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Optical Design & Engineering

Software advances human eye modeling

Create personalized computer eye models to simulate an individual's vision quality using optical and mechanical engineering software.
17 July 2007, SPIE Newsroom. DOI: 10.1117/2.1200707.0800
Eye-related industries can benefit from custom computer eye models to simulate visual performance, predict the effects of treatment options, and develop or improve technologies that interface with the eye. For instance, existing schematic paraxial eye models have long been used to estimate refractive corrections or as optical placeholders for an eye.1 However, these models are inadequate for large pupils and wide angles because they use an equivalent refractive index instead of the true gradient index of the natural crystalline lens.2 Furthermore, real eyes vary widely in dimensions and ocular surface profiles and schematic models only generalize for entire populations.

Ocular biometry is concerned with the analysis of eye measurements. It has become increasingly important as a result of the growing number of surgical and non-surgical options now available to correct ocular aberrations. Some biometry also assesses individual ocular elements. For example, corneal topographers map the anterior corneal surface. Ultrasound equipment measures anterior chamber depth, lens thickness, and axial length. Scheimpflug cameras characterize the anterior segment and backscatter from cataract. The eye is also a complete optical system that can be characterized with Shack-Hartmann wavefront sensors.3 This technique measures non-uniform, asymmetric wavefront errors affecting vision. Wavefront biometry helps to construct the ablation profiles used in laser refractive surgery.

Entire personalized computer eye models can now be designed using biometry. For instance, enhanced eye models can simulate visual performance with or without aberrations. I have been developing eye modeling systems since 1998.4 Early models, derived from wavefront biometry, assisted in the development of a confocal adaptive optics scanning laser ophthalmoscope at the University of Houston College of Optometry that is now in use at the University of California at Berkeley.5,6 Subsequent modeling included the gradient index crystalline lens.7,8 Advanced cataractous eye models assisted the development of wavefront sensors that also detected scatter from nuclear cataract.8,9 My current work involves combining biometry, gradient index crystalline lenses, spectacles, contact lenses, intraocular lenses, and intraocular scatter into monocular and binocular eye model systems, culminating in the Advanced Human Eye Models (AHEM) modeling capability.10

Optometrists, ophthalmologists, optical engineers, and vision scientists prescribe or develop spectacles, contact lenses, intraocular lenses, or new ocular technologies. AHEM can simulate visual performance by testing different scenarios. Established methods rely on subjective patient feedback from two retinal images. A clinical setting may consist of an eye chart, a phoropter or trial lens, and the patient's eyes. AHEM can model this setting by imaging the eye chart through a test prescription, through aberrated eyes, and onto binocular retinas.

Aberrations, diffraction, and scatter all degrade retinal image quality.1 In this context, refractive aberration models constructed with Zernike polynomials5 are useful; however, AHEM can be taken a step further. Light scatter occurs from normal aging, cataract, corneal edema, the iris, the retina, and also from corrective optics.11 The Advanced Systems and Analysis Program (ASAP) software of Breault Research Organization can model light scatter based on surface and volumetric particle scatter theory applied to ocular interfaces and volumes.12 ASAP scatter and illumination analyses can then be applied to AHEM retinas to generate retinal image maps and metrics. Retinal image quality can also be simulated by incorporating whatever affects vision in the model. This includes personal biometry, target distance, additional optics, glare sources, source intensity, refractive error, trauma, corneal and cataract scatter, diffraction, contrast, color, and other variable visual conditions.

AHEM can be configured in multiple ways, as for example in the following unaccommodated binocular configuration, featuring a gradient index crystalline lens constructed with Solidworks and Rhinoceros computer-aided design (CAD) software.7,13 The lens was combined with the starting point geometry of an existing standard model eye.1 The corneal surfaces of the eye model with the imported lens were then optimized using Zemax software to yield measured and published retinal modulation transfer functions for various wavelengths, pupils, and field angles.8,14 The resulting geometry was converted to ASAP for additional manipulation, target imaging, stereo integration, and optical analyses.12 Fusion of retinal stereo image displays then produced 3D visualizations of test targets. ASAP software also proved extremely flexible for combining data in multiple input formats.

Engineering software is a valuable tool for the physiological optical engineer. In the AHEM example, CAD software offered detailed surface profiling7,13 while sequential ray tracing optical software provided the required optimization.14 Non-sequential ray tracing software enabled the analysis of the interaction between the imported biometry and other systems inclusive of physical optics and light scattering.12 ASAP can also be used as a standalone vehicle for AHEM.

Advanced Human Eye Models can provide simulations of eye systems including ophthalmic and external optics. They can also predict visual performance with retinal image analyses inclusive of the effects of aberrations, diffraction, and scatter.

William J. Donnelly III
Technical Customer Service
Breault Research Organization
Tucson, AZ
Department of Ophthalmology
College of Medicine
University of Arizona
Tuscon, AZ 

William J Donnelly III is a senior optical engineer at Breault Research Organization. He is also a research lecturer in the department of Ophthalmology at the University of Arizona, and an optics consultant. He publishes frequently and was a graduate student at the University of Houston College of Optometry, where he completed his MS work under Dr Austin Roorda and his PhD work under Dr Raymond Applegate, two major contributors to visual optics. He was also a NASA subsystem manager where he engineered still and motion picture cameras for the space shuttle program after receiving a BS in Imaging Science at the Rochester Institute of Technology.