Bruneni Lecture on Ophthalmic Optics, ASCO2007

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Bruneni Lecture on Ophthalmic Optics, ASCO-2007
Higher-order Aberrations and the Design of
Spectacle Lenses Larry N. Thibos, PhD School of
Optometry, Indiana University, Bloomington, IN
47405 thibos_at_indiana.edu http//research.opt.ind
iana.edu
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Lecture outline

• The premise of wavefront correction
• Magnitude of ocular wavefront errors
• Stability of wavefront errors
• Alignment stability
• Judging success

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The premise of wavefront correction

• The wavefront aberration map is a prescription
  for perfection
• OPD maps show how the wavefront must be reshaped
  to make it perfectly flat.

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Wavefront Phase Map of Optical Path Differences
Advanced phase lt Short optical path

• Reference

Wave-front
Ectasia
Retarded phase lt Long optical path
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Lecture outline

• The premise of wavefront correction
• Magnitude of ocular wavefront errors
• Stability of wavefront errors
• Alignment stability
• Judging success

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Wavefront error depends strongly on pupil diameter
For any given pupil size, wavefront error falls
exponentially with radial order Suggests
diminishing importance of higher-order
aberrations in typical eye.
Thibos, Hong, Bradley Cheng JOSA (2002)
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Normalize wavefront error by pupil area
Wavefront error per unit of pupil area is largely
independent of pupil diameter. Suggests a
comparison with ordinary defocus. All higher
order terms combined are equivalent to 1/4
diopter of defocus clinical tolerance for
prescribing spectacles.
Clinical tolerance
0.25
Message higher-order aberrations are tiny!
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Higher-order aberrations in normal population are
small
In normal eyes, RMS wavefront error for
higher-order is less than RMS for 1/4 D of
defocus. Unprecedented precision is requried to
correct higher-order aberrations that are smaller
than residual defocus and astigmatism in best
corrected eyes.
Thibos, et al. (2002) JOSA- A, 19, 2329
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Lecture outline

• The premise of wavefront correction
• Magnitude of ocular wavefront errors
• Stability of wavefront errors
• Alignment stability
• Judging success

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Stability of wavefront errors

• Ocular wavefront errors vary with
• Time (sec, hours, days, weeks, months, years)
• Accommodation
• Pupil diameter
• Tear film evaporation

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Effect of pupil constriction on Zernike
coefficients
The typical eye has about 0.16?m of Zernike
spherical aberration for a dilated (7.5mm) pupil.
As the pupil constricts, the amount of c40 and
c20 (focus) changes. Expressed in diopters, the
shift in focus varies from 0 to 0.3 diopters.
This hyperopic shift (for positive SA) might be
corrected by accommodation.
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Dynamic Optical Model of Tear Film Thinning
Thin film gt Short OPL gt wavefront advances
Wavefront
Tear film
Rays from point source

Rough refracting surface is covered by tear film
before breakup
Distorted wavefront where tears are thin
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Qualitative Comparison of Techniques
RI image
FL image
SH image
Before Tear Break-up
After Tear Break-up
From Himebaugh, N. (2007)
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Quantitative Analysis of SH data image
Macro-aberration map
Micro-aberration map
Simulated image
20/20
20/20
From Himebaugh, N. (2007)
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Lecture outline

• The premise of wavefront correction
• Magnitude of ocular wavefront errors
• Stability of wavefront errors
• Alignment stability
• Judging success

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The alignment problem (Prentice's Rule)
W(x)ax2
Symmetric wavefront aberration of the spectacle
lens represents pure defocus.
x
x
Well-aligned lens
Line-of-sight
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The alignment problem (Prentice's Rule)
W(x)a(x-x0)2 ax2-2ax0xx02
Asymmetric wavefront aberration of the spectacle
represents defocus prism piston.
x
x0
x
Mis-aligned lens
Line-of-sight
x0
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The alignment problem (Higher-order Prentice's
Rule)
W(x)ax3
Wavefront aberration of the spectacle lens
represents pure coma.
x
W(x)a(x-x0)3ax3-3ax0x23ax02x-ax0
Wavefront aberration of the spectacle represents
coma defocus prism piston.
x0
x
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Displacement of higher-order aberration generates
a cascade of lower-order terms.
0
Defocus
1
Astigmatism
2
Radial order
Coma
3
SA
4
5
0
1
2
3
4
5
-1
-2
-3
-4
-5
Meridional frequency
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Example of alignment problem

• Unlike defocus astigmatism, alignment of eye
  with lens is critical for correcting higher-order
  aberrations.

This movie simulates the effect of gradually
misaligning a lens designed to correct a typical
level of ocular spherical aberration (0.1micron
RMS)..
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Lecture outline

• The premise of wavefront correction
• Magnitude of ocular wavefront errors
• Stability of wavefront errors
• Alignment stability
• Judging success

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Success depends on the goal

• Three potential uses of wavefront aberrometry
• Good correct 2nd order aberrations better
• Better correct some of the eye's HO aberrations
• Best correct all of the eye's HO aberrations

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What does optimum focus mean for an aberrated eye?

• Two popular definitions based on
• Paraxial analysis (Seidel)
• Balanced analysis over whole pupil (Zernike)

War4
Wb(r4-r2)
Which definition do patients use during
refraction?
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My view

• Patients don't care about wavefront aberrations.
• They care about retinal image quality because
  that is what determines visual performance.

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Predicting best focus for monochromatic light
WQ
IQ
VQ
Cheng et al, J. Vision (2004) 4310-321
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Despite small size, aberrations have significant
impact
Very few eyes have optical quality within a
factor 2 of perfect for 6mm pupil. Most eyes
have much to gain in MTF by correcting
aberrations.
20/20
20/10 (neural sampling)
Diffraction limited
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Chromatic aberrations also affect image quality
Polychromatic image quality is determined by
interaction of wavefront errors and chromatic
defocus at every wavelength.
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Conclusions

• Are higher order aberrations relevant to the
  design of spectacle lenses? They clearly affect
  optimum sphero-cyl prescriptions, but it is a
  very complicated story! Introducing higher-order
  aberrations into the picture has clouded it
  considerably.
• Ocular aberrations are
• Relatively small
• Unstable
• Difficult to correct
• A land of diminishing returns for normal eyes
• Abnormally large aberrations will be easier to
  correct with contact lenses or IOLs w/ stable
  alignment than w/ spex.

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The Visual Optics Group at Indiana University
Larry Thibos, PhD Arthur Bradley, PhD Steve
Burns, PhD Ann Elsner, PhD Donald Miller,
PhD Carolyn Begley, OD, MS Rowan Candy, OD
PhD Jacob Rubinstein, PhD Nikole Himebaugh, OD,
PhD Pete Kollbaum, OD, PhD Jayoung Nam,
PhD Charles Coe, OD Haixia Liu, MD Jingyun Wang,
BS Sowmya Ravikumar, BS Toco Chui, MS Xin Wei,
BS Danielle Warren, OD Weihua Gao, BS Kevin
Haggerty, BS
Benno Petrig, PhD Ravi Jonnal, MS Jungtae Rha,
PhD Yan Zhang, PhD Barry Cense, PhD Zhangyi
Zhong, BS Jie Shen, BS Hongxin Song, BS
Support National Institutes of Health /
NEI National Science Foundation Borish Center for
Ophthalmic Research
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The Visual Optics Group at Indiana University
Larry Thibos, PhD Arthur Bradley, PhD Steve
Burns, PhD Ann Elsner, PhD Donald Miller,
PhD Carolyn Begley, OD Rowan Candy, OD PhD Jacob
Rubinstein, PhD Sergio Barbero, Phd Nikole
Himebaugh, OD Charles Coe, OD Haixia Liu,
MD Jingyun Wang, BS Pete Kollbaum, OD Sowmya
Ravikumar, BS Toco Chui, MS Xin Wei, BS Danielle
Warren, OD Weihua Gao, BS Kevin Haggerty, BS
Benno Petrig, PhD Ravi Jonnal, MS Jungtae Rha,
PhD Yan Zhang, PhD Barry Cense, PhD Zhangyi
Zhong, BS Jie Shen, BS Hongxin Song, BS
Support National Institutes of Health /
NEI National Science Foundation Borish Center for
Ophthalmic Research
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Distribution of signed Zernike coefficients (6mm
pupil)
Z-coeffs are randomly distributed about zero for
most higher-order aberrations. Suggests
evolutionary trend towards perfection.
Thibos, Hong, Bradley Cheng JOSA (2002)
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Distribution of un-signed Zernike coefficients
(6mm pupil)
Magnitude of Z-coeffs of higher-order modes is
smaller than residual defocus and astigmatism in
well-corrected eyes. Suggests typical eye has
relatively small amounts of higher-order
aberration.
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Wavefront error per unit of pupil area
Wavefront error per unit of pupil area is largely
independent of pupil diameter.
Clinical tolerance
0.25
0.25
Message higher-order aberrations are tiny!