Advances in dynamic light scattering techniques

Recent developments in processing techniques and detection hardware have opened new horizons for the application of light scattering methods based on the dynamic analysis of coherent scattered light. Increased computational power of modern microprocessors allow real-time data evaluation on standard desktop computers. The continuous improvement of detector arrays, such as cameras based on CCD or CMOS technologies, facilitate space-resolved detection of scattering intensities, which can be used to boost the statistical weight accumulated in a single experiment. New methods and improved accuracy on the other side also provide answers to questions concerning the quantitative data interpretation which were only partially addressed in some of the earlier work. Since the early days dynamic light scattering (DLS) methods have been applied to a wide variety of systems and therefore the further development continued under different names such as DLS, laser Doppler, photon correlation spectroscopy, quasi-elastic light scattering and the family of laser speckle methods. All these techniques rely on the same phenomena, but look at it from different perspectives. Although the importance to exchange ideas and concepts was realized relatively early [1, 2], researchers still prefer to play on their own fields. Here we make an attempt to adopt a slightly more general point of view. Monochromatic light scattered by a set of particles of mesoscopic size produces a random interference pattern on a screen. This pattern in a general case has a form of irregularly spaced and sized bright spots called speckles. The scattered light intensity pattern remains unchanged as long as the particles do not change their position. Particle motion naturally leads to a temporal evolution of the scattered speckle field since one interference pattern is continuously replaced by another. In a single speckle spot this evolution is observed as strong temporal fluctuations of the intensity, with a certain well defined temporal correlation. The time frame of correlated intensity observed in the single spot is called temporal speckle. The intensity fluctuations are inherently linked to the scatterers’ dynamics and therefore the temporal correlation function depends on the particles displacement. Thus the measurable correlation properties of light can be linked to the dynamical properties of particles, which in turn can provide information about flow velocity and direction, particles size, density of moving scatterers, etc. In the scope of DLS temporal fluctuations are usually analyzed by means of the intensity or photon auto-correlation function (ACF, see Fig. 1). In the time domain analysis the correlation function usually decays starting from zero delay time (also called