Fingerprints

Outline  Individuality of Fingerprints  What are fingerprints?  Classification of fingerprint patterns  AFIS  Methods of detecting latent fingerprints

Individuality of Fingerprints  Started collecting fingerprints around 1910 (Civil Service)  Millions of fingerprints have been classified over past 95 years. –No two are identical! –Estimated statistical probability of 1 in 64 billion that two people have identical fingerprints

What are fingerprints?  Combination of oils, salts, proteins, perspiration left behind by the friction ridges on fingertips  Friction Ridge –Epidermis –Dermis –Dermal papillae  Formed in the uterus  Determines pattern of friction ridge  Same shape for life –Only grow in size

Classification  Loop –Ulnar – opens toward pinkie –Radial – opens toward thumb  Arch –Plain –Tented  Whorl –Plain –Central pocket loop –Double loop –Accidental

Tented Arch

Central Pocket Loop Whorl

Double Loop Whorl

Accidental Whorl

AFIS  Automated Fingerprint Identification System –Converts a fingerprint image into digital minutiae  Relative locations of ridge endings  And bifurcations –Thousands to millions of comparisons per second –Comparison must be verified by an expert

How does AFIS compare prints? X Y

Live Scan Technology  Fingers (and thumbs) rolled on a small section of glass  Pressure from ridges fed directly to computer  Fingerprint appears on screen as finger is rolled  Keeps track of which finger is being rolled based on combination  No need to roll in ink then scan with scanner  Coupled directly to AFIS

Types of Prints left at a Crime Scene  Visible –Fingerprints left from fingers that have been in contact with colored material  Blood, paint, grease, ink  Plastic –Ridge impressions left in soft material  Wax, putty, soap, dust  Latent –Invisible prints caused by the transfer of body perspiration or oils

Methods for detecting latent prints  Powders –Black, gray, fluorescent, etc. –Magnetic  Chemical Treatment –Ninhydrin –Silver Nitrate –Super-Glue  Combined with dyes

Chemical Treatment of Latent Prints  Ninhydrin –This chemical reacts with amino-acids on proteins  Turns dark blue –Quite useful on porous surfaces where dusting fails –Can take up to 24 hours for latent print to develop  Heat speeds this up

Chemical Treatment of Latent Prints  Silver Nitrate –Perspiration contains salt (NaCl) –Silver nitrate reacts with chloride to form silver chloride –Silver chloride can be converted to silver metal with UV lamp  Ag + + Cl -  AgCl + UV  Ag (metal)  Good for porous surfaces like paper

Chemical Treatment of Latent Prints  Fuming Super Glue –Hang object in a sealed container  Covered fish tank –Heat a small amount of super glue in a dish on a hot plate  Cyanoacrylate fumes react with components of latent prints in minutes  A white color is formed by this reaction –Good for large areas  Like the inside of a car  Difficult to remove prints once developed with this method  Can treat super-glue print with fluorescent dye

Fuming Chamber

Powders for latent prints  What color powder does one choose? –Black, Gray, Red, etc.  Powder color is chosen to contrast the color of the surface –Black surface, choose a gray powder –Light surface, choose a black powder –Multi-colored surface, fluorescent powder

How to apply powder to visualize a latent print  Never touch the bristles of a fingerprint brush  Twirl or tap to remove powder from previous application  Use a different brush for each powder  Dusting powder in a shallow dish  Dip tip of bristles in powder –Do not allow powder up in the center of bristles  Just graze the surface with a circular or sweeping motion

 Once a print becomes visible –Be careful not to dust with too much powder –Continue as before until the direction of the ridges are visible –Follow the ridge direction with the brush motion –If necessary, remove excess powder with clean brush How to apply powder to visualize a latent print

How to Lift a Print  Peel off 6-7 cm of transparent bookbinding tape –Do not cut  Cover the print by smoothing the tape over the print beginning at free end and working slowly over the print applying pressure with your finger –Do not simply lay tape over print  Air bubbles will destroy it  Once tape covers print –Use roll as handle to peel tape from surface in one continuous motion  Prevents distortion of print  Smooth tape onto card of contrasting color as powder –Cut tape