PubMed Health⌕ Search

Biomedical subjects

Ralf Blendowske

Publications and source records attributed to Ralf Blendowske.

8 recordsLinked to original sources
PubMed2007

Paraxial optics of astigmatic systems: relations between the wavefront and the ray picture approaches.

PURPOSE: The paraxial propagation of astigmatic wavefronts through astigmatic optical systems can be described by the augmented step-along method (ASAM). Its equivalence to the linear ray optics approach is considered in detail. METHODS: The ASAM is exploited to derive paraxial ray paths through a general coaxial astigmatic system. RESULTS: Starting from the information inherent in the ASAM all 2x2 submatrices rendering the general 4x4 transference of linear optics can be generated. This proves the complete equivalence of both approaches. Additionally, we show that the symplectic relations are automatically obeyed in the ASAM. CONCLUSIONS: The ASAM offers a complete alternative to describe the paraxial optics of astigmatic optical systems. According to the ASAM, an optical system is fully characterized by the back vertex vergence and the angular magnification matrix. Hence, a complete description of the paraxial optics of an eye should not only report the state of refraction but the angular magnification matrix as well, although it is not yet very common. The magnification matrix might be important in cases of anisometropia or the design of progressive addition lenses. Yet, a simple clinical procedure to determine the angular magnification matrix is missing.

Astigmatism↗
PubMed2006

Paraxial propagation of astigmatic wavefronts through noncoaxial astigmatic optical systems.

PURPOSE: The paraxial propagation of astigmatic wavefronts through a noncoaxial system is described. METHODS: The augmented stepalong method (ASAM) for vergences is extended to tilted and decentered elements. RESULTS: Equations for the case of tilted and decentered elements are presented in the framework of vergence calculations. All effects according to the perturbation of a centered system can be derived from the vergences provided by the calculations for the unperturbed system. In particular, the shift in image space rendered by the pertubation is simply the sum of additional decenter and tilt terms to the unperturbed system. CONCLUSIONS: Previous work on the description of the propagation of astigmatic wavefronts has been generalized to noncoaxial systems. This completes the wavefront picture built on the paraxial augmented stepalong method.

Astigmatism↗
PubMed2006

An analytical model describing aberrations in the progression corridor of progressive addition lenses.

PURPOSE: In the progression corridor of a typical progressive addition lens (PAL) with an addition of 2.5 D, the power changes by roughly 1/8 D/mm. This renders a power difference of some 0.5 D across a typical pupil diameter of 4 mm. Contrary to this fact, PALs do work well in the progression zone. To explain why, we apply a simple model to derive wavefront characteristics in the progression zone and compare it with recent experimental data. METHODS: We consider a simple analytic function to describe the progression zone of a PAL, which has been introduced by Alvarez and other authors. They considered the power change and astigmatism, which are second-order wavefront aberrations. We include third-order aberrations and compare them with spatially resolved wavefront data from Hartmann-Shack-sensor measurements. RESULTS: The higher-order aberrations coma and trefoil are the dominant aberrations besides astigmatism as given by experimental data. According to our model, the third-order aberrations in the transition zone are strongly coupled to the power change and the cubic power of the pupil radius. Their overall contribution according to experimental data is nicely reproduced by our model. The numeric contribution of higher-order aberrations is small and, for practical purposes, the wavefront can be described locally by the second-order components of sphere and astigmatism only. CONCLUSIONS: We propose a simple analytical model to understand the optics in the progression corridor and nearby zones of a PAL. Our model confirms that for typical pupil sizes, all higher-order aberrations, including the dominant modes of coma and trefoil, are small enough to render an undisturbed vision in the progression zone. Therefore, higher-order aberrations have a minimal impact on the optical performance of these lenses.

Astigmatism↗
PubMed2006

Modified point diffraction interferometer for inspection and evaluation of ophthalmic components.

We demonstrate that a modified point diffraction interferometer can be used to measure the power distribution of different kinds of ophthalmic lenses such as spectacles, rigid and soft contact lenses, progressive lenses, etc. The relationship between the shape of the fringes and the power characteristics of the component being tested is simple and makes the design a very convenient and robust tool for inspection or quality control. Some simulations based on the Fresnel approximation are included.

Journal Article↗
PubMed2005

Paraxial propagation of astigmatic wavefronts in optical systems by an augmented stepalong method for vergences.

PURPOSE: The propagation of astigmatic wavefronts through astigmatic optical systems is reconsidered in the wavefront perspective. METHODS: The stepalong method for vergences, described by 2x2 matrices, is applied and augmented to produce off-axis information like the magnification. This so-called augmented stepalong method (ASAM) is derived by applying the paraxial propagation of astigmatic wavefronts to tilted wavefronts as well. RESULTS: The features of the ASAM are discussed for a single surface, a thick lens, and a general system. CONCLUSIONS: The ASAM provides all necessary information to describe a centered astigmatic optical system in paraxial approximation.

Astigmatism↗
PubMed2004

Tolerating vertex distance changes for spherocylindrical corrections.

Prescriptions depend on the vertex distance. Although weak prescriptions are insensitive to a vertex distance change, stronger ones must be adjusted if the vertex distance is modified by an appreciable amount. The tolerable amount can be specified for pure spherical powers. For spherocylindrical corrections, the concept of dioptric distance is invoked in this article and applied to the propagation of an astigmatic wavefront. This leads to a simple rule that is well suited to decide on the necessity of modifying a prescription.

Astigmatism↗
PubMed2002

Oblique central refraction in tilted spherocylindrical lenses.

Spectacle corrections worn in a frame with a faceform tilt should be used with effective spherocylindrical parameters. For the case of oblique central refraction, Keating presented a procedure to determine these parameters in a third-order approximation. The central equation of his approach, the effective dioptric power matrix, is partly the result of an educated guess. Based on the equations of wavefront tracing, an analytical derivation of his equation, slightly modified however, is given in this paper.

Eyeglasses↗