Laser diffraction

What is laser diffraction used for?

Laser diffraction is a popular way to determine the particle size distribution in liquids and dry powders for quality control and process optimization purposes. It is commonly used for particle size analysis during the production of pharmaceuticals, chemicals, food, and construction materials. Laser diffraction can also be used for environmental monitoring to characterize particulate matter in various media, including soil, sediment, and sludge samples.

How does laser diffraction work?

The operating principle of laser diffraction is based on the indirect calculation of particle sizes from the diffraction patterns of particles. During the analysis, a laser beam is passed through the sample, and the intensity of light scattered from the suspended particles is measured as a function of scattering angle, light wavelength, and light polarization. A mathematical model based on the Fraunhofer and Mie theories is then applied to determine the particle size distribution.

When laser light interacts with particles, it scatters in patterns. According to Fraunhofer diffraction theory, larger particles scatter light with high intensity at smaller angles, while smaller particles scatter light with lower intensity at larger angles. A laser diffraction instrument calculates the particle size distribution by measuring the intensity and angles of the scattered light. For a more accurate analysis, especially for small particles, Mie theory can be applied to account for refraction and reflection in addition to diffraction. To apply Mie theory, the refractive index of the particles and the medium must be known.

Sample selection and preparation 

Laser diffraction is suitable for wet and dry samples. Dry samples can be analyzed in an air stream, with the equipment’s recirculation system ensuring total homogenization. Alternatively, they can be dispersed in a suitable, non-solvating liquid medium. The droplet size of liquids sprayed from hand pumps, nozzles, and aerosols can also be measured.

Advantages and limitations

Laser diffraction has a wide dynamic range, as it can detect particles from ~0.01 µm to 2,000 µm. It is also a cost-efficient, fast, and reproducible method for routine PSD analysis, and can be conducted according to an international standard (ISO 13320).

The most notable limitation is the assumption that all signals originate from ideally spherical particles, making laser diffraction unsuitable for particle shape determination. Irregularly shaped particles, particularly ones with a high aspect ratio, also induce broad size distributions as both width and length are counted as individual particles.

More suitable methods for non-spherical particles include size and shape analysis with an image analyzer and scanning electron microscopy (SEM).

Need a laser diffraction analysis?

Measurlabs offers particle size analyses using laser diffraction and several other analytical methods, including dynamic light scattering and SEM. Our testing experts can also help you choose the most appropriate method for your sample, so do not hesitate to contact us using the form below.