In the previous post 8 samples were selected for the STD.
First figure shows the selected samples for the STD scanned (raw spectra) in Instrument 1 and 2, and the differences between them.
In a recent article from Mark Westerhaus to NIR News, he advice about the importance to check and review these plots and its shape, in order to apply the best standardization (single or multiple).
Comparing with more details the spectra, there are differences in the instruments not only in the photometric scale, also in the wavelength positions and in the bandwidth, but this differences are not constant all along the wavelength axis.
Actually manufacturers are improving the instruments to match each other, specially in the wavelength axis where more complex agorithms for correction are needed.