Some elements on diffusion of light by particles

When a particle is illuminated by a beam of light, it reflects light in all directions. This is the diffusion phenomenon, involving three mechanisms, more precisely, refraction, reflection and diffraction. 

The interactions between a light beam and a given particle depend mainly on the ratio between the particle size and the wavelength of the beam of light. In order to precise the domain of each phenomenon, a size parameter a is defined [8]  

According to the value of the parameter a, and considering the light sources usually used (UV, visible, near IR), three domains are considered [8]  

A variety of techniques are available for the characterisation of particle size distributions but, because of the large size distribution of solids in water and wastewater, no single analytical method can be used. Besides their physico-chemical properties (stability, settleability, etc.), heterogeneous fractions have interesting optical properties. This is the reason why optical methods take an important place among granulometric methods (Table 5). 

Microscopic analysis, sieves or membrane techniques are also used for the separation of solids in waters [6], These optical, physical or electrical techniques are rather easy to use, but need specific and often expensive instruments. 

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