Jose Carlos Lopez Vazquez

Evaluation of the uncertainty of phase-difference measurements in (quasi-)Fourier transform digital holographic interferometry

Estimation of the uncertainty is an essential requisite for high-end measurement systems. In this communication we derive an expression to evaluate the standard uncertainty of the phase-difference measurements resulting from Fourier and quasi-Fourier transform digital holographic interferometry. We apply the law of propagation of uncertainty, as defined in the “Guide to the expression of uncertainty in measurement” (GUM), to the digital reconstruction of two holograms by Fourier transformation and to the subsequent calculation of the phase change between the holographic reconstructions. The resulting expression allows the evaluation of the uncertainty of the phase difference at every pixel in the reconstruction plane in terms of the measured hologram brightness values and their uncertainty at the whole of the pixels of the original digital holograms. This expression is simplified by assuming a linear dependence between the uncertainty and the local value of the original holograms; in that case, the local uncertainty of the phase difference can be evaluated from the local complex values of the reconstructed holograms. We assess the behavior of the method by comparing the predicted standard uncertainty with the sample variance obtained from experiments conducted under repeatability conditions, and found a good correlation between both quantities. This experimental procedure can be also used to calibrate the parameters of the linear function relating the uncertainty with the local value of the digital holograms, for a given set of operational conditions of the acquisition device.

Transparent object analysis in a Mach-Zehnder interferometer using the multiplicative analogical moire phase-shifting method

Transparent object Wave-Front Distortion has been measured in a Mach-Zehnder interferometer employing as evaluation tool the Multiplicative Analogical Moire Phase-Shifting Method. The measuring procedure is based on the direct superimposition of a transmission grating with a phase modulated high-frequency Mach-Zehnder spatial carrier. Latter, a set of phase-shifted fringe patterns, provided by low-pass filtering of the obtained multiplicative moire patterns, is combined in a Generic Phase-Shifting Algorithm (GPSA). The interesting information is achieved once the previous procedure is carried out with and without the transparent object located at the measuring area. The Wave- Front Distortion can be calculated by subtraction of these results provided by the GPSA or directly if a Differential PSA is employed. A comparison of the results obtained with both algorithms has been done.

Study of plate vibrations by moire holography

Advantages of moire holography with single illumination beam as a real-time method to detect vibrational modes of diffusing objects have been already experimentally demonstrated by several authors. The main obstacle to automatizing the analysis of the moire pattern is the difficulty in filtering the carrier pattern. At the present, it seems affordable to realize filtering in quasi-real time by means of a digital image-processing system. However, several problems arise in this operation, for example, the spread of the spatial frequency spectrum of the carrier fringes. Different filtering algorithms, with and without FFT, are compared. Different ways to combine both carriers are examined.

Transient bending wave analysis by Fourier evaluation of single-pulsed TV holography fringe patterns

In this paper we demonstrate the feasibility of impact- induced transient deformations measurement by single-pulsed subtraction TV holography an the Fourier transform method with contouring fringes as spatial carrier. Fringe formation in single-pulsed subtraction TV holography and phase demodulation by the Fourier transform method are descried. Contouring fringes are proved to be well suited for introducing spatial carrier in the correlation fringe patterns. Experimental results are presented. Finally, the degree of immunity to environmental disturbances of this technique is discussed and improvements are proposed.

Crack detection by TV holography: continuous and pulsed techniques

Two TV-holographic techniques to detect cracks in mechanical elements are demonstrated. One of them employs continuous illumination and is based in the modal analysis of the part. The other one consists in the study by pulsed TV-holography of transient waves induced in the part.

Surface evaluation combining the moire effect and phase-stepping techniques in Fizeau interferometry

The usual phase-stepping algorithms are employed to evaluate the phase of Fizeau interferograms in topographic measurements. The superposition of the interferogram with a Ronchi grid provides a Moire pattern. The phase modulation is obtained by the in plane displacement of the grid, while a sinusoidal intensity pattern is obtained by defocusing the image acquisition system. In this work, the measurement range of the method in the function of the maximum variation of height surfaces is calculated and verified.

Analysis of the measurement range in Fizeau phase-stepping interferometry using the moire effect

In this work, the phase evaluation of multiple-beam Fizeau interferograms is carried out by the usual phase-stepping algorithms by superimposing the interferogram on a Ronchi grid located at the interferometer image plane. The phase modulation, required for phase measurement, is obtained by translating the grid in its own plane, while a slight defocusing in the image acquisition system provides a suitable sinusoidal intensity pattern. An estimation of the measurable range of surface slopes is calculated and experiments on a spherical surface have been realized in order to analyze the validity of the obtained results.

New fiber optic laser probe for the automatic inspection of cracks in the inner side of heat exchanger tubes of nuclear power plants

A novel artificial vision system with fully automatic operation that complements the results obtained by eddy current techniques in the inspection of heat exchanger tubes of nuclear power plants is presented. The system has a specifically developed fiberoptic probe based on a reflectometric principle that uses a linear array of optical fibers to form an image of the inner surface of the tubes, allowing the user to detect and measure defects opened to this surface. The system architecture, principle of measurement, main features and experimental results are presented.

Fiber optic probe for in-situ inspection of surface cracks in steam generator tubes

The automatic inspection of cracks in the inner side of steam generator tubes is routinely carried out using eddy currents techniques. These methods are mature and have quite high performance but the achievable resolution is relatively low (approximately 1 mm) and present drawbacks related to the reliability of the inspection of the areas of tubes at the neighborhood of the supporting plates. Several years ago we have proposed a solution to these limitations based on a new fiberoptic reflectometric technique that has already been theoretically and experimentally analyzed using a first laboratory prototype. In this work we describe a second generation prototype that is integrated in a whole inspection system that could perform an inspection in the field. The results obtained over cylindrical tubes with inner diameter 16.8 mm (corresponding to real steam generator tubes) with cracks created by electrical disintegration and by stress- corrosion and employing conditions close to real operation confirm our previous conclusions: both circumferential and axial cracks of widths as small as 30 micrometer can be detected and measured and typical resolution capability of 10 line pairs/mm can be obtained.

Phase-shifted double single-pulse additive stroboscopic TV holography for the measurement of high-frequency vibrations using low-bandwidth phase-modulation devices

We present a novel technique for the application of stroboscopic additive TV-holography to the measurement of vibrations using temporal phase-shifting. Based on a previous concept - contrived and developed by the same authors - that used two illumination pulses within each vibration cycle and interpulse phase modulation at the same rate that the vibration of the object, this new technique implements and analogous phase modulation scheme but between two swiftly alternating bursts of single pulses with different phase within each video frame, rather than using true double-pulses, thus allowing quantitative measurements to be performed with stroboscopic illumination keeping the characteristics of stability and temporal resolution of the double-pulse additive stroboscopic technique but with the additional benefit of reaching high vibration frequencies with low bandwidth phase modulators.