History

The objects viewed in highly coherent light acquire a peculiar granular appearance named speckle. If the optically rough surface evolves in time, the scattered light gives rise to a dynamic speckle pattern and its variation depends on the activity of the sample. Biological samples and other dynamic processes (blood flow, vibrations, etc.) show this behaviour. Dynamic speckle or biospeckle can also be observed in non-biological industrial processes, including the drying of paint, corrosion and heat exchange. The visual appearance of the speckle diagram is similar to that of a boiling liquid. This activity takes place when the sample changes its properties due to movement of the scattering centers, changes in the optical path due to variations of refractive index, configuration changes or combination of these situations.

The study of the temporary evolution of the speckle patterns may provide an interesting tool to characterize the parameters involved in both biological and industrial transient processes. Since 1996, many efforts have been carried out at CIOp, to assign numbers that characterize this biospeckle activity and that correlate favourably with alternative measurement methods of interest for the experimenter.

Several algorithms to characterize the dynamic speckle pattern activity using contrast analysis, accumulation of differences between images, spatial and temporal speckle pattern analysis, wavelets based entropy, temporal spectral bands, etc. have been developed. In these methods a qualitative display is presented. For quantitative measurements, the temporal history of the speckle pattern (THSP) analysis with the autocorrelation function, the inertia moment of the co-occurrence matrix, the structure function, spatial variance of the phase and the difference histogram methods were employed. Numerical models based on movement of the scattering centres and an estimation of the spatial variance of the phase was also proposed.

Applications of the biospeckle techniques oriented to the characterization of biological tissues and industrial processes were developed at CIOp. For example: seeds viability, fruits damage, parasite motility, fungi detection, bacterial chemotaxis, drying of paint processes, hydroadsorption in gels, evolution of foams, etc.

Some of these researches were conducted in collaboration with groups of the Universities of Rosario (IFIR), Salta, Centro de la Provincia de Buenos Aires (Tandil), CIDEPINT (La Plata), Universities of Lavras and Sao Paulo, Brasil, Católica del Perú (Lima), Antioquia, Colombia and Instituto Politécnico, La Habana, Cuba.

Three published papers that illustrate this research line are shown in the following pages: an application devoted to seeds characterization, a proposed algorithm based on spectral frequency bands and an activity display obtained with a numerical model based on the spatial variance of the phase.

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