Characterization and Energy Absorption Efficiency Determination of LED as an Effective Photothermal Excitation Source in Lock-In Thermography
TL;DR: In this paper, the use of light-emitting diode (LED) as an excitation source for photothermal lock-in experiments was investigated, and the energy absorption efficiency and signal-to-noise ratio of the established setup was investigated.
Abstract: This paper studies the use of light-emitting diode (LED) as an excitation source for photothermal lock-in experiments, and investigates the energy absorption efficiency and signal-to-noise ratio of the established setup. Existing lock-in excitation sources usually use high power (in the range of kilowatt) halogen lamps. The present LED excitation source has an advantage of low power consumption making the setup much more energy efficient. Another advantage of LED in this paper is their low infrared (IR) content, in contrast to halogen lamps, the IR content gets reflected into the IR camera, and leads to an unpredicted change in phase image in lock-in thermography.
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TL;DR: In this paper, the authors proposed a novel pulse compression algorithm, in the context of frequency modulated thermal wave imaging, derived from a predefined reference pixel in a recorded video, which contains direct measurement of the excitation signal alongside the thermal image of a test piece.
Abstract: This paper proposes a novel pulse compression algorithm, in the context of frequency modulated thermal wave imaging. The compression filter is derived from a predefined reference pixel in a recorded video, which contains direct measurement of the excitation signal alongside the thermal image of a test piece. The filter causes all the phases of the constituent frequencies to be adjusted to nearly zero value, so that on reconstruction a pulse is obtained. Further, due to band-limited nature of the excitation, signal-to-noise ratio is improved by suppressing out-of-band noise. The result is similar to that of a pulsed thermography experiment, although the peak power is drastically reduced. The algorithm is successfully demonstrated on mild steel and carbon fibre reference samples. Objective comparisons of the proposed pulse compression algorithm with the existing techniques are presented.
10 citations
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TL;DR: In this paper, a novel frequency-phase modulated (FPM) waveform was introduced, and computationally verified by the current authors, which couples the concept of frequency and phase modulation to each other in view of obtaining an optimized excitation signal for improved thermal wave radar (TWR) imaging.
Abstract: Thermal Wave Radar (TWR) imaging employs the concept of pulse compression in order to obtain an increased probing depth and depth resolution in infrared thermographic testing of materials. The efficiency of the TWR imaging is highly dependent on the nature of the employed excitation signal. Most studies exploit the use of an excitation signal with an analogue frequency modulation (e.g. sweep signal) or a discrete phase modulation (e.g. Barker coded signal). Recently, a novel frequency-phase modulated (FPM) waveform was introduced, and computationally verified by the current authors, which couples the concept of frequency- and phase modulation to each other in view of obtaining an optimized excitation signal for improved TWR imaging. This paper experimentally investigates the performance of the novel optimized FPM waveform for the inspection of glass and carbon fiber reinforced polymer (GFRP and CFRP) composites, using an optical infrared thermography set-up in reflection mode. The response of the halogen lamps to the FPM waveform is measured, and further the influence of the electro-thermal latency of excitation lamps on the applicability of the novel FPM excitation signal is analytically investigated. Then, the performance of the FPM waveform is experimentally investigated for both glass- and carbon fiber reinforced polymers with defects of different depths and sizes. A comparative analysis is performed with amplitude modulated (classical lock-in), frequency modulated (sweep) and phase modulated (Barker coded) excitation, each with the same time duration as the FPM waveform. The novel FPM waveform outperforms these existing waveforms in terms of defect detectability and contrast-to-noise ratio, especially for the deeper defects. Different central frequencies are examined and the improved performance of the FPM waveform in TWR imaging is demonstrated in all cases.
6 citations
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TL;DR: A nonuniform frame capture technique to reduce the memory allocation space of the recorded video, based on varying the sampling rate with a change in instantaneous frequency and is specifically useful for frequency modulated excitation signal is proposed.
Abstract: This paper proposes an energy efficient instrumentation set-up for pulse-compression thermal-wave imaging with a low-power LED excitation source. The set-up consists of three different subsystems that are synchronised in time. The individual systems consist of a LED modulation circuitry, reference signal measurement circuitry, and an IR camera trigger signal generation circuitry for frame capture. A separate reference acquisition circuitry is useful in quantifying defect depth resolution. This paper also proposes a nonuniform frame capture technique to reduce the memory allocation space of the recorded video. The technique is based on varying the sampling rate with a change in instantaneous frequency and is specifically useful for frequency modulated excitation signal. The proposed technique is implemented on a carbon fiber reinforced polymer test-piece. The variation of pulse compression parameters with different defect dimension is studied, and the results are verified with an electro-thermal simulator. Further, an objective comparison of pulse compression experiment for different experiment duration is presented.
4 citations
Cites methods from "Characterization and Energy Absorpt..."
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TL;DR: This paper proposes an idea of employing sparse reconstruction-based technique for thermal imaging defect detection, and describes cross-validation method for optimization of a user parameter in sparse reconstruction method.
Abstract: This paper proposes an idea of employing sparse reconstruction-based technique for thermal imaging defect detection. The implementation of the reconstruction technique is tested on a carbon fiber reinforced polymer test piece with artificially drilled defects and the test results are compared with the established cross correlation method. The two processes are compared in terms of defect detectability, their SNR variation with defect depth and their computation complexity. When compared with cross correlation algorithm, the technique is expected to solve memory space problems by compressing all information from large cross-correlated pulse video into a single reconstructed image as an output. Furthermore, in existing cross correlation methods, the pulse peak time shifts with defect depth. Hence, defect quantification algorithms, such as SNR calculation, require multiple frame analysis. Such algorithms are comparatively simplified in sparse reconstruction technique. This paper explores sparse reconstruction algorithm for resolving close-spaced defects. This paper further describes cross-validation method for optimization of a user parameter in sparse reconstruction method.
3 citations
Cites methods from "Characterization and Energy Absorpt..."
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TL;DR: In this article, a method for improving control of high-power optical and electrical types of excitation sources with possibility of voltage or current stabilization is proposed with the possibility of current stabilization.
Abstract: The aim of the paper is to study the possibilities for using one excitation source for transient as well as lock-in infrared thermography modes. Problems in controlling the high-power optical excitation sources in active thermography are analysed. Special attention is paid to high power LEDs as sources for thermal stimulation in thermography. A method for improving control of high-power optical and electrical types of excitation sources is proposed with possibility of voltage or current stabilization.
Cites background from "Characterization and Energy Absorpt..."
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References
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TL;DR: In this article, a quantitative derivation for the acoustic signal in a photoacoustic cell in terms of the optical, thermal, and geometric parameters of the system is presented. And the theory predicts the dependence of the signal on the absorption coefficient of the solid, thereby giving a theoretical foundation for the technique of photoacoustical spectroscopy.
Abstract: When chopped light impinges on a solid in an enclosed cell, an acoustic signal is produced within the cell. This effect is the basis of a new spectroscopic technique for the study of solid and semisolid matter. A quantitative derivation is presented for the acoustic signal in a photoacoustic cell in terms of the optical, thermal, and geometric parameters of the system. The theory predicts the dependence of the signal on the absorption coefficient of the solid, thereby giving a theoretical foundation for the technique of photoacoustic spectroscopy. In particular, the theory accounts for the experimental observation that with this technique optical absorption spectra can be obtained for materials that are optically opaque.
2,186 citations
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TL;DR: In this paper, the authors combined thermal wave and thermography to provide a short-time low-frequency phase angle image where non-thermal features can be suppressed using lock-in data analysis.
Abstract: Thermography and thermal wave techniques can be combined to provide in a short‐time low‐frequency phase angle images where nonthermal features can be suppressed. The principle is optical thermal wave generation simultaneously on the whole sample surface and sequential monitoring of all pixels using both thermographic techniques and lock‐in data analysis. Due to parallel stationary excitation one can use low modulation frequencies allowing for a depth range that is of relevance for applications.
597 citations
"Characterization and Energy Absorpt..." refers background in this paper
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Book•
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01 Jan 1996
TL;DR: In this article, the photothermal effect is used to generate thermal wave non-destructive evaluation, which is then used for the measurement of semiconducting materials and their properties.
Abstract: Introduction. History. Overview. Thermal waves. Optical generation of thermal waves - photothermal effect. Instrumentation and detection techniques. Transient thermal phenomena. Photoacoustic spectroscopy and its applications. Thermal wave non-destructive evaluation. Characterization of semiconducting materials. Thermal property measurement. Monitoring and measurement of gases and the atmosphere.
506 citations
"Characterization and Energy Absorpt..." refers background in this paper
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TL;DR: In this paper, several experimental investigations of newly developed thermographic NDT techniques based on the transient temperature measurements are described and several experimental results of the pulse heating NDT and the lock-in NDT are shown.
Abstract: This paper describes several experimental investigations of newly developed thermographic NDT techniques based on the transient temperature measurements. Several experimental results of the pulse heating thermographic NDT and the lock-in thermographic NDT are shown.
161 citations
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TL;DR: In this article, the defect detection capabilities of pulse transient thermography and lock-in thermography were compared using equal excitation energies, and a signal-to-noise ratio analysis was performed on defect images obtained by the two techniques.
Abstract: The defect detection capabilities of pulse transient thermography and lock-in thermography were compared using equal excitation energies. A signal-to-noise ratio analysis was performed on defect images obtained by the two techniques. The test piece imaged by both techniques was a carbon fibre composite plate containing back-drilled flat-bottomed hole artificial defects of 4, 6 and 12 mm diameters at depths ranging from 0.25 to 3.5 mm. Similar limits of defect detection were found for the two techniques. Lock-in thermography phase images were found to exhibit anomalous switches between positive and negative phase values for different modulation frequencies and for different combinations of defect diameter and depth. This effect resulted in values of defect phase response that differed substantially from the one-dimensional thermal wave interpretation of lock-in thermography imaging. The consequences of this anomalous effect and its physical origins are discussed.
136 citations
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