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
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
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
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
- With both images in the working window, select the image
with the scale.
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
- 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.
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.
visual image must completely disappear from the “loud” background
prior to taking the second photograph.
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.
Gary John, CLPE, CSCSA is the Crime Lab Manager with the
Canyon County Sheriff’s Office in Caldwell, ID.
Naccarato DDS, CCSI is a Criminalist and Forensic Odontologist
with the Canyon County Sheriff’s Office
in Caldwell, ID.
L. Petersen BA, CCSI is a Criminalist with the Canyon County
Office in Caldwell, ID.
Office, 1115 Albany Street, Caldwell, Idaho 83605, 208-454-7528.
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