Ocular Wavefront Error Representation (ANSI Standard) Jim Schwiegerling, PhD Ophthalmology & Vision Sciences Optical Sciences University of Arizona

Transcription

1 Ocular Wavefront Error Representation (ANSI Standard) Jim Schwiegerling, PhD Ophthalmology & Vision Sciences Optical Sciences University of Arizona

2 People Ray Applegate, David Atchison, Arthur Bradley, Charlie Campbell, Larry Horowitz, David Loshin, Scott MacRae, Susana Marcos, Dan Neal, Austin Roorda, Tom Salmon, Jim Schwiegerling, Larry Thibos, Rob Webb, David Williams and others

3 Reasons for Standardization Zernike polynomials have traditionally been used to represent wavefronts in optical systems with circular pupils. First developed by Fritz Zernike in the 1930s for phase contrast microscopy. Multiple ways of scaling, orienting and ordering the basic polynomials have evolved, which can lead to confusion. The standard helps to remove ambiguity to facilitate clarity and exchange between all users. Lessons learned from Corneal Topography standards in the 1990s.

4 References Applegate RA, Thibos LN, et al. Reference Axis Selection: Subcommittee report of the OSA working group to establish standards for measurement and reporting of optical aberrations of the eye. J Refract Surg 16; S656-S658:2000. Thibos LN, Applegate RA et al. Standards for reporting the optical aberrations of eyes. J Refract Surg 18; S652-S660:2002. Appendix 1 of both Customized Corneal Ablation: The Quest for Supervision. Ed. MacRae SM, Krueger RR, Applegate RA (Slack, Thorofare, NJ, 2001) and Wavefront Customized Visual Correction: the Quest for Supervision II. Ed. Krueger RR, Applegate RA, MacRae SM (Slack, Thorofare, NJ, 2001). ANSI Z Methods for Reporting Optical Aberrations of Eyes. ISO/DIS (Draft currently being voted on 2007)

5 Wavefront Error Measurement

6 Coordinate System y Polar Coordinates: r θ r max x r ranges from [0, r max ] θ ranges from [0, 360 ]

7 Normalized Coordinate System y ANSI STANDARD Normalized Polar Coordinates: ρ θ 1 x ρ ranges from [0, 1] θ ranges from [0, 360 ]

8 Another Coordinate System y NON-STANDARD Normalized Polar Coordinates: φ 1 ρ x ρ ranges from [0, 1] θ ranges from [-180, 180 ]

9 The Third Dimension The z-axis is perpendicular the and x and y-axes and coincides with the Line of Sight. The Line of Sight connects the point of fixation to the center of the entrance pupil, which in turn connects the center of the exit pupil to the fovea.

10 Zernike Polynomials Radial Polynomial, ρ Azimuthal Frequency, θ

11 Complex Wavefront In general, wavefronts can take on a complex shape and we need a way of breaking down this complexity into more simpler components.

12 Zernike Decomposition x Spherical Refractive Error = x Cylindrical Refractive Error x Coma

13 Wavefront Error Representation The wavefront error can be represented by n = 0, 1, 2 ; m = -n, -n +2 n-2, n

14 Generalized Zernike Polynomials Double Index n is radial order m is azimuthal frequency Normalization Radial Component Azimuthal Component

15 ANSI Standard Zernikes constant that depends on n & m Kronecker delta function e.g.

16 ANSI Standard Zernikes only depends on m (i.e. same for both sine & cosine terms Constant that depends on n and m Powers of ρ Factorial Function - a! = a (a-1) (a-2) (3) (2) (1) e.g.

17 Example Zernike Polynomial Oblique Trefoil

18 Zernike Polynomials - Single Index Radial Polynomial, ρ Z 3 Z 1 Z 0 ANSI STANDARD Z 2 Z 4 Z 5 Starts at 0 Left-to-Right Top-to-Bottom Z 6 Z 7 Z 8 Z 9 Z 10 Z 11 Z 12 Z 13 Z 14 Azimuthal Frequency, θ

19 Other Single Index Schemes NON-STANDARD Z 1 Z 2 Z 3 Starts at 1 cosines are even terms sines are odd terms Z 4 Z 5 Z 6 Z 7 Z 8 Z 9 Z 10 Z 11 Z 12 Z 13 Z 14 Z 15 Noll, RJ. Zernike polynomials and atmospheric turbulence. J Opt Soc Am 66; (1976). Also Zemax Standard Zernike Coefficients

20 Other Single Index Schemes Z 1 Z 2 Z 3 Z 4 Z 5 Z 6 NON-STANDARD Starts at 1 increases along diagonal cosine terms first 35 terms plus two extra spherical aberration terms. No Normalization!!! Z 7 Z 8 Z 10 Z 11 Z 9 Z 12 Z 13 Z 17 Z 18 Zemax Zernike Fringe Coefficients Also, Air Force or University of Arizona

21 Other Single Index Schemes Born & Wolf NON-STANDARD Malacara Others??? Plus mixtures of non-normalized, coordinate systems. Use two indices n, m to unambiguously define polynomials. Use a single standard index if needed to avoid confusion.

22 Examples Example 1: 4m 4mm 0.25 D of myopia for a 4 mm pupil (r max = 2 mm)

23 Examples Example 2: 1m 2mm 1.00 D of myopia for a 2 mm pupil (r max = 1 mm) Same Zernike Expansion as Example 1, but different r max. Always need to give pupil size with Zernike coefficients!!

24 Orders of Zernike Polynomials Radial Polynomial, ρ Low Order Azimuthal Frequency, θ Higher Order

25 Summary Use the two-index scheme n, m to unambiguously define the Zernike polynomials whenever possible. If needed, use the standard single index top-to-bottom, left-to-right notation. Always include the size of the pupil with the Zernike coefficients. Be aware that other schemes exists in the literature and in other fields of optics.