Holographic inter­ferometry is the technique of measuring stress, strain, and vibration with light. It is defined by the wavelength of light, finding flaws in structural bonds. It makes full use of a hologram's ability to reproduce the optical field reflected or trans­mitted by an object. Holographic inter­ferometry utilizes two unforeseen capabilities of a hologram. First, a hologram can record incoherent fields and reconstruct them coherently so that they can interfere in its recon­structed field. Secondly, a hologram can reconstruct the optical field from an object. It can be remade with such fidelity that it inter­feres with the original field from the object when relocated from its original position.

Now, Karl Stetson of Karl Stetson Associates has recounted the discovery of holo­graphic inter­ferometry, discussed its development, and itemized some of its major appli­cations. He assessed the original experiments and what might happen in the future. Stetson was inspired by Holographic Visions: A History of New Science by Sean Johnston, yet felt a profound sense of dissatis­faction. He worked to rethink the approach to many of the experi­ments described in the book. It allowed him to evaluate everything with fresh eyes and test the theories.

One example was the inter­ference fringes on the edges of specific holographic recon­structions. The author explained how his team worked together to change their metho­dology, compared to many decades before. They found that it was essential to assess properly how these fringes came about. Through their experi­mentation, the team found that it was most helpful in measuring real-time vibration on reflective surfaces.

Measuring real-time vibrations on reflective surfaces is vital for many industries. While the team first used tin cans and other simple forms, they struggled to find a sector that could take full advantage of holo­graphic inter­ferometry. However, the airline industry came on board very quickly, with the support of professors of acoustics. Holographic testing required a significant initial investment, adding consi­derable cost to products. Airlines would invest in this, as any failure would be larger than the original cost. Jet engines vibrate a lot, and high cycle fatigue is a major source of failure for the blades. They are designed to have reso­nances that are not excited at the rotation speeds of operation. Holo­graphy offered a way to visualize the mode shapes and confirm mathematical analyses.

The high efficiency of a jet engine depends upon the narrow gap between blade tips and the engine casing. An abradable material is bonded to the engine casings. If the rotating blades contact the casing from any defor­mation, they will merely scrape away some material and not break off. If there are places where the seal is not well bonded to the casing, these may be knocked out by the scraping blades and result in poorer engine efficiency. Ultrasonic noise shows disbanded areas as dark spots during holographic testing of a casing segment.

The move to digital holo­graphic inter­ferometry has been inevitable, given the techno­logical developments of the last thirty years. There is no doubt that there will be further develop­ments in this field in the future, but the Stetson cannot be sure where it will take us. (Source: LPC / CAS)

Reference: K. A. Stetson: The discovery of holographic interferometry, its development and applications, Light: Adv. Manufac. 3, 2 (2022); DOI: 10.37188/lam.2022.002

Link: Karl Stetson Associates LLC, Coventry, USA