Speckle-visibility spectroscopy: A tool to study time-varying dynamics

Abstract

We describe a multispeckle dynamic light scattering technique capable of resolving the motion of scattering sites in cases that this motion changes systematically with time. The method is based on the visibility of the speckle pattern formed by the scattered light as detected by a single exposure of a digital camera. Whereas previous multispeckle methods rely on correlations between images, here the connection with scattering site dynamics is made more simply in terms of the variance of intensity among the pixels of the camerafor the specified exposure duration. The essence is that the speckle pattern is more visible, i.e., the variance of detected intensity levels is greater, when the dynamics of the scattering site motion is slow compared to the exposure time of the camera. The theory for analyzing the moments of the spatial intensity distribution in terms of the electric-field autocorrelation is presented. It is tested for two well-understood samples, a colloidal suspension of Brownian particles and a coarsening foam, where the dynamics can be treated as stationary and hence can be benchmarked by traditional methods. However, our speckle-visibility method is particularly appropriate for samples in which the dynamics vary with time, either slowly or rapidly, limited only by the exposure time fidelity of the camera. Potential applications range from soft-glassy materials, to granular avalanches, to flowmetry of living tissue. We describe a multispeckle dynamic light scattering technique capable of resolving the motion of scattering sites in cases that this motion changes systematically with time. The method is based on the visibility of the speckle pattern formed by the scattered light as detected by a single exposure of a digital camera. Whereas previous multispeckle methods rely on correlations between images, here the connection with scattering site dynamics is made more simply in terms of the variance of intensity among the pixels of the camera for the specified exposure duration. The essence is that the speckle pattern is more visible, i.e., the variance of detected intensity levels is greater, when the dynamics of the scattering site motion is slow compared to the exposure time of the camera. The theory for analyzing the moments of the spatial intensity distribution in terms of the electric-field autocorrelation is presented. It is tested for two well-understood samples, a colloidal suspension of Brownian particles and a coarsening foam, where the dynamics can be treated as stationary and hence can be benchmarked by traditional methods. However, our speckle-visibility method is particularly appropriate for samples in which the dynamics vary with time, either slowly or rapidly, limited only by the exposure time fidelity of the camera. Potential applications range from soft-glassy materials, to granular avalanches, to flowmetry of living tissue.

Acute Functional Recovery of Cerebral Blood Flow after Forebrain Ischemia in Rat

“…Note that equation (1) describes the relationship between speckle contrast and correlation time more accurately (Bandyopadhyay et al, 2005;Wang et al, 2007), compared with the formula widely used in laser speckle literature (Dunn et al, 2001;Durduran et al, 2004;Ayata et al, 2004). However, no significant discrepancies were found between the two formulae, when used for relative blood flow measurements (Wang et al, 2007).…”

“…The basics of the LSI technique have been described in detail earlier (Goodman, 1985;Briers, 2001;Dunn et al, 2001;Bandyopadhyay et al, 2005). Briefly, when coherent laser light is shone on a turbid medium, such as biologic tissue, photons traveling from different scattering paths form random speckle patterns on the surface of the medium, which can be captured by a CCD (charge-coupled device) camera.…”

“…(s) and the mean (/IS) of the intensity within a square window, for example, 5 Â 5 pixels. The correlation time (t c ) of the tissue, which is inversely proportional to the mean velocity of the scattering particles (Ayata et al, 2004), is related to the speckle contrast by a simple formula (Goodman, 1985;Bandyopadhyay et al, 2005),…”

Speckle-Visibility Spectroscopy of Depolarized Dynamic Light Scattering

“…Theory and experiments were found to be in very good agreement. To the best of our knowledge, this approachbeing analogous to SVS 4 and a viable alternative to differential dynamic microscopy 44 is new to depolarized DLS and can provide benefits. Finally, the theory is also valid for standard polarized scattering if the corresponding relationships are used.…”

“…The statistical properties of these random temporal fluctuations carry quantitative information about the Brownian dynamics. 3,4 The optical anisotropy of particles adds another level of complexity to DLS, which can be exploited in depolarized dynamic light scattering (also referred to as dynamic depolarized light scattering, DDLS). 5 Depolarization is a direct consequence of anisotropic polarizability.…”