Jorge L. Flores

Color-fringe pattern profilometry using a generalized phase-shifting algorithm.

In order to overcome the limitations of the sequential phase-shifting fringe pattern profilometry for dynamic measurements, a color-channel-based approach is presented. The proposed technique consists of projecting and acquiring a colored image formed by three sinusoidal phase-shifted patterns. Therefore, by using the conventional three-step phase-shifting algorithm, only one color image is required for phase retrieval each time. However, the use of colored fringe patterns leads to a major problem, the color crosstalk, which introduces phase errors when conventional phase-shifting algorithms with fixed phase-shift values are utilized to retrieve the phase. To overcome the crosstalk issue, we propose the use of a generalized phase-shifting algorithm with arbitrary phase-shift values. The simulations and experimental results show that the proposed algorithm can significantly reduce the influence of the color crosstalk.

Edge enhancement and image equalization by unsharp masking using self-adaptive photochromic filters

A new method for real-time edge enhancement and image equalization using photochromic filters is presented. The reversible self-adaptive capacity of photochromic materials is used for creating an unsharp mask of the original image. This unsharp mask produces a kind of self filtering of the original image. Unlike the usual Fourier (coherent) image processing, the technique we propose can also be used with incoherent illumination. Validation experiments with Bacteriorhodopsin and photochromic glass are presented.

On the modeling of microemulsion polymerization. Experimental validation

A recently developed model for o/w microemulsion polymerization, tested only with styrene, is validated here by comparing its predictions with experimental data from the polymerization of three monomers with different chemical structures and water solubilities. The model reproduces well the kinetic data as well as the characteristics of the latex and polymer (particle size, average molar mass, number of polymer chains per particle, etc.) for the three monomers. The model predicts only two reaction rate intervals and that the conversion at which the maximum reaction rate occurs, depends only on the type of monomer and reaction conditions, in agreement with experimentally reported data. In the model, both micellar and homogeneous nucleation mechanisms are permitted, even for low water-solubility monomers such as styrene, because of the large amounts of surfactant typically employed in microemulsion polymerization. The role of these two mechanisms in the polymerization process is examined here.

Color encoding of binary fringes for gamma correction in 3-D profiling.

Three-dimensional profiling by sinusoidal fringe projection using PSI-algorithms are distorted by the nonlinear response of digital cameras and commercial video projectors. To solve the problem, we present a fringe generation technique that consists of projecting and acquiring a temporal sequence of strictly binary color patterns, whose (adequately weighted) average leads to sinusoidal fringe patterns with the required number of bits, which allows for a reliable three-dimensional profile using a PSI-algorithm. Validation experiments are presented.

Phase retrieval from one partial derivative

Phase objects can be characterized using well-known methods such as shear interferometry and deflectometry, which provide information on the partial derivatives of the phase. It is often believed that for phase retrieval it is strictly necessary to have knowledge of two partial derivatives in orthogonal directions. In the praxis, this implies that the measurements have to be performed along two dimensions, which often requires a rotation of the object or rotation of the shear direction. This is time consuming and errors can be easily generated from the process of rotation, especially for image registration in the axial direction. In the present Letter, we will demonstrate that only one partial derivative often suffices to recover the phase, and we will discuss under which conditions that is possible. Simulations and validation experiments are presented.

Orientation-selective edge detection and enhancement using the irradiance transport equation

We present a method for orientation-selective edge detection and enhancement based on the irradiance transport equation. The proposed technique distinguishes the sign of the derivative of the intensity pattern along an arbitrarily selected direction. The method is based on the capacity of liquid-crystal displays to generate simultaneously a contrast reverted replica of the image displayed on it. When both images (the original one and its replica) are imagined across a slightly defocused plane, one obtains an image with enhanced first derivatives. Unlike most Fourier methods, the proposed technique works well with a low-coherence light source, and it does not require precise alignment. The proposed method does not involve numerical processing, and thus it could be potentially useful for processing large images in real-time applications. Validation experiments are presented.

Generation of phase-shifting algorithms with N arbitrarily spaced phase-steps

Phase-shifting (PS) is an important technique for phase retrieval in interferometry (and three-dimensional profiling by fringe projection) that requires a series of intensity measurements with known phase-steps. Usual PS algorithms are based on the assumption that the phase-steps are evenly spaced. In practice, however, this assumption is often not satisfied exactly, which leads to errors in the recovered phase. In this work we present a systematic algebraic approach for generating general PS algorithms with N arbitrarily spaced phase-steps, which present advantages (e.g., the PS error can be avoided) over known algorithms that assume equally spaced phase-steps. Simulations are presented.

Generalized phase-shifting algorithms: error analysis and minimization of noise propagation

Phase shifting is a technique for phase retrieval that requires a series of intensity measurements with certain phase steps. The purpose of the present work is threefold: first we present a new method for generating general phase-shifting algorithms with arbitrarily spaced phase steps. Second, we study the conditions for which the phase-retrieval error due to phase-shift miscalibration can be minimized. Third, we study the phase extraction from interferograms with additive random noise, and deduce the conditions to be satisfied for minimizing the phase-retrieval error. Algorithms with unevenly spaced phase steps are discussed under linear phase-shift errors and additive Gaussian noise, and simulations are presented.

Analog image contouring using a twisted-nematic liquid-crystal display

We present a novel image contouring method based on the polarization features of the twisted-nematic liquid-crystal displays (TN-LCDs). 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, i.e., the borders between illuminated and dark areas are enhanced. The proposed method is quite robust and does not require precise alignment or coherent illumination. Since it does not involve numerical processing, it could be useful for contouring large images in real-time, which presents potential applications in medical and biological imaging. Validation experiments are presented.

Verdet constant dispersion measurement using polarization-stepping techniques

We present a novel method for measuring the Verdet constant dispersion. The proposed method involves spectral polarimetric measurements using three (or more) polarization steps. The procedure has formal similarity with the phase-shifting interferometry (PSI). Thus the Verdet constant in the desired spectral range can be retrieved using well-known PSI algorithms. Validation experiments are presented.