Jesus Caum

Shack-Hartmann sensor based on a cylindrical microlens array

We present a Shack-Hartmann wavefront sensor (SHWS) based on a cylindrical microlens array as a device for measuring highly aberrated wavefronts. Instead of the typical spot pattern created by a conventional SHWS, two orthogonal line patterns are detected on a CCD and are superimposed. A processing algorithm uses the continuity of the focal line to extend the dynamic range of measurement by localizing the line, even if it leaves the CCD area confined by the corresponding microcylinder. The measurement of a wavefront from a progressive addition lens with an 80 lambda peak-to-valley value reveals the capabilities of the sensor.

A micrometric non-contact profiler for optical quality surfaces

A non-contact technique for obtaining accurate profiles of optical quality surfaces with micrometric accuracy has been developed. The technique is based on the Ronchi test principle, that is, on the study of the interaction of a wavefront reflected on the surface to be profiled with a square-wave transmittance ruling. From the resultant fringe pattern and some basic geometrical optics principles it is possible to measure the local normal to the surface being tested at a set of given points. This local normal map may then be integrated, yielding the surface profile. By use of a theoretically expected surface shape, the main parameters of the surface may then be determined by surface fitting of the measured data to that expected surface shape. Results of the profilometric measurements both of a spherical and of a toroidal surface are presented. The measured profiles are validated by comparison of the radii of curvature obtained using a high precision radioscope with the ones obtained by surface fitting of the measured profiles to their expected surface shapes. Additionally, subtracting the best-fit theoretical surface from the measured profile allows the observation of surface deviations from the theoretical shape to within some tenths of a nanometres.

Current Developments on Optical Feedback Interferometry as an All-Optical Sensor for Biomedical Applications

Optical feedback interferometry (OFI) sensors are experiencing a consistent increase in their applications to biosensing due to their contactless nature, low cost and compactness, features that fit very well with current biophotonics research and market trends. The present paper is a review of the work in progress at UPC-CD6 and LAAS-CNRS related to the application of OFI to different aspects of biosensing, both in vivo and ex vivo. This work is intended to present the variety of opportunities and potential applications related to OFI that are available in the field. The activities presented are divided into two main sensing strategies: The measurement of optical path changes and the monitoring of flows, which correspond to sensing strategies linked to the reconstruction of changes of amplitude from the interferometric signal, and to classical Doppler frequency measurements, respectively. For optical path change measurements, measurements of transient pulses, usual in biosensing, together with the measurement of large displacements applied to designing palliative care instrumentation for Parkinson disease are discussed. Regarding the Doppler-based approach, progress in flow-related signal processing and applications in real-time monitoring of non-steady flows, human blood flow monitoring and OFI pressure myograph sensing will be presented. In all cases, experimental setups are discussed and results presented, showing the versatility of the technique. The described applications show the wide capabilities in biosensing of the OFI sensor, showing it as an enabler of low-cost, all-optical, high accuracy biomedical applications.

Roughness measurement of paper using speckle

We present a method of measure of the roughness of the paper based on the analysis of a speckle pattern on the surface. Images of speckle over the surface of paper are captured by means of a simple configuration using a laser, beam expander, and a camera charge-coupled device (CCD). Then we use the normalized covariance function of the fields, leaving the surface to find the roughness. We compare the results obtained with the results obtained with a confocal microscope and the Bendtsen method that is a standard of the paper industry. This method can be considered as a noncontact surface profiling method that can be used online.

Progressive addition lenses power map measurement using Ronchi test techniques

Progressive ophthalmic lenses involve high resolution technological manufacturing processes, due to the particular non-symmetrical, aspherical form of their convex surface. However, the testing of the surfaces is not usually performed using whole-field methods because of the high dynamic range and resolution required for the slope changes in the convex surface. Applying some simple enhancements, a robust Ronchi test technique can be used to obtain accurate power distribution maps of commercial progressive lenses. In this technique, a CCD detector is used as a high resolution slope map, which combined with multiple acquisition techniques, allow for high resolution measurements of both lateral displacement (10-4 m) and slope measurements (10-5 rad) with the required dynamic range in slope measurements for progressive power lenses. The power maps of different types of progressive lenses are presented, showing the differences between lens design and manufacturing. The ability of the enhanced Ronchi test technique to quantitatively map power variations with important slope changes is demonstrated.

Measurement of the photometric distribution of light sources by deflectometric techniques

This paper proposes an instrument allowing the accurate characterization of the energetic distribution of a given source at a distant plane positioned at will. This is achieved by measuring the light pattern leaving the source by applying deflectometric techniques and photometric measurements, and then applying raytracing algorithms in order to propagate this distribution to a distant plane. The techniques allow a very compact setup, and a prototype of the optomechanical system has been implemented and tested. The system is intended to substitute the photometric tunnels now in use for testing, for instance, automotive reflectors.

Design of adaptive digital filters for phase extraction in complex fringe patterns obtained using the Ronchi test

A powerful technique is presented for processing complex fringe patterns with high noise levels and arbitrary distributions of spatial frequencies, which can successfully extract the phase information. Artifacts that arise from phase extraction in local filtering approaches are avoided by using a simple design and implementation strategy for the adaptive filter, based on the theory of digital filter design used in electronics, and applied to pixel rows (or columns) in the fringe-pattern. The filter designed in this manner is then applied to phase extraction in an experimental fringe pattern measured in a digital Ronchi test setup using a Carré phase-shifting procedure. The filtering strategy has a very low computational cost and allows phase extraction in noisy ronchigrams regardless their spatial frequency distribution, provided the fringes are still visible.

Generalized ray tracing method for the calculation of the peripheral refraction induced by an ophthalmic lens

Abstract. Peripheral refraction, the refractive error present outside the main direction of gaze, has lately attracted interest due to its alleged relationship with the progression of myopia. The ray tracing procedures involved in its calculation need to follow an approach different from those used in conventional ophthalmic lens design, where refractive errors are compensated only in the main direction of gaze. We present a methodology for the evaluation of the peripheral refractive error in ophthalmic lenses, adapting the conventional generalized ray tracing approach to the requirements of the evaluation of peripheral refraction. The nodal point of the eye and a retinal conjugate surface will be used to evaluate the three-dimensional distribution of refractive error around the fovea. The proposed approach enables us to calculate the three-dimensional peripheral refraction induced by any ophthalmic lens at any direction of gaze and to personalize the lens design to the requirements of the user. The complete evaluation process for a given user prescribed with a −5.76D ophthalmic lens for foveal vision is detailed, and comparative results obtained when the geometry of the lens is modified and when the central refractive error is over- or undercorrected. The methodology is also applied for an emmetropic eye to show its application for refractive errors other than myopia.

Comparison of B-spline and Zernike fitting techniques in complex wavefront surfaces

Zernike polynomial fitting has been the commonplace alternative for assigning a measured wavefront a given shape. However, Zernike polynomials have intrinsic limitations under given conditions, mainly in complex wavefronts with for instance decentered double-peaks or with relevant undulations. The main goal of this paper is analyzing an alternative to Zernike fitting based in B-Spline fitting, comparing the strengths and weaknesses of each representation when applied to wavefront fitting. Simple and complex wavefront cases will be presented and studied, and the quality of their fitted representations using Zernike and B-Spline polynomials will be compared, presenting the main factors relevant in their comparison. Moreover, the case of random white noise added to the estimated data will allow an insight into the expected behavior of both representations when applied to experimental data.

Analysis and characterization of surface defects in ophthalmic lenses

A device has been designed based on the diffraction that will permit to analyze in an objective and quick form, the quality of ophthalmic lenses. This device situated in the line of production will improve the process of fabrication. The device is based on the phenomenon of the diffraction that takes place in the defects when impacting the light of the laser. The device consists of an optical system, in charge of driving the light of the laser, under good conditions, on the lens to analyze, a sensor, adapted to the wavelength of the laser, that detects the presence of the defect through the produced diffraction and a mechanism in which the lens to inspect is located assures that the laser sweeps the whole surface of the lens. A control system connected to the previous systems regulates the whole process The image obtained can be used to analyze and characterize the type of defect. Using image processing we segment the images in order to classify the defects that appear in the surface of the lens.