$laser diffraction particle sizer$

Furthermore, laser diffraction is not able to determine the exact size of very fine particles. For example, laser diffraction is not designed for analyzing ultra-low or ultra-high concentrated samples because of the interferences caused by multiple scattering. Thus Mie theory is well-suited for the analysis of small particles.Īs for most established methods for particle characterization, laser diffraction also has clear limits and many disadvantages. Scattered or refracted light can be measured very well, while reflected or absorbed light might influence the measurement results negatively, so that their influence must be considered for the measurement and calculation. Using the Mie theory for laser diffraction analysis instead, diffraction phenomena like the refractive index or the absorption index are also taken into account. For the use of the Fraunhofer approximation no knowledge of the refractive index is needed, but transparent or very small (smaller than 50 µm) particles negatively influence the measurement results. Two theories are used to analyze these diffraction phenomena: The Fraunhofer theory (also called Fraunhofer diffraction) describes the intensity distribution of the diffraction angles and is well-suited to measuring large or opaque particles.

$laser diffraction particle sizer$

To do this job, an algorithm compares the measured values with the expected theoretical values of different size categories to estimate the relation between the size categories in the entire sample volume. When we analyze particles with laser diffraction, the individual scattering patterns overlap and are presented in one total pattern, so they need to be distinguished. The diffraction angles are determined by the wavelength of the light and by the size of the particle: big particles show a small diffraction angle and a strong intensity, while small particles have a big diffraction angle and a weak intensity. These phenomena can be monitored by laser diffraction, as light spreads in ring-shaped waves when it reaches a spherical particle. Whenever laser light encounters a particle or another barrier diffraction phenomena occur. In the last step, detectors measure the scattered light on a wide angle range through a Fourier lens (a system of lenses) and determine the particle size distribution provided by the measurement data of the angles and the intensity of the scattered light. When the beams collide with the particles they are reflected, scattered, refracted or absorbed. By using lenses or optical fibers the laser beams are additionally broadened to maximize the measurement area. Each laser beam measures different sized particles. The velocity of this transport is not an issue.

$laser diffraction particle sizer$

The laser beams are directed through a dispersed sample so that the particle stream is transported diagonally through the first laser beam. These light sources have different wavelengths. The starting point of a laser diffraction analysis is typically formed by two light sources (although someties there is just one or even three). In order to achieve this, a laser diffraction instrument consist of various components. For smaller particle sizes the laser diffraction method has some problems because of the low intensity of the scattered light.īasically you can divide the workflow of a laser diffraction analyzer into two steps: Firstly, it measures the angles and the intensity of the light scattered by the particles in the sample, afterwards the gathered data is converted into particle size distributions. The particle sizing range of the laser diffraction method is impressive, as it can measure particles between 10 nm and 3 mm, although ideal measuring results for particle size starts at around 100 nm. It is even possible to use laser diffraction for the analysis of particles in an air stream. The samples may be dispersed in different media.

$laser diffraction particle sizer$

Thus a laser diffraction analyzer does not measure the particle size but the angle and the intensity of the scattered light in the measured sample. It makes use of the principle that light, when hitting a particle, scatters more intensively and in smaller angles the bigger the particle is. Over the past few years laser diffraction has become established as one of the leading methods in laboratory environments for analyzing suspensions, emulsions, granulates and fine powders. There are a number of measurement technologies for the determination of particle size distributions.

$laser diffraction particle sizer$

Laser diffraction particle sizer