Fingerprints

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Fingerprints
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Outline

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

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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

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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

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Classification

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

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Tented Arch
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Central Pocket Loop Whorl
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Double Loop Whorl
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Accidental Whorl
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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

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How does AFIS compare prints?
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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

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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

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Methods for detectinglatent prints

• Powders
• Black, gray, fluorescent, etc.
• Magnetic
• Chemical Treatment
• Ninhydrin
• Silver Nitrate
• Super-Glue
• Combined with dyes

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Chemical Treatmentof 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

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Chemical Treatmentof 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

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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 10-15 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

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Fuming Chamber
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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

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How to apply powderto 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

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How to apply powderto visualize a latent print

• 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

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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