Aberrometry for the Common Joe

Thomas O. Salmon, OD, PhD, FAAO

8/19/11 Revision

Basic teaching objectives1. What are aberrations?2. What are aberrometers?3. How do they work?4. How do we interpret the data?5. How can you diagnose abnormal values?

1. What are aberrations?• Aberrations = refractive errors• Myopia, hyperopia, astigmatism =

LOA• But there are others …• Higher-order aberrations (HOA)

Coma Spherical aberration Trefoil Others

Clarification … which aberrations?

• Monochromatic• Not chromatic• Not Seidel

Coma Spherical aberration Oblique astigmatism Petzval Curvature of field

Pupil size, pupil size, pupil size, …

Courtesy of Dr. Donald Miller, Indiana UniversityRetinal Imaging and Vision at the Frontiers of Adaptive Optics, Physics Today (Jan 2000)

20/200sized letter

Summary 1

Q. What are aberrations?A. Refractive errors

Q. What are higher-order aberrations?

A. Refractive errors beyond sphere and cyl.

2. What are aberrometers?• Instruments that measure

…• Refractive errors• (super auto-refractors)• Sphere, cylinder + HOAs• (Higher-order aberrations)• ≠ corneal topography! COAS

K topography vs aberrometry

High-resolutionkeratometry

High-resolutionauto-refraction

• 1970s and 80s• Strategic defense• Refraction through the

atmosphere• Shack-Hartman wavefront sensor• Astronomy• Adaptive optics (AO)

History of HO aberrometry

Reagan

Shack

Ocular HO aberrometry research

• 1960 MS Smirnov• 1977 Howard Howland• 1990 Josef Bille and …• Junzhong Liang• J Opt Soc Am A, July 1994• Mid 1990s - other labs

Howland

Liang

Bille

IU Shack-Harmann Wavefront Sensor (catch a wave!)

Clinical HO aberrometry

• Laser refractive surgery

• Large HOAs• Clinical aberrometry• Wavefront-guided

LASIKCOAS

3. How do they work?

• Light is projected in.• Reflect off the retina• Light passes through

the eye’s optics.• Catch the light.• Analyze it.• Reconstruct the

optical wavefront’s shape

Courtesy of Alcon

Shack-Hartmann aberrometry

+y}

}x

Light exiting the eye

Analyze each dot’s position

Reconstruct wavefront

shape

What does the wavefront tell you?

-0.5

-0.5

-0.5

1.00.5

0.0

-2 -1 0 1 2

210

-1-2

pupil (mm)

pupil (mm)

-1.0 -0.5 0.0 0.5 1.0 1.5 2.0Wavefront error (um)

Topographic map Surface plot

• Flat wavefront = perfects optics (no aberrations)• Elevations = wavefront errors• Wavefront distortions reveal refractive errors.

Higher-order wavefront maps

Normal eye Post LASIK

Besides refractive errors …

• Optical quality metrics Modulation transfer function (MTF) Point spread function (PSF) Strehl ratio

• Visual performance metrics• Simulate the patient’s vision!• Design the ideal optical correction for

the eye W-guided RS, CL, spectacles, IOL, etc.

Simulated retinal image emmetropia

Simulated retinal image for AI

-1.00 -0.50 x 100 HO RMS 0.51 µm (6.0-mm pupil)

Simulated retinal image emmetropia

Simulated vision for TO

-3.00 -2.00 x 170 HO RMS 0.38 µm 6.0-mm pupil

Summary 3Q. How do

aberrometers work?A. Measure light that

has passed through the eye's optics.

Q. Why? (Who cares?)A. The wavefront

provides a wealth of info about the eye's optics and vision.

4. How do we interpret wavefront data?

• Does this eye have good or bad optics?

• How good or bad is the optical correction?

• Are HO aberrations the problem?• Specifically:1. Which aberrations does this eye

have?2. How bad are they?

Which aberrations are present?

• Wavefront - distorted by all the aberrations combined

• Specifically, which ones are present?• Need Zernike analysis to break it

down.• Zernike system = hierarchy of

aberrations• Each aberration causes a particular

shape of wavefront distortion.

sphere astigmatism trefoil 7Z-3

Some example wavefronts

Zernike analysis breaks it down

+ + + . . .

sphere astigmatism trefoil

coma sphericalaberration

+ + +=

+

Z44 Z42

Zernike modes

0

1

2

3

4

5

0

0

0

1

1

1

-1

-1

-1

-2

-2

2

2

-3

-3 3

3

4-4

5-5

order (n)

Znm

HOA

or Z(n,m)

CombinedZernike modes1

2

3

order (n)

Z1-1 Z1

1

Z11Z2

-2 Z22Z2

0

Z22

Z3-3

Z3-1 Z3

1

Z33

Z31

Z33

Magnitude

& axis form

0

1

2

3

4

order (n)

Z0

Z40

Z31

Z20

Z11

Z22

Z33

Z44Z42

prism

sphere astigmatism

coma trefoil

sphericalaberration quadrafoil

piston

secondary astigmatism

Zernike analysis tells us …• Which aberrations are present.• Breaks the wavefront down into the

Standard Zernike modes or … Magnitude & axis form

• Each Zernike mode = one aberration• But, … how bad are the aberrations?• Z analysis also provides a value for

each.

Zernike coefficients• One for each Zernike mode• Units in microns• ± sign• Must specify pupil size• Absolute Zernike coefficient =

magnitude

Conventional Rx:

Mode: Z2-2 Z2

0 Z22 Z3

-3 Z3-1 Z3

1 Z33 Z4

-4 Z4-2 Z4

0 Z44Z4

2

Coefficient (µm):

.56 .27 .64 -.03 .07 -.05 .06 .03 .04 .11 0 -.08

2nd order

3rd order 4th order

Zernike coefficients

Rx: +0.19 - 0.67 x 111 Pupil diameter: 5.6 mm Total RMS: 0.76 µm Higher-order RMS: 0.51 µm

+0.25 -0.75 x 111

Unit =µm + or -values

RMS wavefront error• The basic data - individual Zernike

coefficients• How bad are combined aberrations?• Total aberrations (LOAs + HOAs)• Just higher-orders (HOA RMS)• Or, just third-order aberrations, etc.

€

RMS = Znm( )

2+ ...+ Zn

m( )2

Summary 4. Interpreting the data

• Aberrometers measure wavefronts• Wavefront - distorted by aberrations• Zernike analysis - which aberrations are

present• Zernike coefficients - how bad they are• Data in microns, with ± signs• RMS - magnitude of grouped aberrations• Pupil size, pupil size, pupil size !

5. Diagnosis - what’s normal?• Aberrometry - diagnoses abnormal

optics• Ideal eye = zero aberrations, but

…• every eye has some aberrations.• So, are those Zernike or RMS

values good or bad?• Need reference norms

OCO Norms

• JCRS Dec 2006• 2,560 normal eyes• 9 sites• Zernike & RMS

norms• Data on www• Google “Dr. Salmon”

Downloadable info• Full article in PDF• Norms table - PDF & Excel• Signed Zernike coefficients• Absolute values• Combined (polar) Zernike modes• RMS for HOA and orders 3, 4, 5, 6• http://arapaho.nsuok.edu/~salmonto

HOA results

• Prominent individual HOAs (6.0-mm pupil)

• Z3-1 (vertical coma) = 0.14

• Z40 (spherical aberration) = 0.13

• Z3-3 (oblique trefoil) = 0.11

Pupil diameter Mean (µm) 2x mean

6.0 0.33 0.665.0 0.19 0.384.0 0.10 0.20

Summary 5

Q. How can you know what’s normal?

A. Compare the data to norms

If more than double the norms … (for that pupil size) suspect

abnormal optics.

Summary – the basics• Aberrations = refractive errors• Aberrometers measure wavefronts.• Zernike analysis tells which

aberrations• Zernike coefficients & RMS tell how

bad the aberrations are.• Compare values to norms• Mean HO RMS (6 mm) = 0.33 µm

And ...

Dont’ forget pupil diameter!

Case example• 34 yof, complained of shadows,

diplopia, glare, eye strain, especially at night

• RK OU at age 21• Spec Rx = plano, 20/20- OU• Aberrometry confirmed large

HOAs

HOA wavefront maps (5.0 mm)

-2 -1 0 1 2

2

1

0

-1

-2

mm

mm

-2.50 -1.25 0.00 1.25 2.50Wavefront error (um)

OD

-2 -1 0 1 2

2

1

0

-1

-2

mm

mm

-2.50 -1.25 0.00 1.25 2.50Wavefront error (um)

OS

RMS=0.89 µm RMS=0.92 µm

Zernike coefficients

0.00

0.10

0.20

0.30

0.40

0.50

0.60

Z(3,-3)Z(3,-1)Z(3,1)Z(3,3)Z(4,-4)Z(4,-2)Z(4,0)Z(4,2)Z(4,4)Z(5,-5)Z(5,-3)Z(5,-1)Z(5,1)Z(5,3)Z(5,5)

5.0-mm pupil

OD

OS

Normal

Abs Zernike coefficient (um)

Zernike mode

HO RMS & pupil size

0

0.2

0.4

0.6

0.8

1

2.02.53.03.54.04.55.05.5

Pupil & RMS dataOD

OS

Normal

HO RMS (um)

Pupil diameter (mm)

5.0 mm3.0 mm

Aberrometry & refractive surgery

• Replace conventional auto-refractor• Verify pre-op refraction• Guide choice of laser procedure• Data sent to laser for custom

correction• Evaluate quality of the correction

Eye research

• Optics of the eye• Optical

corrections• Visual perception• Optics-related

phenomena-0.02

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0 10 20 30 40 50 60

Time (seconds)

1- Day Acuvue MoistProclear 1 DayEye without SCL

Change in HO RMS (um)

Northeastern State University

ご清聴ありがとうございました