Damage Detection in Transparent Materials Using Non-Contact Laser Excitation by Nano-Second Laser Ablation and High-Speed Polarization-imaging Camera

Although transparent materials with birefringent properties (e.g., solar panels and separator films for secondary cells) are common, damage detection during the manufacturing process is crucial to economically realize high-quality materials. Herein a method using a pulsed-laser and a high-speed polarization-imaging camera is proposed to rapidly detect damage, including scratches and dents, in transparent materials. Specifically, as stress waves, which are generated by a non-contact impulse excitation from laser ablation, propagate through a material, the stress concentrations induced around damage are measured as the two-dimensional birefringent phase differences using a high-speed polarization-imaging camera with a microsecond-order temporal resolution. When stress is dominant, the distribution of the measured birefringent phase difference can be considered the relative distribution of stress. Using acrylic plates as a representative transparent material with several hundred micrometers of damage (e.g., a dent or a scratch), we demonstrate that the proposed method detects damage in a very short timeframe of several microseconds.