| Author | Message | ||
     Atif (Atif) Member Username: Atif Post Number: 1 Registered: 04-2015 |
From: (Journalism Student at a northeastern-US University) Date: 2005/04/14 Thu PM 01:58:53 EDT Subject: Fingerprint IDs I am a journalism student… and I am very interested in forensic science. I have read your web page and I was hoping you might …answer a few questions about fingerprint identifications. For example. Why is it necessary to identify points in the Level two analysis. Why can't you overlay a print from a crime scene onto one from a fingerprint card and see if they match? Also are there established categories used for the characteristics used in the Level three analysis (for example standard terms to describe shape or edge contours)? _______________________ Atif | ||
| asdf qwe (Sarakhan) Member Username: Sarakhan Post Number: 1 Registered: 11-2014 |
Now, why would a ten year old's fingerprints be in a national database: immigration mostly. __________________________ sara | ||
| Webservant |
The below questions were emailed to me recently by a journalism student. Although answered elsewhere in this forum using different wording, this posting may help others with similar questions. Also, some of my brother and sister experts can hopefully supplement my answers with important information and concepts that I omitted. Cheers, --Webservant From: (Journalism Student at a northeastern-US University) Date: 2005/04/14 Thu PM 01:58:53 EDT Subject: Fingerprint IDs I am a journalism student… and I am very interested in forensic science. I have read your web page and I was hoping you might …answer a few questions about fingerprint identifications. For example. Why is it necessary to identify points in the Level two analysis. Why can't you overlay a print from a crime scene onto one from a fingerprint card and see if they match? Also are there established categories used for the characteristics used in the Level three analysis (for example standard terms to describe shape or edge contours)? Regards, (Journalism Student) --- Short answers to your questions: Q1. Why is it necessary to identify points in the Level two analysis. A1. Assuming your use of the word identify does not mean individualize, but means “use,” then my answer is as follows: Fingerprint experts look first at the Level 1 detail overall ridge flow and shape (no need to compare a circular-shaped whorl pattern with a finger that has only flat ridges flowing from side-to-side of the impression)… then they look for Level 2 ridge features (such as ending ridges and bifurcations) as sort of a road map to navigate and detect corresponding or dissimilar areas. In theory, skeletonized images represented by lines only one pixel thick could be used for the preliminary filtering steps involving Level 1 and Level 2 detail. When Level 1 or Level 2 detail is different, an elimination may be effected. However, when Level 1 and Level 2 detail is similar between images, a meaningful latent print comparison decision would require images representing actual ridge shapes (Level 3 detail) before an identification decision can be made. Q2. Why can't you overlay a print from a crime scene onto one from a fingerprint card and see if they match? A2. You can perform such overlays, but the flexibility of friction ridge skin means that even among multiple impressions you make from just one finger, there will be great variability that negates direct overlay comparison. In the past 125 years of research and development, that approach has been a common attempt that was abandoned once real world tests were commenced. Q3. Are there established categories used for the characteristics used in the Level three analysis (for example standard terms to describe shape or edge contours)? A3. There are articles and books giving word description examples for Level 3 detail, but there are no formal, established categories of Level 3 descriptive wording because word descriptions are not used by Latent Print Examiners to effect identifications. We look at images and actual impressions to effect friction ridge comparisons. Some of us use similar word descriptions to describe features we observe, but none of us base identifications on words. You are able to positively identify your mother or your spouse when you see a good quality photo of her/him… even to the point that you would be willing to swear in court to the identification. However, even if you have special artistic training you probably cannot compile a word picture that would incorporate the same words another artist would use to describe the photo of your mother/spouse… and probably neither your word compilation nor another artist’s words could suffice to enable a third person to positively identify your mother (to the exclusion of all other persons) based only on words. --- Longer Answers to your questions… You are probably aware that most latent fingerprint units worldwide incorporate the use of computers for matching against large repositories. All large scale AFIS systems use 1 through 3 as follows: 1. General ridge flow and ridge pattern shape (Level 1 detail) to filter searches (binning) and reduce the amount of computer records to be matched. 2. Ridge ending and bifurcation (Level 2 detail) X, Y, theta and ridge-count-to-nearest-neighboring ridge ending or bifurcation. This step is accomplished typically using a skeletonization process that reduces ridge detail to a narrow line excluding most or all Level 3 detail of ridge shape. 3. A candidate list of potentially matching fingerprint records with associated fine resolution (500 ppi or 1000 ppi) images of fingerprints to enable a human to compare images. Universally, nobody uses only the skeleton image (Level 2 detail only) from step 2, above, for side-by-side comparison decisions. The reason they do not use the skeleton image is that even those persons lacking training to explain the nuances of Level 1, Level 2 and Level 3 detail, know that they must be able to look at the actual shapes of the ridges to effect a meaningful comparison. Persons with lesser training (lesser than many well-trained Latent Print Examiners) may not be able to explain Level 1, Level 2 and Level 3 detail, but those same persons would ask you if you are crazy if you were to attempt to have them make comparisons from a latent print candidate list using only skeletonized versions of known and questioned impressions … they would want to see the actual ridges because that is what they use for comparisons (not just Level 2 detail). Your question about overlaying fingerprints indicates that you may not understand the flexibility of friction ridge skin. Imagine that instead of fingerprints, you are talking about a rubber stamp that has your name and address. Also imagine that instead of being relatively hard rubber, the name stamp is made of especially soft and flexible material that moves very easily... as easily as you can push around the flexible skin on your fingertips when you roll them against a desk. Also imagine that instead of being mounted on a flat surface like most name stamps, your special stamp is attached to a rounded dowel rod. Thus, when you push the special name stamp against a surface you will have infinite variability insofar as what portion of your name stamp comes into contact with the receiving surface (sometimes getting more or less of individual letters) and because the rubber in your special stamp is much more flexible than normal stamps, the letters can lean over against each other, can twist, can flatten out, or exhibit other signs of pressure distortion due to the three dimensional height of the extra soft rubber characters. Add to these variables the fact that the matrix (paint, ink, blood, oil, dust, three-dimensional impression in wax or tar, faint or strong eccrine gland or sebaceous gland deposits, etc.) of the evidentiary impression can be very different from the nature of images captured from live scan or inked prints... and you can see how your special, soft rubber stamp can leave impressions that would seldom overlay exactly with one another. Two impressions that can be exactly overlaid should cause suspicion that one is a duplicate impression of the other (a fabrication?) because the variables involved would make it highly unlikely that in a given investigation there would exist two fingerprints where all the pressure distortion, matrix appearance, beginning and ending of ridge detail areas around the perimeter of the impression, etc., would be close enough for an exact "overlay match." If you ponder the special, soft rubber stamp example even more, you can probably imagine that depending on the nature of the matrix, amount of distortion and amount of the rubber stamp that actually made contact, there could be a great variety of quantity and quality of information representing your name and address in different impressions. To be of value to determine enough information is represented in an impression to convince you it is from your name stamp versus someone else’s who has some of the same letters, portions of first, last, street and city names, street number, zip code, etc., you would not need all of the information all of the time. When the quality of the matrix recording the stamp detail is poor (less distinct or sharp) or when there is considerable distortion, a greater amount of the overall stamp impression would need to be recorded (represented in the impression) to convince you it is your name stamp. Conversely, if the quality of the impression is high, you should be able to pick out the small nicks and cuts unique to individual letters on your stamp (although not precisely the same as Level 3 detail, these fine detail (accidental characteristics occurring through use) nicks and cuts are what Questioned Document Examiners use to identify rubber stamps, along with overall lettering, font shape, horizontal and vertical spacing, etc.). Thus, with fingerprint comparisons, rubber stamp comparisons, and most similar visual comparison activities, there is a variable balance between quality and quantity of information required in order to effect an identification. A full-face photograph of your mother is not required (maybe only 40% is sufficient depending on the area represented) when the photograph of her includes finely focused and legible detail. Conversely, even a full-face image may be insufficient if the image is out of focus or was taken so far away that pixel spatial (similar to film grain) resolution reduces legibility. Hopefully you can grasp the concept that you are able to positively identify your mother when you see an image that includes sufficient quality and quantity of information… but that the infinite variables of radiometric, spatial and spectral resolution in digital imaging require that you actually examine the image in question (and evaluate the area and amount of her face represented, along with the quality of the image) before you can make the determination as to whether or not the image can be identified. In considering how and why fingerprints have such variable procedures/wording/definitions nationwide and worldwide, imagine what it would be like if fingerprints were not studied and used in practical applications for the past hundred years in a wide variety of large and small government operations with varying scientific aptitude, cultures, rules, interface with other organizations/countries, etc. If instead of fingerprints, iris images had been adopted as the most common means of identifying persons worldwide during the past 125 years, and fingerprints were only just now coming into vogue for novel identification usage, undoubtedly fingerprints today would require nearly perfect images of complete impressions for their use at all (just as iris scan now requires nearly perfect, high resolution images of the iris before an image is considered useful). With time, and after decades of study and practical application, it could be determined that nearly perfect and complete fingerprint impressions are not always required and that with a proper balance of quality and quantity, identifications can be reliably effected on less than perfect finger or palm impressions. In 1943, that is what happened in America when the FBI and overall US law enforcement abandoned the minim point rule (points being what we now call Level 2 detail) commonly cited in US literature before then. With careful study and practical application over many years, iris identification will also evolve in a manner resulting in positive identification decisions based on iris images now normally considered unsuitable. Such endeavors may already be underway in surveillance identification studies using telescopic lenses that capture less than perfect iris images due to movement (walking and other normal head movement) and due to the distance from the person (introducing reduced spatial resolution - similar to film grain problems). The adoption of less stringent and modified standards is a natural evolution for scientific disciplines based on extensive observations that include millions and millions of real world practical applications over decades. I hope that some of my layperson examples are able to communicate the nature (complexity and simplicity) of latent print examination procedures and why the current state of the science is exactly as it should be given the practical and scientific evolution occurring during the past 100+ years. Cheers, --Webservant |
