Diffraction

Laser Diffraction (LDS) also called forward or low angle light scattering (LALLS) is now the first choice technique for sizing particles in liquid suspension or as aerosols in the micron size range.

Early instruments used Fraunhofer model of optical scattering to calculate size distributions because its simplicity could be modelled by the processors,  memory resources and software available in earlier computers. Basically we lived with the restrictions like size >> wavelength because we had to. The Fraunhofer approximation was usually undertaken with selected range lenses matched to a narrow set of forward detector angles.

The adoption of the comprehensive Mie theory description of optical scattering and using single lens optics has  much improved the quality of results and the range of materials that can be sized. This is clear in ISO 13320-2009 where Mie approach gained over Fraunhofer when very wide angles of scattering allowed sub micron particles to be characterised.

5 micron laser diffraction pattern   optics on particle      MS3000 optics redmie vs fnhf plot

products   technology
 Mastersizer Fraunhofer versus Mie discussion
Particle Sizing
ISO 13320
Laser Diffraction web page