What are fingerprints?
• Impressions left on any surface composed of
patterns made by the friction ridges
– Same definitions could apply to toe, foot, or palm
prints
• Three types:
– Latent
– Patent
– Plastic
Plastic Prints
• Impressions left in
something pliable
like clay, wax, or even
paint
– Some paintings are
authenticated by
fingerprints of the
artist found in the
paint
Patent Prints
• Visible prints left on a
surface with blood, ink,
or some other liquid
• “Known” prints are
patent and usually
made of ink on a
fingerprint card
Latent Prints
• Invisible or transparent impressions;
unintentionally left
• Must have transfer medium to leave print
• Primarily left by the oils, perspiration, salts,
and proteins of the body
– Made primarily of water
• May or may not be absorbed by the item on
which it is left
How can you find an invisible print?
• Latent prints are processed based on the
substrate on which they are left:
– Porous: absorbs the transfer medium
• Chemical processing
– Non-porous: does not absorb
• Physical & chemical processing
• And on the condition of the evidence
– Photographs are taken to preserve
Chemical Processing
• Cyanoacrylate ester or super glue
– Reacts with moisture, polymerizing the ester, & depositing it
• Amino acids, fatty acids, and proteins in the print
• Water vapor in the air
– Can be accelerated with circulation, heating, & water vapor
• Ninhydrin
– Reacts with amino acids (any amino acids) to produce a purple
color
– Accelerate with heat & humidity (ie, an iron); photograph
Physical Processing
• Alternate Light Sources (ALS)
– Different wavelengths of light that excite components of
some powders or dyes to better enhance prints
• Powders
– Made of a resin that adheres to the print and color to
visualize it
– May be black, white, fluorescent, or magnetic
– Black powder mixed with water can be used to enhance
prints on adhesive surfaces
• Dyes
– Many different kinds and colors
– Used after Superglue fuming with ALS
Fundamental Principles of Fingerprint
Identification
1. Fingerprints are individual
•
•
•
Unique to a single & specific person
No two fingerprints have been found to have identical
minutiae
Mathematic calculations support the very low likelihood
of having identical fingerprints
2. Fingerprints remain unchanged during a lifetime
•
Barring injury, disease, or decomposition
3. Fingerprints have ridge patterns that allow for
classification
Comparison
• Analysis
– Look at ridge flow, orientation, & minutiae
– Determine if print has enough detail or value for
comparison
• Compare
– Minutiae between two prints
– Are the characteristics & orientation identical?
• Evaluation
– Is there enough minutiae in common to determine
identity?
• Verification by another examiner
– Have another examiner review the prints without
knowledge of your conclusion
Analysis
Compare & Evaluate
Verify
AFIS
• Automated Fingerprint Identification Systems
– Before AFIS, complicated Henry system used to assist in
manual comparisons
• Computer system turns print pattern into skeletal
image
• Ridge endings & bifurcations used as identifiers
– Gives placement & orientation of minutiae
• A search algorithm creates a geometric pattern from
these points & compares it
AFIS
• After a print is searched:
– A fingerprint examiner
reviews the candidate list
produced by AFIS
– As needed the 10-card or evidence for the candidate is
requested
– The searched print is manually compared to the candidate
print
• The examiner with follow ACE-V
• The results are registered as a hit or no hit
Important to remember
• Fingerprints can not tell when someone was
somewhere
– Children’s prints are especially difficult to recover
• Prints are compared manually with plastic,
patent, photographed, or visuallized
• At no point does the computer overlay
something and flash “MATCH”