The employed measurement system shown in Fig. 1 comprises a commercial NIR spectrometer (NIR-512L-1.7T1, 901 to 1685 nm, Control Development, South Bend, Indiana), a commercial VIS spectrometer (AvaSpec-2048-TEC-FT, 177 to 1098 nm, Avantes, Apeldoornseweg, the Netherlands), and a fiber optic probe (Avantes, FCR-19IR200-2-ME, 2 readouts and 17 illumination fibers, 200-μm core diameter, , pairwise source–detector separations: , , , ), delivering 8.5 mW of broadband optical power (Avantes, Avalight-HAL, tungsten halogen source). Outer diameter of the probe was 6.35 mm with an effective pressure area of . The probe was attached to a stainless steel arm equipped with a temperature-compensated silicon piezoresistive force sensor fabricated in UL-FE, LMSE (Ljubljana, Slovenia). The acquisition of contact pressure and spectra was synchronized by a custom logic. Spectrometer exposure time was set to 3 ms, while the force was measured at a rate of 100 kHz. For each acquired spectrum, the contact pressure was calculated as the quotient between the mean force and the probe surface, providing submilligram-force resolution. The spectrometer readout time was about 15 ms, limiting the acquisition rate to 55 Hz. Precise and accurate control of the applied contact pressure in the range from 0 to 150 kPa was ensured by a motorized linear stage and the custom control logic.