Longitudinal Spherical Aberration (LSA)
The longitudinal spherical aberration (LSA) of the human eye changes as a function of its accommodation level. It is common for current commercially available simultaneous vision bifocal contact lenses for example, to be designed with incorporated spherical aberration to alter the depth of focus of the eye. At the Contact Lens and Visual Optics Laboratory we have studied the effect of various levels of positive and negative spherical aberration upon the depth of focus of the eye. Some of the results of this study have indicated that the natural spherical aberration of the eye changes during accommodation (+ve LSA for far accommodation and ve LSA for near accommodation). These changes tend to increase the depth of focus of the eye towards the direction of intermediate focal distance. However, the use of spherical aberration in bifocal contact lenses to make use of this causes significant visual acuity loss for only a modest improvement in this depth of focus.
The following figures show firstly how depth of focus changes with the alteration of vergence and LSA, and secondly how the MTF (Modulation Transfer Function) alters with the change in vergence of a system that incorporates -2.34 dioptres of LSA. An MTF shows the ratio of the image contrast to object contrast. The PSF (pupil spread function) shows how a point source of light is changed by the vergence optics of the system and the MTF and displays how that point source's image would appear at the fovea. So that in this case, the PSF is optimal with +0.50 dioptres of vergence.
Read More:
Collins MJ, Franklin R, and Davis, BA. (2000) Optical considerations in the contact lens correction of infant aphakia. Optometry and Vision Science 79(4):234-240.
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Dynamic Aberrations
The optical aberrations of a human eye are not static. Current techniques used for analysis of the dynamic changes in aberrations include simple Fourier transform based methods, which are found to have limitations in this particular application. We developed a set of signal processing tools for time, frequency, and time-frequency analyses of aberration measurements, combination of which provides a better insight into the sources of these microfluctuations. The methodology includes extraction of artifacts form potentially significant eye movements, filtering, optimal parametric signal modeling, and frequency and time-frequency representations.
| Time, frequency, and time-frequency representations of the Zernike comatic term |
Read More:
- D. R. Iskander, M. J. Collins, M. R. Morelande and M. Zhu, “Analyzing the dynamic wavefront aberrations in the human eye”, IEEE Transactions on Biomedical Engineering, 51(11): 1969-1980, November, 2004.
- M. Zhu, M. J. Collins, and D. R. Iskander, “Microfluctuations of wavefront aberrations of the eye”, Ophthalmic and Physiological Optics, 24:562-671, 2004.
- M.J. Collins, B. Davis and J. Wood,"Microfluctuations of steady-state accommodation and the cardiopulmonary system", Vision Research, 35: 2491-2502, 1995.