Electronic speckle pattern interferometry in long wave infrared: a new technique for combining temperature and displacement measurements: application in thermo-mechanical assessment of structures

Abstract

Holographic and speckle based techniques are well known for measuring full-field displacements and performing non destructive testing on solid objects [1]. The measurement range and accuracy of these techniques are related to the wavelength of the laser. In visible light they are often not well suited for field applications or for large displacement measurement due to the short wavelength used, which imposes high stability constraints. One way to decrease this sensitivity is to use a longer wavelength. Our purpose is to investigate holography in the Long Wave InfraRed (LWIR) range and more specifically at 10 μm with CO2 lasers. This provides a typical 20 factor of decrease of the stability requirements; meantime the range of measurement is increased by the same factor. We present here advanced results obtained within the FANTOM FP7 project. Besides the lower sensitivity to external perturbations, another interesting feature that is put forward in this project is the possibility to develop a sensor which combines displacement by holography/speckle and thermography measurements. Here we take advantage to the fact that the necessary observation device for the holographic signal is a thermographic camera. This has an interest in various applications such as analysis of thermo-mechanical behaviour of structures or non destructive testing, where often displacements and temperature fields need to be observed and correlated one to another. We already presented preliminary results on out-of-plane and in-plane ESPI at 10.6 μm, and also on digital holographic interferometry [2], however without combining thermography. Recently we presented an improved digital holographic interferometry set-up [2], and a study of the scattering problems arising when working at such long wavelengths [3]. In this paper we present for the first time to our knowledge the combination of full-field temperature and displacements, with a single sensor.