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Stanford University Psych 221 / EE 362 Winter 2002-2003 Zernike Polynomials and Their Use in Describing the Wavefront Aberrations of the Human Eye Psych 221/EE362 Applied Vision and Imaging Systems Course Project, Winter 2003 Patrick Y. Maeda [email protected] Stanford University Patrick Y. Maeda 3/10/03 Introduction and Motivation Stanford University Psych 221 / EE 362 Winter 2002-2003 Great interest in correcting higher order aberrations of the eye Laser eye surgery (PRK, LASIK) – Currently, only defocus and astigmatism being corrected (2nd order aberrations) – Improve vision better than 20/20 – Correct problems caused or induced by current generation of laser surgery Imaging of the retina and other structures in the eye using adaptive optics Correction requires measurement of optical aberrations Defocus and astigmatism can be determined using sets of lenses Measurement of higher orders require more sophisticated techniques – Measurement of the wavefront aberration with Shack-Hartmann Wavefront Sensor Mathematical description of the aberrations needed Accurate description of wave aberration function Accurate estimation of wave aberration function from measurement data Patrick Y. Maeda 3/10/03 Project Outline Stanford University Psych 221 / EE 362 Winter 2002-2003 Introduction/Motivation General Optical System Description Monochromatic Wavefront Aberrations PSF and MTF calculations Why Use Zernike Polynomials? Definition of Zernike Polynomials Describing Wave Aberrations using Zernike Polynomials Simulating the Effects of Wave Aberrations Wavefront Measurement and Data Fitting with Zernike Polynomials Conclusion, References, Source Code Appendix Patrick Y. Maeda 3/10/03 Stanford University Psych 221 / EE 362 Winter 2002-2003 Coordinate Systems Object Plane y Object h Height x Pupil Coordinate System y Optical System y Image Plane Optical Axis Normalized Pupil Coordinate System y x θ x h’ ρ r y a θ x x 1 z Image Height x = r cos(θ) y = r sin(θ) θ = tan-1(x/y) r = (x2+y2)1/2 x = ρ cos(θ) y = ρ sin(θ) θ = tan-1(x/y) ρ = r/a = (x2+y2)1/2 Patrick Y. Maeda 3/10/03 Stanford University Psych 221 / EE 362 Winter 2002-2003 Wave Aberration Exit Pupil Wave Aberration W(x,y) 2 i W ( x, y ) 1 PSF ( x, y ) 2 2 FT p( x, y ) e d Ap MTF ( s x , s y ) 2 fx x y , fy d d FT PSF FT PSF s x 0, s y 0 y Aberrated Wavefront Reference Spherical Wavefront Image Plane x z The wavefront aberration, W(x,y), is the distance, in optical path length (product of the refractive index and path length), from the reference sphere to the wavefront in the exit pupil measured along the ray as a function of the transverse coordinates (x,y) of the ray intersection with the reference sphere. It is not the wavefront itself but it is the departure of the wavefront from the reference sphere. Patrick Y. Maeda 3/10/03 Describing Optical Aberrations Stanford University Psych 221 / EE 362 Winter 2002-2003 Optical system aberrations have historically been described, characterized, and catalogued by power series expansions Many optical systems have circular pupils Application of experimental results typically require data fitting It is, therefore, desirable to expand the wave aberration in terms of a complete set of basis functions that are orthogonal over the interior of a circle Patrick Y. Maeda 3/10/03 Why Use Zernike Polynomials? Stanford University Psych 221 / EE 362 Winter 2002-2003 Zernike polynomials form a complete set of functions or modes that are orthogonal over a circle of unit radius Convenient for serving as a set of basis functions Expressible in polar coordinates or Cartesian coordinates Scaled so that non-zero order modes have zero mean and unit variance – Puts modes in a common reference frame for meaningful relative comparison Other power series descriptions are not orthogonal Wave aberrations in an optical system with a circular pupil accurately described by a weighted sum of Zernike polynomials The Orthonormal set of Zernike polynomials is recommended for describing wave aberration functions and for data fitting of experimental measurements for the eye7 – Terms are normalized so that the coefficient of a particular term or mode is the RMS contribution of that term Patrick Y. Maeda 3/10/03 Mathematical Formulae 3 Stanford University Psych 221 / EE 362 Winter 2002-2003 The Zernike polynomial s are defined as 3 : Z nm ( , ) N nm Rn ( ) cos( m ) m N nm Rn ( ) sin( m ) m for m 0 , 0 1 , 0 2 for m 0 , 0 1 , 0 2 for a given n : m can only take on values of n, n 2, n 4, , n N nm is the normalizat ion factor N nm 2(n 1) 1 m0 m 0 1 for m 0 , m 0 0 for m 0 factorial.m Rn ( ) is the radial polynomial m R () m n ( n m ) 2 s 0 zernike.m (1) s (n s )! n2 s s ! 0.5(n m ) s ! 0.5(n m ) s ! Patrick Y. Maeda 3/10/03 List of Zernike Polynomials 7, 9,10 Stanford University Psych 221 / EE 362 Winter 2002-2003 mode order frequency j n m Z nm , Meaning 0 1 2 0 1 1 0 -1 1 1 2 sin ( ) 2 cos( ) Constant t erm, or Piston Tilt in y - direction, Distortion Tilt in x - direction, Distortion 3 2 -2 4 2 0 3 2 2 1 Field curvature, Defocus 5 2 2 6 2 cos (2 ) Astigmatis m with axis at 0 or 90 6 3 -3 8 3 sin (3 ) 7 3 -1 8 3 1 8 3 3 2 cos( ) 9 3 3 8 3 cos (3 ) 10 4 -4 10 4 sin (4 ) 11 4 -2 12 4 0 13 4 2 10 4 4 3 2 cos( 2 ) 14 4 4 10 4 cos (4 ) 6 2 sin (2 ) 8 3 3 2 sin( ) 10 4 4 3 2 sin( 2 ) 5 6 4 6 2 1 Astigmatis m with axis at 45 Coma along y - axis Coma along x - axis Secondary Astigmatis m Spherical Aberration , Defocus Secondary Astigmatis m Patrick Y. Maeda 3/10/03 Wave Aberration Description Stanford University Psych 221 / EE 362 Winter 2002-2003 The wave aberration is expressed as a weighted sum of Zernike polynomial s 7 : k W ( , ) n W n m n m n Z nm ( , ) n 1 m m m m Wn ( N n Rn ( ) sin( m )) Wnm ( N nm Rn ( ) cos( m )) n m n m 0 k W ( x, y ) j max W Z j 0 j j ( x, y ) WaveAberration.m Patrick Y. Maeda 3/10/03 Stanford University Psych 221 / EE 362 Winter 2002-2003 Double-Index Zernike Polynomials Radial Order, n 0 -6 Z m n -5 -4 , Azimuthal Frequency, m -3 -2 -1 0 1 2 3 4 5 6 ZernikePolynomial.m Common Names7 Piston 1 Tilt 2 Astigmatism (m=-2,2), Defocus(m=0) 3 Coma (m=-1,1), Trefoil(m=-3,3) 4 Spherical Aberration (m=0) 5 Secondary Coma (m=-1,1) 6 Secondary Spherical Aberration (m=0) Patrick Y. Maeda 3/10/03 Stanford University Psych 221 / EE 362 Winter 2002-2003 Double-Index Zernike Polynomial PSFs Radial Order, n 0 -6 Z m n -5 -4 , Azimuthal Frequency, m -3 -2 -1 0 1 2 3 4 5 Common Names7 6 ZernikePolynomialPSF.m Piston 1 Tilt 2 Astigmatism (m=-2,2), Defocus(m=0) 3 Coma (m=-1,1), Trefoil(m=-3,3) 4 Spherical Aberration (m=0) 5 Secondary Coma (m=-1,1) 6 Secondary Spherical Aberration (m=0) Patrick Y. Maeda 3/10/03 Double-Index Zernike Polynomial MTFs Radial Order, n -6 -5 Azimuthal Frequency, m -3 -2 -1 0 1 2 3 -4 4 5 Stanford University Psych 221 / EE 362 Winter 2002-2003 Common Names7 6 1 0.9 0.8 Pupil Diameter = 4 mm 0 to 50 cycles/degree = 570 nm RMS wavefront error = 0.2 0.7 0 ZernikePolynomialMTF.m 0.6 0.5 0.4 0.3 0.2 0.1 0 Z 1 , m n 20 30 0.8 0.8 0.7 0.7 0.6 0.6 0.5 0.5 0.4 0.4 0.3 0.3 0.2 0.2 4 50 10 20 30 40 0 50 0 10 20 30 40 1 1 1 0.9 0.9 0.8 0.8 0.8 0.7 0.7 0.7 0.6 0.6 0.6 0.5 0.5 0.5 0.4 0.4 0.4 0.3 0.3 0.3 0.2 0.2 0.1 0.1 0 10 20 MTFy MTFx 0.1 0 0.9 0 3 40 1 0.9 0.1 2 30 40 0 50 50 0.1 10 20 30 40 0 50 0 10 20 30 40 1 1 1 1 0.9 0.9 0.9 0.9 0.8 0.8 0.8 0.8 0.7 0.7 0.7 0.7 0.6 0.6 0.6 0.6 0.5 0.5 0.5 0.5 0.4 0.4 0.4 0.4 0.3 0.3 0.3 0.3 0.2 0.2 0.2 0.1 0.1 0.1 0 0 0 0 10 20 30 40 50 0 10 20 30 40 50 50 Coma (m=-1,1), Trefoil(m=-3,3) 0.2 0.1 0 10 20 30 40 0 50 0 10 20 30 40 1 1 1 1 0.9 0.9 0.9 0.9 0.9 0.8 0.8 0.8 0.8 0.8 0.7 0.7 0.7 0.7 0.7 0.6 0.6 0.6 0.6 0.6 0.5 0.5 0.5 0.5 0.5 0.4 0.4 0.4 0.4 0.4 0.3 0.3 0.3 0.3 0.3 0.2 0.2 0.2 0.2 0.1 0.1 0.1 0.1 0 10 20 30 40 0 50 0 10 20 30 40 0 50 0 10 20 30 40 0 50 50 Spherical Aberration (m=0) 0.2 0.1 0 10 20 30 40 0 50 0 10 20 30 40 1 1 1 1 1 1 0.9 0.9 0.9 0.9 0.9 0.9 0.8 0.8 0.8 0.8 0.8 0.8 0.7 0.7 0.7 0.7 0.7 0.7 0.6 0.6 0.6 0.6 0.6 0.6 0.5 0.5 0.5 0.5 0.5 0.5 0.4 0.4 0.4 0.4 0.4 0.4 0.3 0.3 0.3 0.3 0.3 0.3 0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.1 0.1 0.1 0 0 0 0 0 0 10 20 30 40 50 0 10 20 30 40 50 0 10 20 30 40 50 0 10 20 30 40 50 50 Secondary Coma (m=-1,1) 0.2 0.1 0 10 20 30 40 0 50 0 10 20 30 40 1 1 1 1 1 1 1 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.2 0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0 0 10 20 30 40 50 0 0 10 20 30 40 50 0 0 10 20 30 40 50 0 Tilt Astigmatism (m=-2,2), Defocus(m=0) 0.2 0 1 0 6 10 0.9 0 5 0 1 Piston 0 10 20 30 40 50 0 0 10 20 30 40 50 0 50 Secondary Spherical Aberration (m=0) 0.2 0.1 0 10 20 30 40 50 0 0 10 20 30 40 50 Patrick Y. Maeda 3/10/03 Double-Index Zernike Polynomial MTFs Radial Order, n -6 -5 Azimuthal Frequency, m -3 -2 -1 0 1 2 3 -4 4 5 Stanford University Psych 221 / EE 362 Winter 2002-2003 Common Names7 6 1 0.9 0.8 Pupil Diameter = 7.3 mm 0 to 50 cycles/degree = 570 nm RMS wavefront error = 0.2 0.7 0 ZernikePolynomialMTF.m 0.6 0.5 0.4 0.3 0.2 0.1 0 Z 1 , m n 20 30 0.9 0.8 0.8 0.7 0.7 0.6 0.6 0.5 0.5 0.4 0.4 0.3 0.3 0.2 0.2 4 50 10 20 30 40 0 50 0 10 20 30 40 1 1 1 0.9 0.9 0.8 0.8 0.8 0.7 0.7 0.7 0.6 0.6 0.6 0.5 0.5 0.5 0.4 0.4 0.4 0.3 0.3 0.3 0.2 0.2 0.1 0.1 0 10 20 MTFy MTFx 0.1 0 0.9 0 3 40 0.9 0.1 2 30 40 0 50 50 0.1 10 20 30 40 0 50 0 10 20 30 40 1 1 1 1 0.9 0.9 0.9 0.9 0.8 0.8 0.8 0.8 0.7 0.7 0.7 0.7 0.6 0.6 0.6 0.6 0.5 0.5 0.5 0.5 0.4 0.4 0.4 0.4 0.3 0.3 0.3 0.3 0.2 0.2 0.2 0.1 0.1 0.1 0 0 0 0 10 20 30 40 50 0 10 20 30 40 50 50 Coma (m=-1,1), Trefoil(m=-3,3) 0.2 0.1 0 10 20 30 40 0 50 0 10 20 30 40 1 1 1 1 0.9 0.9 0.9 0.9 0.9 0.8 0.8 0.8 0.8 0.8 0.7 0.7 0.7 0.7 0.7 0.6 0.6 0.6 0.6 0.6 0.5 0.5 0.5 0.5 0.5 0.4 0.4 0.4 0.4 0.4 0.3 0.3 0.3 0.3 0.3 0.2 0.2 0.2 0.2 0.1 0.1 0.1 0.1 0 10 20 30 40 0 50 0 10 20 30 40 0 50 0 10 20 30 40 0 50 50 Spherical Aberration (m=0) 0.2 0.1 0 10 20 30 40 0 50 0 10 20 30 40 1 1 1 1 1 1 0.9 0.9 0.9 0.9 0.9 0.9 0.8 0.8 0.8 0.8 0.8 0.8 0.7 0.7 0.7 0.7 0.7 0.7 0.6 0.6 0.6 0.6 0.6 0.6 0.5 0.5 0.5 0.5 0.5 0.5 0.4 0.4 0.4 0.4 0.4 0.4 0.3 0.3 0.3 0.3 0.3 0.3 0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.1 0.1 0.1 0 0 0 0 0 0 10 20 30 40 50 0 10 20 30 40 50 0 10 20 30 40 50 0 10 20 30 40 50 50 Secondary Coma (m=-1,1) 0.2 0.1 0 10 20 30 40 0 50 0 10 20 30 40 1 1 1 1 1 1 1 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.2 0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0 0 10 20 30 40 50 0 0 10 20 30 40 50 0 0 10 20 30 40 50 Tilt Astigmatism (m=-2,2), Defocus(m=0) 0.2 0 1 0 6 10 1 0 5 0 1 Piston 0 0 10 20 30 40 50 0 0 10 20 30 40 50 0 50 Secondary Spherical Aberration (m=0) 0.2 0.1 0 10 20 30 40 50 0 0 10 20 30 40 50 Patrick Y. Maeda 3/10/03 Stanford University Psych 221 / EE 362 Winter 2002-2003 Simulation based on Human Eye Data Mode j Coefficient (m) RMS Coefficient (m) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 0 0 0 1.02 0 0.33 0.21 -0.26 0.03 -0.34 -0.12 0.05 0.19 -0.19 0.15 0 0 0 0.416413256 0 0.134721936 0.074246212 -0.091923882 0.010606602 -0.120208153 -0.037947332 0.015811388 0.084970583 -0.060083276 0.047434165 Total RMS Wavefront Error (m) MTF of Zero Aberration System, 5.4mm pupil 1 0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2 0 0.484608089 MTF of Zero Aberration System, 5.4mm pupil 1 0 10 20 30 s x (cycle/deg) 40 50 0 0 10 20 30 s y (cycle/deg) 40 50 MTF of Aberrated System, Wrms = 0.85012 MTF of Aberrated System, Wrms = 0.85012 1 1 0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2 0 0 10 20 30 s x (cycle/deg) 40 50 0 0 10 20 30 s y (cycle/deg) 40 50 WaveAberrationMTF.m WaveAberration.m WaveAberrationPSF.m Patrick Y. Maeda 3/10/03 Stanford University Psych 221 / EE 362 Winter 2002-2003 Measurement Setup y Pupil Iris x z Incoming Light Beam Retina Ideal Real Planar Aberrated Wavefront Wavefront Patrick Y. Maeda 3/10/03 Shack-Hartmann Sensor Layout PBS Stanford University Psych 221 / EE 362 Winter 2002-2003 Pupil Relay Optics CCD Lenslet Array Light Source Patrick Y. Maeda 3/10/03 Stanford University Psych 221 / EE 362 Winter 2002-2003 Shack-Hartmann Wavefront Sensor Lenslet Array y(x1, y1) Aberrated Wavefront y(x1, y2) y(x1, y3) y(x1, y4) Focal Length f Patrick Y. Maeda 3/10/03 Data Fitting with Zernike Polynomials Stanford University Psych 221 / EE 362 Winter 2002-2003 W ( x, y ) x( x, y ) x f W ( x, y ) y ( x, y ) y f W ( x, y ) W j Z j ( x, y ) j W j is the coefficien t of the Z j mode in the expansion W j is equal to the rms wavefront error for that mode Z j ( x, y ) W ( x, y ) W j x x j Z j ( x, y ) W ( x, y ) W j y y j Z j ( x, y ) x( x, y ) W j f x j (14) Z j ( x, y ) y ( x, y ) W j f y j (15) Equations (14) and (15) can be used to determine the Wj’s using Least-squares Estimation Patrick Y. Maeda 3/10/03 Stanford University Psych 221 / EE 362 Winter 2002-2003 Least-squares Estimation Let, x( x, y ) y ( x, y ) b( x, y ) and c ( x, y ) f f Z j ( x, y ) Z j ( x, y ) g j ( x, y ) and h j ( x, y ) x y Equations (14) and (15) can be expressed in matrix form b( x1 , y1 ) g1 ( x1 , y1 ) g 2 ( x1 , y1 ) b( x , y ) g ( x , y ) g ( x , y ) 2 1 2 1 2 1 1 2 b( x k , y k ) g1 ( x k , y k ) g 2 ( x k , y k ) c( x1 , y1 ) h1 ( x1 , y1 ) h2 ( x1 , y1 ) c ( x , y ) 1 2 h1 ( x1 , y 2 ) h2 ( x1 , y 2 ) c( x k , y k ) h1 ( x k , y k ) h2 ( x k , y k ) g j max ( x1 , y1 ) g j max ( x1 , y 2 ) W1 g j max ( x k , y k ) W2 h j max ( x1 , y1 ) h j max ( x1 , y 2 ) W j max h j max ( x k , y k ) 2k 2 j max or The Least - squares estimate of is given by : LS ( T ) 1 T where T is the matrix tra nspose of Patrick Y. Maeda 3/10/03 Benefits of Orthogonality Stanford University Psych 221 / EE 362 Winter 2002-2003 Since the Z j ' s are orthogonal : Their partial derivative s in x are orthogonal Their partial derivative s in y are orthogonal Therefore the columns in are orthogonal T D1 where D1 is a diagonal matrix wit h non - zero diagonal elements LS D2 T where D2 is a diagonal matrix The wave aberration coefficien ts are obtained by projection of the data onto the partial derivative s of the Zernike polynomial s and multiplica tion by a diagonal matrix Note that a non - orthogonal set of basis functions may result in the inversion of an ill - conditione d matrix, ( T ) 1 Patrick Y. Maeda 3/10/03 Conclusions Stanford University Psych 221 / EE 362 Winter 2002-2003 Zernike Polynomials well suited for Describing wave aberration functions of optical systems with circular pupils Estimation of wave aberration coefficients from wavefront measurements Able to integrate Psych 221 learning with material from optical systems and Fourier optics courses Linear systems theory make image formation and image quality evaluation straightforward Suggestions for future work Extend simulation to incorporate chromatic effects Investigate the how wave aberration changes with accommodation Conduct simulations on a wide set of patient data Simulate the higher order aberrations induced by the PRK and LASIK Research some of the new wavefront technologies like implantable lenses Patrick Y. Maeda 3/10/03 References Stanford University Psych 221 / EE 362 Winter 2002-2003 [1] MacRae, S. M., Krueger, R. R., Applegate, A. A., (2001), Customized Corneal Ablation, The Quest for SuperVision, Slack Incorporated. [2] Williams, D., Yoon, G. Y., Porter, J., Guirao, A., Hofer, H., Cox, I., (2000), “Visual Benefits of Correcting Higher Order Aberrations of the Eye,” Journal of Refractive Surgery, Vol. 16, September/October 2000, S554-S559. [3] Thibos, L., Applegate, R.A., Schweigerling, J.T., Webb, R., VSIA Standards Taskforce Members (2000), "Standards for Reporting the Optical Aberrations of Eyes," OSA Trends in Optics and Photonics Vol. 35, Vision Science and its Applications, Lakshminarayanan,V. (ed) (Optical Society of America, Washington, DC), pp: 232-244. [4] Goodman, J. W. (1968). Introduction to Fourier Optics. San Francisco: McGraw Hill [5] Gaskill, J. D. (1978). Linear Systems, Fourier Transforms, Optics. New York: Wiley [6] Fischer, R. E. (2000). Optical System Design. New York: McGraw Hill [7] Thibos, L. N.(1999), Handbook of Visual Optics, Draft Chapter on Standards for Reporting Aberrations of the Eye. http://research.opt.indiana.edu/Library/HVO/Handbook.html [8] Bracewell, R. N. (1986). The Fourier Transform and Its Applications. McGraw Hill [9] Mahajan, V. N. (1998). Optical Imaging and Aberrations, Part I Ray Geometrical Optics, SPIE Press [10] Liang, L., Grimm, B., Goelz, S., Bille, J., (1994), “Objective Measurement of Wave Aberrations of the Human Eye with the use of a Hartmann-Shack Wave-front Sensor,” J. Opt. Soc. Am. A, Vol. 11, No. 7, 1949-1957. [11] Liang, L., Williams, D. R., (1997), “Aberration and Retinal Image Quality of the Normal Human Eye,” J. Opt. Soc. Am. A, Vol. 14, No. 11, 2873-2883. Patrick Y. Maeda 3/10/03 Appendix I Stanford University Psych 221 / EE 362 Winter 2002-2003 Matlab Source Code Files: zernike.m ZernikePolynomial.m ZernikePolynomialPSF.m ZernikePolynomialMTF.m WaveAberration.m WaveAberrationPSF.m WaveAberrationMTF.m Patrick Y. Maeda 3/10/03