Practical Research Of Fundus Camera Based-on The Liquid Crystal Adaptive Optics

The eye was the only organ which could be imaged optically and non-traumaticly, so the eye fundus examination is significant for diagnoses the eye diseases and other relative diseases (such as diabetes, nephropathy and so on). However, due to aberration of human eyes, the resolution of the common inspection equipment was only 10-15μm, which can not meet the demand for early diagnosis. The application of adaptive optics could resolve the limit of human eye aberration. With AO, the imaging resolution could reach 3μm, which is the optical diffraction limit for 6mm pupil diameter, and each single visual cell could be identified. Although deformable mirror was widely employed by a number of study teams, but it is less practicality because of its large size, costly and small correction range. In the paper, the engineering of eye fundus high-resolution imager was studied, and the wavefront corrector we used is the high-resolution liquid crystal spatial light modulator.Since the effective reflection of the fundus is as low as 10-5~10-3 and the eye has large axial chromatic aberration, the illumination light source must be quasi- monochromatic. Therefore, on the comprehensive consideration of the safety of eye fundus illumination, the spectrum transmission of the eye, the spectrum reflection of the fundus and spectrum absorbency of blood, 808nm and 561nm laser was chosen as the illumination light for imaging the cell layer and blood vessel. Because the laser speckle deteriorates the image severely, we used a fast rotating milk glass to reduce the speckle. And in order to improve the efficiency of lighting energy and the illumination uniformity, a high transmission of the random phase plate was used instead of the milk glass and a multi-mode fiber was employed to uniformed lighting further. Through theoretical calculation and simulation by Tracepro software, it is determined that the obscuration diameter of annular stop was at least 1.5-2mm in order to eliminate stray light reflected by the cornea. What’s more, the shape of compensation lens is optimized, and it is determined that the curvature diameter of the compensation lens should be smaller than 120mm, then the stray light reflected from the lens surface could be obscured totally.It is put forward using an alterable stop to extend the imaging field after the aberration correction. The small diameter for the small hole and large hole is 1mm and 6mm respectively. And the form of the alterable stop is optimized to reduce the switch time between large and small illumination field less than 25ms, so the correction of the aberration and the imaging of the fundus could be completed within 50ms. It has been proved that the fluctuation of the eye aberration was less than 0.07λ(RMS), so the dynamic fluctuation of the aberration could be neglected.Furthermore, the large field of non-corrected imaging system was designed and integrated with the AO imaging system. So we can firstly get a large field (about 12 deg) observation of the fundus to search the pathological focus. And the visual target was optimized as a dynamic cross line chart, which could improve the position accuracy and stability of eye gazing. At last the assembling of the imager system is optimized.Based on the above study, the prototype of the high-resolution eye fundus imager was developed successfully. It is an important step for realizing the practicality of the high-resolution eye fundus imager based-on liquid crystal adaptive optics.