Friction ridges on the underside of fingers, hands, toes and soles of the feet form unique patterns that may be found on surfaces of objects touched by a person of interest in a crime investigation. These unique patterns are almost always invisible to the naked eye, and for that reason are called latent (hidden) prints. Latent prints are made visible by two methods. The first method is to very lightly dust the print. The dust adheres to skin secretions (amino acid). The dusting powder color contrasts with the color of the surface upon which the print rests. Dusting may be done with a brush, lightly applied, that makes the print visible. The next step is to photograph the print.
The second method is to use an aerosol can to very lightly spray the latent print with a chemical, such as ninhydrin, which turns skin secretions to the color of purple and therefore visible.
A print collection method that has been reported to be successful in lifting prints from a skin surface uses cyanoacrylate (Super Glue). The opportunities for use of this method are limited.
After a print is made visible and photographed, it is lifted onto a latent print lift card. A lift card, which comes in a variety of shapes and sizes, consists of a clear plastic tape (think Scotch Tape). The tape is adhered to a moderately flexible card. The clear plastic is removed from the card; it is carefully placed on top of the print; the print is lifted by the clear plastic from the surface it is upon; and the clear plastic is placed on the card. The color of the card will provide a contrasting background to the color of the dust that was used to make the print visible. For example, a print that has been dusted with white powder is placed on a black card. With ninhydrin, which produces a purple color, a good color choice of the card might be white.
Latent print lift cards are transferred (using chain of custody) to a forensic laboratory where they are examined and compared to prints that are on file in a fingerprint identification database. The Automated Fingerprint Identification System (AFIS) is a national database. Many states have separate databases but also send prints to AFIS. A “hit” on a database means that the prints collected at the scene perfectly or closely match fingerprints of a person or persons who may be connected to the crime.