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A Test-Bed for Vision Science
Based on Adaptive Optics
Scott C. Wilks
Charles A. Thompson, Scot S. Olivier, Brian J. Bauman,
Lawrence Flath, and Robert Sawvel
Adaptive Optics Group
Lawrence Livermore National Laboratory
and
John S. Werner and Thomas Barnes
Center for Neuroscience
University of California, Davis 95616
This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory
under Contract No. W-7405-Eng-48.
Diffraction-Limited Adaptive Optics and
the Limits of Human Visual Acuity
normal
vision
supernormal
vision
• New advances in ophthalmology may enable correction of
high-order aberrations in the eye.
– Advances in laser eye surgery, contact and interocular lenses.
• Improved aberration correction could provide supernormal
vision - better than 20/10 visual acuity, more than a factor of 3
increase in contrast sensitivity.
• Psycho-physical effects of aberration-free eyesight on visual
performance are not known.
• We are using unique LLNL expertise in adaptive optics to enable
detailed scientific studies of the visual performance benefits of
improved aberration correction for the general population.
New advances in ophthalmology may
enable SUPERNORMAL VISION
• Normal human visual acuity is 20/20 on the Snellen scale
after correction for defocus and astigmatism.
• The physiology of the average human eye can support better than
20/10 visual acuity if higher-order aberrations are corrected.
Wavefront of
distorted image
Wavefront of
perfect image
Psf of 6.8 mm
Pupil w/ AO on/off
Supernormal
vision
Eye
Imperfect
Cornea and Lens
normal
vision
• New advances in laser refractive surgery and contact
lenses may enable correction of high-order aberrations.
Rms wavefront error (µm)
Types of aberrations in population
1.4
Mean of 63 eyes
5.7 mm pupil*
1.2
1
0.8
0.6
0.4
0.2
0
Z2,0 Z2,-2 Z2,2 Z3,-1 Z3,1 Z3,-3 Z3,3 Z4,0 Z4,2 Z4,-2 Z4,4 Z4,-4 Z5,1 Z5,-1 Z5,3 Z5,-3 Z5,5 Z5,-5
Defocus
Astigmatism
Regular
eyewear
Coma
Spherical Aberration
Zernike Modes
Uncorrected high order aberrations:
LASIK, custom-made contact lenses
*J. Porter, private communication
High-resolution adaptive phoropter combines ophthalmic
wavefront sensor with liquid crystal wavefront corrector
Conventional Phoropter
Wavefront sensor
Liquid crystal corrector
• Ultimately, a clinical ophthalmic adaptive optics system could be
used to replace the phoropter in order to allow optometrists to
assess high-order aberrations in the eye while the patient directly
observes the visual benefit of correction.
– Permanent correction of high-order aberrations could then be
accomplished with custom laser eye surgery or contact lenses.
Aberration-free vision
Eye
Diffraction-limited
image on retina:
resolution only limited
by pupil size
Photoreceptors sample
image 1-to-1:
optical resolution matches
retinal resolution
E
Perfect lens
Eye chart
20/8 “supernormal” vision!
Can we really see the “E”?
Hamamatsu optically addressed nematic liquid crystal
spatial light modulator - operational principles
LCD
( phase intensity
map Optically
written here)
LC
( phase map
Optically written
here)
Spatial
Light
Modulator
Imaging
Lens
Imaging
Lens
Aberated
Process
Beam
LCD
( desired phase
intensity map
electrically
written here)
Backlighting
Laser Diode
apprx. 30mw
C orrected
Process
Beam
Imaging
Optic
Read Beam
Write Beam
5Vpp
ITO
Electrode
Layer
LC
Dielectric
Mirror
ITO
Electrode
Layer
Read
Beam
Write
Beam
Optical
Glass
Substrate
Alignment Alignment S:H
Optical
Layer
Layer
Glass
iphotoconductor Substrate
SLM stroke vs Voltage shows us where to
operate device, to maximize stroke.
SLM Stroke: Voltage, frequency space
1200
450 Hz
500 Hz
550 Hz
600 Hz
650 Hz
1000
Stroke (nm)
800
600
400
200
0
0
0.5
1
1.5
2
2.5
Voltage (Volts)
3
3.5
4
4.5
5
The SLM response is slightly uneven over the
face of the device.
We really only care about the PV that gives us the phase
we want to wrap at. This means we want to wrap on a
Surface, and not just at a pixel value (say 150.)
Plot of the response of the SLM (Stroke) versus grey
value (0-256) for our optimal values, 600 Hz, 3.6 Volts.
We write a pattern to the SLM, correcting for
the abberations inhernet in the device.
Peak to Valley of ~ 400 nm (surface)
The LC-SLM is perfect for phase wrapping, effectively increasing
stroke.
Incoming light
looks like this
for all 3 cases
below.
h(x)
l
0
h(x)
0
l/4
0
Reflected light, for 3 different phase lags.
l/2
We can apply phase wrapping on the flat file.
By wrapping at PV approximately 150, we can take out the
0.7 micron abberation, using only 0.3 microns of stroke!
Phase wrapping the flat file.
Either this:
both give this flatness of reflecting surface:
0 < PV < 254
or this, written to SLM…
0 < PV < 150
(corresponding to ½ wave
Of 630 nm light in WYKO)
Computationally, what does phase wrapping
look like?
function wrap_phase_new, input
f(x)
; scale numbers up, so we wrap at 150, not 256
output = byte(in)
output = output*(150.0/256.0)
return,output
end
150
0
;This function wraps values > 150
in=input*(256.0/150.0)
300
300*(256/150)
f(x)
150*(256/150)
0
f(x)
256
0
f(x)
0
150
Phase wrapping a gaussian.
PV = 254
0 < PV < 254
PV = 300
0 < PV < 150
(corresponding to ½ wave
Of 630 nm light in WYKO)
Slice across center
Phase wrap the 633 nm, but not the 785nm.
Write pattern to SLM
785 nm
Far field spot
633 nm
Far field spot
Grey bars have
Pixel Value = 150
633 light sees “flat” surface, while 785 sees a grating.
Now, phase wrap the 785 nm, but not the 633 nm.
785 nm
Far field spot
633 nm
Far field spot
Write pattern to SLM
White bars have
Pixel Value = 254
633 light sees “flat” surface, while 785 sees a grating.
Prototype adaptive phoropter using liquid
crystal spatial light modulator
Prototype adaptive phoropter using liquid
crystal spatial light modulator
Far field off SLM no aperture, in testbed.
reference
SLM unpowered
SLM with flat file
Control Hardware Integration Effort
Dalsa CA-D6
256x256, 8 bit Camera
Adaptive Optics Associates
200mm pitch, 5mm f.l. Lenslet Array
Hamamatsu SLM
Matrox Pulsar
Frame Grabber / VGA (SLM) Driver
Current Status:
•All pieces have been procured
•Dalsa and Pulsar have been successfully
run with Dell PC.
•Software modifications in progress
Software
Dell PC
Summary: There are many technical challenges
in using SLM’s for Vision Correction.
• Hamamatsu LC SLM
– We found a voltage-frequency combination that maximizes stroke.
– Stroke is still limited stroke < 1mm: Solution? Use phase wrapping.
– SLM has much finer resolution than wavefront sensor – thus, using
smaller aperture still gives high resolution, as well as flatter SLM.
– Chromatic dispersion (different response at different wavelengths)
is consistent with advertised values: 2 color solution.
• Phase Wrapping
– Principles of phase wrapping shown to work (2 color experiment.)
– Two color correction (close loop at one color, correct at another) will
be our next test.