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Visualizing and Retrieving Fingerprints

Prints that are left at a crime scene can be deposited in different ways. Some are visible but others can only be seen through enhancement.

Types of Prints

One type of visible print is made when a finger touches a soft material such as putty, wet paint, or soap and leaves behind a 3-dimensional indentation. These are known as plastic prints.

plastic print in a ball of putty

Another type of visible print is created when the finger encounters a colored material and then transfers it to another surface, creating a patent print. Patent prints can be made from blood, dirt, paint, or ink.

bloody fingerprints on a white wall

Both plastic and patent prints are collected using photography. The prints are photographed in high resolution with a measurement scale included for reference. Sometimes additional lighting, chemicals or dyes may be used but this is typically not necessary for visible prints.

Most prints, however, are not visible without enhancement. These are known as latent prints and are caused by the transfer of body oils and sweat to the surface of an object. There are several ways to detect and collect these prints. In general, the smoother and less porous a surface is, the more likely it is that prints can be successfully found and developed.


Dusting with fingerprint powder reveals a handprint on this mirror.

Collecting Latent Prints

Dusting

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.

Latent print on standard printer paper developed using magnetic black powder and photographed with a ring light. Photo by Jack Spades | Public domain

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

It is becoming more commonplace for investigators to examine any likely surfaces (doors, doorknobs, windows, railings, etc.) with an alternate light source (ALS). 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.

Latent print developed on a $1 bill using fluorescent magnetic power and photographed using UV light. Photo by Jack Spades | Public domain

Knowledge Check #1

Which of the following, if available, should be performed first?

  1. Dust for fingerprints.
  2. Use alternate lighting to reveal fingerprints.

Answer: b. Use alternate lighting to reveal fingerprints. Dusting can destroy evidence and eliminate the opportunity for further investigation of that material.

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.

Image "a" depicts the cartridge casing treated with vitamin B after being superglue fumed under ambient lighting. Image "b" illustrates the developed fingerprint under alternate light sources. "Figure 3" in "Exposure Basics" by Roberto Alfaro is licensed under CC BY-NC-ND 4.0.

This video shows how Denver Police utilize superglue processing. Watch from the beginning to 1:18.

Open Inside the Crime Lab: Latent Print Unit in a new tab

Knowledge Check #2

From the video, how is the evidence treated following the superglue chamber?

  1. Cleaned with alcohol
  2. Sprayed with a stain
  3. Viewed under a microscope

Answer: b. Sprayed with a stain

Chemical Developers

Porous surfaces such as paper are typically processed with chemicals, including ninhydrin and physical developer, to reveal latent fingerprints. These chemicals react with specific components of latent print residue, such as amino acids and inorganic salts.

  • Ninhydrin causes prints to turn a purple color, which makes them easily photographed.
  • DFO (1,2-diazafluoren-9-one) is another chemical used to locate latent fingerprints on porous surfaces; it causes fingerprints to fluoresce, or glow, when they are illuminated by blue-green light.

Paper treated with ninhydrin reagent reveals latent prints after being processed with a household steam iron.

Paper treated with ninhydrin reagent reveals latent prints after being processed with a household steam iron. Courtesy of NFSTC.

Other Collection Methods

In addition to the methods previously identified, there are special techniques for capturing prints from skin, clothing, and other difficult surfaces.

  • Amido Black, a non-specific protein stain that reacts with any protein present, is typically used for developing or enhancing bloody impressions on human skin.
  • To reveal prints on clothing, high-tech methods such as vacuum metal deposition using gold and zinc are showing promise for the investigator.

Chemical Processing

Chemical Processing for Latent Prints from the National Forensic Science Technology Center summarizes a variety of the chemicals used to process latent prints, describing the surfaces that the chemicals work well on, what type of alternate light source (ALS) is required, as well as the light filter that should be used for photographing the prints. This processing is typically done in the laboratory, as many of the chemicals are flammable or toxic. These precautions are included in the table.

Knowledge Check #3

According to the Chemical Processing for Latent Prints chart, which chemical is best to use to develop prints on a plastic bag?

  1. Small particle reagent
  2. Sudan black
  3. Rhodamine 6G
  4. It is not possible to process prints on a plastic bag.

Answer: b. Sudan black

Knowledge Check #4

According to the Chemical Processing for Latent Prints chart, what precautions should be taken when working with amido black?

  1. Gloves
  2. Eye protection
  3. Mask
  4. Fume hood, if available
  5. All of the above should be used when working with amido black.

Answer: e. All of the above should be used when working with amido black.

Like fingerprint powders, chemical processing can reduce the investigator's ability to perform other techniques that could reveal valuable information. Therefore, any nondestructive investigations are performed before the evidence is treated with chemicals. For example, a ransom or hold-up note will be examined by a questioned documents expert before being treated with ninhydrin, since some formulations of ninhydrin will cause certain inks to run, thus destroying the writing.

New Methods

Scientists are continually working to develop new methods to visual prints with even greater detail. This video shows one such method, inspired by the intricate eyes of a fly.

Open A New Way to See Fingerprints in a new tab

Knowledge Check #5

How is CTF better than current methods to develop fingerprints?

  1. The machine is smaller and portable.
  2. The process is quicker.
  3. More ridges and details are revealed.
  4. It can process fingerprints at the scene of the crime.

Answer: c. More ridges and details are revealed.

Credits

NFSTC. "How It's Done: Fingerprint Analysis." A Simplified Guide to Crime Scene Investigation, National Forensic Science Technology Center, September 2013, http://www.forensicsciencesimplified.org/prints/how.html.