Laser speckle is an interference phenomena which is always present when a laser is used. Time-average speckle interferograms of the left and right side of the chest of a normal healthy subject were recorded at the Fourier transform plane on a fine grain photographic plate, while the subject held his breath. The interferogram was analysed by the point wise method and a graph was drawn between the position and the displacements at corresponding points on the chest wall. The associated displacements of the chest wall due to heart action are correlated with cardiac function.
B. M. Sivaram,
"Cardiac Monitoring Using Laser Speckle", Proc. SPIE 0211, Optics and Photonics Applied to Medicine, (29 May 1980); doi: 10.1117/12.958364; https://doi.org/10.1117/12.958364