This mathematics operation was originally invented to reduce spectral
noise and eliminate background effects of NIR data. From NIR technique
non-specific scattering of radiation at the surface of particles, variable
spectral path length through the sample and chemical composition of the
sample typically cause baseline shifting or tilting. The influence is
larger at longer wavelengths. Such multiplicative interference of scatter
and particle size can be eliminated or minimized by applying a standard
normal variate correction.
In addition to the standard normal variate correction very often a detrending
is applied in order to remove offset and tilting more thoroughly. Both
operations can be applied at once using the Linear least squares Baseline
Correction of the software.
Multiplicative
Scatter Correction is another method for noise reduction!
Please review the chapter "Multiplicative
Scatter Correction" for details.
Standard normal variate algorithm is designed to work on individual
sample spectra. The transformation centres each spectrum and then scales
it by its own standard deviation:
where
i
= spectrum counter
j
= absorbance value counter of ith
spectrum
Aij
(SNV) = Corrected absorbance value
Aij
= measured absorbance value
xi
= is the mean absorbance value
of the uncorrected ith spectrum
SDev = Standard deviation of the absorbance
values of ith spectrum
Spectra treated in this manner have always
zero mean value and a variance equal to one and are thus independent of
original absorbance values.
