*2.2. Hyperspectral Imaging (HSI)*

Hyperspectral images of the prints were acquired using a Resonon Pika II hyperspectral camera measuring between 400 and 900 nm with 2 nm spectral resolution. Only data between 410 and 900 nm were used for identification due to noise affecting lower wavelength channels. Hyperspectral data analysis was performed using the method and code provided in [25]. The method uses the Uniform Manifold Approximation and Projection (UMAP) algorithm to reduce the dimension of spectral data and then clusters hyperspectral data to produce characteristic spectra, called endmembers, for each color in each print.
