Stimulated Infrared Thermography (TIS) is a fast and global method for NonDestructive Evaluation (NDE). Among the recently emerging NDE methods, it is probably the less intrusive one since it really needs no contact at all with the tested structure. A new inversion technique, using an early detection of the contrast, is presented to demonstrate how it recovers the depth of the defects with accuracy and partially removes the effects produced by the lateral heat diffusion. l . INTRODUCTION The main ONERAfL3C contribution in the Non Destructive field lies in the techniques of the Stimulated Infrared Thermography (TIS) which consist in a pulse uniform illumination of the surface of the studied specimen, followed by the analysis of its InfraRed (IR) emission as a function of time /I/. Section 2 of this paper is devoted to the recent improvements brought by our group to this analysis and which allows to strongly decrease the effects of the lateral heat diffusion (3-D effects). As a consequence, the method leads to a high accuracy as regards the identified in-depth position of the defects, and to a better identified defect shape (see section 3). The advantages of this new inversion will be assessed, by performing it on artificial and natural impact defects present in composite materials. 2. TIS EARLY DETECTION 2.1. Industrial set up The set-up used in the experiments described in this paper, corresponds to the front surface configuration, i.e. illumination and detection on the same surface. The pulsed sources available in our system can be either flashes (pulse duration 4 ms) or continuous IR lamps (48 kW), the pulse being achieved, in that case, by the opening/closing of mechanical shutters electromechanically driven (the smallest pulse duration obtained with this system is 200 ms). An AGEMA 880LW records the surface temperature as a function of time during periods of times up .to 10 mn after the pulse illumination. For instance, when placed at 1.2 m from the sample surface, the system allows to obtain a field of view of 20 cm and a spatial resolution of 2 mm with the objective of 12' used for these experiments. 2..2. Inversion using the early detection The detection and the characterization of the resistive subsurface defects is achieved by seeking local emergence of thermal contrast after the pulse illumination, i.e. an increase of the local temperature above the defect with respect to the temperature in a sound region, (figure la). Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1994712 C7-48 JOURNAL DE PHYSIQUE IV Generally, the depth and the resistance of the defect are deduced from the value and the time of the maximum contrast /l/. This search is made pixel by pixel and finally leads to two synthetic images, respectively for the local depths and resistances of the subsurface defects. An alternative consists in the detection of the time corresponding to half the development of the contrast 121. The deeper the defects are, the longer these characteristic times are, with an important lateral heat diffusion as a consequence. Under these conditions, the inversion provides an underestimation of both the depths and the thermal resistances of the detected defects. a) Schematic explanations of the two detections b) Definition of the detection threshold, dT being the noise