Beckman Coulter LS 13 320, User Manual

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PN B05577AC 1-1
CHAPTER 1
Theory
Light scattering is a common technique for measuring particle size distribution. The practice
involves exposing particles to a beam of light, and analyzing the patterns of scattered light
produced. The technique is fast and flexible, and also provides precise and easily adaptable
measurements.
The LS 13 320 series of instruments analyze the patterns of scattered light produced to provide
precise and reproducible particle size distributions.
Theoretical Background
Light Scattering
Scientists often describe the scattering phenomena in terms of diffraction, reflection, refraction,
and absorption. When light illuminates a particle with a dielectric constant different from that of
the medium, depending on the wavelength of the light and the optical properties of the particle,
light scatters in a unique way. The intensity of the scattering is a function of the complex refractive
index ratio between the material and its surrounding medium. This intensity falls within the regime
of Rayleigh scattering and is inversely proportional to the fourth order of the light wavelength, i.e.,
the shorter the wavelength, the stronger the scattering.
Think of the color of the sky. A shorter wavelength of sunlight (blue) is scattered during daytime
given the path length of the sunlight through the atmosphere. One sees a longer wavelength of
sunlight (red) transmitted through long optical paths through the atmosphere during dawn and
dusk. Utilizing this wavelength dependence, Beckman Coulter uses red as the color for the stoplight
and for all traffic control warning signs because red light has the least scattering power in the visible
light spectrum. This allows the transmitted light to go through fog, rain, and dust particles and
reach the intended detector: in this case, the human eye.
Most materials exhibit strong absorption in the infrared and ultraviolet regions, which greatly
reduces scattering intensity. Therefore, most light scattering measurements are gathered using
visible light at wavelengths of 350 nm to 900 nm. Several technologies make use of light scattering
to obtain information about materials. Among these technologies, elastic light scattering (ELS) is
the main method for the characterization of particles ranging in size of microns to millimeters.
In ELS, scattered light has the same frequency as the incident light, and the intensity of scattered
light is a function of the particle's optical properties and dimensions. In general, the scattered light
intensity of a particle is a function of the following variables: particle dimension, particle refractive
index, medium refractive index, light wavelength, polarization, and scattering angle. The scattered
intensity from a particulate sample is, in addition to the above variables, a function of particle
concentration and particle-particle interaction. Some of the variables are constants in a particular
experimental setup, such as light wavelength and the particle refractive index.