The right-hand column above shows a typical profile for a hollow cone nozzle.
Here the highest concentration of spray is observed close to the edge of the
plume. A low concentration region exists towards the centre.
Optical models
This section contrasts the Fraunhofer approximation used in some instruments
with the Mie theory which underpins Spraytec operation.
Fraunhofer approximation
Older instruments and some existing instruments rely on the Fraunhofer
approximation only. This assumes that:
.
The particle is much larger than the wavelength of light employed. ISO
13320 defines this as being greater than 40ë (25µm when a He-Ne laser is
used).
.
All sizes of particle scatter with equal efficiencies.
.
The particle is opaque and transmits no light.
These assumptions are incorrect for many materials and for small particles they
can give rise to errors approaching 30%, especially when the relative refractive
index of the material and medium is close to unity, or when the particles are
transparent, as is the case for water droplets. When the particle size approaches the
wavelength of light the scattering becomes a complex function with maxima and
minima present.
Mie theory
Spraytec uses the full Mie Theory which completely solves the equations for
interaction of light with matter. This allows completely accurate results over a
large size range (0.02 - 2000µm typically).
Requirements for using Mie
The Mie Theory assumes the volume of the particle, as opposed to Fraunhofer
which is a projected area prediction. The penalty for this complete accuracy is that
the refractive indices for the material and medium must be known and the
absorption part of the refractive index must be known or estimated. However, for
the majority of users this will present no problems as these values either will be
known or can be measured.
CHAPTER 10
Spraytec
Page 10.16 MAN 0368
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