High Resolution Retinal Imaging System Based On Liquid Crystal Adaptive Optics

The eye is important sense organ of the human; it is also a "window" to humanhealth. Retina microscopic image is very important in vision science and will aid forthe very early clinic diagnosis of the diseased that influence the retina (such ashypertension, diabetes, etc.). However, the resolution of retinal image is limited byocular aberration, the resolution of traditional fundus imaging device is only15-20μm.To overcome this obstacle, adaptive optics technology was incorporated into fundusimaging system. The resolution of the fundus image was increased to diffractionlimited level and the single photoreceptor could be resolved. As for compensator,deformable mirror (DM) is the first choice for AO system. However, its application inophthalmology is restricted by the high price and bulk mechanical structure. In thisthesis, liquid crystal on silicon was used as wavefront corrector, and the highresolution retinal imaging system based on liquid crystal adaptive optics is studied.As a wavefront corrector, liquid crystal wave-front corrector have a number ofattractive advantages, such as small size, low cost. However, it also has somelimitations, the phase modulation performance of LCOS depends on the wavelengthof the incident light because of the dispersion of the liquid crystal. Therefore, Thephase modulation properties of the LCOS must be appraised and calibrated beforeusage. In the thesis, two-dimensional phase grating method was proposed to calibratethe phase modulation properties. This method is different with interferencemeasurement method has lower requirements on the experimental environment. Moreover, it can be performed in the adaptive optics system. It can reduce thecomplexity of experiment and meet the needs of onlinemeasurement.Due to the low reflectivity of the human eye, a high quantum efficiencyback-thinned CCD camera was used as the imaging device. But in the output imagewill appear striped background because of the etaloing effect of thinning CCD chip,and it reducing image quality severely. In the thesis, we analyzed the distribution ofstriped background according to the principle of multi-beam interference and thenproposed flat-field correction method to eliminate the striped background. Thedistribution of striped background will vary from eye to eye, so we take the fundusimage without aberration correction as reference image of flat field and got goodresults on photoreceptor image.In addition to take the high quantum efficiency CCD as imaging device, profoundresearch has been made on improving the energy efficiency of the system. First, amethod based on Fourier Optics is proposed to simulate the imaging process ofShack-Hartmann Wavefront Sensor (SH-WFS), and the sampling rate of SH-WFS isfixed. To avoid the unnecessary illumination during data processing period, doublepulse illumination mode was proposed, It could reduce the time of eye suffer laserirradiance effectively. And then we test the polarization state of fundus backscatteredlight when the illumination light is linearly polarized light. The experimental resultindicated that the fundus backscattered light has high degree of polarization. Based onthis properity, the double polarized light source illuminated open-loop liquid crystaladaptive optical system was proposed, and the energy efficiency was increased to70%from50%.At last, the robust for the vivo eye of this system was discussed. Firstly, thelimitions of the eye accommodation are analyzed. To solve this problem, the loworder aberrations of human eye are compensated by trial lens. In order to eliminate thestray light reflected by the lens surface, the curvature of the trial lens should besmaller than150m, when the diameter of the circular obscuration is equal to2.67mm.In the vivo eye experiment, it is difficult to obtain a clear fundus image; it is causedby multilayer reflection of retina. In order to obtain the clear image, a continuous focusing control sequence is proposed, the imaging CCD was moving axially when itcaptured the image.On the basis of the foregoing research, and double polized light illuminationopen-loop liquid crystal adaptive optics system was established. Firstly the correctionaccuracy of this system was tested. The residual error is approximately0.07λ, and theStrehl ratio was greater than0.8. The imaging system could reach diffraction limiteresolution. At last, three subjects with different myopia were tested, the highresolution images of photoreceptor were obtained.