It assumes that the particles being measured are opaque and scatter light at narrow angles. The Fraunhofer approximation was used in early diffraction instruments. Traditionally, two different models are used: the Fraunhofer approximation and Mie Theory. Particle Size Calculations In laser diffraction, particle size distributions are calculated by comparing a sample's scattering pattern with an appropriate optical model. The wavelength of light used for the measurements is also important, with smaller wavelengths (e.g., blue light sources) providing improved sensitivity to sub-micron particles. The dynamic range of the measurement is directly related to the angular range of the scattering measurement, with modern instruments making measurements from around 0.02 degrees through to beyond 140 degrees (Figure 3).
#LASER DIFFRACTION EXPERIMENT DISCUSSION SERIES#
A typical system consists of a laser, to provide a source of coherent, intense light of fixed wavelength a series of detectors to measure the light pattern produced over a wide range of angles and some kind of sample presentation system to ensure that material under test passes through the laser beam as a homogeneous stream of particles in a known, reproducible state of dispersion. It is this behavior that instruments based on the technique of laser diffraction exploit in order to determine particle size. Large particles therefore scatter light at narrow angles with high intensity, whereas small particles scatter at wider angles but with low intensity (Figure 2). Scattering intensity is also dependent on particle size, diminishing with particle volume. As particle size decreases, the observed scattering angle increases logarithmically. It relies on the fact that particles passing through a laser beam will scatter light at an angle that is directly related to their size. Laser Diffraction Laser diffraction has become one of the most widely used techniques for particle size analysis in the coatings industry, with applications from product development through to production and quality control. In addition, the technique needs to be reliable, simple to use and capable of generating reproducible data, if acceptance and usefulness are to be maximized. It is clear then that any instrument or technique selected for particle size analysis needs to generate data in a form that is relevant to the process. A catalyst engineer, for example, may be particularly interested in surface area, as this influences reaction rate, and might therefore prefer a technique that generates surface-area-based data. Each representation is equally valid, although they are not equally relevant to any given process. This would lead to the diameter of the sphere that has the same volume as the measured particle being reported as the particle size. In each case, the reported diameter will be dependent on the physical property measured using the chosen technique.įor example, a technique could measure the mass or volume of the particle.
Here the spherical equivalent diameters, reported using different techniques for the same particle, are shown. Laser diffraction can be used for the non-destructive analysis of wet or dry samples, with particles in the size range 0.02 to 2,000 microns, and has inherent advantages that make it preferable to other options for many different materials.Īn example of the application of the equivalent sphere approximation is shown in Figure 1. Many different techniques have been devised for determining particle size distribution, but for a wide range of industries laser diffraction has become the preferred choice. It is therefore clear that manufacturers intent on optimizing product quality need an effective and reliable method of particle size analysis. Optical properties, such as opacity, tinting strength, undertone, film appearance and weather resistance, as well as bulk properties, including dispersion and flocculation characteristics, and viscosity, are all, to some extent, a function of particle size. Particle size is a variable of significant interest to coatings manufacturers, as it has a direct impact on the quality of the finished product.
Here he discusses laser diffraction, one of the most widely used technologies for particle size measurement, its applications and benefits. PCI, March 2005 ), Paul Kippax looked at the significance of particle size and its relationship to the performance of both raw materials and final products.
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