A unique study performed by the U.S. Secret Service shows that printers and copiers
actually leave reproducible physical impressions on paper which can be detected
using a technology known as electrostatic detection. First marketed by Foster
and Freeman, Ltd., (U.K.) as the electrostatic detection apparatus (ESDA), ESDA
provides forensic document examiners with a reliable way to identify indentations
on documents.
The Secret Service study (J Forensic Sci, May 2004, Vol. 49, No. 3) examined
the feasibility of using ESDA to identify individual characteristics that can
be used to associate a document in question with a specific office machine.
“
The findings of this study have been promising based on observations and theory,” said
Gerry LaPorte, a forensic scientist in the Secret Service's Forensic Services
Division. “Indeed, it has been shown that printers and copiers can impart
reproducible physical impressions on paper which are detectable using ESDA.”
The study noted that “documents produced on office printers and copiers
are often associated with a variety of crimes involving counterfeit identification
documents, counterfeit financial obligations (e.g. currency and business checks),
threatening letters, contracts, wills, financial accounts, and criminal record-keeping.”
“Forensic document examiners now have another use for the ESDA/EDD and
another technique to use in identifying a particular printer,” said Arizona
forensic document examiner Sandra Ramsey Lines, editor of the Journal of the
American Society of Questioned Document Examiners.
Paper Chase
The LaPorte study proposed to answer five questions:
Do printers and/or copiers impart physical impressions on documents?
If physical impressions are present, are they detectable using ESDA?
Are the impressions reproducible?
Can the detectable impressions be used as class characteristics?
Can individual characteristics be determined using this methodology?
Traditionally, ESDA is used to detect indented impressions of handwriting on
paper. The LaPorte study was to determine whether impressions left by printer
and copier hardware are likewise visible.
Prior to performing an ESDA, LaPorte fed blank sheets of paper dusted with
black magnetic powder through the printers so places where the machine contacted
the paper could be easily seen.
In the study, LaPorte “observed impressions made by parts of the feeding
mechanism (e.g. roller wheels and picker bars) on the front and back of the
paper. Moreover, the markings on the front were different than those on the
back, indicating the importance of performing ESDA examination on both side
of a questioned document.”
“The presence of fine striations and minutiae is essential to the assessment,” LaPorte’s
paper stated. “It therefore cannot be emphasized enough that minimal,
careful handling of documents is necessary since it is quite easy to impart
artifact impressions that are detectable following an ESDA examination.”
LaPorte stressed the need for familiarization with the hardware and an understanding
of the components. For example, three Epson Stylus Color printers (models 600,
740, and 900) are constructed on the same platform, but there are additional
star wheels on the different models. Contact does not always occur between
the paper and the star wheel. Instead, physical contact is dependent on whether
the tray is pulled out to catch the paper, he noted.
“Identification of class characteristics on documents can be valuable
information to corroborate and support additional investigative findings, but
identifying individual characteristics is tremendously more substantive,” LaPorte
stated in his paper. “Individual characteristics found on documents produced
from a printing system will allow the forensic examiner to definitively link
two or more questioned documents to each other or to a suspect office machine.”
“Computer printers have replaced typewriters in today’s world,
so being able to link computer printers to a document generated by a laser
printer or inkjet would give valuable assistance to a criminal investigation,” said
Howard Seiden, a questioned document examiner in the Broward (County) Sheriff's
Office, Fort Lauderdale, FL. “This paper gives a new direction to the
examination of computer printers that document examiners have written off as
too generic.”
Seiden said more could eventually be done to expand the number of printers
and suggested that a database could be compiled to provide the document examiner
a guide to compare ESDA characteristics of various printers or photocopiers.
Common Touch
There are numerous areas within a printer or photocopier that can physically
touch the paper causing disturbances that are detectable by ESDA, although
it may not always be obvious what specific source in the printer is creating
the detectable markings. It may therefore be necessary to physically examine
the printer after the document was created to evaluate the ESDA results.
Document examiners greeted LaPorte's intriguing use for ESDA with cautious
approval. “While further studies should be pursued, perhaps the present
techniques will better serve in elimination purposes,” said Jim Blanco,
a San Francisco forensic document examiner.
Blanco advised caution, however. Suppose you have a situation where a document
has been printed, say, by an HP LaserJet 1100, then signed and sent off into
commerce. Later, this same document may be put through a copy machine with
an automatic feeder which could add that particular copy machine's impressions
to the document where the grabbing elements/rollers of the copier press and
track upon the paper.
If, in such a case, the original HP 1100 printer were to leave very faint marks
so that the marks themselves may not reveal during an ESDA exam, yet the grabber/feeder/rolling
element of the automatic copier would leave prominent marks, a questioned document
examiner conducting examinations of these latent marks may be led down the
wrong road of identification or elimination of perhaps the very machine that
really created the document, Blanco said.
Yellow Flags
There are other caveats. LaPorte offered a number of procedural and interpretive
areas that examiners should consider. These include the fact that even documents
that are produced on the same machine may have additional physical markings.
For example, documents may have been processed in a mail facility where additional
rollers or wheels have left markings. Differences in paper or the manufacturing
of the hardware could also be the cause of differences. Additionally, an
original equipment manufacturer will produce machines for a number of vendors.
These
may produce the same markings but have a different brand name.
The method outlined in LaPorte’s study enhances the methods a document
examiner has at his disposal, but does not replace existing chemical analysis.
As LaPorte’s study notes, chemical examination of inks and toners is
still an invaluable tool for comparing questioned documents. Examiners should
consider using LaPorte’s method of physical examination to corroborate
these ink findings.
Douglas Page is a Science/Medical Writer and editor. He can be reached at
douglaspage@earthlink.net.
