Ultrasonic optical flaw detection enables the visualization of internal flaws (at a depth of about 1 mm) that are hard to find using conventional ultrasonic testing. It can easily detect flaws in joints and bonding surfaces in research and development processes involving multi-materials, which are created by combining different materials to increase strength and reduce weight.
https://www.shimadzu.com/an/products/materials-testing/ultrasonic-optical-flaw-detector/miv-x/index.html