Research On Vision Correction With Wavefront Data And On Neural Characteristics Of Human Eye

The refractive system of eye is complicated, and there exist defocus, astigmatismand high-order aberrations which play a role in human vision. As the development ofthe wave-front technique, optometry is not only able to obtain the information of thedefocus and astigmatism, but also the information of the high-order aberrations, suchas spherical aberration, coma and so on. And correcting high-order aberrations of eyesbecomes possible as the development of the measurement technique of eye’swavefront aberrations. The visual function of human eye can be divided into twocascading processes, one is the refraction of the eye’s optics and the other is theretina–brain neural course. Segmentation evaluation the image quality of the entirevisual system is the precondition and the objective of the vision correction. Neuralcontrast sensitivity function can provide some predictions for lots of eye problems,which is beneficial for timely clinical treatment.This dissertation mainly concentrates on the study on optimized correcting theaberrations of the eyes. Based on the wavefront technology and individual eye model,optical designing method for the correction instruments of spectacle lens and contactlens and the evaluation method for the visual function are both investigated asfollows:The wavefront-guided sphero-cylindrical spectacles are designed. By taking intoaccount the2~6order wavefront aberrations, the individual eye model is constructedin optical design software ZEMAX. On the basis of that, the sphero-cylindricalspectacles are optimized and the prescriptions of the spectacles are provided.Compared with the data resulting from other method, it is justified to be correct.Through setting the individual eye model, this lens design is fit to analyze theimaging quality of the lens-eye system for various viewing fields and rotationalangles of the eye. By modeling in ZEMAX for randomly chosen50eyes, it is foundthat36eyes among them perform well in visual quality with sphero-cylindricalcorrection only and14eyes perform unsatisfactorily with sphero-cylindrical correction and need further correction.With the measured wavefront aberrations data, the Liou eye model is modifiedby means of transferring the defocus into the crystalline lens and the vitreous bodywhile transferring the astigmatism and high-order aberrations into the cornea. Afterthat the individual eye model is set up with the accommodation characteristics in agood agreement with natural eye. Aiming on correcting the high-order aberrations ofvisions which can not be realized by sphero-cylindrical spectacles, the new type ofspectacles is designed. The viewing characteristics are took into consideration bysetting the lens-eye system as multi-configurations, corresponding to the case ofeyeball’s rotations, and by setting certain fields of view for each configuration,corresponding to the case of certain viewing field. The spectacles are respectivelydesigned into the aspheric-toric form and the freeform-toric form for eyes withdifferent features of wavefront aberrations. It is demonstrated that for the eyes withhigher astigmatism and spherical aberration, the aspheric lens is needed to yield amuch better vision correction especially for larger field of view, while for the eyeswith larger high-order aberrations, such as coma and trefoil, the freeform lens caneliminate the influence of these aberrations on the vision and provide goodperformance either for larger field of view or for central filed of view. Due to theviewing characteristics settings and special profiles optimization, this lens design hassignificance in improving the vision for some specific eyes with high astigmatism andhigh-order aberrations.The method to acquire the structure of the contact lens is proposed on the basisof the objectively measured ophthalmological data. With the data of the cornealtopography, the anterior corneal surface is fitted with the optimized sphere or toricprofile and then the optical zone of the back surface of contact lens is determined.With the data of the wavefront aberrations of eye and the diffraction theory of angularspectrum, the equivalent wavefront at the front surface of the contact lens is acquiredby a propagation procedure of the ocular wavefront aberrations through tear film andlens medium, and then the optimized toric profile for the front surface of the lens isobtained with the least square fitting method to correct the wavefront aberrations atthat plane. Finally, the equivalent spherical and cylindrical powers as well as the astigmatism angle of the contact lens are calculated with the structures of the contactlens. By comparison of the ocular refraction power with the corrector refractionpower (contact lens plus tear lens), this method is proved to be correct. Comparedwith the accuracy of clinical vision correction of0.125D by traditional methodnowadays, the maximum difference of the refraction power is0.04D for our method.In order to get the neural contrast sensitivity function (NCSF) at temporalfrequencies of human eyes, the contrast sensitivity function (CSF) at temporalfrequencies and the wavefront aberrations of the eye are respectively measured. Theeye’s modulation transfer function (MTF) is obtained by constructing the individualeye model of the subject and then the NCSF at temporal frequencies is calculated byrelating the MTF with CSF. The results demonstrate that the overall value of theNCSF decreases as the temporal frequency increases while the form of NCSF curvetakes on the band-pass shape to the low-pass shape, and then to almost monotonicvariation. The attenuation factor of the NCSF, which represents the sensitivity ofvisual neural system to the temporal frequency, varies little as spatial frequencyincreases and it reflects the resolution characteristics of visual neural system to themotion object.