Francois Galmiche

Advances in pulsed phase thermography

In this paper, fundamental theory of the pulsed phase thermography (PPT) processing method is presented including notes on thermal waves, pulsed thermography and lockin thermography. Moreover, advances in signal inversion for PPT using wavelets are included along with discussions about pulse shaping and polynomial fitting for signal improvement (synthetic data). Results are presented as well.

Pulsed phased thermography with the wavelet transform

Pulsed thermography (PT) is a well established approach in nondestructive evaluation of materials: a short thermal perturbation stimulates the specimen of interest and the subsequent surface temperature evolution reveals specimen subsurface structure and thermal properties. Recently, a novel analysis technique was proposed to process PT information, the pulsed phase thermography (PPT). This technique exploits the Fourier transform of the infrared images in order to analyze the thermal response of the specimen with distinct benefits. In this paper, the PPT technique is extended with the wavelet transform which is a powerful method of signal analysis bearing specific advantages with respect to the classic Fourier transform. Advantages and drawbacks of this extended PPT technique will be presented along with a brief theory of the wavelet transform and comparative experimental results.

Discrete signal transforms as a tool for processing and analyzing pulsed thermographic data

In this paper, we review some of the discrete signal transforms that are in use in the field of thermography for defect detection and/or characterization. Signal transformation is used with the purpose of finding an alternative domain where data analysis is more straightforward. For instance, it is possible to pass from the time domain to the frequency spectra through the one-dimensional discrete Fourier transform (DFT). The DFT constitutes the basis of pulsed phase thermography (PPT), but other transformations are possible such as the discrete wavelet transform (DWT) with the advantage that, in this case, time information is preserved after the transformation. It is also possible to rearrange data into domains others than frequency. For instance, the Hough transform (HT) allows the detection of regular forms (e.g. lines, curves, etc.) in a parameter space. The HT has been used in two different ways in thermography: for the detection of lines in thermal profiles, with the goal of discriminating between defective and non-defective regions; or it can be used to locate the inflection points in phase profiles obtained by PPT to extract the blind frequencies. The Laplace transform can also be used in the time domain to improve flaws detection and their characterization in the transformed space. Eigenvector-based transforms, such as singular value decomposition (SVD), have also been implemented. Principal component thermography (PCT) uses SVD to decompose thermographic data into a set of orthogonal modes. We discuss all these transforms and provide some comparative results.

Thermographic inspection of cracked solar cells

In this paper, a report is presented about the thermographic inspection of photovoltaic solar cells in search for cracks. Theoretical and practical application including experimental results and image processing are included.

Time aliasing problem in pulsed-phased thermography

Pulsed phases thermography (PPT) is a well-known processing method of non-destructive testing (NDT) by infrared thermography. This method is based on the frequency analysis of the images obtained from pulsed infrared thermography. The Fast Fourier Transform (FFT) is generally used for this technique. Such a method faces important time aliasing problems. Particularly important when slow infrared cameras are used and also in case of inspection of high thermal conductivity material like aluminum. Moreover, the oddness of the phase function, which is often the interesting part of the frequency response of the experiment increases dramatically this problem.

Pulse shaping in infrared thermography for nondestructive evaluation

Infrared thermography is a common technique for nondestructive evaluation. In active infrared thermography privileged here, a thermal stimulation is required to generate relevant thermal contrasts on specimens. Among stimulation techniques, pulse heating is one of the most common along with lock-in thermography. In this article, a technique is discussed that shows that by modifying the pulse heating shape, higher thermal contrasts are generated (for a given level of energy injection). In fact, it was found that ideally, it is better to use two pulses separated by a short Δ time interval. Variation of Δ directly influences the thermal contrast. By increasing Δ, the thermal contrast first improves up to a certain level before to reduce below the reference value (single pulse case). This was tested on several materials of low, medium, and high thermal conductivity. Moreover, this was also confirmed by an appropriate thermal model (not discussed in the article). In the text, a theory and experiments are provided.

Active infrared thermography for land mine detection

In this text a novel active infrared thermography approach is proposed for land mine detection. The simple approach consists in spraying the soil to inspect with water having a different temperature than soil temperature. Different temperature zones witness presence of buried mines. Theory related to pulsed thermography, simple simulation of the process, some results and envisaged battlefield deployment are presented.

HIGH EFFICIENCY DIFFRACTIVE OPTICAL ELEMENTS FOR BEAM COUPLING AND ARRAY GENERATION

Diffractive optics elements (DOE’s) are widely used for laser beam focusing, coupling, collimating, shaping and light beam array generation. In those applications specific functionalities of the DOE and diffraction efficiency, instead of aberrations, are the most important issues. The design of a DOE depends on the specifications of the DOE and is subjected to the fabrication constraints. Our objective is to achieve the best performance of the diffractive coupling lenses and array generation gratings under the given fabrication constraints.