A method of obtaining an optical image of a skin-print on an absorbent substrate having a skin-print receiving region comprises: depositing a solution of a silver-containing compound having a solubility in water of at least 10 g/l on the skin-print receiving region of the absorbent substrate such that between 25 µg and 300 µg per cm2 of the silver-containing compound is deposited, and allowing the substrate to dry. The method further comprises depositing a skin-print on a skin-print receiving region of the substrate such that sodium chloride and water in the skin-print react with the silver-containing compound to form silver chloride exposing the substrate to electromagnetic radiation, e.g. UV radiation, such that the silver chloride decomposes to form metallic silver taking an optical image of the pattern of the metallic silver and releasing a buffer solution onto the substrate. The buffer solution comprises a counterion for silver, which reacts with the silver-containing compound to form a compound of silver having a solubility in water of less than 1 g/I. The absorbent substrate may form part of a lateral flow immunoassay device 1 that has a skin-print receiving region 42, and analysis region 44.
