Wavefront-coding technique for inexpensive and robust retinal imaging

Abstract

We propose a hybrid optical-digital imaging system that can provide high-resolution retinal images without wavefront sensing or correction of the spatial and dynamic variations of eye aberrations. A methodology based on wavefront coding is implemented in a fundus camera in order to obtain a high-quality image of retinal detail. Wavefront-coded systems rely simply on the use of a cubic-phase plate in the pupil of the optical system. The 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 postprocessing. Thus, the system can provide high-resolution retinal images, avoiding all the optics needed to sense and correct ocular aberration, i.e., wavefront sensors and deformable mirrors.

Effect of spherical aberration in trefoil phase plates on color wavefront coding

“…It has been recently shown 15) that a phase plate fabricated poly(methyl methacrylate) (PMMA) and polished in trefoil shape with a large sag generates not only trefoil phase but also a significant amount of spherical aberration. The results also showed that the WFC system tolerates spherical aberration through a given defocus range but the effect of the spherical aberration in color images was not studied in depth.…”

“…Here we will use the parameters of the optical system proposed in Ref. 15 to obtain retinal images at 633 nm, and therefore, we will assume α = 152 µm, and the lens that focuses images onto the CCD is placed at the exit pupil of the system, with an aperture of 1 cm and a focal length of 25 cm. Simulations with Zemax 28) for different values of A provides the Zernike coefficients of the wavefront at the exit pupil.…”

“…[5][6][7][8][9][10][11][12][13][14][15][16] Nevertheless, it has been shown that cubic and trefoil-shaped phases are most used to achieve high-quality images when spherical aberration, 10,11) astigmatism 14,15) and high orderaberrations 15) are also present in the system. WFC can be found in many different applications such as in ophthalmic optics, [15][16][17] iris recognition, 18,19) complexity reduction in optical systems, 20) control of thermal defocus in infrared systems, 21) barcode reading, 22) and microscopy, 23) among many others. The DOF in each application can be extended by a trefoil phase at the exit pupil of the optical system, and the trade-off between the DOF and the image resolution properties is dependent on the peak-to-valley value (strength) of the phase.…”

“…Nevertheless, other aberrations that the plate can generate have not been taken into account and considered coded by the system. In a recent work, 15) Arines et al demonstrated that phase plates polished in trefoil shape do also produce a considerable amount of spherical aberration and a negligible amount of astigmatism and coma. The effect of the spherical aberration on color imaging has not been studied and it is the main goal of this work.…”

“…15) Nevertheless, the method presented here can also be applied to the design of other masks for different purposes and therefore improve their performance.…”