Laser Diffractometry (Laser Diffraction)

Laser Diffractometry (Laser Diffraction)

Laser Diffractometry is an optical technique that uses diffraction patterns for particle size analysis of suspensions, colloidal dispersions, dry powders and aerosols. The scattering of an incident laser beam, by particles dispersed in a suitable medium, yields characteristic interference patterns that depend on the size of the particles involved. The intensity of scattered light varies in direct proportion to the particle size. The scattering angle, however, varies inversely with particle size. A Laser Diffractometry measurement is performed by passing a laser beam of specified wavelength through a dispersed particulate sample. The angular variation in intensity of the scattered light is measured, using detector arrays placed at various angles. The intensity and angular patterns are then analyzed, to calculate the size of the particles in accordance with the theoretical frameworks of Mie or Fraunhofer scattering. Particles that are much larger than the incident wavelength display Fraunhofer scattering, while Mie scattering is observed for particle sizes comparable with laser beam wavelength.

A Laser Diffractometer employs a laser light source, an arrangement of lenses to spread the incident beam and collect the scattered light followed by a detector system to capture angular light intensity information. The raw data from the detector is analyzed by a computer, to obtain particle size distributions. Lasers of different wavelengths are used to cover various particle size ranges. The lower size limit of Laser Diffractometry is about 10 nm. The higher size limit is instrument dependent and reported to exceed 3500 nm.

Laser Diffractometry can be applied to wet or dry dispersions. Dry dispersion methods can be used for dry powders and non-agglomerating materials that can be dispersed in air. Wet dispersion of samples in water or non-aqueous solvents can be employed for sticky materials or those that cannot be air dispersed.

Common Uses of Laser Diffractometry

• Soil particle size analysis
• Particle size analysis of powders, suspensions and colloidal dispersions
• Aerosol particle size analysis

 Advantages of Laser Diffractometry

• Applicable over a wide range of particle sizes
• Wet and dry dispersions can be measured

Limitations of Laser Diffractometry

• Optimally dispersed sample is a prerequisite for the reliable determination of particle size
• For smaller particles, the effects of reflection, refraction and absorption of light need to be considered in the calculation as per Mie theory.
• The lower measurable particle size limit is when Mie scattering transitions to Rayleigh scattering.

Industrial Applications of Laser Diffractometry

• Soil particle size analysis
• Glass production raw material analysis
• Paints and inks quality control
• 3D printing
• Biomedical research