Seven Facts About Laser Diffraction

14 Mar, 2011

Laser diffraction is one type of particle size analysis and is a technique known and respected across many applications for its ability to provide fast and reliable particle size data. In this type of particle size analysis, a cloud (or ‘ensemble’) of particles that is representative of the greater collection, travels through a broadened beam of laser light which scatters the light on to a specialised lens. Information about particle size and shape can then be deduced from the scattered pattern of light.

The laser diffraction technique assumes that the particles that pass through a laser beam will scatter light at an angle that directly corresponds with their size. It then follows that as the size of particles decreases, the scattering angle that is observed will increase. Essentially, light scattered at narrow angles with high intensity indicates large particles and particles scattered at wider angles and with low intensity suggest smaller particles.

A laser diffraction system requires the following:

  • A laser – this is necessary as a source of intense and coherent light that is of a defined wavelength
  • A sample presentation system – this ensures that the material being tested successfully travels through the laser beam as a stream of particles that have a known state of dispersion that can be reproduced
  • Detectors – specialised detectors are applied to measure the light pattern produced across a range of angles.

Facts About Laser Diffraction:

  1. Over the last twenty years, laser diffraction has, to a large extent, replaced traditional methods of particle size analysis, such as sieving and sedimentation.
  2. Laser diffraction has replaced microscopy (including optical and electron) for particles that are larger than tens of nanometres.
  3. Laser diffraction offers many advantages, including: efficient and fast operation and ease of use; the capacity to reproduce results; a vast size range that spans up to five orders of magnitude.
  4. Laser diffraction analysers do not only measure simple diffraction effects. Light sources that do not make use of lasers are sometimes used to enhance the primary laser source to reveal extra information about particle size and shape.
  5. Particles that relate to or are measured for particular industries commonly resemble spheres and corners and edges of these particles are generally smoothed as a result of the rolling and turning motion originating from sample circulation as particle size and shape is measured.
  6. While modern equipment can give quite precise results, it can never be assumed that the size of particles (produced through laser diffraction or any other type of particle sizing measurement) will not differ from their true dimension.
  7. The spherical modelling theory remains the only accepted and logical choice used in a commercial device intended to analyse a wide range of samples, regardless of the real particle shape and size.

Laser diffraction is a particle size analysis technique that generates results that are incredibly useful for processes used for research and in various industries. Providing details about particle size and shape, this technology can be used to provide fast and accurate results.