What Is Laser Diffraction? Principles, Methods, and Particle Size Applications
What Is Laser Diffraction?
Laser diffraction — also called static light scattering or low-angle laser light scattering (LALLS) — is the most widely used particle size analysis technique for characterising the size distribution of particles, droplets, and bubbles in the 0.02 µm to 3.5 mm size range. It measures the angular distribution of light scattered by particles as they pass through a laser beam — relating the scattering pattern to particle size using optical theory — and provides fast, reproducible, and statistically representative size distributions from milligrams of sample in minutes.
Physical Principle of Laser Diffraction
When a particle intercepts a laser beam, it scatters (diffracts) light at angles that depend on the particle size relative to the laser wavelength:
- Large particles scatter light predominantly at small forward angles (close to the beam axis)
- Small particles scatter light at large angles, including backscatter
The complete angular distribution of scattered light intensity (the scattering pattern) is detected by an array of photodetectors positioned at multiple angles from the forward axis. Mathematical inversion algorithms (Fraunhofer diffraction for large particles; Mie scattering theory for particles smaller than ~25× the laser wavelength) convert the measured scattering pattern into the particle size distribution (volume-weighted).
Two Optical Models Used in Laser Diffraction
Fraunhofer Diffraction Approximation
Assumes particles are large relative to laser wavelength, opaque, and interact only through diffraction at their edges. Valid for particles >25 µm and appropriate for irregularly shaped, opaque materials like minerals and powders. Computationally simpler but inaccurate for sub-micron particles.
Mie Scattering Theory
The full electromagnetic scattering solution requires knowledge of the complex refractive index (real and imaginary components) of the particles and dispersant. Valid from nanometres to millimetres. Essential for accurate measurement of transparent, translucent, or sub-micron particles (polymer latexes, pharmaceutical active ingredients, emulsion droplets).
Wet and Dry Dispersion Modes
Wet Dispersion
Particles are suspended in an appropriate liquid dispersant (water + surfactant, an organic solvent, or mineral oil) and recirculated through the optical measurement zone by a pump. Wet dispersion achieves excellent individual particle dispersion — critical for agglomerated powders. Applicable to: slurries, emulsions, suspensions, and powders that can be dispersed in a liquid.
Dry Dispersion
A stream of particles is dispersed by high-pressure air through a venturi and passed through the laser beam. Used for dry powders, pigments, cement, minerals, and food powders — when wet dispersion would alter particle size through dissolution or aggregation. Dynamic image analysis can be combined with dry dispersion for shape information.
Industrial Applications of Laser Diffraction
Pharmaceutical and Biomedical
The particle size of active pharmaceutical ingredients (APIs), excipients, and drug-delivery particles (liposomes, nanoparticles, microspheres) governs the dissolution rate, bioavailability, and inhalation deposition efficiency. Laser diffraction per USP <429> and ISO 13320 is the pharmacopoeial method for pharmaceutical particle size determination. Inhalation drug products (metered-dose inhalers, dry powder inhalers) require aerodynamic size distribution measurements by cascade impaction in addition to laser diffraction.
Ceramics and Advanced Materials
Ceramic powder particle size distribution governs packing density, sintering temperature, and final ceramic density. Laser diffraction verifies that powder gradations meet specifications for press-and-sinter, injection moulding, and thermal spray applications.
Coatings and Pigments
Pigment particle size affects colour strength, hiding power, and coating gloss. Laser diffraction characterises dispersion quality in paint and ink production by monitoring grinding and dispersion efficiency throughout the process.
Mining, Minerals, and Aggregates
Particle size distribution of mineral ores, cement clinker, and aggregates governs processing efficiency (comminution, flotation, classification). Laser diffraction is the rapid, automated replacement for traditional sieve analysis for sub-1 mm particles.
Emulsions and Suspensions
Droplet size in emulsions (polymer dispersions, coatings, agricultural spray applications) governs stability, viscosity, and application performance. Laser diffraction provides the sub-micron size resolution needed to characterise fine emulsion droplets not measurable by sieve or optical methods.
Conclusion
Laser diffraction is a fast, reliable, and widely adopted technique for determining particle size distribution across a broad range of materials. Analyzing how particles scatter light at different angles provides accurate, reproducible, and statistically representative data essential for controlling product performance and process efficiency. Its versatility — from sub-micron particles to millimetre-scale materials — makes it a cornerstone analytical method in industries such as pharmaceuticals, coatings, ceramics, and mining, where particle size directly influences functionality and quality.
Why Choose Infinita Lab for Laser Diffraction Analysis?
Infinita Lab provides laser diffraction particle size analysis (wet and dry) per ISO 13320 and USP <429> through our nationwide accredited particle characterisation laboratory network, covering particle sizes from 0.02 µm to 3.5mm using Mie and Fraunhofer optical models.
Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you.
Frequently Asked Questions (FAQs)
What is D10, D50, and D90 in laser diffraction results? D10, D50, and D90 are volume-weighted percentile diameters — the particle diameters at which 10%, 50%, and 90% of the cumulative volume distribution falls below that value. D50 (the median diameter) is the most commonly reported single-value size parameter. D90−D10 span characterises the distribution breadth; (D90−D10)/D50 is the relative span (dimensionless width indicator).
Why is refractive index important in laser diffraction using Mie theory? Mie theory requires the complex refractive index (n + ik, where n is real part and k is the imaginary absorption component) of both the particle and dispersant to accurately predict the scattering pattern for small particles. Incorrect refractive index input can produce significant sizing errors — particularly for sub-10 µm particles where Mie effects are dominant. Refractive index values are typically obtained from literature, Becke line test, or ellipsometry.
What is the difference between laser diffraction and dynamic light scattering (DLS)? Laser diffraction measures the angular distribution of scattered light at a single time point — providing a volume-weighted size distribution from ~20 nm to 3.5 mm. DLS measures temporal fluctuations in scattered intensity from Brownian motion — providing a z-average hydrodynamic diameter for particles typically <5 µm. Laser diffraction covers a wider size range and provides full volume distributions; DLS achieves better sensitivity for sub-500 nm particles and colloidal dispersions.
How does agglomeration affect laser diffraction particle size results? Agglomerated particles scatter light as larger entities — shifting the measured size distribution to larger values than the primary particle size. Adequate dispersion energy (ultrasound, high-shear stirring, surfactant addition) is required to break up soft agglomerates before measurement. The dispersion state must be validated by measuring size as a function of sonication energy — reaching a stable endpoint before reporting results.
What is the obscuration range and why is it critical in laser diffraction? Obscuration is the fraction of laser beam intensity blocked by the particle sample — proportional to particle concentration. ASTM and ISO standards specify acceptable obscuration ranges (typically 2–20% for most instruments) that ensure: (1) sufficient particles for statistical precision and (2) no multiple scattering (where light scattered by one particle is re-scattered by another, distorting the scattering pattern). Outside this range, obscuration must be adjusted by diluting (too high) or concentrating (too low) the sample.
