By: Steven L. Petersen BA, CCSI; Shawn L. Naccarato DDS, CCSI; and Gary John, CLPE, CSCSA, Issue: December 2007/January, 2008 Forensic Magazine

Old technique (iodine fuming) plus new technology (digital subtraction) equals more options for the development and enhancement of latent fingerprints.

The use of iodine fumes to locate latent fingerprints has been available to criminalists for years. Iodine fumes react with sebaceous materials in a fingerprint in a reversible physical process that does not alter or interfere with the subsequent application of other development techniques such as 1, 2 Indanedione, Ninhydrin, DFO, etc.

The visible image of the fingerprint, which arises subsequent to the application of iodine vapors, is a transient image that will fade over time. Therefore, the image must be photographed as soon as possible after it is visualized. Some may consider the disappearance of the iodine vapor image to be a disadvantage or, at the very least, a nuisance and opt to utilize other, more stable methods for developing latent fingerprints on surfaces such as paper, wood, glass, and plastics.

With the advent of Adobe Photoshop, the use of the “older” fuming techniques of iodine vapors when combined with the “newer” Photoshop techniques of digital subtraction can offer another effective tool to develop and enhance latent images on visually difficult or distracting backgrounds. This method offers an easy, cost-effective mechanism for developing and visualizing images on difficult backgrounds, while at the same time maintaining the integrity of the fingerprint for subsequent application of other processing chemicals/techniques.

METHODOLOGY
First, we processed the items of evidence utilizing the traditional iodine fuming technique. For this research, an iodine fuming wand was used. The items were then photographed using a Fuji S2 Pro mounted on a photography table with adjustable height track and standard side-lighting. A scale was used in the first picture.

Next, without moving the item of evidence or changing any of the lighting conditions, we removed the scale and took another picture after the iodine had dissipated. Iodine will dissipate over time, at least 24 hours, or it can be sped up to a matter of minutes by placing a source of ammonia fumes near the item. The primary consideration at this stage involved insuring that the item of evidence had not moved, and that the lighting conditions on the first photograph were the same as those on the second photograph.

After both photographs were taken, they were brought into Adobe Photoshop. For this experiment, Adobe Photoshop CS2 was used. The following procedure and settings can be used to duplicate our methods:

• With both images in the working window, select the image with the scale.
• Select Image > Calculations. A Calculations window will now appear. Source 1 will be the image with no scale and source 2 will be the image with scale. The channel that has worked the best in our research has been the blue channel.
  However, depending on the colors in the background, a different channel might be preferable.
• Blending will be set as Subtract. Opacity should be set to 100%.
• Offset can be set from 65 to 100 depending on the requirements of the image.
• Scale should be set to 1.
• Result should be set to New Channel.
• Once these steps have been completed, click OK and a new channel will appear in the Layers Window.
• Select this channel “Alpha 1,” and then select Image > Mode > Grayscale. At this point, all of the usual enhancements such as brightness, contrast, levels, etc. can be adjusted to the criminalist’s preference.

RESULTS AND DISCUSSION
Traditional iodine fuming followed by digital subtraction can be a useful technique for the development of latent fingerprints on difficult or “busy” visual backgrounds.
Advantages of this method include:

• The technique is relatively easy to learn and utilize.
• Costs associated with the technique are minimal if Adobe Photoshop is already being utilized.
• The latent image remains undamaged by this process so other development processes can subsequently be utilized.

Disadvantages of this method include:

• This technique can be a bit time-consuming as the visual image can take some time to dissipate.
• The visual image must completely disappear from the “loud” background prior to taking the second photograph.
• Lighting and positioning of the evidence item must be strictly maintained and controlled for both the photo with the visible print on the “loud” background and the subsequent photo of just the “loud” background.

Some examples of prints that were visualized using the digital subtraction method are shown in Figures 1, 2, and 3.

Captain Gary John, CLPE, CSCSA is the Crime Lab Manager with the Canyon County Sheriff’s Office in Caldwell, ID.

Shawn L. Naccarato DDS, CCSI is a Criminalist and Forensic Odontologist with the Canyon County Sheriff’s Office in Caldwell, ID.

Steven L. Petersen BA, CCSI is a Criminalist with the Canyon County Sheriff’s Office in Caldwell, ID.

Canyon County Sheriff’s Office, 1115 Albany Street, Caldwell, Idaho 83605, 208-454-7528.

This article originally appeared in the December 2007/January, 2008 Forensic Magazine and reprinted with the permission of the Forensic Magazine and Steven Peterson.
Click here to view original article in the Forensic Magazine