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Automatic Refractometer and it's principles
- 1. S AUTOREFRACTOMETER Bimal Kumar Thakur Consultant Optometrist 1
- 2. 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. 2
- 3. 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. 3
- 4. 4 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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- 6. 6 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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- 8. 8 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.
- 9. 9 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.
- 10. 10 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.
- 11. 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 11
- 12. 12 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.
- 13. 13 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.
- 14. 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. 14
- 15. 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. 15
- 16. 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. 16
- 17. 17 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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- 19. 19 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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- 21. 21 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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- 23. 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 23
- 24. 24 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.
- 25. 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. 25
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- 27. 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. 27
- 28. 28 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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- 30. 30 S The time difference from the slit reaching each of the detectors allows the autorefractor to detect the meridian under investigation.
- 31. 31 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.
- 32. 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. 32
- 33. 33 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
- 34. 34 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
- 35. 35 Basic functioning of Hartmann-Shack Aberrometer
Editor's Notes
- The parallel rays of light entering the eye from a distant object are normally focused on a point on the retina in an emmetropic patient. They are limited to 2 small bundles when double pinhole apertures or a scheiner’s disc is placed in front of the pupil In a myopic eye, the 2 ray bundles cross each other before reaching the retina, and 2 small spots of light are seen. In a hypermetropic eye, the ray bundles are intercepted by the retina before they meet & thus again 2 small spots of light are seen.
- 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.
- Certain autorefractors use an open view to allow patients an unrestricted binocular view of a distance target
- 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.
- 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 surface
- 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.