Showing papers by "José A. Ferrari published in 2011"
TL;DR: This work presents a method that consists in projecting and acquiring a temporal sequence of strictly binary patterns, whose (adequately weighted) average leads to a sinusoidal fringe pattern with the required number of bits.
Abstract: Three-dimensional shape measurements by sinusoidal fringe projection using phase-shifting interferometry algorithms are distorted by the nonlinear response in intensity of commercial video projectors and digital cameras. To solve the problem, we present a method that consists in projecting and acquiring a temporal sequence of strictly binary patterns, whose (adequately weighted) average leads to a sinusoidal fringe pattern with the required number of bits. Since binary patterns consist of “ones” and “zeros”—and no half-tones are involved—the nonlinear response of the projector and the camera will not play a role, and a nearly unit contrast gray-level sinusoidal fringe pattern is obtained. Validation experiments are presented.
33 citations
TL;DR: The proposed technique works under incoherent illumination and does not require precise alignment, and thus, it could be potentially useful for processing large color images in real-time applications.
Abstract: We present a novel optical method for edge enhancement in color images based on the polarization properties of liquid-crystal displays (LCD). In principle, a LCD generates simultaneously two color-complementary, orthogonally polarized replicas of the digital image used as input. The currently viewed image in standard LCD monitors and cell phone’s screens -which we will refer as the “positive image or true-color image”- is the one obtained by placing an analyzer in front of the LCD, in cross configuration to the back polarizer of the display. The orthogonally polarized replica of this image –the “negative image or complementary-color image”- is absorbed by the front polarizer. In order to generate the positive and negative replica with a slight displacement between them, we used a LCD monitor whose analyzer (originally a linear polarizer) was replaced by a calcite crystal acting as beam displacer. When both images are superimposed laterally displaced across the image plane, one obtains an image with enhanced first-order derivatives along a specific direction. The proposed technique works under incoherent illumination and does not require precise alignment, and thus, it could be potentially useful for processing large color images in real-time applications. Validation experiments are presented.
12 citations
TL;DR: Unlike Fourier, the proposal works with incoherent illumination and does not require precise alignment, and thus, it could be a useful tool for edge extraction/enhancement in large images in real-time applications.
Abstract: We present an optical method for nondirectional edge extraction/enhancement in color images. The method is based on the capability of twisted-nematic LCDs to traduce the image information in changes of the state of polarization of light, which allows us to generate simultaneously two replicas of the digital image displayed on the LCD: a true-color (“positive”) image and a complementary-color (“negative”) one. In our setup the imaging system consists of a lens plus a pupil mask formed with concentric apertures and orthogonal polarizers. This layout allows us to simultaneously image a well-focused positive replica (due to the circular aperture) superimposed to a slightly defocused negative one (due to the annular aperture). It is not difficult to demonstrate that this generates a nondirectional (Laplacian) edge enhancement. Unlike Fourier, our proposal works with incoherent illumination and does not require precise alignment, and thus, it could be a useful tool for edge extraction/enhancement in large images in real-time applications. Validation experiments are presented.
8 citations
TL;DR: It is demonstrated that the observed phenomena are not necessarily evidence of the existence of left-handed materials and that they can be fully explained by classical optic concepts, e.g. light propagation in waveguides.
Abstract: Experimental results showing “negative refraction” and some kind of “lensing” −in the microwave-infrared range− are often presented in the literature as undisputable evidence of the existence of composite left-handed materials. The purpose of this paper is to present experimental results on “negative refraction” and “lensing” at visible wavelengths involving a waveguide array formed by a tight-packed bundle of glass fibers. We will demonstrate that the observed phenomena are not necessarily evidence of the existence of left-handed materials and that they can be fully explained by classical optic concepts, e.g. light propagation in waveguides.
7 citations
TL;DR: It is considered that the comparison of the squared binary method (SBM) and the sinusoidal pulse-width modulation (SPWM) method has considerable deficiencies.
Abstract: We comment on a recent Letter [Opt. Lett.35, 4121 (2010)OPLEDP0146-959210.1364/OL.35.004121], in which the authors discuss an optimal pulse-width modulation (OPWM) method for sinusoidal fringe generation. We consider that the comparison of the squared binary method (SBM) and the sinusoidal pulse-width modulation (SPWM) method has considerable deficiencies.
5 citations
TL;DR: In this article, an optical processor capable to perform partial second derivatives of an image working with partially coherent illumination is presented. But the method is based on the capacity of these devices to generate simultaneously a contrast reverted replica of the image displayed on it.
2 citations
TL;DR: In this paper, a new optical current sensor architecture based on a polarimetric configuration and a control system for self-compensation of the Fara day effect taking place at the sensor he ad.
Abstract: We present a new optical current sensor architecture, which is based on a polarimetric configuration and a control system for self-compensation of the Fara day effect taking place at the sensor he ad. After passing through a bulk Faraday sensor head, the light travels through the free space reaching a Faraday modulator placed some distance away from the conductor carrying the current. The firs t device acts a current transducer and the second one acts as a magneto-optical element operated in a closed-loop mode to compensate the angle of rotation of the polarization introduced by the sensor head. The control system operates in closed loop feedback through a simple current-driven solenoid, and this way, the optical output from the current sensor is maintained at a constant intensity. Considering that the optical and electrical parameters of the sensor head and the Faraday modulator are known, the electrical current applied to the solenoid can be measured, and thus the current flowing through the conductor can be calculated. Experimental results demonstrate the feasibility of the proposed device to measure remotely the current carried by the conductor. Key words: Optical current sensor, Faraday effect, closed loop feedback
1 citations
TL;DR: An image segmentation method based on the use of an optical technique for detection of contours (as a preprocessing stage) and a posterior digital segmentation algorithms that has a better performance than purely digital algorithms.
Abstract: We present an image segmentation method based on the use of an optical technique for detection of contours (as a
preprocessing stage) and a posterior digital segmentation algorithms. The contouring technique is utilized to find an
accurate contour and it is based on the polarization features of the twisted-nematic liquid-crystal displays (TNLCDs).
TN-LCDs are manufactured to work between a crossed polarizer-analyzer pair. When the analyzer is at 45 deg (instead
of 90 deg) with respect to the polarizer, one obtains an optically processed image with pronounced outlines (dark
contours) at middle intensity. On other hand, the segmentation algorithms are based on active shape models or snakes.
Experimental results show that the proposed technique yields accurate segmentation for low-contrast images. Also, we
demonstrate that this hybrid-system has a better performance than purely digital algorithms.