Thermonicolet Omnic Software User's Guide 6.1 (PDF) - Charles E ...

The units are described in the next sections.
Kubelka-Munk
Select Kubelka-Munk to convert % reflectance spectra collected using the diffuse
reflection (DR) technique to Kubelka-Munk units (see the following example). Since
these units are similar to absorbance units, the Kubelka-Munk format is useful for
searching diffuse reflectance spectra against absorbance spectra in commercially
available libraries. (Kubelka-Munk spectra are actually searched in absorbance.)
Because of pathlength variation, in many cases the concentration of samples run
using the diffuse reflection technique do not vary linearly with log (1/R) (which is
the equivalent of absorbance for reflection measurements). The Kubelka-Munk scale
was developed to provide a more linear relationship with respect to concentration.
When you develop quant methods for determining component concentrations in
sample mixtures, you can use spectra in log (1/R) or Kubelka-Munk units. Kubelka-
Munk units are normally preferred because they eliminate any wavelengthdependent
specular reflection effects, which reduce the accuracy of the analysis.
However, the Kubelka-Munk model is accurate only if these criteria are met:
• You are using high quality DR spectra.
• The sample is diluted with a nonabsorbing matrix material such as KBr.
• The sample concentration is low (about 1% sample to 99% matrix by weight).
• The sample and matrix material are finely ground (a particle size of 2 to
5 micrometers is recommended).
• The mixture is uniform in particle size and composition.
• The sample thickness must be at least 3 mm.
188 Thermo Nicolet