BUCHI NIRCal - Manual (Page 14)

NIRCal 5.5 Software Manual
14 NIRCal 5.5 Manual, Version A
Figure 3: After transformation with the PCA, the spectra appear as points assorted in clusters,
which are well separated from each other.
* In case of many different categories (properties) more than three delineation axes are required to
show all important differences. It is not necessarily the case, that the first three axes are the most
important ones.
Inspection of the Qualitative Calibration
The attributes of a good calibration are:

all tolerance regions of the single categories (Clusters) are cohesive;

all tolerance regions are convex and engaged consistently;

all calibration and validation spectra are within a valid range and assigned correctly;

all other substance spectra can not be predicted false with a certain calibration;

Q-Value is closest possible to 1.
Application of a qualitative calibration and interpretation of the results
During the application of a qualitative calibration, it is determined whether a new measured spectrum
can be associated with a calibrated category.
A measured spectrum is identified as OK when these three criteria are fulfilled:

the residual is smaller than the allowed limit;

the distance is within the allowed tolerance (the spectrum is in a known cluster),

the identity of the measured substance (substance-ID entry) matches the identity of a
substance used to build up the calibration model.
1.1.4 Quantitative calibration
NIR Spectra can be seen as fingerprints, which are characteristic for a certain substance. When
investigating a substance mixture, different fingerprints are superposed within a complex spectrum.
The concentration rate of the substances is present in the spectra, but cannot be seen. Target of a
quantitative calibration is, to calculate applicable filters that establish the coherence of the measured
intensities and the concentrations of the single components.
The quantitative calibration serves as the determination of parameters such as concentration (e.g.
water content, blending ratio, hydroxyl number, etc.) or physical properties (e.g. density, viscosity).
Choosing the calibration samples
As a basis for the calibration work a set of reference data is used for which the interesting
concentrations have been determined with the referring lab method. For each component (property) at
least 15 samples must be used.