Laser diffraction analysis
Laser diffraction analysis, also known as laser diffraction spectroscopy, is a technology that utilizes diffraction patterns of a laser beam passed through any object ranging from nanometers to millimeters in size to quickly measure geometrical dimensions of a particle. This process does not depend on volumetric flow rate, the amount of particles that passes through a surface over time.
Laser diffraction analysis is based on the Fraunhofer diffraction theory, stating that the intensity of light scattered by a particle is directly proportional to the particle size. 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 accomplished via a red He-Ne laser, a commonly used gas laser for physics experiments that is made up of a laser tube, a high-voltage power supply, and structural packaging. Alternatively, blue laser diodes or LEDs of shorter wavelength may be used. Angling of the light energy produced by the laser is detected by having a beam of light go through a suspension and then onto a sensor. A lens is placed between the object being analyzed and the detector's focal point, causing only the surrounding laser diffraction to appear. The sizes the laser can analyze depend on the lens' focal length, the distance from the lens to its point of focus. As the focal length increases, the area the laser can detect increases as well, displaying a proportional relationship. 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.
Laser diffraction analysis has been used to measure particle-size objects in situations such as:
- observing distribution of sediments such as clay and mud, with an emphasis on silt and the sizes of bigger samples of clay.
- determining in situ measurements of particles in estuaries. Particles in estuaries are important as they allow for natural or pollutant chemical species to move around with ease. The size, density, and stability of particles in estuaries are important for their transportation. Laser diffraction analysis is used here to compare particle size distributions to support this claim as well as find cycles of change in estuaries that occur because of different particles.
- soil and its stability when wet. The stability of soil aggregation (clumps held together by moist clay) and clay dispersion (clay separating in moist soil), the two different states of soil in the Cerrado savanna region, were compared with laser diffraction analysis to determine if plowing had an effect on the two. Measurements were made before plowing and after plowing for different intervals of time. Clay dispersion turned out to not be affected by plowing while soil aggregation did.
Since laser diffraction analysis is not the sole way of measuring particles it has been compared to the sieve-pipette method, which is a traditional technique for grain size analysis. When compared, results showed that laser diffraction analysis made fast calculations that were easy to recreate after a one-time analysis, did not need large sample sizes, and produced large amounts of data. Results can easily be manipulated because the data is on a digital surface. Both the sieve-pipette method and laser diffraction analysis are able to analyze minuscule objects, but laser diffraction analysis resulted in having better precision than its counterpart method of particle measurement.
Laser diffraction analysis has been questioned in validity in the following areas:
- assumptions including particles having random configurations and volume values. In some dispersion units, particles have been shown to align themselves together rather than have a turbulent flow, causing them to lead themselves in an orderly direction.
- algorithms used in laser diffraction analysis are not thoroughly validated. Different algorithms are used at times to have collected data match assumptions made by users as an attempt to avoid data that looks incorrect.
- measurement inaccuracies due to sharp edges on objects. Laser diffraction analysis has the chance of detecting imaginary particles at sharp edges because of the large angles the lasers make upon them.
- when compared to the data collecting of optical imaging, another particle-sizing technique, correlation between the two was poor for non-spherical particles. This is due to the fact that the underlying Fraunhofer and Mie theories only cover spherical particles. Non-spherical particles cause more diffuse scatter patterns and are more difficult to interpret. Some manufacturers have included algorithms in their software, which can partly compensate for non-spherical particles.
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