The reflectance spectra of our manuscripts were recorded with a FieldSpec4 spectrometer (ASDi/PANalytical) with integrated light source and a bifurcated fibre-optic probe kept parallel to the surface normal. When an optic fibre is used, as is the case here, this analytical method is often called 'FORS' - fibre-optic reflectance spectroscopy.

The spectral resolution of our instrument is 3 nm at 700 nm, and 10 nm at 1400 and 2100 nm. The instrument includes a modular Silicon array and two Peltier-cooled InGaAs detectors. The measurement spot size is about 3 mm in diameter. Sixty-four scans were acquired for each spectrum for a total acquisition time of approximately 8 seconds. Calibration to reflectance was obtained using a Spectralon© standard. Spectral interpretation was based on both in-house and published reference databases.

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