Experimental classification of dynamic speckle regimes: insights from controlled rotational diffuser measurements

Dynamic speckle phenomena, arising from coherent light scattering on moving diffuse surfaces, are widely used for motion analysis in fields such as biomedical imaging and industrial inspection. However, the classification of dynamic speckle regimes remains challenging, particularly in understanding their distinct temporal and spatial behaviors across various velocity ranges. In this study, we introduce a comprehensive framework for categorizing speckle dynamics into three regimes: frozen, intermediate, and fully decorrelated. Each exhibits distinct temporal decorrelation properties, with direct consequences for motion quantification. To validate this framework, we designed an experimental setup comprising a coherent laser source, a controlled rotational diffuser as the moving scattering surface, and a high-resolution imaging system. This configuration enables precise control of speckle motion and systematic sampling of a wide velocity range. The experimental results reveal consistent and reproducible transitions between the three regimes, in strong agreement with the predicted contrast-velocity relationships. Our findings underscore the practical significance of this classification. In particular, they demonstrate that system performance depends critically on the regime in which measurements are made. Accurate velocity estimation requires adapting the acquisition strategy to the specific characteristics of each regime, including frame rate and exposure time. The intermediate regime, where contrast varies only weakly with velocity, should be avoided in system design due to its poor sensitivity. In addition to clarifying speckle dynamics, this framework enables the optimization of imaging system parameters by ensuring that measurements are performed in regimes where contrast is most sensitive to motion.