Metrohm NIRS XDS Process Analyzer – MicroBundle - Manual (Page 84)

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10.4 Transmission Probe Tests
The Transmission Probes require special methods of calibration. One
fiber bundle carries “white light” from the instrument to the sample.
The other fiber bundle is used to return light to the instrument, to be
broken down in the monochromator to individual wavelengths, then
measured for spectral response. Absorbance of the sample is then
determined.
10.4.1 Wavelength Linearization, Transmission Probes
The first test to be run is Wavelength Linearization. This provides initial wavelength locations for the
sample spectrum, prior to final wavelength calibration.
Wavelength Linearization uses internal wavelength materials to determine a set of internal, arbitrary
peak positions that the instrument will use to maintain repeatability of wavelength response.
Wavelength Linearization is performed on the internal reference fiber located in the XDS Process
enclosure.
The wavelength positions of
these peaks appear as shown.
The scale of this display is
marked in encoder pulses,
which do not relate to
nanometers directly.
From the peaks, a linearization
is performed, which allows
assignment of nanometer
values. (Actual spectral shape
depends on fiber
configuration.)
The spectrum shown above does not exhibit any noise effect from fiber attenuation. Actual fiber
optic probe spectra will exhibit noise at the lower and upper ends of the spectrum, based upon
length of fiber used. This is normal.
A Wavelength Linearization is performed in the forward direction of grating scan, then again in the
return direction. The resulting information is adjusted by Vision into one corrected spectrum.