#fingerprintdetection

[forensicfield]

Fluorescence Examination:: As early as 1933, fluorescence examination with UV light was suggested as a method of visualizing latent prints dusted with anthracene powder on multicolored surfaces (Inbau, 1934, p 4). Before the late 1970s, UV fluorescent powder was used occasionally and appears to have been the only credible fluorescent method of latent print detection. In 1976, researchers at the Xerox Research Centre of Canada discovered inherent latent print fluorescence via continuous wave argon ion laser illumination. Shortly thereafter, the first latent print in a criminal case was identified, using inherent luminescence via laser excitation (fingerprint on black electrical tape) (Menzel and Duff, 1979, p 96). Since the late 1970s, advancements in the technology of fluorescence detection have greatly aided the hunt for many types of forensic evidence. Today, evidence that would be barely perceptible or even invisible under normal lighting is routinely intensified by fluorescence. Bloodstains, semen, bruises, bone fragments, questioned documents, flammable residues, fibers, and fingerprints all merit examination with a forensic light source or laser.

To visualize latent prints via fluorescence, a specific bandwidth of radiation must be shone on either an untreated latent print or one treated with a fluorescent chemical. The wavelengths chosen will be determined by the chemical involved and the luminescent nature of the substrate. The evidence is then examined through viewing goggles or filter plates that block the incident light from the forensic light source. These goggles act as a barrier filter and are fundamental in separating the incident light generated by the light source and the weak fluorescing signal emitted by the latent print. This separation of incident and emitted light signals gives fluorescence examination its sensitivity.

[forensicfield]

Fingerprint patterns: • Arch: Ridges flow one above the other in a gently arching configuration. There are no deltas or recurves. • Tented arch: Arch pattern with at least one up-thrusting ridge at right angles to the ridges above it. • Ulnar/radial loop: Consists of at least one free recurving ridge and one delta. These patterns are named after the ulna and radius, the bones of the forearm. In ulnar loop, recurves originate from and return to the little finger side of the hand, whereas recurves in radial loops originate from and return to the thumb side of the hand. • Whorl: Consists of at least one recurving ridge and two deltas. A straight line drawn between the points of delta will cross at least one of the ridges revolving around the core. • Central pocket loop: Similar to the whorl, except that none of the recurving ridges will be crossed by a straight line between the points of delta. • Double loop: Consists of two looping formations and two deltas. • Composite: Contains at least two of the previous described basic patterns, excluding the arch. • Accidental: Contains two deltas, one in front of an up-thrusting formation and the other in front of a recurving formation.

[forensicfield]

Chemicals which are used to detect fingerprints.

1.) Ninhydrin is most commonly used to detect fingerprints, as the terminal amines of lysine residues in peptides and proteins sloughed off in fingerprints react with ninhydrin. It is a white solid which is soluble in ethanol and acetone at room temperature.

2.) Zinc Chloride is also used by forensic laboratories for enhanced finger print detection.

3.) 1,8-Diazafluoren-9-one, also known as DFO, is a chemical that is used to find fingerprints on porous surfaces. It makes fingerprints glow when they are lit by blue-green light. DFO reacts with amino acids present in the fingerprint to form highly fluorescent derivatives.

4.) Nonpolar Ninhydrin is used to detect fingerprints because it reacts with amino acids from the proteins in skin cells transferred to the surface by the individual leaving the fingerprint.

5.) 1,2-Indanedione is the result of the search for a low cost substitute for DFO. Like DFO and ninhydrin, LV508 reacts to amino acid residues on porous surfaces, but it rarely produces visible prints. Developed prints do produce strong fluorescence when exposed to comparable alternate light source illumination.

6.) 5-Methylthioninhydrin (5-MTN) is a reagent for fingerprints that combines the favorable characteristics of ninhydrin (strong color) and those of DFO and 1,2-IND (strong fluorescence). 5-MTN-developed fingerprints have a stronger color than those developed with regular ninhydrin. 

7.) 2-Isononylninhydrin (INON, Thermanin) is used to detect fingerprints.

8.) Silver nitrate has been an established agent for the detection of latent fingerprints for some 120 years, and it was one of the few reagents suitable for use on porous surfaces until ninhydrin was introduced in forensics.

[forensicfield]

Question- How is fingerprint matching done? Answer- fingerprints can be compared to each other by examining the minutiae to determine whether: • the same minutiae are present (eg a bifurcation); • the minutiae flow in the same direction (eg the bifurcation is on a ridge running horizontally and the two divided ridges are to the right of the bifurcation); and • the minutiae occupy the same relative positions to each other (eg the bifurcation is separated from an enclosure below it by six intervening ridges). Where minutiae on two different fingerprint impressions meet these criteria, they are referred to as points of similarity. Where minutiae do not meet these criteria, they are referred to as points of dissimilarity. When sufficient minutiae are located in the same true relative sequence or unit relationship, then an identification is assumed, and the points of similarity are referred to as points of identification.

Fingerprints have usually 20 to 30 minutiae and two fingerprints are considered to be matched if they have more than 10 matched minutiae.

Question- How many points are needed for a fingerprint match? Answer- Fingerprint experts can disagree about how many points in common are needed to declare a match between two sets of fingerprints. For example, some experts will declare a match based on only 12 points in common, whereas other experts may require up to 20 points in common before declaring a match.