This work focuses on the characterization of narrow vertical cracks of nite size using optically excited lock-in thermography (OLT). To characterize these cracks, we need to solve an ill-posed inverse problem. As a previous step to the solution of this inverse problem, we propose a sensitivity analysis to quantify the influence that the parameters involved in the model have on the surface temperature. Some of these parameters are estimated at the laboratory and they incorporate uncertainty that may severely aff ect the reconstruction of thin cracks. For this reason, we design a calibration criterion based on the sensitivity analysis to determine which parameters we need to include as unknowns of the inverse problem. We perform this analysis using a numerical discontinuous Galerkin method. Additionally, we propose a theoretical noise model for the thermograms. Then, we use a weighted least square method (WLS) to determine the parameters from the experimental thermograms. We also obtain a theoretical uncertainty of the reconstructed parameters in OLT-WLS fi tting according to the used surface temperature dataset. Finally, we perform a numerical experiment with a 2.4 m-thick vertical crack to show the sensitivities of the surface temperature with respect to the model parameters. We also determine the uncertainty of the parameters under di fferent datasets with known noise characteristics.