Julián Espinosa - Academia.edu (original) (raw)

Papers by Julián Espinosa

Journal of Modern Optics, 2011

Surfaces with radial structure do not fit well to squared detectors or sampling matrices. Cartesi... more Surfaces with radial structure do not fit well to squared detectors or sampling matrices. Cartesian grid sampling provides a different density of nodes in sectors. Zernike polynomials are a complete set of orthogonal polynomials defined on a unit disk often used as an expansion of such surfaces. In the fitting process, the sampling distribution is not usually taken into account and might have undesirable effects on the final parameter estimates. We propose applying weighted least-squares regression that compensates the unequal influence of sectors due to the sampling distribution, assigning a weight function to the nodes grid and thus providing a better fit in the central optical zone.

Journal of Biomedical Optics, 2009

The eye is not a centered system. The line of sight connects the fovea with the center of the pup... more The eye is not a centered system. The line of sight connects the fovea with the center of the pupil and is usually tilted in the temporal direction. Thus, off-axis optical aberrations, mainly coma and oblique astigmatism, are introduced at the fovea. Tabernero et al. [J. Opt. Soc. Am. A 24(10), 3274-3283 (2007)] showed that a horizontal tilt of the crystalline lens generates a horizontal coma aberration that is compensated by the oblique light incidence on the eye. Here we suggest that corneal astigmatism may also play a role in compensation of oblique aberrations, and we propose a simple model to analyze such a possibility. A theoretical Kooijman eye model with a slight (~0.6 D) with-the-rule astigmatism is analyzed. Light rays at different incidence angles to the optical axis are considered, and the corresponding point spread functions (PSFs) at the retina are calculated. A quality criterion is used to determine the incidence angle that provides the narrowest and highest PSF energy peak. We show that the best image is obtained for a tilted incidence angle compatible with mean values of the angle kappa. This suggests that angle kappa, lens tilt, and corneal astigmatism may combine to provide a passive compensation mechanism to minimize aberrations on the fovea.

Construction and Building Materials, 2015

Measurement of concrete strain through non-invasive methods is of great importance in civil 11 en... more Measurement of concrete strain through non-invasive methods is of great importance in civil 11 engineering and structural analysis. Traditional methods use laser speckle and high quality 12 cameras that may result too expensive for many applications. Here we present a method for 13 measuring concrete deformations with a standard reflex camera and image processing for 14 tracking objects in the concretes surface. Two different approaches are presented here. In the 15 first one, on-purpose objects are drawn on the surface, while on the second one we track small 16 defects on the surface due to air bubbles in the hardening process. The method has been tested 17 on a concrete sample under several loading/unloading cycles. A stop-motion sequence of the 18 process has been captured and analyzed. Results have been successfully compared with the 19 values given by a strain gauge. Accuracy of our methods in tracking objects is below 8 m, in 20 the order of more expensive commercial devices. 21 22 23

Journal of Modern Optics, 2011

Spontaneous eye blinking is a complex process, which implies the joint actions of the eyelid musc... more Spontaneous eye blinking is a complex process, which implies the joint actions of the eyelid muscles and the eye globe. These actions provoke the eyelid closure and simultaneous eye retraction into the orbit. Recently, the authors presented a non-invasive technique for measuring both movements that permits calculating several parameters. Following the work from Malbouisson and co-workers, we have fitted the displacement curves of the eyelid to an analytical function and, additionally, we have extended the kinematic model to the eye retraction movements. As a result of the fitting, some dynamic parameters have been presented for the first time in the literature. This study can be easily extended to obtain relevant information about ocular biomechanical properties in physiological optics and provides results that may be of interest in neurophysiology, in order to understand the blinking mechanism.

A knowledge of the shape of the cornea is of major importance for the planning and monitoring of ... more A knowledge of the shape of the cornea is of major importance for the planning and monitoring of surgery, and for the correct diagnosis of corneal diseases. Many authors have studied the geometry of the second corneal surface in the central region and it has been stated that there is a high correlation between the central radii of curvature and asphericities of the two corneal surfaces. In this work we extend this study to a larger, central, 6-mm diameter of the cornea. Surface height data, obtained with an Oculus Pentacam from 42 eyes from 21 subjects, were analysed to yield surface power vectors. Corneal heights of both surfaces were also decomposed into low-order Zernike polynomials and the correlations between each of the power vectors and low-order Zernike coefficients for the two surfaces were studied. There was not only a strong correlation between spherical powers and Zernike defocus coefficients, but also between the astigmatic components. The correspondence between the astigmatisms in both surfaces found here can be of the utmost importance in planning optical surgery, since perfect spherical ablation of the first surface does not assure total correction of corneal astigmatism.

Optics Letters, 2010

We present a noninvasive technique for high-speed measuring of eye retraction and eyelid position... more We present a noninvasive technique for high-speed measuring of eye retraction and eyelid position during blinking. The anterior chamber of the eye is illuminated by the slit lamp of a biomicroscope and eye dynamics during a blinking sequence are captured with a high-speed camera working at 500 frames per second. Digital image processing allows quantitative analysis of cornea and eyelid positions during the closing and opening phases of the blinking process. Our method allows simultaneous measuring of corneal retraction, duration of down and up phases, total blinking duration, and average and peak speeds of the eyelids in both phases, thus providing a complete analysis of the blink's transversal motions.

Optik, 2000

Even with fixed gaze, the human eye is under steady motion (tremor micosaccades and drifts). Dete... more Even with fixed gaze, the human eye is under steady motion (tremor micosaccades and drifts). Detection of these movements requires of invasive techniques or expensive devices with sophisticated detection methods. In this paper we present a technique for pupil segmentation and contour analysis which will provide valuable information about fixational eye movements. The method is based on fitting an ellipse to the pupil contour. Pupillary hippus, microsaccades and drifts are obtained as well as cyclotorsional movements. The method is simple and the experimental requirements are easily available since just a biomicroscope and a digital videocamera are required.

Research in Nondestructive Evaluation, 2011

High speed cameras are often used for monitoring impacts and fast dynamic processes on structures... more High speed cameras are often used for monitoring impacts and fast dynamic processes on structures. However, quantitative information about these processes is usually obtained through other means like accelerometers or Doppler vibrometers. In this paper, we show that a proper arrangement of the experiment and the camera allows non contact measurement of the characteristics of the main displacement mode (amplitude, frequency and attenuation). An application is given for the analysis of structure damages after low speed car impacts. The method is low cost, fast and accurate, and it permits direct visualization and measurement of the movement of the vibrating body.

In this communication, the authors have determined the longitudinal chromatic aberrations in real... more In this communication, the authors have determined the longitudinal chromatic aberrations in real eyes. The method that has been used combines real data of corneal morphology, central thickness of crystalline lens and biometric measures of axial length together with numerical calculation of the propagation process. The curvature of the crystalline lens has been adjusted to different curvature models and refractive index distributions. The wavelength dependence of all ocular media has been modelled through the Cauchy formula. Propagation through anterior and posterior chambers has been accomplished through numerical calculation of diffraction integral instead of classical ray-tracing approach. This imposes serous restrictions on the number of samples that are needed for a full propagation process. If we are only interested in amplitude calculations the method consists of evaluating propagation from cornea to crystalline lens with a spectrum propagation method. Propagation from the lens to the best image plane is accomplished by a direct calculation of Fresnel integral. With this model, we have obtained the refraction chromatic difference in diopters for several eyes. Results are compared with real measures of the chromatic aberration, showing a good agreement with numerical calculations. The capabilities of the technique have been demonstrated by applying the method to the study of the chromatic aberration of a keratoconus.

Journal of Biomedical Optics, 2009

The eye is not a centered system. The line of sight connects the fovea with the center of the pup... more The eye is not a centered system. The line of sight connects the fovea with the center of the pupil and is usually tilted in the temporal direction. Thus, off-axis optical aberrations, mainly coma and oblique astigmatism, are introduced at the fovea. Tabernero et al. ͓J. Opt. Soc. Am. A 24͑10͒, 3274-3283 ͑2007͔͒ showed that a horizontal tilt of the crystalline lens generates a horizontal coma aberration that is compensated by the oblique light incidence on the eye. Here we suggest that corneal astigmatism may also play a role in compensation of oblique aberrations, and we propose a simple model to analyze such a possibility. A theoretical Kooijman eye model with a slight ͑ϳ0.6 D͒ with-the-rule astigmatism is analyzed. Light rays at different incidence angles to the optical axis are considered, and the corresponding point spread functions ͑PSFs͒ at the retina are calculated. A quality criterion is used to determine the incidence angle that provides the narrowest and highest PSF energy peak. We show that the best image is obtained for a tilted incidence angle compatible with mean values of the angle kappa. This suggests that angle kappa, lens tilt, and corneal astigmatism may combine to provide a passive compensation mechanism to minimize aberrations on the fovea.

Journal of Modern Optics, 2008

In this communication, we report a new method to determine the wavefront aberration function of p... more In this communication, we report a new method to determine the wavefront aberration function of personalized eyes. We obtain corneal surface data and axial lengths for different eyes. With these data, we construct a diffractive model of a human eye, which permits calculation of propagated light distributions at any distance inside the eye. We obtain the field at the exit of crystalline lens and extract the phase information. To unwrap this phase we use a path-following method and with the reconstructed phase we obtain the wavefront aberration function that we analyze using Zernike polynomials.

Journal of Modern Optics, 2006

Numerical calculation of Fresnel patterns through fast Fourier transforms usually requires an ext... more Numerical calculation of Fresnel patterns through fast Fourier transforms usually requires an extremely large number of samples in order to fulfil the Nyquist sampling condition. In many applications, the cut-off frequency of the system is much below the limit fixed by our calculations. As a consequence of this, correct sampling may result in heavy processes that produce results of useless accuracy. Unfortunately, subsampling may introduce aliasing that may distort the final appearance of the diffracted pattern. In this paper, we present a simple method that permits subsampling the Fresnel pattern while maintaining the Nyquist condition, and thus preventing the appearance of aliasing effects in the calculation. Secondary effects of the subsamplings are rescaling the illuminating wavelength and the introduction of an effective low-pass filter. Some applications relative to the propagation of the light inside the human eye are also suggested in the text.

Optics Express, 2007

The paper presents the polychromatic analysis of two diffractive optical elements with extended d... more The paper presents the polychromatic analysis of two diffractive optical elements with extended depth of focus: the linear axicon and the light sword optical element. Chromatic aberration produces axial displacement of the focal segment line. Thus, we explore the possibility of extending the focal depth of these elements to permit superposition of the chromatic foci. In the case of an axicon, we achieve an achromatic zone where focusing is produced. In the case of the light sword element, we show that the focusing segment is out of axis. Therefore a superposition of colors is produced, but not on axis overlapping. Instead, three colored and separated foci are simultaneously obtained in a single plane. Three dimensional structures of the propagated beams are analyzed in order to provide better understanding of the properties and applications of such elements.

Optics, Photonics, and Digital Technologies for Multimedia Applications II, 2012

ABSTRACT The behaviour of a construction safety net and its supporting structure was monitored wi... more ABSTRACT The behaviour of a construction safety net and its supporting structure was monitored with a high speed camera and image processing techniques. A 75 kg cylinder was used to simulate a falling human body from the upstairs floor of a building under construction. The cylinder rolled down over a ramp until it reaches the net. The behaviour of the net and its supporting structure was analysed through the movement of the cylinder once it reaches the net. The impact was captured from a lateral side with a high speed camera working at 512 frames per second. In order to obtain the cylinder position each frame of the sequence was binarized. Through morphological image processing the contour of the cylinder was isolated from the background and with a Hough transform the presence of the circle was detected. With this, forces and accelerations applying on the net and the supporting structure have been described, together with the trajectory of the cylinder. All the experiment has been done in a real structure in outdoors location. Difficulties found in the preparation on the experiment and in extracting the final cylinder contour are described and some recommendations are giving for future implementations.

Optics letters, 2008

Numerical calculation of convergent Fresnel patterns through fast Fourier transform usually requi... more Numerical calculation of convergent Fresnel patterns through fast Fourier transform usually requires a large number of samples to fulfill the Nyquist sampling condition around the focus. From polynomial decomposition of the wavefront it is possible to determine which polynomial orders are the main contributors to the number of samples. This information can be used to properly modify the initial wavefront and relax the Nyquist condition thus giving a more efficient numerical algorithm.

Journal of biomedical optics

Videokeratometers and Scheimpflug cameras permit accurate estimation of corneal surfaces. From he... more Videokeratometers and Scheimpflug cameras permit accurate estimation of corneal surfaces. From height data it is possible to adjust analytical surfaces that will be later used for aberration calculation. Zernike polynomials are often used as adjusting polynomials, but they have shown to be not precise when describing highly irregular surfaces. We propose a combined zonal and modal method that allows an accurate reconstruction of corneal surfaces from height data, diminishing the influence of smooth areas over irregular zones and vice versa. The surface fitting error is decreased in the considered cases, mainly in the central region, which is more important optically. Therefore, the method can be established as an accurate resampling technique.

Optics Express, 2013

In this paper, we demonstrate the use of a video camera for measuring the frequency of small-ampl... more In this paper, we demonstrate the use of a video camera for measuring the frequency of small-amplitude vibration movements. The method is based on image acquisition and multilevel thresholding and it only requires a video camera with high enough acquisition rate, not being necessary the use of targets or auxiliary laser beams. Our proposal is accurate and robust. We demonstrate the technique with a pocket camera recording low-resolution videos with AVI-JPEG compression and measuring different objects that vibrate in parallel or perpendicular direction to the optical sensor. Despite the low resolution and the noise, we are able to measure the main vibration modes of a tuning fork, a loudspeaker and a bridge. Results are successfully compared with design parameters and measurements with alternative devices.

Medical Imaging, 2005

In this communication, the authors have determined the longitudinal chromatic aberrations in real... more In this communication, the authors have determined the longitudinal chromatic aberrations in real eyes. The method that has been used combines real data of corneal morphology, central thickness of crystalline lens and biometric measures of axial length together with numerical calculation of the propagation process. The curvature of the crystalline lens has been adjusted to different curvature models and refractive

Biophotonics: Photonic Solutions for Better Health Care III, 2012

ABSTRACT Retinal image quality is usually analysed through different parameters typical from inst... more ABSTRACT Retinal image quality is usually analysed through different parameters typical from instrumental optics, i.e, PSF, MTF and wavefront aberrations. Although these parameters are important, they are hard to translate to visual quality parameters since human vision exhibits some tolerance to certain aberrations. This is particularly important in postsurgery eyes, where non-common aberration are induced and their effects on the final image quality is not clear. Natural images usually show a strong dependency between one point and its neighbourhood. This fact helps to the image interpretation and should be considered when determining the final image quality. The aim of this work is to propose an objective index which allows comparing natural images on the retina and, from them, to obtain relevant information abut the visual quality of a particular subject. To this end, we propose a individual eye modelling. The morphological data of the subject's eye are considered and the light propagation through the ocular media is calculated by means of a Fourier-transform-based method. The retinal PSF so obtained is convolved with the natural scene under consideration and the obtained image is compared with the ideal one by using the structural similarity index. The technique is applied on 2 eyes with a multifocal corneal profile (PresbyLasik) and can be used to determine the real extension of the achieved pseudoaccomodation.

Biophotonics: Photonic Solutions for Better Health Care III, 2012

We present a new algorithm to process captured images of reflected Placido rings. Up to our knowl... more We present a new algorithm to process captured images of reflected Placido rings. Up to our knowledge, conventional topographers transform from Cartesian to polar coordinates and vice-versa, thus extrapolating corneal data and introducing noise and image artefacts. ...

Journal of Modern Optics, 2011

Surfaces with radial structure do not fit well to squared detectors or sampling matrices. Cartesi... more Surfaces with radial structure do not fit well to squared detectors or sampling matrices. Cartesian grid sampling provides a different density of nodes in sectors. Zernike polynomials are a complete set of orthogonal polynomials defined on a unit disk often used as an expansion of such surfaces. In the fitting process, the sampling distribution is not usually taken into account and might have undesirable effects on the final parameter estimates. We propose applying weighted least-squares regression that compensates the unequal influence of sectors due to the sampling distribution, assigning a weight function to the nodes grid and thus providing a better fit in the central optical zone.

Journal of Biomedical Optics, 2009

The eye is not a centered system. The line of sight connects the fovea with the center of the pup... more The eye is not a centered system. The line of sight connects the fovea with the center of the pupil and is usually tilted in the temporal direction. Thus, off-axis optical aberrations, mainly coma and oblique astigmatism, are introduced at the fovea. Tabernero et al. [J. Opt. Soc. Am. A 24(10), 3274-3283 (2007)] showed that a horizontal tilt of the crystalline lens generates a horizontal coma aberration that is compensated by the oblique light incidence on the eye. Here we suggest that corneal astigmatism may also play a role in compensation of oblique aberrations, and we propose a simple model to analyze such a possibility. A theoretical Kooijman eye model with a slight (~0.6 D) with-the-rule astigmatism is analyzed. Light rays at different incidence angles to the optical axis are considered, and the corresponding point spread functions (PSFs) at the retina are calculated. A quality criterion is used to determine the incidence angle that provides the narrowest and highest PSF energy peak. We show that the best image is obtained for a tilted incidence angle compatible with mean values of the angle kappa. This suggests that angle kappa, lens tilt, and corneal astigmatism may combine to provide a passive compensation mechanism to minimize aberrations on the fovea.

Construction and Building Materials, 2015

Measurement of concrete strain through non-invasive methods is of great importance in civil 11 en... more Measurement of concrete strain through non-invasive methods is of great importance in civil 11 engineering and structural analysis. Traditional methods use laser speckle and high quality 12 cameras that may result too expensive for many applications. Here we present a method for 13 measuring concrete deformations with a standard reflex camera and image processing for 14 tracking objects in the concretes surface. Two different approaches are presented here. In the 15 first one, on-purpose objects are drawn on the surface, while on the second one we track small 16 defects on the surface due to air bubbles in the hardening process. The method has been tested 17 on a concrete sample under several loading/unloading cycles. A stop-motion sequence of the 18 process has been captured and analyzed. Results have been successfully compared with the 19 values given by a strain gauge. Accuracy of our methods in tracking objects is below 8 m, in 20 the order of more expensive commercial devices. 21 22 23

Journal of Modern Optics, 2011

Spontaneous eye blinking is a complex process, which implies the joint actions of the eyelid musc... more Spontaneous eye blinking is a complex process, which implies the joint actions of the eyelid muscles and the eye globe. These actions provoke the eyelid closure and simultaneous eye retraction into the orbit. Recently, the authors presented a non-invasive technique for measuring both movements that permits calculating several parameters. Following the work from Malbouisson and co-workers, we have fitted the displacement curves of the eyelid to an analytical function and, additionally, we have extended the kinematic model to the eye retraction movements. As a result of the fitting, some dynamic parameters have been presented for the first time in the literature. This study can be easily extended to obtain relevant information about ocular biomechanical properties in physiological optics and provides results that may be of interest in neurophysiology, in order to understand the blinking mechanism.

A knowledge of the shape of the cornea is of major importance for the planning and monitoring of ... more A knowledge of the shape of the cornea is of major importance for the planning and monitoring of surgery, and for the correct diagnosis of corneal diseases. Many authors have studied the geometry of the second corneal surface in the central region and it has been stated that there is a high correlation between the central radii of curvature and asphericities of the two corneal surfaces. In this work we extend this study to a larger, central, 6-mm diameter of the cornea. Surface height data, obtained with an Oculus Pentacam from 42 eyes from 21 subjects, were analysed to yield surface power vectors. Corneal heights of both surfaces were also decomposed into low-order Zernike polynomials and the correlations between each of the power vectors and low-order Zernike coefficients for the two surfaces were studied. There was not only a strong correlation between spherical powers and Zernike defocus coefficients, but also between the astigmatic components. The correspondence between the astigmatisms in both surfaces found here can be of the utmost importance in planning optical surgery, since perfect spherical ablation of the first surface does not assure total correction of corneal astigmatism.

Optics Letters, 2010

We present a noninvasive technique for high-speed measuring of eye retraction and eyelid position... more We present a noninvasive technique for high-speed measuring of eye retraction and eyelid position during blinking. The anterior chamber of the eye is illuminated by the slit lamp of a biomicroscope and eye dynamics during a blinking sequence are captured with a high-speed camera working at 500 frames per second. Digital image processing allows quantitative analysis of cornea and eyelid positions during the closing and opening phases of the blinking process. Our method allows simultaneous measuring of corneal retraction, duration of down and up phases, total blinking duration, and average and peak speeds of the eyelids in both phases, thus providing a complete analysis of the blink's transversal motions.

Optik, 2000

Even with fixed gaze, the human eye is under steady motion (tremor micosaccades and drifts). Dete... more Even with fixed gaze, the human eye is under steady motion (tremor micosaccades and drifts). Detection of these movements requires of invasive techniques or expensive devices with sophisticated detection methods. In this paper we present a technique for pupil segmentation and contour analysis which will provide valuable information about fixational eye movements. The method is based on fitting an ellipse to the pupil contour. Pupillary hippus, microsaccades and drifts are obtained as well as cyclotorsional movements. The method is simple and the experimental requirements are easily available since just a biomicroscope and a digital videocamera are required.

Research in Nondestructive Evaluation, 2011

High speed cameras are often used for monitoring impacts and fast dynamic processes on structures... more High speed cameras are often used for monitoring impacts and fast dynamic processes on structures. However, quantitative information about these processes is usually obtained through other means like accelerometers or Doppler vibrometers. In this paper, we show that a proper arrangement of the experiment and the camera allows non contact measurement of the characteristics of the main displacement mode (amplitude, frequency and attenuation). An application is given for the analysis of structure damages after low speed car impacts. The method is low cost, fast and accurate, and it permits direct visualization and measurement of the movement of the vibrating body.

In this communication, the authors have determined the longitudinal chromatic aberrations in real... more In this communication, the authors have determined the longitudinal chromatic aberrations in real eyes. The method that has been used combines real data of corneal morphology, central thickness of crystalline lens and biometric measures of axial length together with numerical calculation of the propagation process. The curvature of the crystalline lens has been adjusted to different curvature models and refractive index distributions. The wavelength dependence of all ocular media has been modelled through the Cauchy formula. Propagation through anterior and posterior chambers has been accomplished through numerical calculation of diffraction integral instead of classical ray-tracing approach. This imposes serous restrictions on the number of samples that are needed for a full propagation process. If we are only interested in amplitude calculations the method consists of evaluating propagation from cornea to crystalline lens with a spectrum propagation method. Propagation from the lens to the best image plane is accomplished by a direct calculation of Fresnel integral. With this model, we have obtained the refraction chromatic difference in diopters for several eyes. Results are compared with real measures of the chromatic aberration, showing a good agreement with numerical calculations. The capabilities of the technique have been demonstrated by applying the method to the study of the chromatic aberration of a keratoconus.

Journal of Biomedical Optics, 2009

The eye is not a centered system. The line of sight connects the fovea with the center of the pup... more The eye is not a centered system. The line of sight connects the fovea with the center of the pupil and is usually tilted in the temporal direction. Thus, off-axis optical aberrations, mainly coma and oblique astigmatism, are introduced at the fovea. Tabernero et al. ͓J. Opt. Soc. Am. A 24͑10͒, 3274-3283 ͑2007͔͒ showed that a horizontal tilt of the crystalline lens generates a horizontal coma aberration that is compensated by the oblique light incidence on the eye. Here we suggest that corneal astigmatism may also play a role in compensation of oblique aberrations, and we propose a simple model to analyze such a possibility. A theoretical Kooijman eye model with a slight ͑ϳ0.6 D͒ with-the-rule astigmatism is analyzed. Light rays at different incidence angles to the optical axis are considered, and the corresponding point spread functions ͑PSFs͒ at the retina are calculated. A quality criterion is used to determine the incidence angle that provides the narrowest and highest PSF energy peak. We show that the best image is obtained for a tilted incidence angle compatible with mean values of the angle kappa. This suggests that angle kappa, lens tilt, and corneal astigmatism may combine to provide a passive compensation mechanism to minimize aberrations on the fovea.

Journal of Modern Optics, 2008

In this communication, we report a new method to determine the wavefront aberration function of p... more In this communication, we report a new method to determine the wavefront aberration function of personalized eyes. We obtain corneal surface data and axial lengths for different eyes. With these data, we construct a diffractive model of a human eye, which permits calculation of propagated light distributions at any distance inside the eye. We obtain the field at the exit of crystalline lens and extract the phase information. To unwrap this phase we use a path-following method and with the reconstructed phase we obtain the wavefront aberration function that we analyze using Zernike polynomials.

Journal of Modern Optics, 2006

Numerical calculation of Fresnel patterns through fast Fourier transforms usually requires an ext... more Numerical calculation of Fresnel patterns through fast Fourier transforms usually requires an extremely large number of samples in order to fulfil the Nyquist sampling condition. In many applications, the cut-off frequency of the system is much below the limit fixed by our calculations. As a consequence of this, correct sampling may result in heavy processes that produce results of useless accuracy. Unfortunately, subsampling may introduce aliasing that may distort the final appearance of the diffracted pattern. In this paper, we present a simple method that permits subsampling the Fresnel pattern while maintaining the Nyquist condition, and thus preventing the appearance of aliasing effects in the calculation. Secondary effects of the subsamplings are rescaling the illuminating wavelength and the introduction of an effective low-pass filter. Some applications relative to the propagation of the light inside the human eye are also suggested in the text.

Optics Express, 2007

The paper presents the polychromatic analysis of two diffractive optical elements with extended d... more The paper presents the polychromatic analysis of two diffractive optical elements with extended depth of focus: the linear axicon and the light sword optical element. Chromatic aberration produces axial displacement of the focal segment line. Thus, we explore the possibility of extending the focal depth of these elements to permit superposition of the chromatic foci. In the case of an axicon, we achieve an achromatic zone where focusing is produced. In the case of the light sword element, we show that the focusing segment is out of axis. Therefore a superposition of colors is produced, but not on axis overlapping. Instead, three colored and separated foci are simultaneously obtained in a single plane. Three dimensional structures of the propagated beams are analyzed in order to provide better understanding of the properties and applications of such elements.

Optics, Photonics, and Digital Technologies for Multimedia Applications II, 2012

ABSTRACT The behaviour of a construction safety net and its supporting structure was monitored wi... more ABSTRACT The behaviour of a construction safety net and its supporting structure was monitored with a high speed camera and image processing techniques. A 75 kg cylinder was used to simulate a falling human body from the upstairs floor of a building under construction. The cylinder rolled down over a ramp until it reaches the net. The behaviour of the net and its supporting structure was analysed through the movement of the cylinder once it reaches the net. The impact was captured from a lateral side with a high speed camera working at 512 frames per second. In order to obtain the cylinder position each frame of the sequence was binarized. Through morphological image processing the contour of the cylinder was isolated from the background and with a Hough transform the presence of the circle was detected. With this, forces and accelerations applying on the net and the supporting structure have been described, together with the trajectory of the cylinder. All the experiment has been done in a real structure in outdoors location. Difficulties found in the preparation on the experiment and in extracting the final cylinder contour are described and some recommendations are giving for future implementations.

Optics letters, 2008

Numerical calculation of convergent Fresnel patterns through fast Fourier transform usually requi... more Numerical calculation of convergent Fresnel patterns through fast Fourier transform usually requires a large number of samples to fulfill the Nyquist sampling condition around the focus. From polynomial decomposition of the wavefront it is possible to determine which polynomial orders are the main contributors to the number of samples. This information can be used to properly modify the initial wavefront and relax the Nyquist condition thus giving a more efficient numerical algorithm.

Journal of biomedical optics

Videokeratometers and Scheimpflug cameras permit accurate estimation of corneal surfaces. From he... more Videokeratometers and Scheimpflug cameras permit accurate estimation of corneal surfaces. From height data it is possible to adjust analytical surfaces that will be later used for aberration calculation. Zernike polynomials are often used as adjusting polynomials, but they have shown to be not precise when describing highly irregular surfaces. We propose a combined zonal and modal method that allows an accurate reconstruction of corneal surfaces from height data, diminishing the influence of smooth areas over irregular zones and vice versa. The surface fitting error is decreased in the considered cases, mainly in the central region, which is more important optically. Therefore, the method can be established as an accurate resampling technique.

Optics Express, 2013

In this paper, we demonstrate the use of a video camera for measuring the frequency of small-ampl... more In this paper, we demonstrate the use of a video camera for measuring the frequency of small-amplitude vibration movements. The method is based on image acquisition and multilevel thresholding and it only requires a video camera with high enough acquisition rate, not being necessary the use of targets or auxiliary laser beams. Our proposal is accurate and robust. We demonstrate the technique with a pocket camera recording low-resolution videos with AVI-JPEG compression and measuring different objects that vibrate in parallel or perpendicular direction to the optical sensor. Despite the low resolution and the noise, we are able to measure the main vibration modes of a tuning fork, a loudspeaker and a bridge. Results are successfully compared with design parameters and measurements with alternative devices.

Medical Imaging, 2005

In this communication, the authors have determined the longitudinal chromatic aberrations in real... more In this communication, the authors have determined the longitudinal chromatic aberrations in real eyes. The method that has been used combines real data of corneal morphology, central thickness of crystalline lens and biometric measures of axial length together with numerical calculation of the propagation process. The curvature of the crystalline lens has been adjusted to different curvature models and refractive

Biophotonics: Photonic Solutions for Better Health Care III, 2012

ABSTRACT Retinal image quality is usually analysed through different parameters typical from inst... more ABSTRACT Retinal image quality is usually analysed through different parameters typical from instrumental optics, i.e, PSF, MTF and wavefront aberrations. Although these parameters are important, they are hard to translate to visual quality parameters since human vision exhibits some tolerance to certain aberrations. This is particularly important in postsurgery eyes, where non-common aberration are induced and their effects on the final image quality is not clear. Natural images usually show a strong dependency between one point and its neighbourhood. This fact helps to the image interpretation and should be considered when determining the final image quality. The aim of this work is to propose an objective index which allows comparing natural images on the retina and, from them, to obtain relevant information abut the visual quality of a particular subject. To this end, we propose a individual eye modelling. The morphological data of the subject's eye are considered and the light propagation through the ocular media is calculated by means of a Fourier-transform-based method. The retinal PSF so obtained is convolved with the natural scene under consideration and the obtained image is compared with the ideal one by using the structural similarity index. The technique is applied on 2 eyes with a multifocal corneal profile (PresbyLasik) and can be used to determine the real extension of the achieved pseudoaccomodation.

Biophotonics: Photonic Solutions for Better Health Care III, 2012

We present a new algorithm to process captured images of reflected Placido rings. Up to our knowl... more We present a new algorithm to process captured images of reflected Placido rings. Up to our knowledge, conventional topographers transform from Cartesian to polar coordinates and vice-versa, thus extrapolating corneal data and introducing noise and image artefacts. ...