Biomedical subjects

Jorge Ares

Publications and source records attributed to Jorge Ares.

5 recordsLinked to original sources

PubMed2006

Estimation-induced correlations of the Zernike coefficients of the eye aberration.

The statistical properties of the estimated eye aberrations differ in general from the actual ones owing to the cross coupling and aliasing generated in the estimation process. In particular, the estimated Zernike aberration coefficients may show fictitious correlations that are not present in the incoming wavefronts. This fact should be taken into account when analyzing estimated aberration datasets, searching for correlations related to physiological eye features. We give an analytical model for this effect and evaluate its magnitude for a particular example, assuming that the true aberration statistics follows a Kolmogorov power law.

Journal Article↗

PubMed2006

Direct transformation of Zernike eye aberration coefficients between scaled, rotated, and/or displaced pupils.

In eye aberrometry it is often necessary to transform the aberration coefficients in order to express them in a scaled, rotated, and/or displaced pupil. This is usually done by applying to the original coefficients vector a set of matrices accounting for each elementary transformation. We describe an equivalent algebraic approach that allows us to perform this conversion in a single step and in a straightforward way. This approach can be applied to any particular definition, normalization, and ordering of the Zernike polynomials, and can handle a wide range of pupil transformations, including, but not restricted to, anisotropic scalings. It may also be used to transform the aberration coefficients between different polynomial basis sets.

Computer Simulation↗

PubMed2005

Presbyopia compensation with a quartic axicon.

PURPOSE: The purpose of this study was to evaluate the performance of quartic axicons for presbyopia compensation. The working principle relies on profiting the high depth of focus of the axicons to supplement the reduced accommodation amplitude of presbyopes. METHODS: We present the design equations of a particular kind of axicon to compensate a general presbyopia condition using simultaneous vision. A rotationally symmetric polynomial of fourth-order, corresponding to the well-known Seidel spherical aberration term, was chosen as its refractive profile. To validate its performance, we computed the retinal images with Stiles-Crawford apodization for a presbyopic eye compensated with this quartic axicon and compared them with those obtained without compensation or with other available solutions based on the simultaneous vision principle. RESULTS: The quartic axicon provides an important improvement of the image quality for intermediate distance vision in comparison with conventional bifocal and trifocal solutions. The image quality, however, is still not optimum for all distances. CONCLUSIONS: The results show the usefulness of the proposed approach and point out the need for developing further adapted optimizations.

Accommodation, Ocular↗

PubMed2004

Influence of thresholding on centroid statistics: full analytical description.

The centroid method is a common procedure for subpixel location that is applied to a large number of optical sensors. In practice, it is always accompanied by thresholding algorithms used to eliminate undesirable background that may decrease precision. We present a full analytical description of the interaction between centroiding and thresholding applied over an intensity distribution corrupted by additive Gaussian noise. An in depth analysis of the most outstanding statistical properties of this relation (mean and variance) is also presented by means of simulated and experimental data. This work provides fundamental concepts to the designers of sensors that are based on centroid measurements to allow them to use thresholding correctly before centroid computation.

Journal Article↗

PubMed2003

Time and space issues in ecotoxicology: population models, landscape pattern analysis, and long-range environmental chemistry.

Advances in ecotoxicology addressing problems of time and spatial scales are presented and interpreted in the frame of concepts on population/community dynamics and landscape pattern analysis. Example deterministic/probabilistic modeling experiments are used to illustrate key concepts. Space and time scales analyzed are single and multigenerations of local populations, metapopulations, community, and ecosystem/landscape. Most population models used in recent ecotoxicology studies are deterministic and do not include a formal treatment of spatial processes, like migration or local random extinction. Some metapopulation models have been applied with success. Upscaling of ecotoxicological results at the community level is less developed, probably because of the inherent complexity of indirect and direct coactions among organisms. Community and ecosystem toxicity end points that could find a broad use in regulatory applications have not yet been identified. Some practical issues like the estimation of the potential for the natural attenuation of toxicity and the transport of contaminants along food chains must be addressed at these scales/levels of biological complexity. The estimation of ecotoxicological effects has been increasingly evolving to integrate modeling and monitoring contaminant transport and fate, landscape pattern analysis, and spatially explicit population dynamics (including direct and indirect communal interactions).

Agriculture↗