“Fingerprinting” with Photography
Written by Amy L. Schaefer & Detective Donald J. Frost II   

As technology continues to evolve, the standards used by most medical examiner’s or coroner’s offices to establish positive identity of decedents has been concurrently reshaped. With an increase in the use of biometrics, a scientific method of identity has become more common rather than relying on circumstantial variables. Traditional methods of identification—such as inked prints, dental charting, and radiographic images—are used a majority of the time. Although these are most often considered the quickest and easiest sources to use, these methods are not always possible to obtain in a timely manner.


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When this occurs, photography offers an alternative method as a means for scientific identification. A photograph clearly depicting the friction ridge detail of the fingertips (inner surface of distal phalange) can be manipulated to mimic inked fingerprints and then compared with known prints on file with a local police agency or digital database.

There are many circumstances in which the consideration of photography would be a more feasible method of obtaining identification. Photographs of the ridge detail of a finger can be useful in a case where preservation of evidence is of the utmost importance. This allows identification to be obtained prior to the collection of evidence without any disturbance to the decedent.

Photography also serves as a helpful tool for capturing friction ridge detail when decomposition (desiccation/putrefaction) has set in. Desiccation causes the fingertips to become dry and hardened, often making them not pliable enough to obtain a good inked print. In putrefaction, the breakdown of tissue and gases deters the ink from adhering to the skin of the finger, again making a usable inked print next to impossible to acquire.

Timing is an additional factor to be considered. Often, dental and radiology records are only available during regular office hours, making scientific identifications during evenings, weekends, and holidays unobtainable. In all of these cases, photography can be an excellent alternative.

Probably one of the greatest challenges to the photographer in attempting to photograph the friction ridge detail of fingertips is effective manipulation of lighting. To achieve a usable image for comparison purposes, the photography technique must create sufficient lighting contrast between the friction ridges of the finger and the valleys between the ridges, across a sufficient area. The fingertip is a substantially convex surface and the photography of the friction ridge detail on the finger often utilizes varying strengths of oblique lighting (that is, lighting across the surface at an angle nearly parallel to the surface being photographed) in concert with varying strengths of diffused lighting (the diffused lighting can be from a separate light source or available ambient light or a combination of both).


Strong oblique lighting may properly illuminate only a portion of the friction ridges while simultaneously under-illuminating some portions... and yet over-illuminating other portions.

The friction ridge structures capture the light while the valleys between the ridges typically do not, producing the desired visual contrast required for comparison. As curvature of a fingertip increases, oblique lighting will only illuminate sections of the surface at a time. This creates an effect of under-illumination of portions of the surface, resulting in insufficient contrast in those areas; and over-illumination of the other portions, resulting in washing out the detail in those areas in the captured image. Proper use of the diffused lighting can help minimize the luminance extremes of the oblique lighting. Occasionally, the lighting angle may need to be less “oblique” (moving from parallel with the surface to more perpendicular to the surface). In some instances, you may also need to reduce the intensity of the light as the angle becomes less oblique. Any reduction in lighting intensity will require compensation from the camera in increased exposure time.

Every photography situation is unique in terms of factors affecting friction ridge documentation. In some cases, the ridges may be large with clearly defined valleys in between, producing vivid visual contrast—while in others, the ridges may be extremely narrow with less visual contrast. The skin may also change color during the decomposition process and could even become slightly translucent. The difficulty in the manipulation of the decedent for the photography presents additional challenges along with the fact that ambient lighting at the medical examiner’s or coroner’s facility will be varied and unique to each office. Compound those factors with the varying models of cameras available for the task and it makes the experience truly unique for each agency.

The best formula of luminance and lighting angles for each individual office will usually be derived from trial and error and will be narrowed down with each finger photographed. Knowing the factors involved that affect photography can greatly increase the chances of obtaining sufficient detail for a fingerprint comparison to a known standard or for an AFIS entry for a digital search.

Fingers undergoing decomposition can present even more of a challenge, as a wrinkled finger may possess a random arrangement of convexes and concaves simultaneously. One way to attempt to minimize this challenge is to cast the fingertip with a polyvinylsiloxane (or similar) casting material. This method should be considered as a last resort, and only with permission of the medical examiner or coroner, since it physically interacts with the body of the deceased and could destroy evidence not previously collected. When casting fingertips for photography, it is best to use a color normally utilized for casting tool marks since you are attempting to capture similar detail (brown is a common color). If a thin layer of casting material can be successfully applied to the fingertip, it can be neatly removed upon curing (usually in five to seven minutes). The trick is to get the material as thin as possible, as a thick application will simply mimic the original topography as well as the desired detail and will not be able to be flattened upon removal. Some casting systems provide the use of a special spreader tip that can lay down a flat ribbon of the casting material onto the finger. Once a fingertip has been successfully cast, the casting material can then be laid flat for photography, reducing the convex and concave topography of the substrate. Since this surface will be relatively flat, less oblique lighting may be required and in some cases the light may actually need to be more diffused and perpendicular to the surface.

Once one or more viable images of the fingertip are successfully captured, they can be manipulated to represent the equivalent of an inked fingerprint through a combination of color inversion and reversal as needed. As we mentioned earlier, the ridges will catch the light during photography, allowing them to appear lighter, and the valleys will be darkened during photography. Given that in an inked fingerprint the ridges produce the black portion of the print, and the areas of the valleys are left white, we see the need to reverse the color luminance (essentially converting the image to a “negative” of itself: the dark areas become light and the light areas become dark).


Image of decomposed finger


Luminance inversion of image


Reversal ("mirror") of image

Another factor to consider is that direct photography of the fingertip will produce a “mirror” image of the fingerprint as it relates to inked prints for comparison purposes. In other words, the detail on the left of the image will be on the right of the inked print being compared and vice-versa. This highlights another advantage of casting prints. Photography of a cast of the fingerprint negates the “mirror” issue, as the cast is already a left-right reversal of the finger. Unfortunately, photography of a cast does not negate the issue of luminance inversion (the negative of an image). The ridges of a fingertip are typically wider than the valleys in between. As such, even though the valleys of a cast print protrude from the cast in a manner similar to ridges, they are extremely narrow, and they tend to cast shadows. Conversely, the recesses of a cast produced by the ridges tend to catch light due to their surface area and appear lighter.

Once the image (whether of the fingertip or cast) is properly manipulated in terms of luminance and left-right reversal, the comparison by a latent print examiner can proceed. Ultimately, the comparison process will be a cooperative effort between the photographer from the medical examiner’s or coroner’s office and the latent print examiner (typically associated with a local, county, or state law enforcement agency) with each side of the partnership contributing their own strengths to the process.

With the abundance of current technology, it has never been easier to secure identification through scientific means. Although basic methods are often considered first, it is certainly beneficial to be aware of additional techniques, such as photography, to assist in identification when deemed appropriate. A quality photographic image of the friction ridge detail of a fingertip that is manipulated to mimic an inked fingerprint can be easily compared to and matched with a known inked or digital fingerprint standard for identification. Widespread use of digital cameras and photographic editing software makes this process ideal because it is readily accessible to most offices. With a little patience, time, and practice, photography as an additional method for identification can be a simple yet advantageous tool.


About the Authors

This e-mail address is being protected from spam bots, you need JavaScript enabled to view it is a forensic investigator for the Summit County Medical Examiner’s Office where she has worked for the past 21 years. She holds a bachelor’s degree in Criminal Justice from Kent State University and is certified as a Diplomate of the American Board of Medicolegal Death Investigators.

This e-mail address is being protected from spam bots, you need JavaScript enabled to view it is a sworn officer, full-time crime scene detective, and Master Evidence Technician with the Akron (Ohio) Police Department Crime Scene Unit. He has been a guest instructor with the Ohio Peace Officer Training Academy in London, Ohio for eight years and is currently a guest instructor with the Akron Police Department Training Bureau for police academies.

 
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