![]() 11, 30 This discrepancy was avoided by simply introducing more realistic conic surface models. However, spherical surfaces were found to predict much greater spherical aberration and oblique astigmatism than that actually measured. The corneal surface was thus described by a single variable, its radius of curvature R. 6 Historically, modeling of the eye was limited to first-order Gaussian optics, meaning that all optical surfaces were considered as spheres. Its anterior surface is the interface between the air and the eye, and is by far the surface wirh the highest refractive power in the human eye, about 48 D (diopters). The two main optical elements –cornea and lens– are analyzed in separate sections and then the complete optical system is considered along with a final section of concluding remarks. The goal of this review is to extract underlying optical design principles in the human eye from the analysis of models and data. Thus, while canonic or generic eye models are important to understand the optical design of the human eye, personalized eyes are essential to develop clinical applications, such as custom treatments. Therefore, these correspond to ideal human body designs rather than to real human beings. Each represented part of the body or feature was canonic (obtained by some sort of average) and as a result, these statues display a superb magnificence. This is similar to statues in the classic period in ancient Greece. Above all, any model must account for short-term (accommodation) and long-term (ageing) changes.įinally, generic (average) models are of greatest importance to understand the optical design of the eye, but they do not correspond to real individual eyes. Such variety of models reflects the fact that all models are incomplete, since the real human eye contains both cornea and lens (anatomical), works with polychromatic light, it is known to have a limited optical performance (aberrations), it has a very wide visual field, it uses accommodation to focus near objects and it evolves throughout life span. In fact, most of the crystalline's models consider the GRIN structure within the lens. 24 However, the GRIN lens distribution or structure has attracted the interest of numerous authors. Other relevant aspects, such as intraocular scattering, have been incorporated only in a few models. Generic 4, 21 versus custom or personalized. An initial overview of the literature shows a great variety of models, mainly attending to the following features: To understand the optical design of the eye we need models of each component (cornea, lens) and from that we can construct a model of the complete optical system. Thus models make our ideas evolve and models progress with new experimental knowledge. It is crucial to realize that models and underlying hypotheses affect not only the way we understand the eye, but everything involved in the study of the eye from measurement instruments, to data analysis and interpretation of results. One interesting example is the intra-capsular mechanism of accommodation hypothesized by Gullstrand 1 to explain discrepancies between geometry and power of his accommodated eye model (this example will be analyzed below.) The analysis of these discrepancies leads to the formulation of new hypotheses therefore, a new approach should be undertaken. In the general case some agreement, and also important discrepancies, are obtained. The testing stage (4) compares model predictions to experimentally assessed optical performance. Each approach consists of (1) some starting hypothesis based on previous knowledge (2) a set of experimental data and (3) a model relating those data and the hypothesis. Inverse problems are difficult in general and must be solved by successive approaches. The optical design of the eye is already given by nature (optimization through evolution), so its study can be seen as an inverse engineering problem: to unravel such design. ![]() The study of the optical system of the eye has similarities, but also remarkable differences with optical design and testing. Optical testing is also necessary for the verification and validation of designs. ![]() Optical design plays a central role here since this branch of science and technology deals with finding the best combinations of optical elements (lenses, etc.) to obtain a desired function, with optimal performance. Our understanding of the optical system of the eye is evolving quite rapidly due to the combined effort of new experimental methodologies and advanced modeling.
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