Clinical Trusit Dave PhD, BSc, MCOptom, FAAO Automated refraction Design and applications relax accommodation. All autorefractors now use the fogging technique to relax hen the first autorefractor was developed over 30 years accommodation prior to objective ago, many optometrists were concerned about the impact refraction. Practitioners may recall in the such devices would have on the profession. Today, those W past patients stating that the target is concerns are all but forgotten, with the eyecare profession blurred prior to measurements being taken positively embracing objective refraction technology. – this is the effect of the fogging lens. The reason for its increasing popularity is that practitioners are faced with the However, even with this fogging technique, primarily that automated refraction challenge of completing all these tasks micro fluctuations in accommodation devices offer speed, reasonable accuracy within a fixed time frame. An autorefractor occur up to 0.50DS5. Some of this effect is and repeatability. Indeed, there are will, therefore, increase the speed and counteracted by averaging multiple publications to support the notion that efficiency of the refraction process. readings – however, the error is not autorefractors are more accurate and Academic studies require unbiased eliminated. The Shin Nippon NVISION-K repeatable than retinoscopy1,2. However, refractive data. The refraction produced by 5001 (Figure 1) uses an open view to one should not forget that retinoscopy some autorefractors has been shown to be allow patients an unrestricted binocular provides certain information not provided more repeatable than retinoscopy, and as view of a distance target, e.g. a distance by conventional autorefractors. For repeatable as subjective refraction in object. example, it informs the practitioner about cyclopleged subjects2. The use of these Virtually all autorefractors have a Badal media opacities and significant ocular instruments in delivering repeatable, optometer within the measuring head. The aberration. This article describes the unbiased data is invaluable in studies Badal lens system has two main technology employed by various investigating myopia development. advantages. Firstly, there is a linear autorefractors, and considers aspects such relationship between the distance of the as direct prescribing and where these Basic design Badal lens to the eye and the ocular instruments are potentially inaccurate. Autorefractors basically comprise of an refraction within the meridian being infrared source, a fixation target and a measured. Secondly, with a Badal lens Why the need? Badal optometer. An infrared light source system, the magnification of the target The need to deliver a comprehensive eye (around 800-900nm) is used primarily remains constant irrespective of the examination (in terms of detection and because of the ocular transmission and position of the Badal lens. Figure 2 diagnosis of disease) means that many reflectance characteristics achieved at the illustrates the basic principle of the practitioners will benefit from additional sclera3. At this wavelength, light is reflected autorefractor. This type of design was information that provides a valuable basis back from the deeper layers of the eye incorporated by the Dioptron autorefractor upon which to conduct a subjective (choroid and sclera4) and this, together (Coopervision) in the 1970s and refraction. A comprehensive eye with the effects of longitudinal chromatic developed by Charles Munnerlyn who also examination means a complete symptoms aberration, means that a systematic error happens to be one of the pioneers of the and history, ophthalmic investigation of approximately -0.50DS must be added excimer laser. (including subjective refraction) and to compensate for ocular refraction with Infrared light is collimated and passes finally and most importantly, a discussion visible light. through rectangular masks housed in a of the findings. All this, together with new A variety of targets have been used for rotating drum. The light passes through a guidelines on shared care with diabetic, fixation ranging from less interesting ‘stars’ beam splitter to the optometer system. glaucoma and cataract protocols, means to pictures with peripheral blur to further This system moves laterally to find the Figure 1 Figure 2 The Shin Nippon NVISION-K 5001 Basic principle of the autorefractor Infrared light Condensing lens Rotating chopper Polarised filter Light sensor Slit mask Badal lens To PC Slit more minus 0 more plus image 28 | June 4 | 2004 OT Clinical optimal focus of the slit on the retina. Three types of autorefractors profile tells the autorefractor that the Optimal focus is achieved when a peak Fundamentally, there are three types of Badal lens is not in the correct position to signal is received from the light sensor. autorefractors which derive objective correct the meridional power. When the The polarising beam splitter effectively refraction by: intensity profile reaches a peak, the Badal removes reflected light from the cornea optometer reading is taken to signify the whereas the slit image on the retina passes • Image quality analysis power of the meridian being measured. through the polarised beam splitter. The • Scheiner double pin-hole refraction Once this is performed for three system measures at least three meridians • Retinoscopy meridians, the sine-squared function is of the eye in order to derive the refractive used to derive the sphero-cylindrical power of the eye using the sine-squared Each of these will now be discussed in prescription. function6. more detail. Perrigin et al7 compared the refractive The sine-squared function of ocular data from the Dioptron Nova with astigmatism describes the variation of Image quality analysis subjective refraction in a clinical setting for meridional astigmatic power. Thus, for any This method is not used very much in 236 patients. Dioptron and subjective data given prescription sph/-cylxθ, the power modern-day autorefractors. It was had an agreement of ±0.50 for 74% of eyes along any given meridian is given by the originally used in the Dioptron with respect to mean spherical equivalent formula sph+(cyl x sine2θ). Figure 3 autorefractor. However, for completeness, power. Mailer8 compared the accuracy of illustrates the sine-squared function for it will be discussed here. the Dioptron II pre and post cycloplegia the prescription +2.00/-5.00x90. In Figure 2, the basic design of the with subjective refraction in 84 patients. Autorefractors only need to calculate autorefractor is described. Here, the There was 46% agreement to ±0.25DS for the power at three chosen meridians in optimal position of the Badal optometer spheres, 51% for ±0.25D cylinders and order to calculate the sphero-cylindrical lens was determined by the output signal 44% for mean spherical equivalent. After prescription using the sine-squared of the light sensor. The rotating drum cycloplegia, there was 47%, 51% and 51% function. Basically, the three power effectively produces a light/dark agreement respectively. Furthermore, measurements at the three respective alternating target. The light sensor matches cylinder axis agreement was 46% without, meridians provide three points on the the intensity profile of the incoming light and 29% with cycloplegia for ±5 degrees sine-squared function graph. From this, from the eye, to the light intensity pattern axis error. The author concluded that the the rest of the curve can be extrapolated in from the rotating slit drum. Dioptron provided a useful “starting order to calculate the maximum and Figure 4 shows how the image analyser point” to subjective refraction8. Similar minimum power values, i.e. the principal determines the optimal position of the conclusions have been drawn in other focal planes. Badal optometer lens. A low intensity studies9,10. Signal Signal to eye to eye Signal to Signal to light sensor light sensor Sub-optimal position of Badal optometer lens Optimal position of Badal optometer lens Result in: low detector output Result in: high detector output Figure 3 Figure 4 The sine-squared function describes the meridional power variation Autorefraction using the image analysis principle of sphero-cylindrical refractive error Distance light Scheiner double Semi-silvered mirror source pin-hole Narrow pin-hole EYE CROSSED Dual LEDs DIPLOPIA Condensing lens Objectives NO Lateral movement of LEDs to enable correct alignment at DIPLOPIA the photodetector Dual photodetector UNCROSSED DIPLOPIA Figure 5 Figure 6 Optical principles of the Scheiner double pin-hole Principle of the Scheiner double pin-hole based autorefractors 29 | June 4 | 2004 OT Clinical Trusit Dave PhD, BSc, MCOptom, FAAO Study Comparison Subjects Results Conclusion Kinge et al, 1996 Subjective refraction vs. 448 eyes subjective refraction Both autorefractors over minus Valuable complement to BJO Nidek AR-1000 & 448 eyes Humphrey 500 Humphrey 500 by R -0.23D subjective refraction – Humphrey 500 160 eyes Nidek AR-1000 L -0.20 not a replacement. Cycloplegia Nidek AR -1000 by R -0.13, Nidek better on spherical Mean age 20.1 (SD 1.1) L -0.11. equivalent; Humphrey Both p<0.0005 better for astigmatism Gwiaza & Weber, Canon R1 vs. Grand Seiko REs of 50 subjects 92% measures with ±0.25 for cyl Consider the agreement 2004 WR-5100K & Nidek ARK Mean age 30.5 (range 17-59) power (Nidek vs. Seiko). 42% for between autorefractor Optom. Vis. Sci. 700A No cycloplegia Seiko vs. Canon. 40% for Nidek results as different &