & results obtained30

Autorefractors based on the Scheiner PrincipleMost of the latest autorefractors used in practice today use the Scheiner principle.Implementation of this technology in autorefractors is somewhat different to that used by Scheiner in his double pin-hole experiment.Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi31

Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi32In 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.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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Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi34As the LED system is moved back and forth, the separation of the two images varies on the photodetector. 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.In the case of astigmatism, four LEDs are used and the power perpendicular to the meridian under test is measured.

Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi35It is apparent that alignment of the photodetectors in such a system is important. 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.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 PrincipleSome autorefractors (eg. Welch Allen Suresight and Power Refractor II) use infra-red videorefractionA grating, or slit, is produced by a rotating drum.Similar principles to retinoscopy are used where the speed of the reflex is used as an indicator of the patients refraction.Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi36

Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi37It is also referred to as The Knife Edge Principle.Knife-edge refractors use the concept of optical reciprocity such that radiation from the fundus reflex is returned to the primary source.The slit or knife is used to determine the refractive power of the eyeThe speed and direction of the movement of the reflex is detected by photodetectors and computed to derive the meridional power


Knife edge test for myopic eye. The motion of the reflex across the detector provides information on the nature of the refractive error. The speed of the reflex describes the magnitude of refraction.

Knife edge test for an emmetropic eye. The reflex on the detector moves over most of the surface38

Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi39The time difference from the slit reaching each of the detectors allows the autorefractor to detect the meridian under investigation.

Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi40The 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.

The oblique slit will like wise initiate a different time dependent response from the detectors, and thus derive the power within the oblique meridian.

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Autorefractors Currently in UseAutorefractors 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 topgrahers have been developed in which Corneal Topography can also be performed.Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi41

Portable AutorefractorsSince 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.Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi42

Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi43The 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 doctors 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. Tech-nicians or doctors can position themselves to make them work on bedridden patients.


Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi45Intraoperative RefractionA 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.Intraoperative aphakic refraction is performed after cataract extraction using a portable autorefractor & IOL power calculated using specific formulas for the same.There are certain publications on this technique but more studies are required before it can become main stream.

Recent Advances in Automated RefractionAutomated 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.Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi46

Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi47Automated Refraction SystemThe standard components of an automated refraction system:An AutorefractorAn electronic and motor driven auto-phoropter, used to present powered lenses in front of the patient's eyesA hardware or software-driven controller that changes the lenses in the phoropter for the subjective portion of the testing.An Eye Chart to aid in the determination of visual acuity during the testAn autolensmeter that measures the powers of the patient's current pair of glasses or contact lenses.

Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi48Automated Refraction System VideoDisclaimer-Video incorporated for example purpose only. No commercial interest intended


Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi50Advantages 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

Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi51Recently, a tool has been developed which works by combining a simple optical attachment with software on a smartphone which enables assessment of Refractive Error.

NETRA51

Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi52Additionally, 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

Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi53Several 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.

Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi54Wavefront technology, or aberrometry, diagnoses both lower- and higher-order vision errors represented by the way the eye refracts or focuses light.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.

Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi55Wavefront analysis is approximately 25-50 times more accurate than the autorefractometerNow 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.


Basic functioning of Hartmann-Shack Aberrometer

A thin laser beam enters the eye and is focused on the retina. As the emerging rays reflect off the macula and refracts out of the eye through each part of the optical media, they are focused onto n lens array & are captured by a CCD-camera which quantifies their deviation, and creates the wavefront pattern from the recorded deviation.56

Prescribing directlyfrom autorefractorsAlthough 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.Strang-et al conducted an interesting study to investigate patient tolerance to autorefractor prescriptions.Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi57

1996 Optician57

Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi58Strang et al StudyForty-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.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.


Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi60The authors concluded that prescribing purely from the autorefractor prescription was unfeasible in practice.

Similar studies need to be conducted with modern-day autorefractors and instruments capable of automated subjective refraction

Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi61 Autorefraction in Irregular eyesCorneal 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.

Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi62In 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.

ConclusionAutorefractors are 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 and the Retinoscopic principle.Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi63

Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi64Improvements 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.Aberrometers may help to provide a better starting point for refraction in these instancesUnfortunately, 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.

Wednesday, May 13th, 2015Department of Ophthalmology, JNMC, Belagavi65Thank You For Your Attention

REFERENCESClinical Optics by Andrew R. Elkington 3rd EditionOphthalmology 4th edition- Yanoff & DukerAutomated refraction Design and applications, Optometry TodayTheory & Practice of Refraction by AK Khurana 3rd EditionInternet


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Autorefractometer