Abstract: To investigate the internal defects in PC glued sheet, the specimen was heated by pulse flash thermal excitation. Meanwhile infrared test system monitored the specimen surface temperature field in real time. According to the difference in surface temperature of the specimen, the internal defects in specimen could be displayed. However, the obtained contrast of thermal images was poor. In order to extract defects accurately, histogram equalization was used to enhance contrast. The defects size was measured and measuring error was analyzed. The results showed that infrared thermography could intuitively display defects which were more than 4 mm, and that the detecting precision was higher for defects which were more than 6 mm,and that measuring error could be controlled within 10%. Due to the strong edge effect, the measuring error was larger for defects closing to the edge.