Laser diffraction is one of the most popular particle sizing techniques for a variety of industries. Though the generation of data seems easy enough, how does laser diffraction actually work? The physics behind laser diffraction centers around light’s behavior.
Light is a phenomenon that we encounter and use in our daily lives, but did you know light is an extremely powerful analytical tool as well? Different types of light can do everything from playing music on the radio to powering an X-ray. To fully understand how light can be used analytically, we first must understand the basics of light.
Light
Light is a type of electromagnetic radiation composed of elementary particles called photons that move through space. Photons are known to exhibit wave-particle duality, meaning light acts as both a wave and a particle when interacting with a medium. The “wave” aspect refers to the frequency of light, which can be used to explain the energy it carries: shorter wavelengths correspond to higher energy, while longer wavelengths correspond to lower energy1.
Radio waves, for example, are a form of light with the longest wavelength, meaning it is very low energy. X-rays, on the other hand, are a form of light with a very short wavelength, making them high in energy and dangerous for humans to be exposed to. The high energy associated with short wavelengths can cause damage to human DNA2.
Visible light makes up only a tiny fraction of the electromagnetic spectrum, but this range is critical for particle size measurement using laser diffraction. Light naturally vibrates in multiple directions, yet it can be filtered so it vibrates in a single plane; a process called polarization3. This principle is commonly found in polarized sunglasses, where the tint of the sunglasses acts as a filter that blocks certain light orientations, making it easier to see the world around you.
In laser diffraction, the polarization of the laser or diode ensures consistent scattering patterns, which allows us to analyze how light interacts with particles through four mechanisms: diffraction, reflection, refraction, and absorption.
Diffraction
Diffraction4, the namesake of the technique, refers to the bending of light around the shape or contour of an object, such as a particle. Have you ever noticed how CDs sometimes have a rainbow shine to them? CDs contain microscopic grooves on their surface in which light diffracts around, creating that rainbow appearance.
FIGURE 1. Diffraction of Light Around a Particle.
Reflection
Reflection4 is a change in the direction of light when interacting with a medium. The most common example of reflection is a mirror. When looking into a mirror, the light hits at a particular angle, known as the angle of incidence. That light then bounces back at the same angle, known as the angle of reflection. Reflection is the determining factor as to what color different objects appear to the observer; if an object cannot absorb a certain wavelength, or “color”, it will appear as that color (see absorption).
FIGURE 2. Reflection of light off a particle.
Refraction
Refraction4 refers to the bending of light when going from one medium to another. The index of refraction (or refractive index) is a numerical measure of how much light bends. If two things, say a carrier fluid and a particle, have the same index of refraction, then the light will not scatter as the angle at which light is being diffracted will not change. That is not to say laser diffraction as a technique will not work, as diffraction, reflection, and absorption still occur.
FIGURE 3. Refraction of light through a particle.
Absorption
Absorption4 is the reduction of light when hitting an object. Per the law of conservation of energy, energy cannot be created or destroyed. Therefore, when light is absorbed, it must be converted into another form of energy, such as potential, kinetic, or chemical. Have you ever noticed how hot asphalt can get on a summer day? That is because the asphalt absorbs light from the sun and converts it into thermal energy. Different materials have different absorptions that change depending on the wavelength of the light. For example, leaves of a plant appear green because chlorophyll found in their leaves absorbs most wavelengths of light except for light with wavelengths around 490-580 nm, the wavelength of green light. Darker materials tend to absorb the most light, which is why asphalt heats so easily.
FIGURE 4. Absorption of Light by a Particle.
Most laser diffraction instruments account for all four light interaction behaviors – reflection, refraction, absorption, and diffraction by applying mathematical models such as Mie theory. While too complex a topic for this blog, these models use a complex refractive index that considers the particle and carrier refractive indices as well as the imaginary component to ensure accurate particle size calculations. These values are typically entered by the user during method setup. The absorption component is often represented by a numerical value, though some systems may describe absorption qualitatively using terms instead of numbers. To learn more about optical models, see our other blog post: Laser Diffraction: The Importance of Optical Properties.
Instrument Setup
Laser diffraction instruments typically consist of a monochromatic light source (a laser or diode), light detectors, an optical system, a sample feeder, and a calculation system. While each manufacturer designs these components differently, there are only two main optical configurations: Fourier and Reverse-Fourier. The choice between them depends on user priorities. Fourier systems generally provide a higher angular resolution for fine particles, whereas Reverse-Fourier systems offer a more compact instrument footprint and a broader measurable size range.
Fourier Lens Arrangement
A Fourier lens arrangement7 refers to a system where the lens is found posterior to the measurement zone containing particulate matter, relative to the light source. With this arrangement, the light source first travels through beam expansion and collimation lenses. The beam expansion lens first increases the diameter of the rays of light, then limits the divergence of these rays, essentially ensuring that the light travels on a straight path. Once the light is traveling straight, it passes through the measurement zone where the particulate is suspended in a medium. The light source hits these particles as an ensemble (or as a group of particles) and scatters through absorption, refraction, diffraction, and reflection. That scattered light travels through the Fourier lens, which focuses the light so that they converge together and hits forward and backscattering detectors placed at different angles.
Reverse Fourier Lens Arrangement
A reverse Fourier lens arrangement7 is similar to a Fourier lens arrangement in that all components within the system are the same; the lens is found anterior to the measuring zone, relative to the light source. The light source is first expanded and collimated before passing through the Fourier lens, where it becomes converged. After being converged, the light passes through the measurement zone containing the suspended ensemble of particles. The light is scattered in this measuring zone, and the scattered light hits detectors at different angles. The reverse Fourier arrangement also contains detectors in front of the measuring zone, but behind the Fourier lens.
Data Calculation
All laser diffractors have a proprietary, complex, and efficient system of gathering scattering data, but what does it do with that data to create a particle size distribution? These instruments calculate results based on the light scatter of a known sphere by utilizing the angle at which the light is scattered to perform deconvolution. Deconvolution is a mathematical calculation that inverts a signal; light that is scattered at large angles corresponds to a smaller particle size, and light scattered at a small angle corresponds to a larger particle size. The instrument software considers the angle of the scattered light, size and position of the detectors, and the focal distance of the lens to perform a deconvolution to obtain a particle size distribution. Optical models (refractive index and absorption) are also taken into consideration in the mathematical calculation if Mie theory is used.
PTL’s Laser Diffraction Capabilities
Particle Technology Labs is currently equipped with two different brands of laser diffraction instruments: the Sympatec HELOS/BR and Malvern Panalytical Mastersizer 3000. The Sympatec HELOS/BR utilizes the Fourier lens arrangement and has an analytical range of approximately 0.18 µm to 875 µm spread across five lenses. Each lens has a different analytical range, which provides more resolution but can be tricky for polydispersed material. The Malvern Panalytical Mastersizer 3000 utilizes the reverse Fourier lens arrangement and has an analytical range of approximately 0.01 µm to 3,500 µm.
Whether it’s a one-time particle size test, method verification, or validation, PTL is here to help. Contact PTL today for all of your particle sizing needs.
By Emerson Bella, Particle Characterization Chemist III.
Citations:
¹Blair, Bill. The Basics of Light. Blair.pha.jhu.edu, blair.pha.jhu.edu/spectroscopy/basics.html
2National Cancer Institute. Electromagnetic Fields and Cancer. National Cancer Institute. http://www.cancer.gov/about-cancer/causes-prevention/risk/radiation/electromagnetic-fields-fact-sheet
3Adams, Mitzi. IXPE: Polarization Creation. Ixpe.msfc.nasa.gov. ixpe.msfc.nasa.gov/creation.html
4International Standard ISO 13320: Particle Size Analysis — Laser Diffraction Methods.
5Hall, Nancy. First Law – Conservation of Energy. Glenn Research Center | NASA. www1.grc.nasa.gov/beginners-guide-to-aeronautics/first-law-conservation-of-energy/
6National Geographic. Chlorophyll | National Geographic Society. Education.nationalgeographic.org. education.nationalgeographic.org/resource/chlorophyll/
7US Pharmacopeia <429> Light Diffraction Measurement of Particle Size.
