Download Unwrapping Hartman-Shack Images from Highly Aberrated Eyes

Unwrapping Hartman-Shack Images
from Highly Aberrated Eyes Using
an Iterative B-spline Based
Extrapolation Method
Linda Lundstrom & Peter Unsbo
Royal Institute of Technology (KTH), Sweden
Hartman-Shack Wavefront Sensor
• Measures the aberrations of the eye, widely
used to evaluate the optical quality of normal,
abnormal and post-surgery eyes (JZ Liang et
al.,JOSA A 1994)
• Has the potential of doing objective refraction
and giving prescription for patients in seconds (L
Thibos et al., J of Vis 2004)
• Is the key component for adaptive optics which
can improve retinal image to the highest
resolution (JZ Liang et al., JOSA A 1997)
Aberrations of the eye
Why are we interested in the
aberrations of the eye?
1. To understand better how the eye transform optical
information into biochemical and neuronal signals;
2. To improve the design of visual instrumentation
(microscopes, telescopes, display systems…);
3. To improve the quality of visual corrections for people with
visual problems (spectacles, contacts, laser refractive
surgery, IOL implant…);
4. To understand better normal and abnormal eye development
(emmetropization, myopia, anisometropia…) ;
5. To “see” what happens in the retina in vivo with high
resolution retinal imaging systems.
Principle of wavefront sensing
From Thibos, Principles of Hartmann-Shack
Aberrometry, Wavefront Sensing Congress 2000
Principle of wavefront sensing
From Thibos, Principles of Hartmann-Shack
Aberrometry, Wavefront Sensing Congress 2000
Principle of wavefront sensing
By comparing the reference pattern and the observed
pattern, the offsets of the focus spots of the lenslets Δx
and Δ y can be calculated. Then the wavefront W(x, y)
can be computed with the following equations.
Wavefront sensor in ophthalmic
instruments
Limitations of SH WF sensing
• Can’t measure chromatic aberration;
• Light scatter and other noises affect the
precision of the measurement;
• In the case of big distortion of wavefront,
focus spots might overlap, and ambiguity
exists for the reconstruction. As a result,
the measurable aberrations are relatively
small. (Two approaches to this problem:
optical manipulations and software-based
algorithms)
The purpose and the results
• Purpose:
To extend the dynamic range of HartmanShack wavefront sensor using B-spline
based extrapolation
• The results:
The dynamic range of a typical HS sensor
increases 3.5 to 13 times compared with a
simple unwrapping algorithm
Methods
Inputs
HS spot positions in the HS image
Positions of the projected lenslet centers
B-spline basis functions used
Results
Simulations
showed that
compared with
direct method,
the proposed
algorithm can
unwrap 3.5 –
13 times more
aberrations.
Successful examples form human eyes
Disadvantages and future work
• Error propagation. One wrong assignment
at the early stage will affect all following
solutions.
• Continuity assumption. The algorithm
depends on the assumption that
aberrations are continuous over the whole
pupil, but for some dramatically abnormal
eyes, the sampling can cause huge
discontinuity.