Principles and Applications of Laser Diffraction Technology

Light scattering as the basis for size determination of many objects has been explored for a long time. It was Gustav Mie, author of the Mie Scattering Theory, who studied gold nanoparticles in pursuit of his doctoral degree. While such analysis was then limited to labs which had built customized equipment for this purpose, several new developments have led to laser diffraction becoming a part of the arsenal of research laboratories and production processes all over the world.

How Laser Diffraction Works

The principle of laser diffraction is the relationship that exists between light scattering (its angle and intensity) and particle size. The larger the particle, the smaller the angle and the higher the intensity of the scattering. This is made use of in every analyzer ever made, from the basic prototype to the latest LA-960 particle size analyzer.

The device does not directly measure the size of the particle but the angle and intensity of light scattering from the particles. This data is entered into an algorithm that uses the Mie scattering theory to yield information about the particle size from the data on light scattering.

Much work has gone into updating the equipment and the software that are necessary to measure particle size. This has resulted in increased precision, dependable performance, and convenient use. The LA-960 is a tenth-generation laser diffraction analyzer from HORIBA, with each new series being an improved and very different set of instruments from the last. The rest of the article explains how these devices are designed for easy and reliable measurement.

Laser Diffraction Principles

The fundamental part of this method is the way light responds when it strikes a surface, or in this case, a “particle”, such as:

  • Diffraction
  • Refraction
  • Reflection
  • Absorption

Diffraction is a phenomenon that occurs at the edges of a light beam, and is therefore also called "edge diffraction".

The refraction of light happens when light travels through a particle and alters its course thereby.

Both these interactions are important when obtaining data about particle size from the angle and intensity of light scattering. On the other hand, reflection and absorption of light are counterproductive for such measurements and must be compensated for during the measurement and calculation phases.

If a particle exceeds a critical diameter, most of the light that strikes it undergoes scattering due to diffraction, with a comparatively high intensity and small angle due to the larger size of the particle. This size can be found to be a multiple of the wavelength of incident light on the particle, and 20 microns is a useful approximation.

For particles larger than this diffraction, giving the most information about its size rather than refraction, means a refractive index will not help to interpret refracted light with much accuracy in such processes.

If the particle being measured is less than 20 microns, refraction becomes more and more important as a source of information required to deduce the particle size accurately. Such particles scatter light at lower intensities and wider angles compared to larger particles. It is necessary to use the refractive index along with the Mie scattering theory for maximum accuracy of size determination. For this reason, all devices in the HORIBA range of laser diffraction analyzers use the Mie theory by default, while users can input any desired refractive index.

Features of a Laser Diffraction Analyzer: Optical System

A laser diffraction particle size analysis has a two-part workflow:

  • Measurement of light scattering angle and intensity
  • Use of this data to derive a distribution of particle size

The design of the analyzer is crucial to the quality of measurement. This in turn depends upon the quality of the components, advanced engineering and a core design which is based upon sound principles. Increasing the quality of the parts will lead to better performance of the whole.

The parts of a laser diffraction particle size analyzer include:

  • Paired light sources which emit light at different wavelengths
  • More than 80 light detectors that span a range of angles, from 0 degrees to 170 degrees approximately
  • Lenses, mirror and glass measurement cells of an extremely high quality

The LA-960 is HORIBA’s latest laser diffraction particle size analyzer of the tenth generation, representing the fruit of several hundred improvements on the fundamental design. All of these lead to high performance as well as convenience of use. Some of these may be mentioned:

  • An optical bench which requires no maintenance and is sealed to be dust-free
  • Post-less cast aluminum mountings which ensure great stability and excellent alignment
  • A tilted measurement cell used to reduce the entry of stray light which could cause “noise” in the readings
  • Solid-state light sources as well as photodetectors built for a very long working life

Size measurement can only be reliable if the data on light scattering is good. The analyzer is therefore designed to acquire the best data from what is available, which is then sent to the algorithm that makes use of it to produce a particle size distribution.

This information has been sourced, reviewed and adapted from materials provided by HORIBA Scientific.

For more information on this source, please visit HORIBA Scientific.

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