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## A New Definition of Refraction: Basics and Beyond

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**A New Definition of Refraction:Basics and Beyond**Austin Roorda, Ph.D. Unversity of Houston College of Optometry**First use of Optics as Vision Aids**reading stones from1000 AD**1284 Salvino D'Armate built the first spectacles which were**used as reading glasses 1451 Nicholas of Cusa uses negative lenses to cure myopia (nearsightedness) 1604 Kepler describes the optics of near and far sightedness Kepler 1268 Roger Bacon discussed possibilities of magnification with lenses Roger Bacon**1827: Sir George Biddel Airy invents spectacles for**astigmatism 1748: Benjamin Franklin invents the bifocal lens**Aberrometers**VISX Wavescan B&L Zywave Summit Autonomous Custom Cornea Wavefront Sciences Complete Optical Analysis System TraceyTech Tracey 1**What is the Wavefront?**parallel beam = plane wavefront converging beam = spherical wavefront**What is the Wavefront?**parallel beam = plane wavefront ideal wavefront defocused wavefront**What is the Wavefront?**parallel beam = plane wavefront ideal wavefront aberrated beam = irregular wavefront**What is the Wavefront?**diverging beam = spherical wavefront aberrated beam = irregular wavefront ideal wavefront**What is the Wave Aberration?**diverging beam = spherical wavefront wave aberration**Wavefront Aberration**3 2 1 mm (superior-inferior) 0 -1 -2 -3 -3 -2 -1 0 1 2 3 mm (right-left) Wave Aberration: Defocus**Wavefront Aberration**3 2 1 0 mm (superior-inferior) -1 -2 -3 -3 -2 -1 0 1 2 3 mm (right-left) Wave Aberration: Astigmatism**Wavefront Aberration**3 2 1 mm (superior-inferior) 0 -1 -2 -3 -3 -2 -1 0 1 2 3 mm (right-left) Wave Aberration: Coma**Wavefront Aberration**3 2 1 mm (superior-inferior) 0 -1 -2 -3 -3 -2 -1 0 1 2 3 mm (right-left) Wave Aberration: All Terms**How Do We Interpret the**Wave Aberrations?**Wave Aberration Contour Map**2 1.5 1 0.5 mm (superior-inferior) 0 -0.5 -1 -1.5 -2 -2.5 -2 -1 0 1 2 mm (right-left)**Breakdown of Zernike Terms**Coefficient value (microns) -0.5 0 0.5 1 1.5 2 1 2 astig. 3 2nd order defocus 4 astig. 5 trefoil 6 3rd order coma 7 coma 8 Zernike term trefoil 9 10 11 4th order spherical aberration 12 13 14 15 16 5th order 17 18 19 20**Root Mean Square**…… trefoil term defocus term astigmatism term astigmatism term**Point Spread Function vs. Pupil Size**1 mm 2 mm 3 mm 4 mm pupil images followed by psfs for changing pupil size 5 mm 6 mm 7 mm**Simulated Images**20/20 letters 20/40 letters**Strehl Ratio**diffraction-limited PSF Hdl actual PSF Heye**Area under the MTF**Modulation Transfer Function 1 0.9 20/20 20/10 0.8 0.7 0.6 contrast 0.5 0.4 0.3 0.2 0.1 0 0 50 100 150 spatial frequency (c/deg)**Can we Build a Better Autorefractor?**choose the spectacle correction that minimizes the second order Zernike terms Coefficient value (microns) -0.5 0 0.5 1 1.5 2 1 2 astig. 3 2nd order defocus 4 astig. 5 trefoil 6 coma 3rd order 7 coma 8 trefoil 9 Zernike term 10 11 4th order spherical aberration 12 13 14 15 16 5th order 17 18 19 20**Can we Build a Better Autorefractor?**choose the spectacle correction that maximizes the strehl ratio perfect PSF defocus/astigmatism**Can we Build a Better Autorefractor?**choose the spectacle correction that maximizes the area under the MTF 4.5 4 3.5 3 Area under MTF (0 - 50 c/d) 2.5 2 1.5 1 0.5 0 0 1 -1 0.2 0.4 0.6 0.8 -1.4 -1.2 -0.8 -0.6 -0.4 -0.2 Defocus (D)**Can we Build a Better Autorefractor?**• Challenge • RMS, Strehl, and MTF area are not correlated • Solution • population data (Rochester, Indiana) • early results suggest maximizing metrics like the Strehl ratio works best**Aberrometric Refraction**• conventional autorefractors are accurate to ~ 0.5 D • aberrometric autorefractor should achieve ~ 0.1 D accuracy ** ** ± 0.25 D is an acceptable correction error**Post - RK**Post - LASIK