Scott C. WilksCharles A. Thompson, Scot S. Olivier, Brian J. Bauman,
Lawrence Flath, and Robert SawvelAdaptive Optics Group
Lawrence Livermore National Laboratoryand
John S. Werner and Thomas BarnesCenter 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.
A Test-Bed for Vision Science Based on Adaptive Optics
normal normal visionvision
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.
Diffraction-Limited Adaptive Optics and the Limits of Human Visual Acuity
• 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 than20/10 visual acuity if higher-order aberrations are corrected.
normalvision
Super-normalvision
EyeImperfectCornea and Lens
Wavefront of distorted image
Wavefront of perfect image
• New advances in laser refractive surgery and contact lenses may enable correction of high-order aberrations.
Psf of 6.8 mmPupil w/ AO on/off
New advances in ophthalmology may enable SUPERNORMAL VISION
Zernike ModesDefocus
AstigmatismComa Spherical Aberration
0
0.2
0.4
0.6
0.8
1
1.2
1.4
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
Rm
s w
avef
ront
err
or (
µm
)
Mean of 63 eyes5.7 mm pupil*
Uncorrected high order aberrations:LASIK, custom-made contact lenses
Regular eyewear
*J. Porter, private communication
Types of aberrations in population
Conventional Phoropter Wavefront sensor
• 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.
Liquid crystal corrector
High-resolution adaptive phoropter combines ophthalmic wavefront sensor with liquid crystal
wavefront corrector
Aberration-free vision
EDiffraction-limitedimage on retina:resolution only limitedby pupil size
Photoreceptors sampleimage 1-to-1:
optical resolution matchesretinal resolution
20/8 “supernormal” vision!
Can we really see the “E”?
Perfect lens
Eye
Eye chart
AberatedProcess
Beam
C orrectedProcess
Beam
SpatialLight
Modulator
LCD( desired phaseintensity map
electricallywritten here) Backlighting
Laser Diodeapprx. 30mw
Write Beam
Read Beam
ImagingLens
ImagingLens
ImagingOptic
LCD( phase intensity
map Opticallywritten here)
LC( phase map
Optically writtenhere)
Hamamatsu optically addressed nematic liquid crystalspatial light modulator - operational principles
SLM stroke vs Voltage shows us where to operate device, to maximize stroke.
SLM Stroke: Voltage, frequency space
0
200
400
600
800
1000
1200
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Voltage (Volts)
Stroke (nm)
450 Hz500 Hz550 Hz600 Hz650 Hz
The SLM response is slightly uneven over the face of the device.
We really only care about the PV that gives us the phasewe want to wrap at. This means we want to wrap on aSurface, 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)
0
hx
0
The LC-SLM is perfect for phase wrapping, effectively increasing stroke.
0
hx
Incoming lightlooks like thisfor all 3 casesbelow.
Reflected light, for 3 different phase lags.
By wrapping at PV approximately 150, we can take out the0.7 micron abberation, using only 0.3 microns of stroke!
We can apply phase wrapping on the flat file.
Phase wrapping the flat file.
0 < PV < 254
0 < PV < 150(corresponding to ½ waveOf 630 nm light in WYKO)
both give this flatness of reflecting surface:
Either this:
or this, written to SLM…
Computationally, what does phase wrapping look like?
function wrap_phase_new, input
;This function wraps values > 150
; scale numbers up, so we wrap at 150, not 256
in=input*(256.0/150.0)
output = byte(in)
output = output*(150.0/256.0)
return,output
end
0
300x
0
300*(256/150)
x150*(256/150)
0
x256
0
x150
150
Phase wrapping a gaussian.
0 < PV < 150(corresponding to ½ waveOf 630 nm light in WYKO)
0 < PV < 254
PV = 254 PV = 300 Slice across center
Phase wrap the 633 nm, but not the 785nm.
Write pattern to SLM785 nmFar field spot
633 nmFar field spot
Grey bars havePixel Value = 150
633 light sees “flat” surface, while 785 sees a grating.
Now, phase wrap the 785 nm, but not the 633 nm.
Write pattern to SLM
785 nmFar field spot
633 nmFar field spot
633 light sees “flat” surface, while 785 sees a grating.
White bars havePixel Value = 254
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
Dalsa CA-D6256x256, 8 bit Camera
Adaptive Optics Associates200mm pitch, 5mm f.l. Lenslet Array
Matrox PulsarFrame Grabber / VGA (SLM) Driver
Dell PC
Software
Current Status:•All pieces have been procured•Dalsa and Pulsar have been successfully run with Dell PC.•Software modifications in progress
Hamamatsu SLM
Control Hardware Integration Effort
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 < 1m: 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.