Time-averaged holographic interferometry
Holographic interferometry - Principles

This method is often combined with the previous one for vibration analysis. It allows you to visualize not only the object's displacement map during periodic vibration, but also its nodal lines. The principle is simple.

In a conventional holographic setup, the hologram of the vibrating object is recorded with an exposure time that is long compared with the vibration period. In general, the natural frequencies are determined by holographic interferometry in real time, using a set-up like the one shown in figure