Research On Chromatic Aberration Of Human Eye Based On Individual Eye

The human eye as an optical system suffers from chromatic aberration limiting spatial vision. Especially in the well-focused eye, the chromatic aberration becomes a major optical factor damaging the image quality. With the development of wavefront technology, many researches about various kinds of monochromatic aberrations have been extensively studied. However, there are still some sticking points requiring further research about the chromatic aberration, especially the transverse chromatic aberration (TCA).Based on the wavefront technology and individual eye model involving the angle between visual axis and optical axis, this thesis mainly concentrates on the study on the characteristic and correction of the chromatic aberration of human eye.Applying the individual eye model involving the angle between visual axis and optical axis, the statistical distribution of TCA over the visible spectrum is achieved. The two-dimensional and two-color vernier alignment technique is traditionally used to measure TCA in previous research, which would introduce unavoidable experimental error. In this thesis, the angles between the optical axis and visual axis of human eye are calculated from the corneal topography maps for 35 eyes, which are used to construct the individual eye models combining with the other optical parameters obtained from objective measurements. With the analysis of the chief ray of these eye models, the mean value of the statistical absolute magnitude of foveal TCA across the spectrum from 470 nm to 650 nm is calculated as 1.00±0.35 arcmin. Nearly 40% of our statistical population focuses on the range of the magnitude of TCA from 0.675 to 0.975 arcmin. The magnitude of the horizontal TCA is larger than that of the vertical TCA for most of the eyes. Meanwhile, the striking position of the chief ray with short wavelength on the retina is generally closer to the nasal side than that with long wavelength in horizontal direction. The results also indicate that the variation of TCA as wavelength is nonlinear, with the slope smaller at the end of long wavelength than that at the end of short wavelength. In order to evaluate the influence of the longitudinal chromatic aberration (LCA) and TCA on the vision performance, the image quality of the individual eye model involving the angle between visual axis and optical axis is analyzed under three different conditions:LCA but TCA existence, both LCA and TCA existence and both LCA and TCA corrected. It is found that foveal TCA is less detrimental than LCA in visual performance, and so the correction of LCA is more important than the correction of foveal TCA. The research also shows that the correction of peripheral TCA of human eye strongly depends on the location of the achromatizing element, which provides a useful basis for the correction of chromatic aberrations.The displaced artificial stop will introduce additional TCA. After setting an artificial pinhole before the individual eye model in ZEMAX software, we trace the ray passing through the center of the pinhole to investigate the TCA. As a result, the TCA increases linearly with the displacement of the pinhole. There is slight difference of the TCA among the 8 eyes with the maximum of 5.95 arcmin/mm and the minimum of 5.46 arcmin/mm, while the mean growth rate is 5.7 arcmin/mm.To study the variation of the TCA with eccentricity, the peripheral TCA at nasal and temporal field (5°,10°,15°,20°) is analyzed based on the individual eye model considering the visual axis. The peripheral TCA increases linearly with eccentricity. There are small differences among the 8 eyes with an average rate of 0.36 arcmin/deg. The TCA changes of the growth rate faster at the end of short wavelength than that at the end of long wavelength at an identified eccentricity. The growth rates of TCA with wavelength are different at different eccentricities. The growth rate becomes smaller as the incidence light close to the optical axis.To correct the chromatic aberration of human eye, an innovative chromatic corrector with two refractive-diffractive elements for human eye is proposed, which is based on the dispersion property of the diffractive element. Combining with the wide-angle schematic eye model, the performance of the chromatic corrector is evaluated in a comparison with the corrector with single refractive-diffractive element. Both of the two chromatic correctors can compensate for the LCA of human eye greatly. However, the corrector with single refractive-diffractive element yields a rapid increase in the eccentric TCA as field-of-view, while the chromatic corrector with two elements performs very well in the correction of TCA over a wide field-of-view, improving retinal image quality greatly.Wavefront-guided refractive surgery has become an increasing popular alternative surgery for vision correction. So the characteristic of human eye after the surgery is a hot topic in current research. For the first time, we construct the pre-operative and post-operative individual eye models involving the angle between the optical axis and visual axis to investigate the variation of the TCA. It is shown that the mean magnitude of TCA for 15 eyes is reduced from 1.89 arcmin to 1.37 arcmin, which is a 0.52 arcmin decrease. The variation of TCA is largely determined by the degree of the symmetry of the cornea. This research provides a certain reference in the evaluation of the post-operative vision performance.