Application Note Laser Particle Size Analysis

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Principle Of Laser Diffraction

ANALYSETTE 22 NanoTec

ANALYSETTE 22 MicroTec plus with Wet Dispersion Unit

ANALYSETTE 22 measuring range

In the 1920s Swiss astronomer R.J. Tümpler observed that stars appeared darker than expected. He figured that part of the starlight must be lost on its way to earth. Later the American astronomer E.P. Hubble noted that interstellar dust interfered with the observation of visible galaxies. The interstellar dusts consist mostly of very small particles - their typical diameter is between 0,1 and 1µm - which scatter and absorb the starlight. The theories of light scattering researched by astronomers can be used in the laboratory where the elements can be controlled. Monochromatic laser light can replace Starlight and the composition of the particles is often very well known.

From these basic theories instruments for measuring the particle size distribution using laser light scattering have been designed.

Basically the design is always the same: A light beam, usually a laser, shines through the sample to be measured and behind it the intensity distribution caused by the scattering is picked up with a detector. Then the measured intensity distribution shows a system of numerous, more or less concentric rings, where their spacing correlates with the particle size. Large particles create tightly spaced rings; small particles create rings further apart. By measuring the distance of each ring, the particle size can be calculated from the results.

However this simple principle has many complications due to the physical processes involved.

When illuminating a particle with light, different factors lead to a reduction or extinction of the incident light. The extinction is basically the sum of scattering and absorption.

Scattering identifies everything that deflects the incident light off its original direction. It can be divided in three parts, first: the reflection, second the refraction and third the diffraction.

Reflection occurs mostly on the surface of particles and is described according to the law “angle of incidence equals angle of refection”. Reflection on the surface of a sphere provides a very smooth intensity distribution scattering. Reflections on transparent materials can also occur on inner surfaces - Refraction.

In order to understand diffraction, one has to imagine the light beam as a wide wave front, which hits a particle and partially encircles it, similar to a water wave, which meets an obstacle. A characteristic interference pattern behind the particle is formed by the diameter of the particles. The Fraunhofer Theory describes this.

In relation to absorption, the absorption co-efficient of the material must be known in order to link the absorption and particle size. When electromagnetic light waves hit atoms and molecules in a particle they oscillate. This oscillation generates electromagnetic waves, which are emitted as light waves of the same wavelength and are radiated in all directions. These waves then form characteristic intensity distribution patterns.

At the beginning of the 20th century, Gustav Mie developed a complete theory to describe the effects during light scattering in colloidal metal solutions. This Mie Theory is used to interpret the data recovered from modern Laser Particle size analysers.

For more information on analysing particles between 0.8µm and 2mm on the FRITSCH MicroTec plus - Click here.

For more information on analysing particles between 0.02µm and 2mm FRITSCH NanoTec - Click here.