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Magnified images of latent fingermarks stained by NaYF4:Yb,Er UCNPs ((a), (b)) and micron-sized NaYF4:Yb,Er UC powders ((c), (d)), and then imaged on the surface of glass: ((a), (c)) Images in bright field without 980 nm NIR excitation and ((b), (d)) UC fluorescent images formed owing to the excitation of UCNPs by a 980 nm NIR radiation. The scale bar corresponds to 2.5 mm.
