Laser Diffraction Methods

In various chemical

or physical and technical processes particles of the order of 1 μm up to a few mm in size appear, whose size distribution is decisive or important for the process. There are many technical examples like in food production, pharmacy, process engineering, fuel burning in turbines, motors etc., slurry burning, and cosmetics.

The laser diffraction particle sizer is capable to measure a particle size distribution from a collective of particles like a spray and to appropriately process, describe, and present it. The particle material may be solid or liquid in a gas or gas like air bubbles in a liquid. For the measurement it is only important, that the optical properties of the continuous and the dispersed phase are different.

The laser diffraction particle sizer offers the advantage of a fast and non invasive measurement in a wide range of particle sizes. Especially in case of sprays or suspensions this method has become a standard measurement tool.

The particle analysis with laser diffraction methods is described in the international standard ISO 13320-1:1999(E). This provides a guidance on the measurement of size distributions of particles in any two-phase system, for example powders, sprays, aerosols, suspensions, emulsions, and gas bubbles in liquids, through analysis of their angular light scattering patterns. This part of ISO 13320 is applicable to particle sizes ranging from about 0,1 μm to 3 mm.

The image below shows a schematic of a typical optical set up of a laser diffraction particle sizer. The monochromatic beam of the laser (1) – typically a low power He-Ne-laser – is expanded within the beam expander unit (2) and then made parallel using the lens (2). Between this lens and the Fourier lens (4) the particle collective traverses the expanded laser beam (3). The distance IF-I denotes the working range of the Fourier lens and its focal length. The Fourier lens makes sure, that the diffraction pattern of a particle of certain size independently of its position within the measurement volume will be always imaged at the same position on the ring detectors (8). The diffracted light of the particles creates a radial symmetrical interference pattern, which represents a measure of the particle size distribution.

In our technical department we have different optical measurement techniques in order to measure the properties relevant for the particle dynamics characterization.