Light Scattering DLS Photon Correlation Spectroscopy PCS

Dynamic Light Scattering (DLS) Photon Correlation Spectroscopy (PCS) 

DLS is a method that measures the time-dependent fluctuation of scattered intensity, and is also referred to as quasi-elastic light scattering or photon correlation spectroscopy. The latter term is the most commonly used for describing the process, since most dynamic scattering techniques employ autocorrelation. 

In a system where the Brownian motion is not intermpted by sedimentation or particle-partide interaction, the movement of particles is random. Hence, the intensity fluctuations observed after a large time interval do not resemble those fluctuations observed initially, but rather represent a random distribution of particles. Consequently, the fluctuations observed at a large time delay are not correlated with the initial fluctuation pattern. However, when the time differential between the observations is very small (a nanosecond or a microsecond), both positions of particles are similar and the scattered intensities will be correlated, and when the time interval is increased then the correlation will be decreased. The decay of correlation is particle size-dependent that is, the smaller the particles the faster the decay. 

The fluctuations in scattered Hght are detected by a photomultiplier and recorded the data containing information on particle motion are then processed by a digital correlator. The latter compares the intensity of scattered light at time t, I(t), to the intensity at a very small time interval r later, /(t + r), and constructs the second-order autocorrelation function G2(r) of the scattered intensity. 

The experimentally measured intensity autocorrelation function Gjir) depends only on the time interval r, and is independent of t, the time when the measurement 

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