LS Instruments

DWS is a multiple scattering technique and therefore requires a sample to be sufficiently turbid. An important parameter of a sample under study is thus the optical turbidity. It allows quantifying to which degree the collected light is multiply scattered and determines whether an acquired correlation function corresponds to a DLS or DWS experiment or to an intermediate type. To perform such an assessment in DWS we use the l*, which is the mean distance the photon travels in the sample before its propagation direction is randomized.

The figure below depicts a diffuse transmission through a turbid sample and illustrates the photon transport as a random walk.

Fig. 6: Schematic representation of the diffuse transmission through a turbid sample and illustration of the photon transport as a random walk.

The L/l* is measured by the DWS RheoLab^TM prior to any measurement: l* is obtained by measuring the count rates of the transmission and backscattering signals and then using calibration data specific to the instrument used. More information can be found in the reference below:

Improved diffusing wave spectroscopy based on the automatized determination of the optical transport and absorption mean free path.