In recent years the use of Infrared Thermography (IRT) has reached a special importance in the industrial sector as a method of Non-Destructive Testing (NDT), for inspection of surface and subsurface defects of different types of materials. In order to determine the size of such defects by implementing the technique of Infrared Active Thermography, a Plexiglass sample (PMMA) with circular defects of different diameters and depths was designed and built. The sample was thermally stimulated by a continuous radiation from a halogen lamp, and subsequently the thermographic images were acquired by using the time-resolved infrared radiometry technique (TRIR). Images were obtained using Fluke TI-30 and TI-32 cameras, and a program was designed in Matlab for its further processing. Through a user interface, the program can display and filter the image; then it chooses a particular defect and calculates the diameter with their respective uncertainties. The best results for the calculated diameter were obtained with Ti-32 camera. The estimated uncertainties were below of 1.2mm, regard of defect diameter. Importantly, uncertainties grew when the diameter/depth ratio was 1 for depths of 4.0mm.