Abstract
Using raytracing simulation to study the effect of corneal imaging metrics for different aperture sizes as a function of object distances with different schematic model eyes. This raytracing simulation determined the best focus (with the least root-mean-square (rms) ray scatter) and the best wavefront focus (with least rms wavefront error) for four schematic model eyes (Liou-Brennan (LBME), Atchison (ATCHME), Gullstrand (GULLME) and Navarro (NAVME)) with 4 aperture sizes (2-5mm) and 30 object distances in a logscale from 10cm to 10m plus infinity. For each configuration, 10,000 rays were traced through the cornea, and the aperture stop was located at the lens front apex plane as described in the model eyes. The wavefront was decomposed into Zernike components to extract the spherical aberration term. The focal distance with respect to the corneal front apex increases from around 31mm for objects at infinity to around 40mm for objects at 10cm. The best (wavefront) focus was systematically closer to the cornea compared with the paraxial focus, and the overestimation of focal length with the paraxial focus was larger for large aperture sizes and small object distances. The rms ray scatter and wavefront error were both systematically larger with large aperture and small object sizes. At best focus the rms wavefront error was systematically larger, and the rms ray scatter was systematically smaller compared to the best wavefront focus. Spherical aberration varied more with GULLME than with LBME or NAVME, and increased strongly at smaller object distances. The imaging properties of the cornea, especially spherical aberration, increase strongly as the object distance decreases. This effect should be considered, especially when considering aberration correcting lenses for near vision such as multifocal or enhanced depth of focus lenses.
Highlights
In recent decades, a range of new intraocular lens (IOL) designs have been proposed
The spherical aberration term Z40 extracted from the wavefront error within a reference circle of 6 mm at the corneal front surface plane is shown in Figure 5 for all four schematic model eyes (5a: LBME, 5b: ATCHME, 5c: GULLME, 5d: NAVME) and for 2–5 mm aperture sizes as a function of object distance from 10 cm to 10 m, respectively
Most of the controversial discussions on aspheric lenses concern the amount of spherical aberration that should be corrected for comfortable vision; some researchers postulate that corneal spherical aberration should be fully corrected, while others propose a moderate correction of corneal aberration to maintain the robustness of the lens to displacement and tilt
Summary
A range of new intraocular lens (IOL) designs have been proposed. In addition to classical lenses with a spherical optic design on each side, aspheric lenses that address the spherical aberration of the cornea have been introduced. Multifocal and EDOF lens designs typically correct some degree of spherical aberration, either the intrinsic spherical aberration of the IOL or even the spherical aberration of the cornea.[1] Given that such multifocal or EDOF lens designs are used to maintain intermediate or near vision, the corneal aberrations when imaging objects at near distances must be considered.[2,3] For example, a cornea based on the Liou-Brennan schematic model eye[4] shows around 0.26 μm of spherical aberration over a 6 mm central zone. This is true for EDOF designs used for far and intermediate vision; here, the optical aberrations of the cornea when imaging far and near or far and intermediate objects must be considered.[3] The purpose of this simulation study was to analyse the imaging properties of the cornea using several schematic model eyes, including the Liou-Brennan, Atchison, Gullstrand and Navarro. The terms of the analysis are best focus and best wavefront position, and to derive the change in root-mean-squared (rms) spot size, rms wavefront error and spherical aberration with various pupil sizes while imaging objects from infinity to intermediate and near distances using raytracing strategies
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