Effects of Diffuse and Specular Reflections on Detecting Embedded Defects of Foams With a Bifocal Active Imaging System at 0.22 THz

We show the diffuse and specular reflection images of defective foams placed on iron plates, which are obtained by a bifocal active imaging system at 0.22 THz. Foam materials studied are extruded polystyrene boards and acetate copolymer boards with artificially embedded defects. In the field of view of approximately 50 × 90 cm ² , the incident angle of the emitting and receiving cochannel system affects the reflected signal intensity and the imaging method. We build a light reflection model, which corrects the angle-dependent intensity behavior, to make the imaging results more intuitive. Experimental results show that large-angle imaging, which collects mainly diffuse reflection light, can be used to detect defects and is actually more suitable for detecting small defects than small-angle imaging, which mainly collects specular reflection light.