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Latent Print Examination » Homework Questions and Answers - Elementary through University » What Surfaces are Good for Prints? How are FP's stored digitally? Do computers alone make Identifications? « Previous Next »

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Posted on Tuesday, August 07, 2001 - 12:21 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

Ernie Hamm, thanks for the input. Those are very accurate reminders about the nature of latent prints (the impression matrix) in real casework.

My tiny-inking-stamp scenario (for the first question below) is an attempt to enable John Q. Public to imagine whether or not a surface is smooth enough and large enough to possibly bear an identifiable latent print.

--Ed German
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Ernie Hamm
Posted on Sunday, August 05, 2001 - 11:24 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

I would like to add a comment about surfaces from which latent prints can be found. It must be remembered that a latent print is deposited upon a surface because of some sort of contaminant/residue/substance on the friction ridge surface at the time a surface or object is touched. There must be SOMETHING on the friction ridges at the time an object is touched or something on the surface that is removed by the contact. While the successful transfer of the friction ridge details are dependent upon the surface and its condition at the time of contact, the reproduction is made by a transferring medium or a removal of substances. In the Webmaster’s analogy, the transfer is made by ink from a rubber stamp. With this type of transferring medium, a latent print could be readily deposited on many, many types of surfaces. It is not so much the surface that is touched, but the substance that causes a reproduction of the friction ridge detail AND the application of appropriate technique(s) to enhance or visualize the latent print. The significance of the transferring medium and the enhancement/visualization technique should not be confused. I know of an individual that derived some notoriety for ‘developing a latent print on human skin with an alternate light source (ultra violet)”. In fact, the latent print was made with blood (an excellent transferring medium not unlike ink) and could have ultimately been visualized with any sort of light source, but an ALS was the first technique employed. The surface (skin) and technique (ALS/UV) in that instance were not really factors; the transferring medium (blood) was the primary determinant. I agree with Webmaster’s assessment of the negative chances on the various surfaces, but as a latent print examiner, I would (as he would) like to see the object(s) and ‘give it a try’ because of the varying factors and conditions that can have a bearing on the successful recovery of a latent print.
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Posted on Sunday, August 05, 2001 - 10:19 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

First question: From what surfaces can latent finger or palm prints be lifted... and from what surfaces can they not be lifted?

Answer: Imagine that instead of latent finger or palm prints, you are considering a rubber stamp with small letters the size and thickness of normal newspaper column printing... and the rubber stamp is about the size of a pencil eraser. Next imagine that you pressed that rubber stamp against an inking pad and you are ready to begin leaving your new ink stamp impressions on all kinds of surfaces. In general, almost any surface you could stamp and leave a "legible" impression would also be a suitable candidate for latent finger or palm prints. Examples:
Wire window screen - no
Metal frame around wire window screen - yes
Imitation leather grain dashboard - yes (if you press with enough pressure)
Rounded teeth of a hair comb - no
Flat edge of a hair comb - yes
Small wire the thickness of pencil lead - no
Flat plug on the end of a small wire - yes
Burlap bag cloth - no
Big round button attached to burlap bag material - yes
Banana, apple, grape and other fresh fruit - yes
Rotten, slimy and gooey fruit - no
Smooth rock - yes
Single paper match - yes (it's been done in a real case)
Small map pin flag - yes (it's been done in a real case)
Live lizard - yes (if you can catch it)
Forged check which has been handled by many people after it was negotiated - yes (the prints of the forger can very often still be developed despite excessive handling)
Human skin - yes (although seldom attempted, it has been achieved under ideal circumstances in real cases)

Your home, car or business which is now a crime scene - Yes, but the police would have to balance various factors such as their manpower resources overall and during that day... your tolerance for expensive damage or destruction to a room or vehicle if every possible chemical technique is to be used to develop every possible latent print... and whether not-so-reliable, quick-and-dirty brush and powder processing is appropriate (which can solve crime sometimes and is also good for public relations because you will see they are making some attempt).
Your question specified "lifting," but I have answered as if you asked about surfaces on which fingerprints can be developed. Lifting typically refers to dusting with a brush and powder and then "lifting" a developed impression with adhesive tape or a special adhesive rubber (or other material) lifter. Using only a brush and powder will miss probably 50% of the identifiable latent prints present and thus "lifting" is not the best method for recovering all possible latent finger and palm prints related to a crime.

Second question: How are fingerprint records stored digitally?

Answer: Fingerprint records in modern AFIS (Automated Fingerprint Identification Systems) are typically stored in three different file types linked together:
Text data file - the ASCII letters and numbers which link the record to a specific person's name or to a specific investigation or police file number. Typically it includes name, date of birth, sex, race, fingerprint classification, and similar reference data which can help link the record or which can be used to help filter record searches.

Fingerprint image file - typically a 500 pixel per inch (ppi) or 1000 ppi digital image at least 256 shades of gray. Known prints from persons may be stored in compressed image files, but latent prints from crime scenes or from evidence are usually not compressed or compressed with lossless compression.

Fingerprint minutiae file - this file is generally only readable (understandable) by the computer software used to compare questioned finger or palm prints against millions of others in a large database. It often includes information such as whether the minutia is a ridge ending or bifurcation, the x and y axis coordinates of the minutia, the theta (angle) of the direciton of the minutia, the number of intervening ridges between the minutia and its nearest three or nearest eight neighbors. The minutiae file information is normally automatically extracted from inked or live scan finger/palm prints. Latent prints, however, due to the frequent presence of background (non-finger or palm print) interference are often manually extracted by a fingerprint expert.
Third question: Before a court case, are the actual prints still compared or do experts go strictly by the computer record of the print?

Answer: They key word in your question is “experts.” Experts always make a manual comparison before going to court. Experts only identify latent prints manually, though they may receive thousands of candidates for comparison either from AFIS or from hard-working investigators.

However, in the instance of fingerprint identification records substantiating a criminal arrest history, there may not be any manual verification involved in the AFIS ten-print-to-ten-print identification process. If the defendant challenges the fingerprint-based arrest history, then manual comparison will certainly occur before a fingerprint expert goes to court to testify. If there is no contest of the record, it is quite possible that the “rap sheet” (arrest and conviction history) may be admitted in court based at least partially on automated computer comparisons and identifications never verified by a human expert. The approximately eight states (as of August 2001) currently using such “lights out” (no human verification) ten-print-to-ten-print identification processes permit it only when there is an extremely high computer matching score based on fingerprint minutiae and only after years of evaluation to select a matching threshold score so high that they have never known a “non-ident” to occur with such numbers (although an identification may often occur with much lower matching scores).
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Posted on Thursday, August 02, 2001 - 10:48 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

My intrest is as john q public. What surfaces can print be lifed form and what surfaces can't they be lifted from? How are fingerprint records stored digitally? Before a court case are the actual prints still compared or do experts go strictly by the computer record of the print?

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