Malvern Mastersizer 2000, User Manual

Page: 87 / 154

Understanding the views Chapter 6
Mastersizer 2000 Page 6-15
When the particle size approaches the wavelength of light the scattering efficiency
becomes a complex function with maxima and minima present.
Mie theory
The Mastersizer 2000 uses the full Mie theory which completely solves the equa-
tions for interaction of light with matter. This allows completely accurate results
over a large size range.
Requirements for using Mie
The Mie theory assumes the particle is spherical, 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.
A standard set of particles is available for selection in the SOP (presented in the first
Analysis dialogue). More can be defined by a user, but the following parameters
must be specified accurately:
„
Refractive Index (Real) – describes the amount of scattering that takes place
as a result of light interacting with the particle.
„
Refractive Index (Imaginary) – the imaginary or complex refractive index.
This describes the amount of absorption that takes place as the light enters the
particle.
„
Density – the density in g/cm
3
. This is used to calculate the Specific Surface
Area ( SSA ). Users who want to include the SSA as a derived parameter must
complete this field.
The residual shown in the Measurement Parameters view indicates how well
the calculated data fitted the measurement data.
Refractive index of medium
This value is needed by both the Fraunhofer approximation and the Mie theory.
MAN0384-1.0 Mastersizer 2000.book Page 15 Tuesday, March 20, 2007 8:56 AM