The ASD High-Resolution FieldSpec 4 spectroradiometer (Analytical Spectral Devices Inc., Boulder, USA) is equipped with the ASD probe for remote mode operations. The light source consists of a halogen lamp (ASD Iluminator) placed at 30 cm of distance from the samples. The angle between the incident light and the sample and that between the sample and the detector fibre is 45° in both cases. The fibre has an acceptance cone with an angle of 25°. This device works in the 350- to 2500-nm wavelength range and uses three detectors for the different spectroscopic ranges: 350–1000 nm (silicon photodiode array for Vis–NIR), 1001–1800 nm (InGaAs photodiode for SWIR1), and 1801–2500 nm (InGaAs photodiode for SWIR2).

The spectral resolution is 3 nm in the visible region and 6 nm in the near infrared region. Sensor optimization and calibration are set with a Spectralon® white reference scanned once before each sample. Spectra are collected with RS3 software (ASD Inc.), analysed with ViewSpecPro software (ASD Inc.), and compared with those found in the bibliography.