Dynamic Light Scattering (DLS) is a technique commonly used to determine the size of small particles in a solution. It is also known as Photon Correlation Spectroscopy (PCS) or Quasi-Elastic Scattering (QELS). The measurement of the light intensity scattered by particles is used to evaluate their size.
In applications where the scattered intensity is low, single-photon sensitivity is required.

Shining a monochromatic light beam onto a solution with particles causes a light scatter. When the particles are much smaller than the incoming wavelength, the light intensity is scattered uniformly in all directions (Rayleigh scattering), independent of the wavelength. However this scattered intensity fluctuates with time. The time-dependent fluctuation occurs because the particles undergo Brownian motion and the distance between them is constantly varying. Analysing the time dependence of the scattering intensity fluctuation and knowing the viscosity of the medium, yield to the diffusion coefficient of the particles from which the particle diameter can be calculated. The experimental arrangement is shown in the following figure:

A 632nm HeNe laser is used for this measurement. The beam passes through collimation lenses, allowing the light to be focused onto the solution. The scattered light is then detected by the id100-MMF50 single photon detector.

The output signal from id100-MMF50 is sent to a correlator that computes the number of single photons detected as a function of time. The auto-correlation function is then calculated from the detected photon statistics. The particle size can then be calculated by fitting the data with mathematical models based on assumed particle size distributions

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