A New Definition of Refraction: Basics and Beyond

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A New Definition of RefractionBasics and Beyond

• Austin Roorda, Ph.D.
• Unversity of Houston College of Optometry

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First use of Optics as Vision Aids
reading stones from 1000 AD
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1268 Roger Bacon discussed possibilities of
magnification with lenses
Roger Bacon
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1748 Benjamin Franklin invents the bifocal lens
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Retinoscopy
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Phoropter
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Autorefractor
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Refraction Systems
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Aberrometers
VISX Wavescan
BL Zywave
Summit Autonomous Custom Cornea
Wavefront Sciences Complete Optical Analysis
System
TraceyTech Tracey 1
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What is the Wavefront?
parallel beam plane wavefront
converging beam spherical wavefront
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What is the Wavefront?
parallel beam plane wavefront
ideal wavefront
defocused wavefront
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What is the Wavefront?
parallel beam plane wavefront
ideal wavefront
aberrated beam irregular wavefront
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What is the Wavefront?
diverging beam spherical wavefront
aberrated beam irregular wavefront
ideal wavefront
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What is the Wave Aberration?
diverging beam spherical wavefront
wave aberration
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Wave Aberration Defocus
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Wave Aberration Astigmatism
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Wave Aberration Coma
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Wave Aberration All Terms
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(No Transcript)
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How Do We Interpret the Wave Aberrations?
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Wave Aberration Surface Map
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Wave Aberration Contour Map
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1.5
1
0.5
mm (superior-inferior)
0
-0.5
-1
-1.5
-2
-2.5
-2
-1
0
1
2
mm (right-left)
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Breakdown of Zernike Terms
Coefficient value (microns)
-0.5
0
0.5
1
1.5
2
1
2
astig.
3
2nd order
defocus
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astig.
5
trefoil
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coma
3rd order
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coma
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Zernike term
trefoil
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10
11
4th order
spherical aberration
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13
14
15
16
5th order
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18
19
20
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Root Mean Square

trefoil term
defocus term
astigmatism term
astigmatism term
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Point Spread Function
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Point Spread Function vs. Pupil Size
1 mm
2 mm
3 mm
4 mm
pupil images followed by psfs for changing
pupil size
5 mm
6 mm
7 mm
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Simulated Images
20/20 letters
20/40 letters
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Strehl Ratio
diffraction-limited PSF
Hdl
actual PSF
Heye
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Modulation Transfer Function
1
0.9
20/20
20/10
0.8
0.7
0.6
contrast
0.5
0.4
0.3
0.2
0.1
0
0
50
100
150
spatial frequency (c/deg)
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Applications for Aberrometry
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Can we Build a Better Autorefractor?
choose the spectacle correction that minimizes
the second order Zernike terms
Coefficient value (microns)
-0.5
0
0.5
1
1.5
2
1
2
astig.
3
2nd order
defocus
4
astig.
5
trefoil
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coma
3rd order
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coma
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trefoil
9
Zernike term
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11
4th order
spherical aberration
12
13
14
15
16
17
5th order
18
19
20
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Can we Build a Better Autorefractor?
choose the spectacle correction that maximizes
the strehl ratio
perfect PSF
defocus/astigmatism
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Can we Build a Better Autorefractor?
choose the spectacle correction that maximizes
the area under the MTF
4.5
4
3.5
3
Area under MTF (0 - 50 c/d)
2.5
2
1.5
1
0.5
0
0
1
-1
0.2
0.4
0.6
0.8
-1.4
-1.2
-0.8
-0.6
-0.4
-0.2
Defocus (D)
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Can we Build a Better 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

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Aberrometric 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
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Diagnosis of High-order Aberrations
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Post - RK
Post - LASIK
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Keratoconus