**How a VISAR Works**

The Velocity Interferometer System for Any Reflector (VISAR) was developed by Barker and Hollenbach at Sandia National Laboratories in 1972.

The technique works by using an interferometer to measure the minute Doppler shift in light frequency given to a laser beam as it is reflected from a moving specimen surface. The Doppler shift produces light fringes in the interferometer, and the number of fringes is proportional to the surface velocity.

VISAR data consists of recordings of the light intensity outputs from the VISAR interferometer as a function of time, in which each complete oscillation of the light intensity corresponds to one light fringe. VISAR data reduction consists of counting the number of light fringes up to any given time, and multiplying by the Velocity-Per-Fringe constant of the interferometer. Repeating this procedure for each time increment results in a complete velocity vs. time record of the specimen surface. Using a data reduction program, the fringe count can usually be determined to ± 2% of a fringe or better.