Laser diffraction instruments for particle size analysis are based on the fact that when particles pass though a laser beam, they will scatter light at a specific angle depending primarily on their size. These instruments do not measure particle size directly; they rather measure the scattering angle and intensity, after which they calculate particle size distribution by using an optical model. This can be done since observed scattering angle is logarithmically dependent of particle size. Additionally, scattering intensity is inversely proportional to particle volume.
As an approximation, small particles will scatter light with lower intensity but at wider angles, while large particles are characterized by a narrow angle scattering at higher intensities.
Typical instruments for laser diffraction consists of a laser, detectors and a sample delivery system. In order to obtain accurate and reproducible data, testing material passes under a laser beam in a uniform and concise stream. Here, particles are scattered by intense laser light of fixed wavelength, and their scattering angles are measured by a series of detectors. The angular range of detectors determines the dynamic range of the instrument, while different beam wavelengths used can provide better sensitivity to particles of different sizes. Most modern systems provide measurements in a range from 0.02 micron to a few millimetres without changing their optical configuration.
Laser diffraction technique of particle size analysis is a non-destructive method which gives highly reproducible results for both dry and wet samples. It generates volume-based particle size distributions, which are normally equivalent to mass distributions. One of many reasons why this method is most often used is that it requires no external calibration and gives results within a relatively short time.