The TMTF is a tool for thermal NDE (pulsed thermography). It gives a synthetic view of the thermal contrast degradation that is inherent with a reduction of the defect size. It was built, like the MTF in visual or IR imaging, by using a target with a periodic pattern. The TMTF was numerically simulated with a model having a cluster of parallel slots on the rear side. The class of slots having a low relative depth is aimed to simulate corrosion at its onset. For this class, the TMTF is expressed as a unique curve that moves with the time of observation. An inversion method based on the TMTF was proposed: it clears, to some extent, the observed temperature field from the detrimental effect of lateral diffusion.