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Porous surfaces such as paper are typically processed with chemicals, including ninhydrin and physical developer, to reveal latent fingerprints.
These chemicals react with specific components of latent print residue, such as amino acids and inorganic salts.
Ninhydrin causes prints to turn a purple color, which makes them easily photographed.
DFO (1,2-diazafluoren-9-one) is another chemical used to locate latent fingerprints on porous surfaces; it causes fingerprints to fluoresce, or glow, when they are illuminated by blue-green light.