Spherical, non-porous and opaque particles, Diameter d > wavelength l, Particles are distant enough from each other, Random motion, All the particles diffract the light with the same efficiency, regardless of.
Characteristics of the Airy shape
Circular, Consisting in concentric rings I = f (a), Spacing and size of the rings are linked to the particle size, The fist zero angle is related to the diameter d by 1.22 l/d, 75% of the total energy is concentrated in the first lobe.
Aspect of the diffraction pattern with respect to the particle size
for a large particle
for a small particle
The observation of the diffraction pattern at finite distance is done through a lens (L) placed between the laser source and the detector
The diffraction patterns of particles having the same size converge at the same point whatever them location with respect to the lens, The first zero on the detector is 1.22 lf/d where f is the focal length.
The Fraunhofer theory is applicable for large particles compared to the wavelength l (diffusion and absorption are not considered).
For smaller particles, it is appropriate to use Mie Theory.
The Mie model takes into account both diffraction and diffusion of the light around the particle in its medium.
To use the Mie model, it is necessary to know the complex refractive index of both the sample and the medium.
This complex index has a real part, which is the standard refractive index, and an imaginary part, which represents absorption. Complex index = m m = a + b a: real part b: imaginary part
Because of the importance of this model, CILAS has crated a fast algorithm, which enables the user to get, within seconds, diffusion results using Mie theory and taking into account the complex index of the sample.