Written by Lewis Mitchell
IN THIS EVER-PROGRESSING AGE of
technological advancement, more and more we witness the old remnants of
technologies recede into history. It now appears, in the advent of the modern
era, that print is endangered. Personal computers with access to the internet
now permeate the daily life of people across the world, offering easy access to
literature that only decades before would have required printing. Your very
reading of this article demonstrates that point.
Understandably, advancements in the way
we access and handle information has led many people to even question the
future validity of the ultimate written medium, old-fashioned pen and paper. Where
print media is stubbornly falling to alternatives, handwriting persists as the
most convenient and accessible way of conveying information. We can safely say
pen and paper will be sticking around for a few years yet.
This does present some interesting
challenges, notably when it comes to the validation of written works. A prime
example presents itself within the realm of legal documentation—contracts and
agreements. Despite the growth of e-commerce, the norm for signing legal
documents remains a handwritten signature on paper. Moreover, authenticated
documents are required to be signed before a notary public, all seals must be
original, and all certifications must match.
How do we go about validating
these documents, and how can we ensure that a signature is original? Unlike
electronic documents, there are no methods of tracking contributors or
alterations. What if an entrepreneurial rogue changes an initial on a mortgage
agreement, or adds an additional zero on the check they received? For these
reasons, it is vitally important that we can determine the authenticity of
writing. The answer to this challenge comes in one elegant form: spectroscopic
A technique is required to determine ink
composition to establish if a document contains changes made with different
pens. There are many different types of inks. Colors may be the same, but
chemically they can be different. Raman analysis allows for rapid,
non-destructive testing of questioned areas with the specificity to distinguish
similar ink types that may visually look identical.
Recently, researchers have been taking
this investigation further, exploring how to determine the crossing order of
two ink lines originating from different pens. Traditionally this has been a
difficult challenge to approach; the inconsistent background of paper and the
requirement to interpret chemical images introduce large degrees of
uncertainty. How then, can Raman spectroscopy reliably determine the origin of
additional ink lines on paper?
This is possible using a new method to
measure the ink order through the implementation of several techniques. The
premise lies in the analysis of each ink's coverage within the region where
they cross. The crossing region is a mixture of both ink components, logically, where the uppermost ink appears in the largest quantity when analyzing using a laser
impinging on the top surface. This is revealed using false-color images and
process begins by capturing a white-light image of the crossing region and
surrounding area. To capture the composition of the crossing region, the area
is scanned using chemical imaging. This collects spectral information from the
inks with a continually moving line-focused laser. The line focus has a lower-power
density than a traditional spot focus, which allows for a greater laser power
without damaging the ink layer.
the data is gathered, distinct regions of the image are masked and pure
references for each ink are obtained. The masking tool is used to limit the
data to be processed as defined by the thresholding of an image (e.g. white
light or Raman) or by manual selection of an area of the scan.
that we have a reference for each ink, component analysis can be performed to
obtain false-color images that display the distribution of each component on
top of the white-light image. Previously at this stage, the user attempting to
identify the crossing order would threshold the false-color image and establish
the crossing order. As darker areas indicate less similarity to the reference,
the darker ink image in the crossing region was assumed to be on the bottom
layer; however, there was no guarantee two independent users would threshold
the image the same way and arrive at the same conclusion. Depending on the
image, the ink order can potentially be interpreted either way. So, how can we
alleviate this previously encountered pitfall? The answer, of course, lies in
another exclusive investigative tool... introducing concentration estimates.
The concentration estimates tool determines
the total percentage contribution from each ink based on the component-analysis
generated image. The process doesn’t account for any changes to the image
thresholds, therefore ensuring highly consistent results from user to user. The
larger concentration estimate value corresponds to the ink that is more
prevalent in the crossing area being analyzed. The larger the concentration
estimation difference between the two references, the more confidence we can
have in the deposition order.
Similarly, the concentration estimate of
the pure regions indicates the high specificity between the references. In this
way, we have a statistically meaningful value to provide confidence in our
determination of the ink deposition order.
And there you have it: You now know how
disputes of forgery or altercation are spectrographically analyzed and
Lewis Mitchell is an applications
scientist in the Raman spectroscopy department at Renishaw. There, he works on
cutting-edge applications including the development of new hardware and
software with recent work involving auto-focus tracking, univariate stage movement,
and averaged imaging techniques. He graduated Heriot Watt university with a Master
of Science in chemical physics and is now taking his first steps into the world
of Raman spectroscopy.