PURPOSE:To describe the individual virtual eye, a computer model of a human eye with respect to its optical properties. It is based on measurements of an individual person and one of its major application is calculating intraocular lenses (IOLs) for cataract surgery. METHODS: The model is constructed from an eye's geometry, including axial length and topographic measurements of the anterior corneal surface. All optical components of a pseudophakic eye are modeled with computer scientific methods. A spline-based interpolation method efficiently includes data from corneal topographic measurements. The geometrical optical properties, such as the wavefront aberration, are simulated with real ray-tracing using Snell's law. Optical components can be calculated using computer scientific optimization procedures. The geometry of customized aspheric IOLs was calculated for 32 eyes and the resulting wavefront aberration was investigated. RESULTS: The more complex the calculated IOL is, the lower the residual wavefront error is. Spherical IOLs are only able to correct for the defocus, while toric IOLs also eliminate astigmatism. Spherical aberration is additionally reduced by aspheric and toric aspheric IOLs. The efficient implementation of time-critical numerical ray-tracing and optimization procedures allows for short calculation times, which may lead to a practicable method integrated in some device. CONCLUSIONS: The individual virtual eye allows for simulations and calculations regarding geometrical optics for individual persons. This leads to clinical applications like IOL calculation, with the potential to overcome the limitations of those current calculation methods that are based on paraxial optics, exemplary shown by calculating customized aspheric IOLs. (J Optom 2009;2:70-82 ©2009 Spanish Council of Optometry) KEY WORDS: real ray-tracing; Snell's law; corneal topography; wavefront aberration; intraocular lens calculation. RESUMEN OBJETIVO:Describir el ojo virtual individualizado, que es un modelo computacional de ojo humano con respecto a sus propiedades ópticas. Está basado en las medidas realizadas en cada persona, de manera individual. Una de sus principales aplicaciones es el cálculo de lentes intraoculares (LIO) para cirugía de cataratas. MÉTODOS:El modelo está construido a partir de datos de geometría ocular; en particular, la longitud axial y las medidas topográficas de la cara anterior de la córnea. Todos los componentes ópticos de un ojo pseudofáquico se modelan por medio de métodos computacionales científicos. El método de interpolación por splines permite incluir de manera eficiente los datos obtenidos en las medidas de topografía corneal. Las propiedades relacionadas con la óptica geométrica, como el patrón de aberración del frente de onda, se obtienen a partir de la simulación de un trazado de rayos reales, el cual emplea la ley de Snell. Los componentes ópticos se pueden calcular empleando procedimientos computacionales de optimización. Para 32 ojos distintos, se calculó la geometría de la c...

Real ray tracing calculated the exact geometry of custom IOLs to provide the minimum wavefront error, going beyond simple diopter information. Results show spherical aberration can be significantly reduced with aspheric IOLs. However, the limited possible reduction of total HOAs, even perfectly positioned custom aspheric IOLs, may be a reason for the unclear results in studies assessing the potential benefit to visual performance of currently used aspheric IOLs.

PURPOSE: An implementation of real ray tracing based on Snell's law is tested by predicting the refraction of pseudophakic eyes and calculating the geometry of intraocular lenses (IOLs). METHODS: The refraction of 30 pseudophakic eyes was predicted with the measured corneal topography, axial length, and the known IOL geometry and compared to the manifest refraction. Intraocular lens calculation was performed for 30 normal eyes and 12 eyes that had previous refractive surgery for myopia correction and compared to state-of-the-art IOL calculation formulae. RESULTS: Mean difference between predicted and manifest refraction for a 2.5-mm pupil were sphere 0.11±0.43 diopters (D), cylinder -0.18±0.52 D, and axis 5.13°±30.19°. Pearson's correlation coefficient was sphere r=0.92, P<.01; cylinder r=0.79, P<.01; and axis r=0.91, P<.01. Intraocular lens calculation for the normal group showed that the mean absolute error regarding refractive outcome is largest for SRK II (0.49 D); all other formulae including ray tracing result in similar values ranging from 0.36 to 0.40 D. Intraocular lens calculation for the refractive group showed that depending on pupil size (3.5 to 2.5 mm), ray tracing delivers values 0.95 to 1.90 D higher compared to the average of Holladay 1, SRK/T, Haigis, and Hoffer Q formulae. CONCLUSIONS: It has been shown that ray tracing can compete with state-of-the-art IOL calculation formulae for normal eyes. For eyes with previous refractive surgery, IOL powers obtained by ray tracing are significantly higher than those from the other formulae. Thus, a hyperopic shift may be avoided using ray tracing even without clinical history. [J Refract Surg. 2007;23:393-404.]

The increase in oblateness, higher order aberrations, and spherical aberration of real patients' postoperative corneas was not seen in simulations using a Munnerlyn ablation profile. The fluence loss of the laser was one important factor, but did not explain all increasing aberrations observed clinically. We hypothesize that corneal wound healing and biomechanics play a role.

We present a setup to measure biometric data of the eye using Fourier domain interferometry. The measuring depth of a Fourier domain system is basically limited owing to the spectral resolution. Combining two spectral domain interferometers with different wavelength ranges creates two measurement sections and allows for a simultaneous biometric measurement in terms of corneal thickness, anterior chamber depth, and axial length. The necessary offset between both sections in the combined setup was calibrated with a known reference object. The setup was tested by measuring a self-constructed model eye. All biometric data of the model eye can be detected simultaneously. This system has a precision of 13 μm (standard deviation) and a trueness of 46 μm. The signal-to-noise ratio was 98 dB for the anterior part and 76 dB for the posterior part. In contrast to time domain interferometry, this setup does not need any mechanically moving parts. Owing to the short time frame of the biometric measurement, potential eye movements should have no influence on the result. In addition to the fast measurement, this setup provides the possibility to adjust the laser power of both sections independently. This could help in the case of dense cataract.