A Real Time Adaptive Optical System for Vision Science

Transcription

1 A Real Time Adaptive Optical System for Vision Science Li Chen Center for Visual Science, University of Rochester, Rochester, NY, Ben Singer 1

2 Outline Why do we need AO for vision science Rochester AO system design and performance What can AO do for vision science Summary and future AO for vision Why do we need AO for vision science? 2

3 Structure of the human eye Eyelid Retina Fovea Iris Cornea n=1.376 Pupil Optical nerve Disc Lens n=1.41 Ciliary muscle Choroid Sclera The main components of the eye s optics: the cornea, the iris and pupil,the crystalline lens, and the retina. Defects of the eye s optics Sources of retinal blur Scattering Diffraction Aberrations Monochromatic point spread functions Human eye 2.5 mm Perfect eye 6 mm Human eye 6 mm 3

4 A complete description of the eye s optical defects Complete Wave Aberration Zernike Decomposition Low order aberrations 2nd order Defocus Astigmatism 3rd order Higher order aberrations Coma Trefoil 4th order Spherical Other > 5th order RMS wavefront error (µm) Population Statistics of the Eye s Wave Aberration % % 2.7% 1.8%.9% 1.6%.9%.7% Mean of 19 subjects 5.7 mm pupil Z Z Z Z 3Z Z 3 Z 4 Z 2 4 Z Z Z Z 5Z Z 5Z Z 5Z -5 5 Z 2 Z -2 2 Z 2 Z Z 3 Z 3 Z3 Z4 Z4 2 Z -2 4 Z 4 Z Z 5 Z Z 5 Z 5 Z5 Z 5 Defocus Coma Astigmatism Spherical Aberration Porter et al., JOSA A, 21 4

5 Everyone has a different pattern of aberrations Perfect eye (diffrac tion limited) MRB GYY MAK Wave Aberration 5.7 mm pupil Polychromatic Point Spread Function Retinal Image.5 arcmin The wave aberration is not static (HH, 5.7mm pupil, 55 nm wavelength) Wave Aberration Point Spread Function Dynamic correction can increase resolution and contrast Dynamic range of eye s wave aberration required closed loop bandwidth 1~2 Hz to correct temporal fluctuation Hofer. H, Artal P., Singer B., Aragon J. L., and Williams D. R., J. Opt. Soc. Am. A 18, (21) 5

6 Adaptive optics can compensate for monochromatic aberrations Sharp Retinal Image Aberrations in Lens and Cornea Distort Wavefront Sharp Image Deformable Mirror Compensates for Eye s Aberrations Rochester AO system design and performance 6

7 AO for astronomy vs. vision Astronomy Vision Aberration atmospheric imperfect optics Aperture annular circle Light levels photon starved light safety Bandwidth ~1Hz ~2Hz Wavefront abnormal smooth Low order aberration corrector tip-tilt mirror trial lens for correctin g defocus and astigmatism Imaging long exposure short exposure Other issues for vision: fixation target,corneal reflections, pupil camera or pupil tracking, retinal illumination, bite bar or head rest... Adaptive optical system for the eye Eye M 19mm M Fixation target R CCD Pupil camera P Hot mirror P M Beam splitter M P P Off-axis Paraboli c mirrors SLD Hot mirror Deformable Mirror M R P CCD Lenslet array Wavefront sensor 7

8 Wavefront sensing --- Laser beacon qsuperluminescent diode(sld) : 81nm wavelength, 2nm spectral spread qcoherence properties : 3 micron qlaser power : ~5µW Maximum Permissible Exposure(MPE) for ANSI laser safety: MPE =.7854d 2 (1 2(λ.7) )mw where d is the pupil diameter in centimeter, λ is the wavelength in range of.7~1.5micron, for 6.8mm pupil and 81nm wavelength laser beacon, the MPE of laser beacon is 62 µw. Wavefront sensing --- Off-axis method removes the corneal reflection Pin hole Eye Lenslet array Off-axis beam Collimated input beam Laser beacon 8

9 Wavefront sensing---lenslet array Square lenslets (221 of 17 17).4 mm (.384 mm) spacing 24 mm focal length Pupil size for AO 6.8 mm Wavefront sensing---wavefront detector (Roper Scientific: PentaMAX) Cooled CCD camera Frame transfer CCD 15 Hz frame rate with pixels (3Hz frame rate with binning) Pixel size is 15µm pixels per subaperture (11 11with binning) 273, photons per subaperture with 5 µw illumination at 3Hz sampling rate, the SNR is 284:1. 9

10 Wavefront sensing --- Centroid iteration algorithm estimated centroid real centroid Initial centroid box Shrinking centroid box estimated centroid real centroid Shrinking centroid box Last centroid box Advantage: reducing noise effect and improving the centroid precision Wavefront sensing --- Wavefront reconstruction Spot displacement: x k = (x k x k ) y k = (y k y k ) Local slope: s kx = x f Wavefront reconstruction with Zernike polynomials: Zernike coefficient c 1 c 2 C = M c N C = Z + S Zernike inverse matrix z(1,1) x z(2,1) x L z(n,1) x z(1,2) x z(2,1) x L z( N,2) x M M M M z(1, M) x z(2,m) x L z(n,m) x Z = z(1,1) y z(2,1) y L z(n,1) y z(1,2) y z(2,2) y L z( N,2) y M M M M z(1, M) y z(2,m) y L z(n,m) y M: total numbe r of lenslets (221) N: Zernike term (63) N ϕ(x,y) = c i Z i (x, y) i =1 s k y = y f 2M N slope from WFS s 1x s 2x M s M x S = s 1y s 2 y M s M y 1

11 Wavefront correcting--- PMN actuators required in 6.8mm pupil (12 subjects) λ=79nm rms(µm) x11 array (97 actuators).67mm spacing, 4µm stroke strehl ratio x11 array (97 actuators).67mm spacing, 4µm stroke Actuators across 6.8mm pupil Actuators across 6.8mm pupil Chen L., Yoon G., Hofer H., Singer B., Doble N., Por ter J., and Willams D. R., "Adaptive optical system design and optimiz ation for human eye ", SPIE Opto Northest and Imaging, 21. Wavefront correcting---xinξtics 97 channel DM PMN actuators 7 mm (.672 mm) spacing 4 µm stroke Clear aperture 77.4 mm (7.43 mm) Pupil size for AO 7.8mm (6.8 mm) Pupil size for retinal imaging 62.5mm (6 mm) 11

12 Geometry of three conjugate pupil planes PMN actuators 7 mm (.672 mm) spacing 4 µm stroke Clear aperture 77.4 mm (7.43 mm) Pupil size for AO 7.8mm (6.8 mm) Lenslets (221 of 17 17).4 mm (.384 mm) spacing 24 mm focal length Pupil size for retinal imaging 62.5mm (6 mm) Parabolic off-axis mirrors for magnifying and demagnifying Image position On axis 1 degree off axis Wavefront Aberration Rms (µm) Rms for diffraction limit λ/14 =.393 wavelength Benefits of using the off-axis mirrors: more compact, no chromatic aberration and no additional aberration 12

13 Magnification of the AO system Eye lens Off axis DM Off axis lens lenslet WFS mirror mirror CCD Focus length(mm) Pupil plane(mm) Magnification from the eye s pupil to DM is Magnification from the eye s pupil to lenslet is 1.4. Wavefront correcting ----Tip-tilt removed from slope vector Original slope vector S = s 1x s 2x M s M x s 1y s 2 y M s M y x-tilt component y-tilt component S x = S y = M s ix i=1 M M s iy i =1 M S = New slope vector s 1x s 2x s M x s 1y s 2 y s M y S x S x M S x S y S y M S y 13

14 Wavefront correcting----direct slope control algorithm Wavefront Control Reconstruction of the wavefront to determine the actuator commands Slope vector Z + Residual E + Wavefront Reconstruction Voltage vector Direct Slope Control Using wavefront slope as control objective. Slope vector D + Voltage vector Wavefront correcting----direct slope control algorithm V = D + S voltage to update actuators V 1 V 2 V = M V N slope influence function s(1,1) x s(2,1) x L s(n,1) x s(1, 2) x s(2,1) x L s(n,2) x M M M M s(1, M) x s(2, M) x L s(n, M) x D = s(1,1) y s(2,1) y L s(n,1) y s(1,2) y s(2,2 ) y L s(n,2) y M M M M s(1, M) y s(2, M) y L s(n, M) y 2 M N slope from WFS S = s 1x s 2x M s M x s 1y s 2 y M s M y M: total number of lenslets (221) N: total n umber of actuators (97) 14

15 System performance evaluating Wavefront Wavefront error Pupil function Point spread function Modulation transfer function Strehl ratio N ϕ (x, y) = c i Z i (x, y) i =1 N RMS = c i 2 i=1 P(x,y) = A(x,y)exp( i( 2π λ ϕ(x,y)) PSF = FT(P(x,y)) 2 MTF = Re[FT( PSF)] SR = exp[( 2π λ RMS) 2 ] SR = Max(PSFIntensity) Aberratedeye Max(PSFIntensity) Aberrationfree Performance evaluating---spatial performance.1 (12 subjects) rms(µm) x17 array (221 lenslet).38mm spacing Lenslet number across 6.8mm pupil 15

16 Timing sequence in AO system Sampling period T(33ms): Time T 2T 3T 4T Exposure Readout Centroid Voltage V Time delay τ=2t Update mirror Exposure Readout Centroid V Voltage Update mirror Performance evaluating---temporal performance Eye aberration e(s) + - Residual aberration r(s) Sensor noise n(s) + 1 e Ts Ts DM e τs C(s) 1 y(s) Feedback control: V((n)T)= V((n-1)T)+.3Voltage((n)T) C(s)=k c /s Eye aberration e(s) + Residual aberration - r(s) Sensor noise n(s) + 1 e Ts Ts e τs k c s y(s) From this model, the bandwidth is.9hz@ 2Hz sampling rate, and 3Hz sampling rate. Chen L., Yoon G., Hofer H., Singer B., Doble N., Por ter J., and Willams D. R., "Adaptive optical system design and optimiz ation for human eye ", SPIE Opto Northest and Imaging,

17 System performance---dynamic compensation Average wavefront power spectrum Dynamic compensation Static compensation 1 Temporal frequency (Hz) 1 Ratio close d-loop power / open-loop power Model:.9Hz, Experimental data:.85hz Wavefront disturbance rejection data 6 subjects, 2Hz frame rate model Temporal frequency (Hz) Hofer H., Chen L., Yoon G., Singer B., Yumauchi Y., and Williams, D., Optics Express, Vol. 8, No. 11, 21. System performance---aberration correction GYY, 6.8 mm pupil, 55 nm GYY, 6.8 mm pupil, 55 nm RMS (µm) Wave aber ration Point spre ad function 3 Hz, 6.8 mm pupil DM on.1 µm Time (s) 17

18 RMS wavefront error(µm) RMS wavefront error(µm) Static correction vs. dynamic correction Without correction Static correction JP Static correction JC Dynamic correction SR Dynamic correction 1 2 (6.8mm pupil) Strehl ratio Strehl ratio Without correction JP λ=81nm Static correction Dynamic correction JC Static correction λ=81nm 1 2 Dynamic correction AO user interface 18

19 Different pupil size version 6.8mm pupil, 221 lenslets, 97 actuators 6mm pupil, 177 lenslets, 69 actuators 5.1mm pupil, 121 lenslets, 57 actuators 4.3mm pupil, 89 lenslets, 37 actuators Error sources Wavefront sensing error: noise from background, detector dark current, detector read noise, and photon shot noise; Sampling error Temporal error: computational time lag, Electrical output bandwidth, Micro fluctuation Wavefront correcting error: DM fitting error, DM stroke limitation, Mirror hysteresis, Mirror saturation 19

20 What can AO do for vision science? AO for imaging retina Adaptive optical system for retinal imaging Eye M 19mm M Fixation target R CCD Pupil camera P Hot mirror P M Beam splitter M P P Off-axis Paraboli c mirrors SLD Hot mirror Deformable Mirror M R R Interference filter Krypton Flash lamp P R Cold mirror CCD Retinal imaging P CCD Lenslet array Wavefront sensor 2

21 Longitudinal chromatic aberration(lca).8d Bennett A. G. and Rabbetts R., Clinical Visual Optics, Fixation target for imaging different retinal location 21

22 Magnification for imaging retina (1 degree FOV) Eye lens Off axis DM Off axis lens mirror mirror Science camera Focus length(mm) Pupil plane(mm) Retinal plane(µm) Magnification from retina to the retinal image plane at science camera is Retinal illumination Krypton flash lamp delivered 4ms flash to illuminate a retinal disk with 1 degreed in diameter. Imaging detector Cooled back-illuminated CCD science camera(roper Scientific: MicroMAX) , 24µm square pixels, max. frame rate of 3Hz cone size at 1 deg retina with 5 µm in diameter will cover 7pixels. Each pixel of science camera will receive about nm wavelength with Krypton flash lamp output 5.7mw/mm 2 4ms exposure time. The SNR of each pixel is about 41:1. 22

23 Layout of the electrical system Imaging Module Mac G4 Computer PCI Camera Board MicroMax CCD Camera Controller Flashlamp Driver Safety Shutte r Driver Serial Port Velmex Motor Driver AO Module Mac G4 Computer PCI PCI-DIO-96 Board SLD Shutter Driver DM Driver PCI Camera Board Penta Max CCD Camera Controller Pupil camera User interface for retinal image (JP, 1 deg retinal eccentricity) 23

24 Adaptive optics enhances retinal image quality Without adaptive optics (single image) With adaptive optics (single image) With adaptive optics (~1 images) 1 deg retinal eccentricity Increase in the power at the cone spatial frequencies with dynamic correction Ratio of Image Power cones Spatial Frequency (c/deg) The contrast of the cones in the retinal image increased by 3% with dynamic AO correction. This allows cone classing in the retinas of living human subjects 24

25 Color vision depends on three types of cones 1 S M L Absorbance Wavelength (nm) Cone classing with AO YY, 1 deg nasal-superior AP, 1.25 deg nasal MD, 1.25 deg nasal %L = 51 ± 5 JC, 1.25 deg temporal %L = 51 ± 3 JW, 1 deg temporal %L = 6 ± 3 BS, 1.25 deg nasal %L = 66 ± 4 %L = 8 %L = 94 ± 3 Cour tesy Heidi Hofer and Austin Roorda 25

26 Prevailing models in color blindness Replacement Model Loss Model Adaptive Optics has discovered a new form of color blindness MM - Protanope Functional loss of L cones NC - Deuteranope Functional and physical loss of M cones Loss of cones? Missing all L gene(s) Has normal gene array Joe Car roll, Heidi Hofer & D avid Williams, Rochester Jay & Maureen Neitz, Medic al College of Wisconsin 26

27 Retina image of AMD Patient Without AO With AO Courtesy Stacey Choi What can AO do for vision science? AO for psychophysical experim ent 27

28 Visual Benefit of correcting higher order aberrations (Mean of 19 subjects) Modulation transfer mm pupil all monochromatic aberrations corrected only defocus and astigmatism corrected Visual benefit mm pupil 3 mm pupil 5.7 mm pupil Spatial frequency (c/deg) Spatial frequency (c/deg) Guirao, Porter, Williams, Cox, JOSA A, 22 Adaptive optical system for psychophysical experiment Eye M 19mm M Fixation target R CCD Pupil camera P Hot mirror P M Beam splitter M P P Off-axis Paraboli c mirrors SLD Hot mirror Deformable Mirror M R P Cold mirror CCD Retinal imaging P CCD Lenslet array Wavefront sensor Projector Visual stimulus 28

29 Visual acuity with letter E Visual acuity Visual stimulus Subject s task Visual acuity improved with AO.5 (6mm pupil, white light) 2/2 -.5 Log MAR without AO with A O_ static 1 AP GYY HH Subjects 2/1 29

30 Contrast sensitivity improved with AO (6mm pupil, white light) Contrast sensitivity 1 YY Correcting all aberrations 1 Correcting monochromati c aberrations only Correcting defocus 1 and astigmatism only Spatial frequency [c/deg] GYY Spatial frequency [c/deg] Visual benefit 7 YY Correcting all aberrations Correcting higher order monochromatic aberrations Spatial frequency [c/deg] GYY Spatial frequency [c/deg] Yoon, G.Y. and Williams, D.R., J. Opt. Soc. Am. A, 19, (2), , (22) Adaptive optics can generate aberrations Deliberately Blurred Retinal Image Visual Stimulus Deformable Mirror Compensates for Eye s Aberrations and Additional Aberrations Are Added 3

31 AO for accommodation experiment Far step.5 D D Near step -.5 D Without higher order aberrations With higher order aberrations Chen L., Kruger P. and Williams D., "Accommodation without higher order monochromatic aberrations", ARVO 22. DM produces step changes in defocus without changing magnification (Artificial eye, 6mm pupil) Far step Near step Maltese cross (55nm) Defocus (D) Time (sec) 1 deg FOV Defocus (D) Time (sec) 31

32 One subject could not accommodate either with or without higher order aberrations (AP, 6mm pupil, 55nm wavelength) Accommodation to far step Accommodation to near step Defocus (D) Without HO aberrations With HO aberrations Ideal response Time (s) Defocus (D) Ideal response With HO aberrations Without HO aberrations Time (s) Higher order aberrations removed: 95% One subject required higher order aberrations to accommodate (LF, 6mm pupil, 55nm wavelength) Defocus (D) Accommodation to far step Without HO aber rations With HO aberrations Ideal response Time (s) Defocus (D) Accommodation to near step Ideal response With HO aberrations Without HO aberrations Time (s) Higher order aberrations removed: 95% 32

33 4 subjects can accommodate with or without higher order aberrations (JP, 6mm pupil, 55nm wavelength) Accommodation to far step Accommodation to near step Defocus (D) With HO aberrations -.5 Ideal response Time (s) Without HO aberrations Defocus (D) Ideal response Without HO aberrations With HO aberrations Time (s) Higher order aberrations removed: 96% AO for image metrics to predict subjective image quality Requires lengthysubjective procedure Requires algorithm to transform wave aberration into optimum values for sphere, cylinder, and axis Chen L., Por ter J., Singer B., Llorente L., Nagy L., and Williams D. R., "Predicting Subje ctive Image Quality from the Eye s Wave Aberration", ARVO 23. Williams D. R., "Assessment of Optical Aberrations: Wavefront Sensing and Adaptive Optics", ARVO

34 RMS does not capture subjective blur Spherical Aberration =.14µm RMS =.14 µm Spherical Aberration =.14µm Defocus =.25µm RMS =.287 µm Strehl ratio does not capture subjective blur Defocus = -.25 µm Spherical =.14 µm Aberration Rms =.287 µm Strehl ratio =.158 Defocus =.25 µm Spherical =.14 µm Aberration Rms =.287 µm Strehl ratio =

35 AO for image metrics to predict subjective image quality Goal: To develop a metric that predicts the subjective effect of the eye s wave aberration. Methods: Blur matching with adaptive optics Compare subject matches with predictions from different m etrics radial order 2nd AO can present Zernike mode to the eye astigmatism Z -2 2 Z Z 2 2 defocus astigmatism 3rd Z -3 Z -1 3 Z Z 3 trefoil coma coma trefoil 4th Z -4 4 quadrafoil secondary spherical astigmatism Z 4 Z 2 Z Z 4 secondary astigmatism quadrafoil 5th pentafoil Z -5 5 secondary trefoil Z -3 Z -1 Z Z Z 5 secondary coma secondary coma secondary trefoil pentafoil 35

36 Blur matching Standard Aberration Stimulus Test Mode.7.7 Amplitude (µm) Amplitude (µm) Zernike mode -.7 Zernike mode Neither RMS nor Strehl ratio predicts the data.8.7 2nd 3rd 4th 5th Match Value (µm) RMS fitting SR fitting Matching result Zernike modes 36

37 Rochester sharpness metric Retinal Image Formation Neural Processing Max ( ) * = S Point Spread Function Gaussian Neural Point Spread Function σ = 1 Sharpness metric predicts better.8.7 2nd 3rd 4th 5th Match Value (µm) Zernike modes in OSA standard 37

38 Wave aberrations from 4 Lasik patients Wave aberrations generated with adaptive optics in a single eye Subject Adjusts Amount of Defocus to Match the Blur from Each Patient s Wave Aberration Amplitude(µm) Stimulus Patient wave aberration Stimulus Defocus(D) Defocus Zernike mode -.3 Zernike mode 38

39 Comparison of all three metrics (6 Subjects) 1.8 RMS Wavefront Error 1.8 Strehl Ratio 1.8 Rochester Sharpness R Metric prediction (D) y =.633x +.37 y = 1.793x y = x R 2 =.49 R 2 = R 2 R = R 2 = R2= Matching value (D) Does the brain compensate to the eye s optics defects? Though HOA existing the eye always blur the retinal image, our subjective impression is that the vision world is always sharp and clear, suggesting that the brain might compensate their subjective blur influence. Wave Aberration Angle = deg Factor = 1 Stimulus 55nm wavelength 1 degree FOV Rotated Aberration Angle = 45 deg Factor =.6 Artal P., Chen L., Fernánde z E., Singer B., Manzanera S., and Williams D., Adaptive optics for vision:the eye s adaptation to its point spread function, The 4th international Wavefront congress in San Francisc o(23) 39

40 Wave aberration rotated counterclockwise (MC, 6mm pupil) PSF rotated counterclockwise (MC, 6mm pupil)

41 The brain does compensate to the eye s optics defects Subjective blur Blur increased 2~4% Rotated angle Error bar is the standard error from 5 subjects.the subjective blur of the retinal image increased when the PSF was rotated. Summary Rochester s real time AO system for vision science 97 actuator deformable mirror 221 lenslet for wavefront measurement real time compensation up to 3Hz Closed loop bandwidth.85hz This AO system can greatly reduce the aberrations in the eye to achieve nearly diffraction-limited performance. High resolution retinal images can be obtained automatically and quickly AO can improve visual performance There are a number of applications of adaptive optics for basic science (color vision) and medical(laser refractive surgery, and diagnosis and treatment of retinal diseases). 41

42 AO systems for vision in the world GROUP TYPE DM FOV PERFORMANCE IMAGING PSY. EXP. Rochester Conventional 97 Xinetics DM 1 <.1 µm rms, φ = 6.8 mm Yes Yes 37 MEMS ~.17 µm rms, φ = 6.8 mm Yes No Houst on Confocal 37 Xinetics DM 1, 2.5 ~.12µm rms, φ = 7 mm Yes No SLO Indiana Coherence 37 Xinetics DM 1 ~.13µm rms, φ = 6.8 mm Yes No Gated OCT San Diego Conventional 19 Membrane 1 >.5 µm P-V, φ = 8 mm No N/A LLNL Phoroptor BMC MEMS 1 System under testing No Yes LLNL Conventional LC-SLM 1 System under testing No Yes (UCD) Chengdu, China Conventional 37 DM 1.18µm rms, φ = 6mm Yes No Russia Fundus 18 Bimorph 15 <.15µm rms, φ = 5mm Yes No Murcia. Spain Conventional 37 Membrane 1.12 µm rms, φ = 4.3 mm No Yes LC-SLM.1 µm rms, φ = 4.7 mm No N/A Imperial Conventional 37 Membrane 1 >.1µm rms, φ = 4.8 mm N/A N/A College,UK plus tilt mirrors Paris Conventional 13 act. mirror 1 φ = 7mm N/A N/A Future AO systems for vision Large dynamic range, high sensitivity wavefront sensor Low cost, high stroke wavefront corrector Compact system Friendly user interface Combine AO with other imaging modalities 42

43 Retinal Imaging with BMC MEMS Without AO With MEMS AO 6 registered images for each case FOV ~.25, 75µm on the retina Doble N., Yoon G., Chen L., Bierden P., Singer B., Olivier S., and Williams D. R., Optics letters, Vol. 27 No. 17, 22. MEMS in AO Phoropter 43

44 High stroke segmented piston tip/tilt array (Iris AO) A rigid, single-crystal-silicon mirror is assembled onto a surface-micromachined parallel-plate actuator. The actuator is elevated 6 microns above the surface of the substrate because of bending caused by residual stresses in the nickel-polysilicon bimorph flexure. The End 44