Transient thermography (TT) images of impact damaged 3.44, 8.66 and 13.76 mm thick composites, from the impact face and rear face, are compared with the corresponding ultrasonic c-scan (UCS) images. Comparisons of apparent delamination area and size were made using novel image analysis techniques. The defect image sizes were also compared with crack lengths obtained from a selected number of specimens that were sectioned. For all composite thicknesses, the UCS damage area was proportional to the impact energy. The damage area obtained by TT from the front faces of the specimens did not correlate to the UCS damage areas although there was a correlation for the rear face damage on 3.44 and 6.88 mm thick specimens. Crack lengths obtained from sectioning showed that UCS damage length was equal to the maximum crack length and back and front sub surface cracks corresponded to back and front TT images.