Imaging of Subsurface Defects in Bivalve Shells by Photothermal Techniques
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Imaging of Subsurface Defects in Bivalve Shells by Photothermal Techniques O. Gómez-Martínez, M. Zambrano-Arjona and J.J. Alvarado-Gil Applied Physics Department, Cinvestav-Unidad Mérida Carretera Antigua a Progreso km. 6 Mérida, Yucatán, 97310 México
ABSTRACT The potential of photothermal radiometry for subsurface defect imaging in shells of Crassostrea virginica is explored. It is shown that using this technique in the modality of modulated frequency, it is possible to obtain photothermal images in the case of small thermal diffusion lengths. By controlling the modulation frequency, it is shown that the technique can be useful in the study of buried defects in the shell. The application of this technique in monitoring living organisms is discussed.
INTRODUCTION A wide variety of methods for the non-destructive study of subsurface defects and structures in solids have been developed based on thermal excitation. Local changes in density, specific heat, thermal conductivity, etc. can be detected by some type of thermal probe. Some of these techniques are based in the generation of thermal waves inside of the material by a modulated light beam. These thermal waves have the characteristic of being strongly attenuated when traveling along the material, depending on the thermal diffusivity of the material and on the modulation frequency of the incident light [1,2]. Thermal waves can be detected by various means. One of these methods is known as infrared photothermal radiometry. In this case the infrared radiation emitted by the solid as a result of the modulated heating of the impinging light is monitored. Using this technique, the non-destructive and non-contact thermal characterization of a broad range of materials has been performed. It has also been shown that this technique can be used successfully in the development of imaging methodologies in different materials [1]. In the case of biomineralized materials it is very important to measure with techniques that could permit us to evaluate the physical characteristic of the resultant materials. In particular it could be possible to study the characteristics of the growth, anomalies, cracks and damage on the structure due to environmental effects [3-5]. In this paper, it is shown that even for materials as structurally complicated as the oyster mollusk shells, it is possible, using infrared photothermal radiometry, to develop subsurface studies of buried structures [1]. In particular the detailed analysis of natural and artificial cylindrical holes is analysed and the consequence of the variation of the thermal wave penetration on the resultant photothermal signal is reported. HH3.44.1
EXPERIMENTAL DETAILS The experimental system used is shown in Figure 1. A 500 mW Ar ion laser (Ion Laser technology) impinges on the surface of the sample. The beam was modulated using an optical chopper (SR540). The emitted infrared light from the sample was collected and focused onto the sample using two off-axis paraboloidal mirrors (See Figure 2). The detector was a liquidnitroge
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