During previous works, square pulsed thermography was used to carry out non destructive testing of bonding quality of CFRP glued on civil engineering structures during reinforcement operations [1,2]. The use of such wave form excitation was motivated by " on-site " requirements, but also by measurements duration, number of composite layers to test, depth of possible faulting areas versus temperature elevation allowed at composite level according to inner heat diffusion. Nevertheless, square pulsed excitation implies to choose an adapted heat duration. This duration is directly linked to the reliability of the parameter estimator [3]. In fact, after a certain duration the standard deviation of the estimation procedure stagnates. According to these observations, an indicator able to predict the sufficient heating time when the reliability of the parameter estimator reached an asymptotic evolution behavior was studied. Based on the absolute thermal contrast, the proposed indicator Iph is defined with the maximum thermal contrast ∆Tmax and the time delay (tph) between the heating time tc and the appearance of the maximum contrast, as shown in figure 1 (left). A typical evolution of the Iph indicator is proposed in figure 1 (right). This indicator allows to take into acount the detectability as well as the induced flaw temporal effect on the thermal contrast shape evolution. It has been observed that the maximum of Iph is connected with the sufficient heating time when the standard deviation of the estimation procedure tends to be minimized. This paper will present the establishment of this indicator for optimal square heating time and present an analysis of results obtained with numerical simulations and laboratory experiments. Figure 1 Absolute thermal contrast characteristics (left) and indicator Iph evolution with heating time duration References [1] A. Crinière, J. Dumoulin, C. Ibarra-Castanedo and X. Maldague , " Inverse model for defect characterization of externally glued CFRP on reinforced concrete structures: Comparative study of square pulsed and pulsed thermography " ,] L-D. Théroux, J. Dumoulin and X. Maldague , " Square heating applied to shearography and active infrared thermography measurements coupling: from feasibility test in laboratory to numerical study of pultruded CFRP plates glued on concrete specimen " ,Strain journal, Wiley editor, 2014. doi:10.1111/str.12086. [3] V. Feuillet, L. Ibos, M. Fois, J. Dumoulin, Y. Candau, " Defect detection and characterization in composite materials using Square Pulse Thermography coupled with Singular Value Decomposition analysis and thermal quadrupole modeling " , NDT&E International, Volume 51, Octobre 2012, pp 58–67, Elsevier, http://dx.