Eduardo Tepichín-Rodríguez

Comparison of refractive power maps from a reference surface: geometric versus Zernike power polynomials

In recent years there has been many advances in the field of visual optics, such as new technologies to measure and to analyze the wavefront aberration function of the human eye. In this direction, corneal topographers have been extensively used as a tool to obtain related data that can be used to get the refractive power maps of the human cornea in order to characterize the optical function of the eye. On the other hand, it is well known that we can describe the optical aberrations in the human eye as a polynomial expansion of the Zernike polynomials. In this work we present a qualitative comparison of a refractive power map from a reference refractive surface obtained with an alternate form of representation, first proposed in 2007 by Iskander1 et al., of the wavefront aberrations in the dioptric power domain and the usual geometrical representation of the power refractive maps. We present our preliminary results from such comparison.

Dynamic noise elimination on 2D periodic structures using an LCD as an incoherent spatial source

Based on the Lau Effect we propose a system suitable for eliminating non-periodic noise on bi-dimensional periodic structures in which a liquid crystal display (LCD) is used as a dynamic grating source. Experimental results obtained with our proposal are shown.

Application of adaptive photodetectors and the Talbot effect to measure the focal length of a lens

Focal length is a fundamental lens parameter. The main methods to measure the focal length in the optical shop include nodal bench and image magnification. On the other hand, high accuracy methods involve diffraction and interferometric setups. We propose in this work the use of adaptive photodetectors based on the non-steady state photo-electromotive force effect and the Talbot effect to determine the focal length of a lens; in the range of few meters. As the adaptive photodetectors produce an electrical current proportional to the square of visibility of the light pattern which impinges on them, there is no image processing involved in the detection process. Adaptive photodetectors has been used to determine with high accuracy the positions of maximum and minimum visibility of the light patterns diffracted by a vibrating grating (Talbot effect or self-imaging phenomenon). Therefore, if we place the lens under test close to the diffraction grating, the positions of maximum and minimum visibility will be shifted. By measuring the value of this shift the focal length of the lens under test can be readily determined. Preliminary experiments demonstrate that is possible to determine focal lengths up to 3 meters with an uncertainty about 0.1 %.

Detection of low frequency, out-of-plane vibrations by the Talbot effect and adaptive photodetectors

Detection and measurement of low frequency, out-of-plane vibrations play a very important role in several metrological applications. Classical interferometry is well suited for measuring small amplitudes of vibrations, ranging from picometers up to micrometers, but its use is limited to a laboratory environment. Here we consider the Talbot effect and the so-called adaptive photodetectors based on the non-steady-state photo-electromotive force effect for the measuring of low frequency, out-of-plane of vibrations of flat objects with rough surfaces and with amplitudes of vibrations in the order of microns. The adaptive photodetectors produce an electrical current proportional to the square of the visibility of the vibrating intensity pattern impinging on them. In the method here proposed, the vibrating object with rough surface is illuminated with a beam diffracted by a grating (a Ronchi grating), the light reflected by the object is collected by a lens and imaged on to a home-made GaAs adaptive photodetector. The electrical current from the adaptive photodetector is proportional to the instantaneous position of the vibrating object; this signal is monitored and measured with an oscilloscope connected to a lock-in output. The method is very robust and adequate for environments subjected to perturbations and presents the possibility of adjusting its dynamical range by modifying the period of the grating employed. Experimental results that verify our proposal are presented.

Optical performance of a PDMS tunable lens with automatically controlled applied stress

The advances in the field of adaptive optics and in the fabrication of tunable optical components capable to automatically modify their physical features are of great interest in areas like machine vision, imaging systems, ophthalmology, etc. Such components like tunable lenses are used to reduce the overall size of optical setups like in small camera systems and even to imitate some biological functions made by the human eye. In this direction, in the last years we have been working in the development and fabrication of PDMS-made tunable lenses and in the design of special mechanical mounting systems to manipulate them. A PDMS-made tunable lens was previously designed by us, following the scheme reported by Navarro et al. in 1985, in order to mimic the accommodation process made by the crystalline lens of the human eye. The design included a simulation of the application of radial stress onto the lens and it was shown that the effective focal length was indeed changed. In this work we show the fabrication process of this particular tunable lens and an optimized mechanism that is able to automatically change the curvature of both surfaces of the lens by the application of controlled stress. We also show results of a study and analysis of aberrations performed to the Solid Elastic Lens (SEL).

Manufacture and analysis of a refractive surface with variable asphericity to model the human cornea

The cornea contributes substantially to the performance of the human eye and obtaining the shape of the anterior corneal surface is crucial for ophthalmic applications such as the manufacture of contact lenses and visual laser correction. In this direction, there exist a large amount of theoretical models which describe the shape of the anterior corneal surface. A model of the anterior corneal surface using high-order aspherics has been previously reported in the literature, and one of the main features of this model is that it has been shown to accurately reproduce the clinical data. In this work we have designed a refractive surface with variable asphericity adopting the model mentioned above by means of finite-element software, and once the design was obtained we proceeded to manufacture the optical surface made of a polymer known as PDMS. Also, an interferometric analysis with a Mach-Zehnder interferometer was performed in order to obtain its wavefront aberration function. The main application of this optical surface is to be used as a substitute of a corneal surface within an optomechanical system to mimic the performance of the human eye.

Emmetropic eyes: objective performance and clinical reference

The application of the wavefront sensors to measuring the monochromatic aberrations of the normal human eyes has given a new insight in the objective understanding of its performance. The resultant wavefront aberration function can be applied to evaluate the image quality on the retina, which includes the analysis of the higher-order aberrations. Among others, and due to their well-known mathematical properties for circular apertures, the wavefront aberration function is most commonly represented in terms of the Zernike polynomials. The main idea is to have a clinical reference of the objective performance of a set of normal human eyes. However, the high-order aberrations in normal human eyes are different for each person¸ that can be interpreted as that there are many possible solutions for the objective performance of emmetropic eyes. When dealing with the Zernike coefficients and excluding the spherical aberration, higher-order aberrations have a tendency to have a zero mean value. Different proposals have been suggested in the literature to deal with this feature. Moreover, it has been also shown that there is an ethnic dependency in the magnitude of the aberrations. We present in this work the objective performance of a set of uncorrected Mexican eyes, and compare them with other ethnic results published in the literature.

BICAD: Breast image computer aided diagnosis for standard BIRADS 1 and 2 in calcifications

The Breast Imaging Reporting and Data System (BIRADS) was developed by the American College of Radiologists as a standard of comparison for rating mammograms and breast ultrasound images. It sets up a classification for the Level of Suspicion (LOS) of the possibility of a breast cancer. In this paper we present an automated image analyzing system that finds calcifications based on the standard BIRADS 1 and 2. For our goal, we studied the digital mammography database in DICOM format provided by the Department of Radiology of the Hospital Universitario de Puebla. We used The Difference of Gaussian (DOG) filter to find edges of the forms of the different calcifications and a back-propagation Artificial Neural Network (ANN) for the pattern recognition of the BIRADS 1 and 2 cases. This method allowed us to automate the segmentation of the calcifications with a low computational cost. We achieved the pattern recognition with a high level of sensitivity of 0.9629 and specificity of 0.9920.

Reconstruction of tridimensional objects with two different textures using Gaussian model

We present in this work the use of two Gaussian models that describe reflectance of two different textures for the reconstruction of tridimensional objects made of these textures. The textures of the objects are segmented using a combination of image processing techniques. These segmentations are correlated with the Gaussian model for corresponding texture, for the reconstruction of the tridimensional object. We show our preliminary experimental result and we discuss the advantages and disadvantages of the Gaussian models, which are compared with the traditional representation of the inverse square law of light.

Wavefront aberration function from hard contact lenses obtained with two different techniques

The analysis and measurement of the wavefront aberration function are very important tools in the field of visual optics; they are used to understand the performance of the human eye in terms of its optical aberrations. In recent years, we have compared, through two different methods, the wavefront aberration function of a reference refractive surface of 5 mm in diameter and we have demonstrated its equivalence1. Now, we want to extend these results to a set of hard contact lenses. These hard contact lenses have been subjected to different laser ablation techniques which are typically used in refractive surgery. Our goal is to characterize the resultant ablation profile. We show our results obtained for both, a nonablated hard contact lens and the corresponding ablated samples.