Changes in Laser Speckle as a Tool for Vibration Analysis

An object that is vibrating in a resonant mode and has a moderately rough surface illuminated by dispersed laser light shows patterns analogous to Chladni (strewn sand) patterns. The laser speckle patterns differ from the rest of the surface by appearing as regions in which the laser speckle has more dark areas interspersed with brighter light areas. These patterns appear at antinodes and are thus complementary to most Chladni patterns. Some examples of this observation along with Chladni patterns at the same resonant frequency are given. A heuristic theoretical discussion is also presented. [This work was performed under the auspices of the U. S. Atomic Energy Commission.]