Laser Diffraction

Particle size measurement using static light scattering (Particle size
determination by laser diffraction)
The particle size distribution as a parameter to specify a powder or dispersion
plays a central role in many applications. Examples are construction material
(sands, cements), pharmaceuticals, lime stones, ceramics, colored pigments,
fertilizers, emulsions and many more. The range of applications is increasing
permanently and hence the requirements on the measurement methods regarding
size range, measuring time and reproducibility are increasing. Particularly the
precise and reproducible detection of particles with sizes close to the
measuring range limits as well as the simultaneous determination of particle
size using laser of very small particles (nanometer range) as well as large
particles (lower millimeter range) for the characterization of polymodally or
very broadly distributed samples provides a challenge. State-of-the-art laser
diffraction analyzer such as the Bettersizer S3 Plus laser grain size analyzer
solves these tasks by an innovative design of the optical bench for the
detection of backscattered light of very small particles and by detecting large
particles by an integrated high-speed CCD camera or the combination of static
light scattering and automated imaging.
Measuring method
In static light scattering laser light (monochromatic, coherent light)
interacts with the particles, which have to be characterized in terms of
particle size. In dependence of the particles' size, the light waves are
scattered by the particles in a characteristic manner: the larger the particles
are, the greater is the scattering in forward direction. With particles smaller
about 100 nm, the scattering intensity is nearly identical in all directions.

laser-diffraction-at-particles- , with-different-size

Laser diffraction at particles with different size

The scattering intensity is determined by stationary detectors depending on the
angle (light scattering intensity distribution). State-of-the-art laser
diffraction systems such as the Bettersizer S3 Plus laser scattering particle
size analyzer guarantee the determination of scattering intensities in a
continuous angular range of 0.02 – 165°, i. e. in forward, side and backward
direction. This is achieved by means of a so-called double lens design and
oblique incidence optical system (DLOIOS technology): Fourier lenses
(collective lens) are positioned between the laser and particles as well as
between particles and detectors. The particles will interact with the light
within a parallel laser beam. This offers the advantage that the scattered
light can also be detected at very large angles (in backward scattering
direction) and thus even very small particles can be measured precisely. Thanks
to DLOIOS technology, the problems of conventional measurement setups can also
be avoided. Therefore, neither the suitable lenses for the corresponding
particle size measurement range have to be selected prior to the measurement
(in comparison to the Fourier optics), nor do measurement inaccuracies result
from different particle to detector distances, if not all particles lie in one
plane (in comparison to the inverse Fourier optics).

schematic-drawing-of-the-innovativ , e-dloios-technique-of-bettersizer-s3

Schema , tic drawing of the innovative DLOIOS-technique of Bettersizer S3 PLUS and
CCD-camera system (x0.5 and x10)

To calculate the particle size distribution from the measured scattering
spectra, the theory of either FRAUNHOFER or MIE is applied. The FRAUNHOFER
theory is based on the hypothesis of opaque and spherical particles: the
scattered pattern corresponds to a thin opaque two-dimensional plate –
diffraction only occurs at the edges. Therefore no additional optical input
constants of the material are necessary for this calculation.

In contrast the MIE theory uses the hypothesis of virtually translucent and
spherical particles, meaning that the light permeates the matter and is
scattered elastically at the atoms of the particle. The knowledge of the
complex refractive index of the particles and the liquid as well is necessary.
This theory is applicable for particles of all sizes.

The following figure shows an example of a volume-related particle size
distribution of a calcium carbonate powder – measured with a Bettersizer S3
Plus.

bettersizer-s3-plus

Laser diffraction measurement example

The cumulative throughput curve Q3 (blue) and the resulting histogram (q3,
black bar) can be seen.

Literature and norms
ISO 13320 – Particle size analysis – laser diffraction method

Bettersize has become a significant player in the particle sizing business
since 1995. We provide
particle size instrument, particle size measurement units, particle size
analyzer for sale and etc. Want to know more? Please contact us.

Minimum Order: 1 sets

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