A novel approach to control gray level of image invariant during near infrared thermography

Abstract

International audienceKinematic and thermal field measurements can provide rich information in the field of thermo-mechanics of materials and structures (e.g., constitutive model identification, fatigue behavior investigation, cracking detection, etc.). Up to now, silicon based sensor cameras (CCD and CMOS) have been widely used to perform in situ observation of the kinematic field on the material surfaces, mainly thanks to image correlation or interferometry. Thermal fields are usually obtained thanks to dedicated Infrared Cameras. Nevertheless, the acquirement of thermal fields using a silicon based sensor visible camera is possible and suitable for application at high temperature. In this study, a low-cost, high-resolution and contactless field measurement protocol is proposed to in-situ observe temperature distribution. Actually, silicon based sensor visible cameras are also sensitive in the near infrared spectral band (0.7-1.1 μm). In this spectral range, a temperature evolution readily results in a modification of the gray level obtained with the camera. It is usually considered as issues on the acquired images (saturation or poor dynamic range of gray levels) for visible acquisitions. Then adjust the exposure time of the camera with temperature evolution is a suitable method to maintain the gray level of image constant. In this work, we propose an approach to precisely and continuously adjust the exposure time to get a constant gray level of images whatever the temperature evolution. Black body experiments have been conducted to verify the approach