subject: Light Scattering Technique To Measure The Size And Distribution Of Particles [print this page] The scattering of light is one of the most widely used techniques for measuring the size distribution of particles. In practice, the technique is fast and flexible, offering precise measurements that can be easily adapted to samples presented to the analyzer in various forms.
The method involves the analysis (deconvolution) of the patterns of scattered light produced when particles of different sizes are exposed to a beam of light.Some models of particle size analyzerstake advantage of this principle to rapidly provide precise and reproducible particle size distributions.
Light Scattering
When light illuminates a particle having a dielectric constant different from that of the medium, depending on the wavelength of the light and the optical properties of the particle, light will be scattered in a unique way.We commonly describe scattering phenomena in terms of diffraction, reflection, refraction, and absorption.
When light interacts with the electrons bound in the material that re-radiate light, scattering is observed. Because most materials exhibit strong absorption in the infrared and ultraviolet regions which greatly reduces scattering intensity, most light scatteringmeasurements are performed using visible light of wavelengths from 350 nm to 900 nm. The scattering intensity from a unit volume that is illuminated by a unit flux of light is a function of the complex refractive index ratio between the material and its surrounding medium. This intensity falls within the regime of Rayleigh scattering and is inversely proportional to the fourth order of the light wavelength, i.e., the shorter the wavelength, the stronger the scattering.
The reason that the sky is blue at midday and red at sunrise or sunset is that one sees the scattered sunlight duringdaytime and sees the transmitted sunlight during dawn and dusk. Utilizing this wavelength dependence, we use red as the color for the stoplight and for all traffic control warning signs because red light has the least scattering power in the visible light spectrum. This allows the transmitted light to go through fog, rain, and dust particles and reach the intended detector: in this case the human eye.
Several technologies make use of light scattering to obtain information about materials. Among these technologies elastic light scattering (ELS) is the main method for the characterization of particles of sizes ranging from microns to millimeters. In ELS the scattered light has the same frequency as the incident light, and the intensity of scattered light is a function of the particle's optical properties and dimensions.
In general, the scattered light intensity of a particle is a function of the following variables: particle dimension, particle refractive index, medium refractive index,
Mie Theory
The Mie theory describes the interaction of light with a particle of arbitrary size as a function of angle, given that the wavelength and polarization of the light are known and that the particle is smooth, spherical, homogeneous, and of known refractive index. This theory is more complex than the theory set forth by Fraunhofer, in that it accounts for all possible interactions between particles and light, yet it's only applicable to spheres.Spheres produce light scattering patterns that are characterized by the presence of scattering minima and maxima at different locations depending on the properties of the particles. At small angles (typically smaller than 10 degrees) the scattering pattern for spheres is centrally symmetric instead of axially symmetric, i.e., it displays concentric rings in the direction of the incident light. Therefore, large particles produce scattering intensities that are concentrated at small angles and due mainly to diffraction effects from the edge of the particle.
