The median volume-weighted diameter, or D 50.The most widely used numerical laser diffraction results are: The main graphical representation of laser diffraction results is the volume-weighted particle size distribution, either represented as density distribution (which highlights the different modes) or as cumulative undersize distribution. Hence particle shape cannot be determined by the technique. that the particle size result is an equivalent spherical diameter. That the diffracted light is proportional to the particle’s volume also implies that results are assuming particle sphericity, i.e. This is in contrast to counting-based optical methods such as microspcopy or dynamic image analysis, which report the number of particles in the different size classes. This means that the particle size distribution represents the volume of particle material in the different size classes. Particle size distribution (density and cumulative undersize) obtained by laser diffraction Volume-weighted particle size distribution īecause the light energy recorded by the detector array is proportional to the volume of the particles, laser diffraction results are intrinsically volume-weighted. Sprays and aerosols generally require a specific setup. In practical terms, laser diffraction instruments can measure particles in liquid suspension, using a carrier solvent, or as dry powders, using compressed air or simply gravity to mobilize the particles. A computer can then be used to detect the object's particle sizes from the light energy produced and its layout, which the computer derives from the data collected on the particle frequencies and wavelengths. More detector elements extend sensitivity and size limits. Multiple light detectors are used to collect the diffracted light, which are placed at fixed angles relative to the laser beam. As the focal length increases, the area the laser can detect increases as well, displaying a proportional relationship. The sizes the laser can analyze depend on the lens' focal length, the distance from the lens to its point of focus. A lens is placed between the object being analyzed and the detector's focal point, causing only the surrounding laser diffraction to appear. Angling of the light energy produced by the laser is detected by having a beam of light go through a flow of dispersed particles and then onto a sensor. The light source affects the detection limits, with lasers of shorter wavelengths better suited for the detection of submicron particles. Alternatively, blue laser diodes or LEDs of shorter wavelength may be used. Laser diffraction analysis is typically accomplished via a red He-Ne laser or laser diode, a high-voltage power supply, and structural packaging. The lower theoretical detection limit of laser diffraction, using the Mie theory, is generally thought to lie around 10 nm. The model’s main limitation is that it requires precise knowledge of the complex refractive index (including the absorption coefficient) of the particle’s material. Thus, this theory is better suited than the Fraunhofer theory for particles that are not significantly larger than the wavelength of the light source, and to transparent particles. Hence, it is taking into account not only the diffraction at the particle’s contour, but also the refraction, reflection and absorption phenomena within the particle and at its surface. The Mie theory is based on measuring the scattering of electromagnetic waves on spherical particles. For samples of known optical properties, Fraunhofer theory should only be applied for particles of an expected diameter at least 10 times larger than the light source’s wavelength, and/or to opaque particles. Hence is it typically applied to samples of unknown optical properties, or to mixtures of different materials. Its main advantage is that it does not require any knowledge of the optical properties ( complex refractive index) of the particle’s material. Fraunhofer theory only takes into account the diffraction phenomena occurring at the contour of the particle. The Mie scattering model, or Mie theory, is used as alternative to the Fraunhofer theory since the 1990s.Ĭommercial laser diffraction analyzers leave to the user the choice of using either Fraunhofer or Mie theory for data analysis, hence the importance of understanding the strengths and limitations of both models. The angle of the laser beam and particle size have an inversely proportional relationship, where the laser beam angle increases as particle size decreases and vice versa.
Laser diffraction analysis is originally based on the Fraunhofer diffraction theory, stating that the intensity of light scattered by a particle is directly proportional to the particle size. Particles moving through the spread parallel laser beam
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