These measurements can be applied to stress, strain and vibration analysis, as well as to non-destructive testing and radiation dosimetry.
[3] It can also be used to detect optical path length variations in transparent media, which enables, for example, fluid flow to be visualised and analyzed.
Holography is the two-step process of recording a diffracted light field scattered from an object, and performing image rendering.
If, however, a small deformation is applied to the object, the relative phases of the two light fields will alter, and it is possible to observe interference.
[1][4][5][6] While the first observations of phenomena that could be ascribed to holographic interferometry were made by Juris Upatnieks in 1963[7] the essential feature of the process was not understood until the work of Powell and Stetson.
Because these lasers had about three longitudinal modes, their coherence length was periodic, as described by the manufacturer, Spectra Physics in cooperation with the Perkin Elmer Corporation.
Stetson put forth the idea that each mode existed in both the object and in the reference beam, and each pair recorded a separate hologram in the photographic plate.
Powell objected to this idea, because it implied that the hologram had the power to coherently reconstruct fields that were incoherent during its recording.
Later, in April 1965, Stetson and Powell obtained real-time interference patterns between a real object and its holographic reconstruction.
Powell and Stetson have shown that the fringes of the time-averaged hologram of a vibrating object correspond to the zeros of the Bessel function
A slow (e.g. video rate) camera will record time-averaged holographic interferograms which will result in lowpass filtering of the optical fluctuation signal.
This method permits microvascular blood flow imaging,[14] and wide-field measurement of photoplethysmograms by detection of out-of-plane tissue motion.
[15] The wide temporal bandwidth of a high throughput camera can enable wideband detection and analysis of optical fluctuations.