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 fingerprints2. Fingerprints remain unchanged during a lifetime

• Barring injury, disease, or decomposition3. 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”