Basic Principles of Laser Diffraction

Laser diffraction particle size analysis is based on the physical phenomenon that particles in a laser beam scatter laser light at angles that are inversely proportional to the size of the particles [64, 161, 223], Thus, larger particles scatter at small forward 

The fact that the scattering pattern depends on particle size forms the basis for the application of the LD technique for measurement of PSDs. Then, the scattering patterns of single particles in particular the ones for homogeneous, spherical particles are extended to ensembles of particles by assuming that  

In general, the scattering intensity of unpolarized light by a single spherical particle 

The different theories have different advantages and disadvantages. 

The complex Mie coefficients an and bn are obtained from matching the boundary conditions at the surface of the sphere. They are expressed in terms of spherical Bessel functions evaluated at a (i.e., a dimensionless size parameter) and ma. The intensity of the scattered light (13.52) may then be seen as a function of the wavelength of light, A, particle size, dp, and the complex refractive index m, of a particle in a medium  

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