Laser diffraction is a measurement technique for determining the particle size distribution of samples, either dispersed in a liquid or as a dry powder. The technique is based on light waves getting bent when encountering particles in a sample.[15] The measured equivalent spherical diameter is the diameter of a sphere having on the cross-sectional area the same diffraction pattern as the investigated particle.[16] The angle of diffraction depends on the particle size, hence the pattern of diffraction depends on the relative amounts of different particle sizes present in that sample. This diffraction pattern is then detected and analyzed by means of Mie and Fraunhofer diffraction models. The outcome of the measurement is a particle size distribution (PSD).[15] By means of laser diffraction not only the particle size distribution and the corresponding volume weighted D-values can be determined but also the percentage of particles in the main size classes used for the soil classification. Compared to other techniques laser diffraction is a fast and cost-effective method to measure particle size and quickly analyze soil samples. A big advantage is the built-in dispersion (e.g. dispersion by air pressure or ultrasound dispersion) unit of laser diffraction instruments. Therefore, dry samples can be measured without external sample preparation steps, which are required for sieving and sedimentation analysis. Moreover, since the sample can be dispersed properly, there is no need to combine two different measurement techniques to obtain the full range of the particle size distribution, including the silt and clay content. Both Fraunhofer and Mie laser diffraction theories assume that particles are spherically shaped. This results in a small measurement error, since small particles in soil samples, such as clay and silt in particular, are elongated and anisotropic.[17] The particle diameter in the laser diffraction method is determined in relation to their potential volume, which is calculated on the basis of an optical diffraction image at the edges of the particle cross-section. The volume of clay particles is the diameter of the plate’s cross-section, which is treated in the calculations as the diameter of the sphere. Therefore, their dimensions are usually overestimated in comparison to those measured via sedimentation analysis.[17] The error associated with the assumption of the sphericity of particles depends furthermore on the degree of anisotropy. The optical properties of anisotropic particles, such as refractive index and absorption index, change according to their orientation relative to the laser beam which is also variable. Therefore, at different particles orientations different cross-sections will be measured and different diffraction patterns produced. For clays with sizes close to the wavelength of a laser beam, Mie theory would be desirable. This requires precise knowledge of the complex refractive index of the particles’ material, including their absorption coefficient.[18] Because these parameters are often difficult to retrieve, especially the light absorption coefficients for various particles and soil grains, Fraunhofer theory, which only takes into account the light diffraction phenomena at the edge of the particles, is often recommended for natural soils