Patent prints can be found on a wide variety of surfaces: smooth or rough, porous such as paper, cloth or wood or nonporous such as metal, glass or plastic. Latent prints can be found on a variety of surfaces; however, they are not readily visible and detection often requires the use of fingerprint powders, chemical reagents or alternate light sources.
Generally speaking, the smoother and less porous a surface is, the greater the potential that any latent prints present can be found and developed. Patent prints are collected using a fairly straightforward method: photography. These prints are photographed in high resolution with a forensic measurement scale in the image for reference.
One of the most common methods for discovering and collecting latent fingerprints is by dusting a smooth or nonporous surface with fingerprint powder black granular, aluminum flake, black magnetic, etc. If any prints appear, they are photographed as mentioned above and then lifted from the surface with clear adhesive tape.
The lifting tape is then placed on a latent lift card to preserve the print. However, fingerprint powders can contaminate the evidence and ruin the opportunity to perform other techniques that could turn up a hidden print or additional information. Therefore, investigators may examine the area with an alternate light source or apply cyanoacrylate super glue before using powders. Alternate Light Source ALS : It is becoming more commonplace for investigators to examine any likely surfaces doors, doorknobs, windows, railings, etc.
These are laser or LED devices that emit a particular wavelength, or spectrum, of light. Some devices have different filters to provide a variety of spectra that can be photographed or further processed with powders or dye stains. For example, investigators may use a blue light with an orange filter to find latent prints on desks, chairs, computer equipment or other objects at the scene of a break-in.
Using a fluorescent dye stain and an orange alternate light source helps this latent print appear clearly so that it can be documented. Use of various alternate light sources may help enhance the appearance of a fingerprint. Cyanoacrylate: Investigators often perform cyanoacrylate superglue processing, or fuming, of a surface before applying powders or dye stains. This process, typically performed on non-porous surfaces, involves exposing the object to cyanoacrylate vapors.
The vapors fumes will adhere to any prints present on the object allowing them to be viewed with oblique ambient light or a white light source. A chamber specially designed for exposing latent prints to super glue fumes. Failing to do so will reduce the accuracy of the process by about 30 percent, he warned.
Full versus partial The database represents full sets of fingerprints. Criminals usually only leave partial prints, but the system can handle a partial print as long as it is big enough to include five separate distinguishing points, he noted. However, even if the prints are in the system, they cannot always be matched to the evidence print, since the part of the fingerprint on file that would match the evidence print might be blurred or smeared, he added.
He said that about 26 percent of the cases received by the lab include identifiable fingerprints. These often include multiple sets of fingerprints, since the investigators will submit all the prints found at the crime scene. Investigators are asked to submit "elimination prints" of everyone who had legitimate access to the crime scene.
Indeed, criminals themselves were so intimidated by the prospect of being fingerprinted that, in , a suspect arrested by Scotland Yard desperately tried to slice off his own prints while in the paddy wagon. Yet quite apart from these scientific claims, police fingerprinting was also simply prone to error and sloppy work. Depending on what city you were tried in, the standards could vary dramatically. And to make matters more complex, when police lift prints from a crime scene, they are often incomplete and unclear, giving authorities scant material to make a match.
So even as fingerprints were viewed as unmistakable, plenty of people were mistakenly sent to jail. Simon Cole notes that at least 23 people in the United States have been wrongly connected to crime-scene prints. Nonetheless, the reliability of fingerprinting today is rarely questioned in modern courts.
One exception was J. Letts was astounded to hear that the standard for declaring that two prints matched varied widely from county to county. Letts threw out the fingerprint evidence from that trial. The world of DNA identification, in comparison, has received a slightly higher level of skepticism.
When it was first discovered in , it seemed like a blast of sci-fi precision. Alec Jeffreys, a researcher at the University of Leicester in England, had developed a way to analyze pieces of DNA and produce an image that, Jeffreys said, had a high likelihood of being unique.
DNA quickly gained a reputation for helping free the wrongly accused: Indeed, the nonprofit Innocence Project has used it to free over prisoners by casting doubt on their convictions. Yet DNA identification, like fingerprinting, can be prone to error when used sloppily in the field.
Sorting relevant from random is a particular challenge for the simple DNA identification tools increasingly wielded by local police. When investigating a crime scene, local police may not have the training to avoid contaminating their samples. The courts have already recognized the dangers of badly managed DNA identification.
In —only five years after Jeffreys invented the technique—U. Even the prosecution agreed it had been done poorly. It even seems the public has grasped the dangers of its being abused and misused.
In fact, not all 23 were convicted or imprisoned. This story has been edited to correct that fact. Smithsonian regrets the error.
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