In this work active thermography was successfully applied as a quantitative method for the non-destructive evaluation of porosity in carbon fiber reinforced polymers. Results showed that not only the level of porosity, but also the shape of the pores strongly influences the active thermography results, in particular the measured thermal diffusivity. In an experiment the shape of the pores was obtained by means of X-ray computed tomography. Based on this microstructural information, the dethermalization theory can be deduced as the most effective thermal diffusivity model with respect to quantitative porosity determination. Furthermore, the obtained experimental data on active thermography correspond to measurements made using ultrasonic testing with respect to the influence of pore shapes.