laser diffraction

Laser diffraction measures particle size from how particles bend and scatter a beam of light. Shine a laser through a cloud of dispersed powder and each particle casts a pattern of scattered light: large particles scatter light at small angles into a tight ring, while small particles fling it out to wide angles. By recording the whole pattern, the instrument works backward to the mix of sizes that produced it.

The light pattern is interpreted with an optical model, classically Fraunhofer for large particles and the more complete Mie theory for fine ones, which requires knowing the particle's refractive index. The result is a volume-weighted distribution reported as d10, d50 and d90, covering a broad range from roughly the sub-micron up to a few millimetres.

Its appeal is speed, a wide measuring range, and good reproducibility on samples dispersed either in liquid (wet) or in an air stream (dry). A single measurement takes seconds, which makes it a workhorse for routine quality control of drug substances and excipients.

The caveats are important. Laser diffraction assumes particles are spheres, so for needles or plates the reported diameter is an equivalent value, not a true length. Results depend on the chosen optical model and on getting the particles fully dispersed; agglomerates that fail to break up are read as single large particles.