Electronic Speckle Pattern Correlation Interferometry (ESPI) is used as a tool for NDI, in many aspects complementary to x-ray or ultrasonic techniques. Many technical structures are to be inspected from the inside but are not accessible to conventional ESPI. Endoscopes as known from visual inspections lend themselves to circumvent this limitation. The endoscope is used as an 'extended lens' to transfer the object wave to the interferometer. We us the ring shaped illuminating fiber bundle of a rigid endoscope for beam delivery. For observation and object wave transfer the relay optics of the endoscopes is used which results in a very good image quality. In laboratory environment stability and beam quality of this set-up is adequate for NDI-applications in spite of the multi-mode multi-fiber character of the illuminating fiber bundle, since the bundle is mechanically rigid.
Erwin K. Hack,
"Application of endoscopic ESPI in NDI", Proc. SPIE 4420, Laser Metrology for Precision Measurement and Inspection in Industry, (11 September 2001); doi: 10.1117/12.439200; https://doi.org/10.1117/12.439200