*2.2. Image Acquisition*

Digital RGB photographs of section SD17P1 were taken using a 24.3 MP mirrorless camera with a 2.8/16 mm lens (Sony ILCE-6000, Sony SEL-16F28, by Sony, Tokyo, Japan). Multispectral imaging was carried out using a Micro-MCA snapshot camera with six spectral channels (Table 1) by TetraCam (Chatsworth, California, USA). Central wavelengths and FWHM (full width half maximum) of the Sony ILCE-6000 RGB camera have been previously published by Haburaj et al. [12] and used throughout this study.

Using optical measurement systems, the lighting of the profile needs to be controlled and documented. Images were captured under natural diffuse daylight. Apart from smaller diffuse shadows in the corners, the section was captured under uniform lighting conditions. The cameras were placed on a tripod and centred in front of the respective section. Rotation of the cameras was limited to a minimum, and black-and-white reference targets were placed in the section, which allowed us to overlay and merge images manually afterwards while keeping spatial distortions at a minimum.


**Table 1.** Technical specifications of the camera systems used; central wavelengths of the spectral channels blue, green, red, red edge, NIR1, and NIR2.

The RGB images were converted into 8-bit TIF format and normalised to the range [0, 1] via feature-scaling. Six multispectral images were recorded in TetraCam RWS-format (each containing one spectral channel and stored in three 8-bit layers, labelled red, green, and blue) and, to allow further processing, converted into six 10-bit TIF-format images using the formula provided by the manufacturer.

Since the used multispectral camera consists of an array of six separate cameras and the images were acquired from a distance of approximately five metres, a significant spatial offset between the spectral channels was present and required manual band alignment. The proprietary processing software PixelWrench2 contains an option for minimising this offset, which was used successfully by, e.g., Retzlaff [53]. This solution, however, is more suitable for images acquired during remote sensing, as they keep a certain minimal distance from the subject, therefore keeping the spatial offset at acceptable levels. Our ground-based approach, on the contrary, included an offset that was too high and, therefore, was corrected through the pairwise manual selection of reference points. The resulting multispectral images were normalised to the range [0, 1] via feature scaling.
