Measuring Small Particles

Smaller particles pose a real challenge for laser diffraction technology. When illuminated by a laser beam, large particles scatter light at low angles with easily detectable maxima and minima in the scattering pattern. This means detectors placed at low angles relative to the optical path and with sufficient angular resolution can detect these maxima and minima. As particles become smaller, the ratio of particle dimension to light wavelength (d/λ) is reduced, resulting in a smoother and less angular scattering pattern making measurement difficult. In addition, small particles scatter light weakly and maxima and minima can be measured only at very high angles, which affects detection and resolution of the scattering pattern. Different manufacturers use different solutions to address these limitations with varying degrees of success. Most focus on the measurement of back-scattered light.

Bias in Sizing of Non-spherical Particles

Most laser-based particle sizing devices make no allowance for the shape of materials being tested, regardless of particle size. Mathematical models used to calculate distributions are based on spherical systems, so any reported distribution is essentially equivalent to a spherical distribution of the material being analyzed. In most cases this is sufficient as many particles emulate a spherical system closely enough. But for many particles that deviate from perfect sphericity, the size distribution obtained is only apparent or nominal and will be biased. In some extreme cases results using a spherical model on non-spherical particles will be very different from reality. This bias emerges when comparing laser diffraction results with others, such as polarization intensity differential scattering or PIDS.

PIDS vs. Laser Diffraction

PIDS technology is based on the Mie theory of light scattering and relies on the transverse nature of light. With a magnetic and electric vector (at 90°), if the electric vector is up-and-down the light is considered to be vertically polarized. When a sample is illuminated with light of a given polarized wavelength, the oscillating electrical field creates a dipole (or oscillation) of the electronics in the sample. These oscillations are in the same polarization plane as the propagated light source, and oscillating particle dipoles will radiate light in all directions except that of the irradiating light source.
Three wavelengths (450 nm, 600 nm and 900 nm) successively illuminate the sample with vertically and then horizontally polarized light. Scattered or radiated light from the samples is measured over a range of angles. By analyzing the differences for each wavelength, we gain information about the particle size distribution of the sample. What’s being measured is the difference between vertically and horizontally polarized signals, not only the values at a given polarization.

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