Fingerprint Detection and Processing

Fingerprint 2

Fingerprints as Fingerprints as EvidenceEvidence

• Three forms– Visible prints

• Found with the naked eye• Made after ridges come in contact with a colored

material

– Plastic print• Left on a soft material like putty, wax, etc.

– Latent• Left after body perspiration or oils touch the

surface of an object

Processing Processing TechniquesTechniques

• Fingerprint “lifting” consists of many techniques from the simple to the very complex

• Technique used is dependent upon the type of material, quality of print and environment

• Many techniques (esp. chemical) are temporary and must be photographed immediately

Processing Processing TechniquesTechniques

• Powder– Among the most common and easiest to use– Powders come in dark and light forms

• Correct choice depends on the color of the surface

– Uses dusting brush• Lightest touch must be applied• Powder evenly spread over the print• Powder will stick to the oils in the print

– Print is lifted with ordinary cellophane tape

Processing Processing TechniquesTechniques

• Iodine Fuming– Oldest chemical technique– Iodine crystals sublimate in air– Will react with either the

perspiration or fatty oils in the print (still not clear which is correct)

– Very effective especially if heated inside a fuming chamber

– Must be photographed immediately since prints will fade rapidly

• Can be “fixed” with a 1% starch solution

A laboratory-grade fuming chamber

Note: Iodine fumes are poisonous and should not be inhaled

Processing Processing TechniquesTechniques

• Ninhydrin– A chemical that reacts with amino acids

in perspiration• Reveals purple/blue prints• Usually sprayed on the object

– Development may take several hours– Technique has been used successfully

on 15 year old fingerprints– Very good for lifting prints from paper,

cardboard

Note: Ninhydrin solutions are mildly poisonous and exposure should be minimized.

Processing Processing TechniquesTechniques

• Cyanoacrylate– Commercial super glue– Will produce fumes if heated or

dripped on to a cotton ball treated with sodium hydroxide

– Fumes will “stick” to latent prints revealing a white colored print

• Very popular due to ease of obtaining materials

– Especially useful on metals, leather, electrical tape, plastic bags

Note: Super glue fumes are poisonous and flammable in high concentrations.

Processing Processing TechniquesTechniques

• Other techniques– Additional chemical

techniques exist for almost every kind of surface

– Use of polarized light, UV fluorescence

– Lasers