Radius and asphericity of the posterior corneal surface determined by corrected Scheimpflug photography

Dubbelman, M.; Weeber, Henk A; Heijde, Rob G.L. van der; Völker-Dieben, H. J. M.

doi:10.1034/j.1600-0420.2002.800406.x

Cited by 172 publications

(136 citation statements)

References 17 publications

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“…It is important to note that though selective individual ocular component modeling yielded the required levels of myopia and their respective models, all the parameters were found to be within the range of normal values of the population. [53][54][55][56][57][58][59][60] In this experiment, we considered the retinal surface to be a rotationally symmetric aspheric surface for all the myopic models and assumed it to be invariable over the range and types of myopic prescription chosen. The rationale was to limit the degrees of freedom so as to keep the experiment simpler.…”

Section: Discussionmentioning

confidence: 99%

Do Peripheral Refraction and Aberration Profiles Vary with the Type of Myopia? - An Illustration Using a Ray-Tracing Approach

Bakaraju

Ehrmann

Papas

et al. 2009

Journal of Optometry

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Myopia is considered to be the most common refractive error occurring in children and young adults, around the world. Motivated to elucidate how the process of emmetropization is disrupted, potentially causing myopia and its progression, researchers have shown great interest in peripheral refraction. This study assessed the effect of the myopia type, either refractive or axial, on peripheral refraction and aberration profiles. METHODS: Using customized schematic eye models for myopia in a ray tracing algorithm, peripheral aberrations, including the refractive error, were calculated as a function of myopia type. RESULTS: In all the selected models, hyperopic shifts in the mean spherical equivalent (MSE) component were found whose magnitude seemed to be largely dependent on the field angle. The MSE profiles showed larger hyperopic shifts for the axial type of myopic models than the refractive ones and were evident in -4 and -6 D prescriptions. Additionally, greater levels of astigmatic component (J180) were also seen in axial-length-dependent models, while refractive models showed higher levels of spherical aberration and coma. RESUMENSe considera que la miopía es el error refractivo más frecuente a nivel mundial en niños y en adultos jóvenes. Con el objetivo de esclarecer de qué modo se ve perturbado el proceso de emetropización, potencialmente dando lugar a la aparición de miopía y a su progresión, los investigadores se han mostrado muy interesados en estudiar la refracción periférica. Este estudio evalúa en qué medida la refracción periférica y los perfiles de aberración dependen del tipo de miopía predominante, ya sea refractiva o axial. MÉTODOS:Partiendo de modelos de ojo esquemáticos adaptados al caso de la miopía y utilizando un algoritmo de trazado de rayos, se calcularon las aberraciones periféricas, incluyendo el error refractivo, en función del tipo de miopía. RESULTADOS: En todos los modelos estudiados, se observó un cambio o desplazamiento hipermetrópico en el equivalente esférico medio (EEM), cuyo valor parecía depender mayormente del ángulo analizado (excentricidad). En los perfiles de EEM se aprecia que dicho desplazamiento hipermetrópico es mayor para los modelos de miopía de tipo axial que para los de tipo refractivo, diferencia que se hace evidente en los casos correspondientes a -4 D y a -6 D. Además, se observó que el coeficiente asociado al término de astigmatismo (J180) presentaba un valor más alto en los modelos con un mayor peso de la miopía axial, mientras que en los modelos predominantemente refractivos se obtuvieron mayores niveles de aberración esférica y de coma. CONCLUSIONES: Este estudio ha concluido que aquellos ojos miopes donde prima la componente axial pueden presentar un mayor riesgo de progresión que ojos miopes que tengan errores refractivos similares pero donde prime la componente refractiva. Esta predicción surge de los resultados teóricos obtenidos en modelos de ojos utilizando un algoritmo de trazado de rayos, por lo que necesitan ser confirmados experimentalment...

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Section: Discussionmentioning

confidence: 99%

Do Peripheral Refraction and Aberration Profiles Vary with the Type of Myopia? - An Illustration Using a Ray-Tracing Approach

Bakaraju

Ehrmann

Papas

et al. 2009

Journal of Optometry

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“…study of the lens is difficult because the lens has to be imaged through the cornea which causes blur, perspective and optical distorsions. 45 A further difficulty is that shapes in vivo and in vitro cannot be compared since the living lens is under the stress of the ciliary muscle. Even though artificial stretchers have been developed 61 to simulate the action of ciliary forces, it is difficult to make direct comparisons.…”

Section: Optics Of the Crystalline Lensmentioning

confidence: 99%

“…41,43,44 The back surface of the cornea is difficult to measure, but new instruments such as corrected Scheimpflug photography 45 or promising methods such as anterior segment OCT imaging might help to make highly accurate in vivo measurements. Classic values for the posterior curvature radius are lower than the anterior surface, about 6.5 mm, whereas the conic constant seems to be more negative, about -0.66 41 (more prolate ellipsoid).…”

Section: Posterior Surfacementioning

confidence: 99%

The Optical Design of the Human Eye: a Critical Review

Navarro

2009

Journal of Optometry

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Cornea, lens and eye models are analyzed and compared to experimental findings to assess properties and eventually unveil optical design principles involved in the structure and function of the optical system of the eye. Models and data often show good match but also some paradoxes. The optical design seems to correspond to a wide angle lens. Compared to conventional optical systems, the eye presents a poor optical quality on axis, but a relatively good quality off-axis, thus yielding higher homogeneity for a wide visual field. This seems the result of an intriguing combination of the symmetry design principle with a total lack of rotational symmetry, decentrations and misalignments of the optical surfaces (J Optom 2009;2:3-18 ©2009 Spanish Council of Optometry) KEY WORDS: human eye's optical design; eye models; design principles; ocular aberrations; optical quality of the eye. RESUMENEn este trabajo, los modelos de córnea, de cristalino y de ojo se analizan y se comparan con los datos experimentales para evaluar las propiedades y, eventualmente, para desvelar los principios del diseño óptico que afectan a la estructura y al funcionamiento del sistema óptico del ojo humano. A menudo encontramos una coincidencia razonable entre los modelos y los datos experimentales, pero también se ponen de manifiesto algunas paradojas. El diseño óptico parece corresponderse con el de un gran angular. En comparación con los sistemas ópticos convencionales, el ojo tiene una mala calidad óptica en eje pero, por el contrario, presenta una calidad relativamente buena fuera de eje, proporcionando así una gran homogeneidad a una amplia extensión del campo visual. Este hecho parece ser el resultado de una curiosa combinación del principio de diseño simétrico con una total ausencia de simetría de rotación, con descentramientos y con una deficiente alineación de las superficies ópticas. (J Optom 2009;2:3-18 ©2009 Consejo General de Colegios de Ópticos-Optometristas de España) PALABRAS CLAVE: diseño óptico del ojo humano; modelos de ojo; principios de diseño; aberraciones oculares; calidad óptica del ojo.

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“…Therefore, biometrical measurements in the anterior eye segment, such as corneal curvature, changes of lens curvature during accommodation, depth of the anterior chamber and anterior chamber angle, always have to be corrected by specific algorithms. 15,16 The amount of correction to be performed depends on the depth of the layer in question, meaning that each refractive zone adds a small amount of distortion to the path of the light rays. The Pentacam is the only camera whose software includes an algorithm for automatic distortion correction.…”

Section: Pentacam: Basic Considerationsmentioning

confidence: 99%

Pentacam: Principle and Clinical Applications

Jain¹,

Grewal²,

Dada³

et al. 2009

JOCGP

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Radius and asphericity of the posterior corneal surface determined by corrected Scheimpflug photography

Abstract: ABSTRACT.Purpose: To obtain the shape of the posterior corneal surface in a healthy population, using Scheimpflug photography corrected for distortion due to the geometry of the Scheimpflug imaging system and the refraction of the anterior corneal surface.

Cited by 172 publications

References 17 publications

Do Peripheral Refraction and Aberration Profiles Vary with the Type of Myopia? - An Illustration Using a Ray-Tracing Approach

Do Peripheral Refraction and Aberration Profiles Vary with the Type of Myopia? - An Illustration Using a Ray-Tracing Approach

The Optical Design of the Human Eye: a Critical Review

Pentacam: Principle and Clinical Applications

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