Generally, the term "latent print" is used for all types of prints resulting from the transfer of small, hardly visible amounts of skin secretion and dirt. As the prints of the friction ridge pattern on the tip of the finger usually give the most important clues to the investigator, "latent prints" are almost equivalent to the fingerprint itself.
Other prints used as investigative leads are palm prints and foot prints. In some cases even the surface of a glove is known to have left a latent print which led to apprehension of a suspect.
In latent fingerprint investigation the hardly noticeable or completely invisible prints of the ridge pattern of the finger first have to be developed for primary examination. Fingerprints of possible culprits should then be lifted and preserved for identification.
Latent fingerprints usually are a mixture of secretions from the various glands found in the skin. When fresh, they contain a large amount of water (98.5-99.5%), together with small amounts of fatty acids, urea, amino acids, and/or salts. Chemical methods to visualize fingerprints make use of the presence of these components. Ninhydrin, for example, reacts with the amino acids to produce purple compounds. Silver nitrate on the other hand reacts with the small amounts of chloride ion present. Upon reaction the light sensitive silver chloride is formed, which on illumination decomposes to silver, thereby producing a silver image (just as in photography).
Powders do not work by chemical methods but simply adhere to the fingerprint. With fresh finger prints, the water still present in the print contributes significantly to the adhesion of powders. With older fingerprints, powders adhere in principal to the fatty deposits from the sebaceous glands secretions.
The prints of the friction ridges may be transferred by substances other than skin secretion, such as dust or oil. In such cases, the fingerprint often is immediately visible, but for proper examination further development will usually be necessary.
The friction ridge pattern also appears in finger impressions in soft matter or in dust covered surfaces. There exists a basic difference between such an imprinted trace and a normal fingerprint formed by transfer. A print formed by transfer gives a positive image of the friction ridge pattern while a finger impression in soft matter gives a negative image.