The exposure time was 3s/pixel and each spectrum was the average of 2 exposures. The step size was 100 µm, the spectral range was 600-3200 cm -1 with a resolution of 4 cm -1 . Each sample generated a 40 × 40 × 651 cube of data, where 40 was the number of pixels at x and y axis and 651 the number of spectral variables.