Internal target reflections and line-imaging velocimetry

A commonly observed intensity modulation or “beating” in laser velocimetry (VISAR) data is examined and explained. It is found that internal target reflections between two surfaces moving relative to one another produces this intensity modulation in the VISAR steak record. The two partially reflecting surfaces define a Fabry-Perot cavity that creates an intensity interference pattern that is superimposed upon VISAR measurements. Experiments are conducted that demonstrate this phenomenon. Previous VISAR experiments that observe this beating pattern are presented and explained.