A New Definition of A New Definition of Refraction:Refraction:

Basics and BeyondBasics and Beyond

Austin Roorda, Ph.D.

Unversity of Houston College of Optometry

First use of Optics as Vision AidsFirst use of Optics as Vision Aids

reading stones from 1000 AD1000 AD

1268 Roger Bacon discussed possibilities of magnification with lenses

Roger Bacon

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)

Kepler

1604 Kepler describes the optics of near and far sightedness

1748: Benjamin Franklin invents the bifocal lens

1827: Sir George Biddel Airy invents spectacles for astigmatism

RetinoscopyRetinoscopy

PhoropterPhoropter

AutorefractorAutorefractor

Refraction SystemsRefraction Systems

AberrometersAberrometersVISX Wavescan

Summit Autonomous Custom Cornea

B&L Zywave

Wavefront Sciences Complete Optical Analysis System

TraceyTech Tracey 1

What is the Wavefront?What is the Wavefront?

converging beam=

spherical wavefront

parallel beam=

plane wavefront

What is the Wavefront?What is the Wavefront?ideal wavefrontparallel beam

=plane wavefront

defocused wavefront

What is the Wavefront?What is the Wavefront?parallel beam

=plane wavefront

aberrated beam=

irregular wavefront

ideal wavefront

What is the Wavefront?What is the Wavefront?

aberrated beam=

irregular wavefront

diverging beam=

spherical wavefront

ideal wavefront

What is the What is the Wave AberrationWave Aberration??diverging beam

=spherical wavefront wave aberration

-3 -2 -1 0 1 2 3

-3

-2

-1

0

1

2

3

Wavefront Aberration

mm (right-left)

mm

(su

peri

or-in

ferio

r)

Wave Aberration: DefocusWave Aberration: Defocus

Wave Aberration: AstigmatismWave Aberration: Astigmatism

-3 -2 -1 0 1 2 3

-3

-2

-1

0

1

2

3

Wavefront Aberration

mm (right-left)

mm

(su

peri

or-in

ferio

r)

-3 -2 -1 0 1 2 3

-3

-2

-1

0

1

2

3

Wavefront Aberration

mm (right-left)

mm

(su

peri

or-in

ferio

r)

Wave Aberration: ComaWave Aberration: Coma

-3 -2 -1 0 1 2 3

-3

-2

-1

0

1

2

3

Wavefront Aberration

mm (right-left)m

m (

supe

rior

-infe

rior)

Wave Aberration: All TermsWave Aberration: All Terms

How Do We Interpret the How Do We Interpret the Wave Aberrations?Wave Aberrations?

Wave Aberration Surface MapWave Aberration Surface Map

-2 -1 0 1 2

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Wave Aberration Contour MapWave Aberration Contour Map

mm (right-left)

mm

(su

perio

r-in

ferio

r)

Breakdown of Zernike TermsBreakdown of Zernike Terms

-0.5 0 0.5 1 1.5 2123456789

1011121314151617181920

Zer

nik

e te

rmCoefficient value (microns)

astig.defocus

astig.trefoilcomacomatrefoil

spherical aberration

2nd order

3rd order

4th order

5th order

Root Mean SquareRoot Mean Square

2 2 2 22 0 2 12 2 2 3 .......RMS Z Z Z Z

astig

mat

ism te

rm

defo

cus t

erm

astig

mat

ism te

rm

trefo

il ter

m……

Point Spread FunctionPoint Spread Function

Point Spread Function vs. Pupil SizePoint Spread Function vs. Pupil Size

pupil images

followed by

psfs for changing pupil size

1 mm 2 mm 3 mm 4 mm

5 mm 6 mm 7 mm

Simulated ImagesSimulated Images

20/40 letters

20/20 letters

Strehl RatioStrehl Ratio

diffraction-limited PSF

Hdl

Heye

actual PSF

Strehl Ratio = eye

dl

H

H

Modulation Transfer Modulation Transfer FunctionFunction

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 50 100 150

spatial frequency (c/deg)

con

tra

st

Area under the MTF

20/20 20/10

Applications for AberrometryApplications for Aberrometry

Can we Build a Better Can we Build a Better Autorefractor?Autorefractor?

-0.5 0 0.5 1 1.5 2

1

234

567

89

10

111213

141516

171819

20

Ze

rnik

e t

erm

Coefficient value (microns)

astig.

defocus

astig.

trefoil

coma

coma

trefoil

spherical aberration

2nd order

3rd order

4th order

5th order

choose the spectacle

correction that minimizes the

second order Zernike terms

Can we Build a Better Can we Build a Better Autorefractor?Autorefractor?

perf

ect P

SF

choose the spectacle

correction that maximizes the

strehl ratio

defocus/astigmatism

Can we Build a Better Can we Build a Better Autorefractor?Autorefractor?

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

-1.4

-1.2 -1

-0.8

-0.6

-0.4

-0.2 0

0.2

0.4

0.6

0.8 1

Defocus (D)

Are

a un

der

MT

F (

0 -

50 c

/d)

choose the spectacle correction that maximizes the

area under the MTF

Can we Build a Better Can we Build a Better Autorefractor?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 RefractionAberrometric 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

Diagnosis of High-order Diagnosis of High-order AberrationsAberrations

Post - RKPost - RK Post - LASIKPost - LASIK

KeratoconusKeratoconus