Objective Assessment Of Ocular Scatter And Its Influence On Visual Function

The human eye's optics has been quite intelligent after its long-term evolution.However,it is not perfect from an optical point of view.Besides the ocular aberrations,the eye's optics also has lots of complex ocular scatters.Ocular scatter increases with aging and human eye diseases such as cataract.The amount of ocular scatter can reflect the severity of the eye diseases to some extent.It is of great value to quantify ocular scatter in prevention and early diagnosis of the eye diseases.Based on the double-pass(DP)technique,measurement of ocular scatter can be performed,and this method has been applied for estimating ocular scatter in clinic.However,the accuracy of ocular scatter estimation can be influenced by ocular higher-order aberrations(HOAs)and small pupil sizes of the DP system.Therefore,it is interesting to investigate the effect of HOAs on ocular scatter estimation and whether more accurate values of ocular scatter can be achieved by measuring the DP image with HOAs corrected over large pupils.HOAs and ocular scatter lead to the degradation of image quality on the retina,and consequently deteriorate subjective visual performance.Research on the effect of the two factors on visual performance will improve our understanding in visual optics.However,early studies in this direction were all to analyze the relationship between single factor and visual function,not two factors and visual function.Considering that HOAs and ocular scatter affect visual performance simultaneously,research on the combined effect of the two factors on visual performance will provide useful guidance for the optimization of visual correction strategy and for the maintenance of visual performance.In this context,large pupils and adaptive optics(AO)technique are introduced with the DP system to reduce the influence of HOAs and small pupils on ocular scatter estimation and improve the accuracy of ocular scatter estimation.Then the combined effect of HOAs and ocular scatter on visual function are investigated.The main work of this dissertation can be summarized as follows:(1)Simulation on the effect of HOAs on the estimation of ocular scatter.The experimental results suggest that HOAs can effectively reduce the accuracy of ocular scatter estimation.For an AO DP system with a 6-1.5mm annular entrance pupil and a 6-mm circular exit pupil,it is necessary to correct ocular aberrations up to the 6th and 7th Zernike order for normal eyes and diseased eyes,respectively,to reduce the measurement error of obejective scatter index to 0.1 or less.The results provide data foundation for following AO DP system design.(2)Design and development of AO DP measurement system.Based on the simulation results,we set up the correction scheme by using a pair of rotating cylinders in combination with a Badal focus corrector to correct the low order ocular aberrations and using a 145-element piezoelectric DM to correct high order ocular aberrations.The DP system is builded up,and it can achieve 1.1 times diffraction resolution.The system can correct large low order ocular aberrations up to ±6 diopters of defocus and ±5 diopters of cylindrical.Ocular Zernike aberrations up to the 8th order can also be corrected.(3)Investigation on the effect of HOAS on ocular scatter estimation.Based on the AO DP measurement system,DP images and the scatter estimation of each subject are achieved with three different aberration correction strategies.The experimental results suggest that the objective scatter index with only defocus and astigmatism corrected are 1.5~2.7 times larger than that with aberrations corrected up to the 7th Zernike order.This suggests that HOAs can effectively reduce the accuracy of ocular scatter estimation.It also indicates that AO DP system with large pupils can reduce the influence of HOAs and small pupils on ocular scatter estimation and improve the accuracy of ocular scatter estimation.(4)Investigation on the combined effect of HOAs and ocular scatter on the contrast sensitivity function.The AO DP system is modified by incorporating an additional channel for visual testing.Based on this system,the effect of HOAs and intraocular scatter on contrast sensitivity is investigated.Contrast sensitivity measurements are performed with HOAs either retained or corrected by adaptive optics,and with scatter either remaining at the natural eye-induced level or further enhanced by a set of three different scatter filters.Contrast sensitivity is found to be worse when HOAs are uncorrected or scatter increases.Quantitative analysis indicates that the joint effect of HOAs and scatter on contrast sensitivity is not a simple summation of each contributing factor,suggesting a potential compensatory mechanism between HOAs and intraocular scatter on contrast sensitivity.