Laser Diffraction Particle Sizing
The technique of laser diffraction is based around the principle that particles passing through a laser beam will scatter light at an angle that is directly related to their size. As the particle size decreases, the observed scattering angle increases logarithmically. The observed scattering intensity is also dependent on particle sizes and diminishes, to a good approximation, in relation to the particle’s cross-sectional area. Large particles therefore scatter light at narrow angles with high intensity, whereas small particles scatter at wider angles but with low intensity.
The primary measurement that has to be carried out within a laser diffraction system is the capture of the light scattering data from the particles under study. A typical system consists of:
- A laser,to provide a source of coherent, intense light of fixed wavelength
- A sample presentation system to ensure that the material under test passes through the laser beam as a homogeneous stream of particles in a known, reproducible state of dispersion
- A series of detectors which are used to measure the light pattern produced over a wide range of angles.
The size range accessible during the measurement is directly related to the angular range of the scattering measurement. Modern instruments make measurements from around 0.02 degrees through to 135 degrees. A logarithmic detector sequence, where the detectors are grouped closely together at small angles and more widely spaced at wide angles, yields the optimum sensitivity. Finally, the detector sequence is generally set up such that equal volumes of particles of different sizes produce a similar measured signal. This requires the size of the detectors to be increased as the measured scattering angle increases.