Laser Diffraction: Principles, Working and Applications

Laser diffraction (LD), also known as static laser light scattering (SLS) is one of the most popular methods to analyze the size distribution of the particle.

• It is based on the Mie and Fraunhofer scattering theory and has been used to measure the size of the particle. 
• These laser diffraction tools can measure particle sizes ranging from a few nanometers to several microns. 
• Furthermore, the unique characteristic of this technology is not only its wide size range but also its ability to measure dry particles and aerosols.
• Using scattering models, laser diffraction measurements measure scattering intensity based on scattering angle, wavelength, and polarization.
• Laser diffraction has many advantages, including ease of use, quick operation, high reproducibility, and a wide dynamic size range.

Key Terms: Laser diffraction, Fraunhofer theory, Mie theory, Diffraction, LASER, Scattering of light, Intensity of light, Refractive index

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Laser Diffraction

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Laser diffraction is a technique for determining particle size that is commonly used for materials with sizes ranging from hundreds of nanometres to several millimeters. 

• The variation in scattered light intensity as a function of scattering angle is used to calculate particle size. 
• Laser diffraction examines the relationship between particle size, angle, and intensity of dispersed light.
• The scattering intensity of light from large particles is greater and the angle at which it scatters is smaller than that from small particles. 
• Laser diffraction tools can detect particle sizes ranging from 0.02 µm to 2000 mm. 
• The samples are either distributed in air or a suitable liquid medium. 
• As the laser passes through the dispersion medium, it is diffracted by the particles, resulting in a diffraction light pattern that changes with particle size.

Laser Diffraction

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Fraunhofer Theory

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Laser diffraction analysis is based on the Fraunhofer diffraction principle, which states that the brightness of light dispersed by a particle is proportional to the size of the particle. 

• According to this theory, the angle of the laser beam and particle size have an inverse relationship. 
• As particle size decreases, the angle of the laser beam increases, and vice versa. 
• The Mie scattering model, commonly referred to as the Mie theory, has been used as an alternative to the Fraunhofer theory since the 1990s.

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Mie Theory

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Mie theory predicts scattered light intensity depending on the refractive index difference between the particle and the dispersing medium. 

• It also demonstrates how the absorption abilities of the particle impact the amount of light passed through the particle and then absorbed or refracted. 
• The ability to accurately account for the effect of light refraction within particles is especially essential for particles with diameters smaller than 50 m and/or those that are transparent.

The Mie theory is based on the following assumptions:

• The size of spherical particles is being measured.
• Light is dispersed and detected by one particle before it interacts with other particles due to the diluted solution.
• It is possible to determine the optical characteristics of the particles and the medium in which they are suspended.
• The particles are all the same size.

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Principles of Laser Diffraction

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Laser diffraction is based on the indirect calculation of particle sizes based on particle diffraction patterns. 

• The scattering intensity of light from particles in suspension is measured using laser diffraction as a function of scattering angle, light wavelength, and light polarization. 
• The particle size distribution is determined using a mathematical model based on the Fraunhofer and Mie theories.
• When monochromatic, coherent, and polarised light strikes an object, the diffraction phenomenon occurs. 
• Light propagates as spherical wavefronts from the edges of particles, and interactions between the light and the particles cause the diffraction phenomena. 
• The intensity distributions of laser light scatters from multiple particles are detected using laser diffraction equipment at the same time.
• According to Fraunhofer diffraction theory, the intensity of light scattering is directly proportional to particle size. 
• Large particles scatter light at small angles, whereas decreasing particle size logarithmically increases the scattering angle. 
• The intensity of scattering is also affected by particle size. Large particles scatter light with high intensity, and small particles scatter light with low intensity.

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How Laser Diffraction Works

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The connection that exists between light scattering (angle and intensity) and particle size is the principle of laser diffraction. 

• The larger the particle, the smaller the angle and the higher the intensity of scattering. 
• This is used in every analyzer ever built, from the first prototype to the most recent LA-960 particle-size analyzer.
• The device does not directly determine particle size, but rather the angle and intensity of light scattered by the particles. 
• This data is sent into an algorithm, which uses the Mie scattering theory to derive information about particle size from light scattering data.
• Much effort has gone into improving the equipment and software required for measuring particle size. 
• This has resulted in higher precision, dependability, and convenience of use. 
• The LA-960 is a tenth-generation laser diffraction analyzer from HORIBA, with each successive series offering a better and more distinct collection of instruments than the previous one.

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Optical Properties

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Laser diffraction calculates particle size distributions using Mie's theory of light scattering and a volume-equivalent sphere model.

• Mie theory requires knowledge of the optical characteristics of both the sample being examined and the refractive index of the dispersant. 
• Generally, the optical characteristics of the dispersant may be determined from published data.
• In the case of unknown optical characteristics, the user may either measure them or estimate them using an iterative process.

The Fraunhofer approximation, which does not need knowledge of the optical characteristics of the sample, is a simpler technique. 

• For large particles, this can produce precise results. 
• However, it should be used with caution when working with samples containing particles smaller than 50 µm or if the particles are relatively transparent.

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Applications of Laser Diffraction

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The following are the applications of laser diffraction

• It is a useful analytical parameter for research and product development in a variety of sectors, including construction materials, food, medicines, and cosmetics.
• Laser diffraction can be used on both wet and dry samples. Wet samples, such as emulsions, are suspended in liquid, whereas powders are analyzed in an air stream. 
• The recirculation system of the equipment ensures that the sample is completely homogenized.
• The entire sample may be measured using this approach. As the sample passes through the laser beam, diffraction is measured for all particles.
• The results of this method can be obtained in less than a minute. Thus, feedback may be delivered rapidly, and repeat analyses can be conducted quickly as well.
• Laser diffraction particle size analyzers do more than just measure diffraction effects. Non-laser light sources can also be utilized to enhance laser sources in order to show particle data.

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Things to Remember

• Laser diffraction is a technique for measuring the size distribution of particles.
• It is based on the Mie and Fraunhofer scattering theory.
• According to the Mie theory, scattered light intensity depends on particle and dispersion medium refractive index differences.
• Mie theory requires knowledge of both the optical properties of the sample under consideration and the refractive index of the dispersant
• According to Fraunhofer's theory, particle size and laser beam angle are inversely related.
• The Fraunhofer theory does not need knowledge of the optical characteristics of the sample.
• The scattering intensity of light from particles in suspension is measured using laser diffraction as a function of scattering angle, light wavelength, and light polarization. 

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Previous Year Questions

1. The elevation of the boiling point of the solution is 0.75K. The molecular weight of the solute in gmol−1 is
2. 200mL of water is added to a 500mL of 0.2M solution. What is the molarity of this diluted solution?
3. The mass of water in grams present in the solution is (Kf=1.86Kkgmol−1)​
4. what is the weight % and degree of dissociation (in %) of acetic acid in benzene?​
5. What is the molecular weight of the unknown solute?​
6. The degree of dissociation (α) of a weak electrolyte AxBy is related to van?t Hoff factor (i) by the expression​
7. If glucose of 36g weight is dissolved in 2kg of H₂O then, change in boiling point (ΔTb) at 1.013 bar will be (Kb for H₂O is 0.52Kkgmol−1)​
8. Volume of acid required to make 1 litre of 0.1MH₂SO₄ solution is:​
9. With increase in temperature, which one of these changes?
10. Molarity is expressed as​