A method of obtaining a plurality of infrared images of a banknote comprising: simultaneously illuminating the banknote with infrared light at a first wavelength and infrared light at a second wavelength; capturing an image of the banknote with an RGB camera; obtaining from both a first output channel signal and a second output channel signal of the RGB camera sensor the intensity distribution of the infrared light at the first wavelength and the intensity distribution of the infrared light at the second wavelength using a first calibration coefficient and a second calibration coefficient of the RGB camera sensor; and producing separate infrared images of the banknote at the first wavelength and the second wavelength from the respective intensity distributions.