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Eva Acosta, Justo Arines; Optical-digital System Invariant To Eye Aberrations For Retinal Imaging. Invest. Ophthalmol. Vis. Sci. 2012;53(14):3098.
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© ARVO (1962-2015); The Authors (2016-present)
To design a hybrid optical-digital imaging system that provides high resolution retinal images without wavefront sensing or correction of the spatial and dynamic variations of the eye aberrations.
A methodology based on wavefront coding is proposed in order to obtain a high quality image of retinal details. Wavefront coded systems rely simply on the use of aspheric optics (cubic phase plate) in the pupil of the optical system. The cubic phase element is intended to blur images in such a way that invariance to optical aberrations is achieved. The blur is then removed by image post-processing. Thus, the system is free of all the optics needed to sense and correct ocular aberration, i.e., wavefront sensors and deformable mirrors. The key to the method is the joint design of the peak to valley value of the cubic phase mask and signal processing for the specific aim of the optical instrument, in this case the dynamic variation of phase distortions caused by the tear film, cornea and lens.
Numerically computed design of the cubic phase elements are tested using simulation software developed by the authors. The simulations commence with a clear image of the retinal photoreceptors considered as the object. Then, convolution of the retinal image with the PSF corresponding to the sum of the cubic phase plus the eye aberrations for different subjects measured with a Hartmann-Shack sensor and fitted to the first 21 Zernike polynomials (including a dynamic sequence of aberrations for one of the subjects, 59 different eye aberrations). The images are restored by means of a regularized Wiener filter which uses the PSF corresponding to only the cubic phase, and the choice of the peak to valley value for the cubic phase is optimized by minimizing the rms difference between the object and the restored images. The choice of the cubic phase, with 4.5 waves peak to valley, renders high-quality restored images for all the aberrations of all subjects, including a certain degree of randomly added misfocus and astigmatism (up to 1.5 D sphere, positive or negative and up to -1.75D for the cylinder).
The simulations show that the method can provide high quality retinal images under a wide range of eye aberrations while also significantly reducing the complexity, size and cost of some of the current systems by no longer having to sense and correct high order ocular aberrations and allowing a great degree of flexibility for remaining amounts of defocus and astigmatism. The system could potentially be used in a wide variety of real time applications beyond diagnosis.
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