Latent fingerprints are made visible through various techniques, depending on the surface they are deposited on. Generally, these techniques fall into two categories: dusting for non-porous surfaces and chemical treatments for porous surfaces.
Dusting Techniques (For Non-Porous Surfaces)
Dusting is a common method used on hard, non-absorbent surfaces like glass, metal, or painted wood. The process involves applying a fine powder to the surface, which adheres to the oily or sweaty residue left behind by the fingerprint.
- Mechanism: The powder clings to the latent fingerprint residue because it is naturally sticky or oily. Excess powder is then brushed away, leaving a visible print.
- Types of Powders: Various powders are used, including:
- Traditional powders: These are often black, gray, white, or fluorescent.
- Magnetic powders: These powders are applied with a magnetic applicator, reducing the risk of damaging the print.
- Advantages: Dusting is a relatively quick and simple technique.
- Disadvantages: It can be destructive and may not work well on all surfaces.
Chemical Techniques (For Porous Surfaces)
Chemical treatments are used on porous surfaces like paper, cardboard, and unfinished wood, where the fingerprint residue has soaked into the material.
- Ninhydrin: This is a common chemical used on paper. It reacts with amino acids present in fingerprint residue to produce a purple-blue compound called Ruhemann's Purple.
- Diazafluoren-9-one (DFO): DFO reacts with amino acids and is generally used before ninhydrin. It produces a fluorescent print that can be visualized under laser light.
- Silver Nitrate: This reacts with chlorides (from sweat) in the fingerprint residue to form silver chloride. When exposed to UV light, the silver chloride is reduced to metallic silver, making the print visible as a dark gray or black image. This process is destructive and is typically used as a last resort.
- Iodine Fuming: Iodine crystals are heated, and the resulting vapors are absorbed by the oils in the latent fingerprint, making it temporarily visible as a brownish print. It's a non-destructive method, but the prints fade quickly and need to be photographed immediately.
- Small Particle Reagent (SPR): Used on wet surfaces or surfaces that have been exposed to moisture. SPR adheres to the fatty components of the fingerprint residue.
Enhancement Techniques
Sometimes, after initial development with dusting or chemicals, enhancement techniques are used to improve the contrast and clarity of the fingerprint. These can include:
- Photography: Careful photography is crucial to document the developed fingerprint.
- Digital Imaging: Digital image processing can enhance the contrast and remove background noise.
In summary, latent fingerprints are made visible using either dusting techniques for non-porous surfaces or chemical treatments for porous surfaces. The choice of method depends on the surface type and the age of the print.
