Calculating the particle size
The Spraytec software calculates the particle size of sprays by comparing the
acquired light scattering pattern to an optical model which predicts how particle
scattering changes with particle size. To understand the meaning of the results
from the Spraytec, two fundamental concepts require explanation. These are:
.
The theory behind the calculations.
.
Why the result is expressed in terms of equivalent spheres.
Theory
Scientists have for centuries tried to predict the way particles scatter and absorb
light. There are many theories and models that the modern particle size analyst
can use.
One of the simplest theories used is the Fraunhofer approximation. This
model predicts the scattering pattern that is created when a solid, opaque disc of a
known size is passed through a laser beam. This model is satisfactory for large
particles (over 50mm diameter) but it does not describe the scattering exactly. Very
few particles are disc shaped and most particles are transparent.
The accepted theory which accurately predicts the light scattering behaviour of all
materials under all conditions is known as the Mie theory. This was developed to
predict the way light is scattered by spherical particles and deals with the way light
passes through, or is absorbed by, the particle. This theory is more accurate, but it
does assume that the analyst knows some specific information about the particle,
such as its refractive index and its absorption.
The key point about these theories is that if the size of the particle and other
details of its structure are known, the user can accurately predict the way it will
scatter light. Each size of particle has its own characteristic scattering pattern, like
a fingerprint, unlike that for any other size of particle.
Equivalent spheres
Mie theory presumes that the particles being measured are perfect spheres. In
practice they are very rarely so. If the particle is an irregular shape, which
particular dimension should be measured?
As an example, imagine trying to measure the size of a matchbox. An exact
description of the matchbox’s dimensions might be “50mm wide x 25mm long x
10mm high”.
CHAPTER 2
Spraytec Page 2.7
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