The Fundamentals of Laser Diffraction Particle Size Analysis

Laser diffraction is a particle sizing technique that determines a particle size distribution by measuring the angle and intensity of light scattered as a laser beam passes through a dispersed sample. Large particles scatter light at small angles; small particles scatter it at wider angles and lower intensity. It measures sizes from below 100 nm to a few millimetres, across dry powders, suspensions, emulsions, slurries and sprays, which makes it one of the most widely used particle characterisation methods in industry and research.

This guide explains why particle size matters, how laser diffraction works, how to develop a robust and rugged method, and how to interpret the results.

Why does particle size matter?

Nobody measures particle size for its own sake. The goal is to understand how size governs the behaviour, processing or performance of a material. Choosing which size parameter to control depends entirely on the end use.

Particle size directly influences:

• Dissolution rate and bioavailability of pharmaceutical products
• Stability of suspensions and emulsions
• Colour, opacity and gloss of pigments and coatings
• Powder flow and packing behaviour
• Strength and setting of cement
• Energy density and manufacturing consistency of battery electrodes
• Flavour and extraction rate of coffee, and mouthfeel of chocolate
• Flowability and packing in additive manufacturing
• Chemical reaction rates

The trade-offs are what make the choice difficult. If dissolution matters, the amount of fines matters, because smaller particles dissolve first and fastest. But finer particles are also stickier: van der Waals forces are stronger, so they are harder to disperse, particularly when dry. This is where powder rheology, measured on an instrument such as the FT4 Powder Rheometer, becomes useful. For injectable formulations the priority reverses: oversized particles are a serious quality concern, and the method must avoid breaking up agglomerates that need to be detected. The critical size parameter always follows the application.

How does laser diffraction work?

Laser diffraction does not measure particle size directly. It measures the light-scattering pattern produced by particles illuminated in a laser beam, then calculates the size distribution that best explains that pattern. When a laser passes through a suspension, the particles produce a pattern of bright and dark rings at different angles from the incident beam, and the instrument records the scattered intensity as a function of angle.

A useful analogy is driving at night. In rain, your headlights hit large droplets that scatter light forward, away from you. In fog, the same headlights hit much smaller droplets that scatter light in all directions, including back towards you, which is why fog is harder to see through. Large particles produce a bright, narrow forward-scattering pattern; small particles produce a diffuse pattern spread to wide angles at low intensity. A 50 µm particle scatters mostly forward, a 2 µm particle scatters at wider angles, and by roughly 0.05 µm the scattering is nearly uniform in all directions, which sets the practical lower limit of the technique.

In a modern system such as the Malvern Panalytical® Mastersizer 3000, a 633 nm red laser is folded through precision optics into the measurement cell, where the sample is suspended in liquid or air. Detectors on the focal plane capture narrow-angle (large particle) scattering, while side and back-scatter detectors capture the wide-angle scattering from small particles. A second 470 nm blue laser extends the measurement range below 100 nm, because the size at which scattering turns uniform depends on the wavelength of the illuminating light.

The measurement itself follows a set sequence. The instrument initialises and aligns the beam on the centre of the detector so all other angles are correct. It then records a background measurement from the optics and dispersant, which indicates how clean the system is and is subtracted from the sample signal. Sample is added by wet or dry dispersion, the scattering from both the red and blue sources is recorded, and data-quality checks confirm that enough sample has been added for reproducible results. Repeat measurements confirm sample stability before the distribution is reported.

How is the particle size distribution calculated?

The scattering pattern is converted into a size distribution using an optical model. Two models are used in practice: Mie theory and the Fraunhofer approximation.

Mie theory is the preferred model for most applications. It describes the full interaction of light with matter, accounting for light reflected and scattered from the particle surface as well as light refracted and transmitted through the particle, including the secondary scattering that arises when refracted and scattered light interact. It assumes spherical particles, so the result is a volume-equivalent spherical diameter, and it requires the refractive index of the dispersant and the refractive index and absorption of the particles. The Fraunhofer approximation is simpler: it treats particles as opaque discs and models only light diffracted at the surface, ignoring light refracted through the particle. It is reasonable for large particles at small scattering angles with a high refractive-index contrast, but it breaks down for fine particles. For particles smaller than about 50 µm, Mie theory is the recommended general solution, and international standards favour it wherever the optical properties are known [1], [4], [5].

Both models work in the forward direction: given a size distribution, they calculate the expected scattering. The instrument needs the reverse. It takes the measured data, makes an initial guess at the particle size distribution, uses the optical model to calculate the scattering that guess would produce, compares it with the real measurement, and adjusts the distribution iteratively until the difference can no longer be reduced. That final distribution is the result. The mismatch between the modelled and measured data is reported as a residual: the lower the residual, the better the fit. If the wrong optical properties were chosen, the data can be recalculated with the correct model without repeating the measurement.

What do Dv10, Dv50 and Dv90 mean?

Laser diffraction produces a volume-weighted particle size distribution, which makes it particularly sensitive to coarse or oversized particles. Results are usually reported as percentiles of that volume distribution.

• Dv10 is the size below which 10% of the sample volume lies. It tracks the fine end, which drives dissolution, reactivity and suspension stability.
• Dv50 is the median: half the sample volume is finer than this size. It is the most commonly quoted single value.
• Dv90 is the size below which 90% of the volume lies. It tracks the coarse end, contaminants and oversized material.

Two mean diameters are also reported: the surface-area-weighted mean D[3,2], useful for describing fines, and the volume-weighted mean D[4,3], useful for describing the coarse end. Which parameter to use depends on which part of the distribution controls your product’s performance [1].

Why do different particle sizing techniques give different results?

Because different techniques measure different physical properties of the particle. Laser diffraction reports a volume-weighted, volume-equivalent spherical diameter, so it is weighted towards coarser material. Microscopy and image analysis report number-based distributions, so they are more sensitive to fines and to particle shape. Sieving depends on whether a particle can pass through a mesh aperture, and non-spherical particles align to pass through their second-largest dimension, so sieve sizes usually read smaller. For irregular, non-spherical powders the disagreement between laser diffraction and image analysis can be significant, because the volume-equivalent sphere is a poor descriptor of an elongated or plate-like particle [6], [8]. No technique is inherently correct or incorrect. To compare results you must know how each was measured and what it is sensitive to.

How do you develop a robust and rugged method?

Reliable data depends far more on the method than on the instrument. Two concepts define a good method. Robustness is the ability of a method to remain unaffected by small, deliberate variations in its parameters. Ruggedness is the reproducibility of the method across different analysts, instruments, laboratories and time. A method that is both transfers cleanly between sites and holds up in routine quality control.

Once you know why you are measuring, you can define which parameters to control and build a target method around them. Several failure modes recur when they are not controlled.

• Poor choice of optical properties makes the method prone to measuring noise peaks.
• An unstable background produces negative data and can hide genuinely large particles.
• Excessive air pressure in dry dispersion mills friable particles.
• Too high an obscuration causes multiple scattering and generates false fines.

Method development, validation, sampling, dispersion and data quality for laser diffraction are covered by international standards, and following them is the fastest route to a defensible method [1], [2], [3].

Why are sampling and dispersion the biggest challenge?

Because the instrument is rarely the cause when results go wrong; the sample almost always is. Two things determine measurement quality: getting a representative sample, and dispersing it correctly.

A few milligrams presented to the analyser must represent the entire batch, whether that is a ten-tonne truckload waiting outside the lab or a drum of fine powder. As particles get smaller, van der Waals forces make them sticky and prone to segregation, so sampling error grows. Sample-division devices such as spinning rifflers help obtain a representative sub-sample before analysis. Representative sampling is frequently the largest and most overlooked source of error in the whole measurement chain [7]. Agglomerated samples must be dispersed to reveal the primary particles. For wet dispersion this means three things:

1. Choose a dispersant that wets the particles, does not dissolve them, and where needed uses a surfactant. Low concentrations of surfactant (about 0.1 to 4% by weight) reduce the risk of foaming and destabilisation. Common non-ionic surfactants such as Igepal CA-630, Tween 20, Span 20 and Nonidet P-40 adsorb onto particle surfaces, while sodium dodecyl sulfate (SDS) and sodium hexametaphosphate (SHMP) increase repulsion between particles to prevent re-agglomeration in aqueous systems.
2. Add energy through vigorous stirring or ultrasound to break up agglomerates, with additives to prevent them re-forming.
3. Confirm the result with a referee technique such as automated image analysis on a Morphologi® 4.

For dry powders the two critical parameters are air pressure and feed rate. Two venturis are available for the Mastersizer: a standard venturi, and a high-energy venturi with an impaction plate for cohesive powders. The high-energy option is unsuitable for friable material, which it can mill, so air pressure must be controlled and dry results compared against wet dispersion to validate them. Feed rate must be consistent, and dry measurements are best kept lean at around 3% obscuration to avoid multiple scattering.

On the Mastersizer 3000+ series, SOP Architect guides wet-method development through stirrer-speed, ultrasound and obscuration titrations, using the rate of change in the measured size to select appropriate settings and recording them in an SOP file. The aim throughout is to separate particles without damaging them.

How do you validate the method and control quality?

A method must do more than repeat. Accuracy is how close a result is to a target value and, for laser diffraction, can only be quoted against spherical reference materials such as latex spheres or glass beads. Precision is repeatability, and depends on the method, the dispersion state and the sampling. Assess accuracy with a certified reference material, then confirm precision, robustness and reproducibility.

Beyond validation, the real test is whether the method can track and control the process. Can it tell good product from bad across a mill run, and give a sensible answer at every point? A needle-like material measured dry at 4 bar may be perfectly rugged, yet if that pressure mills it to fragments the result no longer reflects the product. Standard operating procedures enforce the same steps and conditions between operators and sites, which is what makes data quality defensible over time.

Where is laser diffraction used?

• Pharmaceuticals. Particle size controls dissolution rate and, for poorly soluble drugs, bioavailability and formulation consistency. Micronisation increases dissolution rate, and further size reduction can raise the effective solubility of poorly soluble actives [10], [12].
• Battery materials. The electrode particle size distribution affects energy density, rate capability and manufacturing consistency in lithium-ion cells [11].
• Mining and minerals. On-line and at-line laser diffraction supports milling efficiency, flotation performance and product specification in mineral slurries [9].
• Food and beverage. Size distribution controls texture, suspension stability and mouthfeel. Higher-quality chocolate is milled to a finer, narrower distribution for a smoother mouthfeel, and finer coffee grounds extract faster and can taste more bitter.
• Paints and coatings. Fine primary pigments scatter light efficiently to give opacity and even coverage, while larger extender pigments influence gloss and durability. As paint dries, gloss falls further for coarser particles as the surface roughens.
• Cement and construction. Cement is typically controlled to roughly 3 to 30 µm. Too many fines below about 2 µm can cause cracking; too many coarse particles reduce strength.
• Additive manufacturing. Powder flowability and packing depend on size distribution, and laser diffraction is used alongside image analysis to specify printable powders [8].

Which international standards apply?

Laser diffraction is supported by recognised standards that govern method development, instrument qualification, sample preparation and data quality:

• ISO 13320:2020, Particle size analysis – Laser diffraction methods (2nd edition). The core methodology standard; Annex A covers the optical theory and Annex G covers robustness and ruggedness [1].
• ASTM E3340-22, Standard Guide for Development of Laser Diffraction Particle Size Analysis Methods for Powder Materials [2].
• ISO/TS 5973:2024, Laser diffraction measurements – Good practice (1st edition) [3].

Frequently asked questions

What particle size range can laser diffraction measure?

Modern laser diffraction systems measure from below 100 nm to several millimetres, one of the broadest ranges of any single particle sizing technique. The Mastersizer 3000+ Ultra has a particle size range of 10 nm to 3.5 mm.

Is laser diffraction suitable for nanoparticles?

It measures larger nanoparticles, roughly above 100 nm. Below that, dynamic light scattering (DLS) is the complementary technique of choice, because it sizes particles from their Brownian motion rather than their scattering angle. The Zetasizer Advanced range measures particles between 0.3 nm to 15 µm (diameter).

How does laser diffraction compare with image analysis?

Laser diffraction is fast and reports a volume-weighted, volume-equivalent spherical diameter across a wide range. Image analysis is slower but measures size and shape directly, which matters for non-spherical particles. The two can disagree for irregular powders, and are often used together.

Does particle shape affect laser diffraction results?

Yes. Laser diffraction reports a volume-equivalent spherical diameter, so highly irregular particles can give different results from shape-sensitive techniques. Automated imaging on a Morphologi® 4 resolves size, shape, transparency and count for dry powders, fibres and suspensions.

How accurate is laser diffraction?

When the method is developed and validated correctly, laser diffraction gives highly repeatable and reproducible results that comply with ISO 13320. Absolute accuracy is quoted against spherical reference materials; day-to-day confidence comes from method robustness and certified reference materials.

Partner with ATA Scientific for particle size analysis

Choosing the right analyser is only part of getting reliable results. Developing a robust method, selecting the correct dispersion approach and interpreting a volume-weighted distribution all take experience. ATA Scientific has supported researchers and manufacturers across Australia and New Zealand for 35 years, since 1991, pairing gold-standard characterisation instruments with local application support, training and lifetime system support.

Whether you are measuring pharmaceutical powders, battery materials, minerals, pigments or advanced materials, our specialists can help you select the right particle sizing technology and build a reliable method for your application. Explore the Mastersizer 3000, or Make a Product Enquiry Today to discuss your application with a particle characterisation specialist.