Applying Anti-Stokes Phosphors in Development of Fingerprints on ...

Technical Note
<strong>Apply<strong>in</strong>g</strong> <strong>Anti</strong>-<strong>Stokes</strong> <strong>Phosphors</strong> <strong>in</strong>
<strong>Development</strong> <strong>of</strong> F<strong>in</strong>gerpr<strong>in</strong>ts on Surfaces
Characterized by Strong Lum<strong>in</strong>escence
Bogdan Drabarek 1
Antoni Siejca 2
Jarosław Moszczyński 3
Barbara Konior 1
Abstract: Us<strong>in</strong>g traditional lum<strong>in</strong>escence methods to develop latent
pr<strong>in</strong>ts becomes problematic when deal<strong>in</strong>g with backgrounds that demonstrate
strong lum<strong>in</strong>escence. In such <strong>in</strong>stances, the application <strong>of</strong>
time-resolved lum<strong>in</strong>escence is considered a good solution. However,
this technique requires the use <strong>of</strong> complicated devices that allow shortlived
background f luorescence to be chopped <strong>of</strong>f from a longer-lived
f<strong>in</strong>gerpr<strong>in</strong>t lum<strong>in</strong>escence. This paper discusses a new and straightforward
technique for the development <strong>of</strong> latent pr<strong>in</strong>ts that <strong>in</strong>volves us<strong>in</strong>g
pigments with upconversion properties (anti-<strong>Stokes</strong> phosphors). The
method requires an illum<strong>in</strong>ation source that emits <strong>in</strong>frared radiation.
Introduction
Lum<strong>in</strong>escence methods that are commonly used to develop
latent pr<strong>in</strong>ts are ma<strong>in</strong>ly based on excitation radiation <strong>in</strong> the
300–700 nm range and optical filtration <strong>of</strong> the observed latent
image by means <strong>of</strong> edge or bandpass filters [1, 2]. This method
is not efficient on surfaces that exhibit strong lum<strong>in</strong>escence.
To overcome this problem, various optoelectronic devices us<strong>in</strong>g
time-resolved lum<strong>in</strong>escence have been designed. Sophisticated
1
Internal Security Agency, Warsaw, Poland
2
Lasar Elektronika, Warsaw, Poland
3
University <strong>of</strong> Warmia and Mazury, Olsztyn, Poland
Received December 9, 2010; accepted March 1, 2011
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devices, requir<strong>in</strong>g the application <strong>of</strong> advanced and usually costly
technology, allow short-lived background f luorescence to be
chopped <strong>of</strong>f from a longer-lived f<strong>in</strong>gerpr<strong>in</strong>t lum<strong>in</strong>escence [3–6].
The proposed method is based on multiphoton absorption, the
upconversion process [7, 8]. It <strong>in</strong>volves the use <strong>of</strong> formulations
conta<strong>in</strong><strong>in</strong>g fluorescent pigments <strong>in</strong>consistent with <strong>Stokes</strong> rule
(e.g., anti-<strong>Stokes</strong> phosphors), as well as <strong>in</strong>frared radiation (IR)
as the excitation source. The method enables the acquisition <strong>of</strong>
lum<strong>in</strong>escent f<strong>in</strong>gerpr<strong>in</strong>ts while elim<strong>in</strong>at<strong>in</strong>g background signals.
This is the effect <strong>of</strong> low-energy IR radiation, which does not
excite lum<strong>in</strong>escence <strong>of</strong> typical dyes.
<strong>Anti</strong>-<strong>Stokes</strong> <strong>Phosphors</strong>
Pigments called anti-<strong>Stokes</strong> phosphors belong to the group <strong>of</strong>
lum<strong>in</strong>ophores consist<strong>in</strong>g <strong>of</strong> fluorides, z<strong>in</strong>c, strontium, or lanthanum
oxides as well as trivalent ion admixtures <strong>of</strong> rare-earth
activated compounds, such as thulium, erbium, ytterbium, and
yttrium. They f<strong>in</strong>d their application <strong>in</strong> optoelectronics, biology,
and <strong>of</strong>ten <strong>in</strong> mark<strong>in</strong>g documents and valuables, such as works <strong>of</strong>
art. These pigments are sold as powders characterized by high
chemical and temperature stability. Their emission frequency is
found <strong>in</strong> the longer absorption frequency, that is, while excit<strong>in</strong>g
with radiation <strong>in</strong> the range 800–1000 nm, they emit visible
light that depends on the phosphor’s composition. This type
<strong>of</strong> location <strong>of</strong> absorption and emission bands is referred to as
anti-Stoke shifts [7].
The mechanism <strong>of</strong> f luorescence <strong>of</strong> the aforementioned
pigments is based on two- or multiphoton absorption. Optically
active ions with relatively constant energy levels placed at
regular <strong>in</strong>tervals may absorb two or more photons <strong>of</strong> the same
wavelength. In the first stage, excitation to a higher metastable
energy level <strong>of</strong> low likelihood <strong>of</strong> emission takes place. The
second stage <strong>of</strong> absorption <strong>in</strong>volves further excitation to a shortlived
level, from where a radial transition to a basic level br<strong>in</strong>gs
about fluorescent radiation <strong>of</strong> a shorter length than <strong>in</strong> the case <strong>of</strong>
excitation radiation. Depend<strong>in</strong>g on the construction and composition
<strong>of</strong> the upconversion pigment, another mechanism is also
possible, which <strong>in</strong>volves transfer <strong>of</strong> energy between pigment
components <strong>of</strong> various ions [8].
The purpose <strong>of</strong> the research performed at the Internal Security
Agency (Warsaw, Poland) was to exam<strong>in</strong>e the potential <strong>of</strong> IR
radiation <strong>of</strong> the 900–1000 nm range and pigments characterized
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y upconversion <strong>in</strong> the development <strong>of</strong> f<strong>in</strong>gerpr<strong>in</strong>ts as well as
the impact on improv<strong>in</strong>g the legibility <strong>of</strong> f<strong>in</strong>gerpr<strong>in</strong>ts developed
through the elim<strong>in</strong>ation <strong>of</strong> background signals.
Materials and Method
Dur<strong>in</strong>g the exam<strong>in</strong>ation, latent pr<strong>in</strong>ts were developed that had
been previously deposited on multicolored plastic surfaces and
showed multirange strong lum<strong>in</strong>escence excited with 240–760
nm radiation. On the other hand, these backgrounds did not show
any lum<strong>in</strong>escence when excited with 900–1000 nm radiation.
In the process <strong>of</strong> the development <strong>of</strong> the latent pr<strong>in</strong>ts, a
pigment show<strong>in</strong>g upconversion characteristics, marked as UP54,
as well as its mixture (1:1 ratio) with white B-400 and B-432
powders (BVDA), were used. UP54 pigment is deprived <strong>of</strong> the
adhesion properties <strong>of</strong> typical f<strong>in</strong>gerpr<strong>in</strong>t powder. It demonstrates
two k<strong>in</strong>ds <strong>of</strong> lum<strong>in</strong>escence:
• <strong>Stokes</strong>, <strong>in</strong> the yellow range <strong>of</strong> white light at UV excitation
<strong>of</strong> approximately 360 nm
• anti-<strong>Stokes</strong>, <strong>in</strong> the green range <strong>of</strong> white light at excitation
with 940–980 nm radiation (near IR)
Latent pr<strong>in</strong>ts were developed only with the UP54 pigment or
its mixture with the previously mentioned f<strong>in</strong>gerpr<strong>in</strong>t powders.
For comparison purposes, the cyanoacrylate method was applied,
and f<strong>in</strong>gerpr<strong>in</strong>ts treated with that method were subsequently
contrasted with fluorescent dyes, such as ardrox, basic yellow,
or safran<strong>in</strong>e.
For the purpose <strong>of</strong> this <strong>in</strong>vestigation, the follow<strong>in</strong>g excitation
generat<strong>in</strong>g devices were used:
• illum<strong>in</strong>ation source for acquisition <strong>of</strong> f<strong>in</strong>gerpr<strong>in</strong>ts
<strong>in</strong> IR Micro Identification Chamber (MIC) (Lasar
Elecktronika, Warsaw, Poland); radiation range <strong>of</strong>
more than 900 nm (Figure 1)
• xenon illum<strong>in</strong>ation source for forensic purposes
(MultiKolor 10Xe, Lasar Elecktronika, Warsaw,
Poland) <strong>of</strong> 350–1800 nm radiation range
A set <strong>of</strong> edge filters (symbols: KG3+T, 1KG3+T, 2KG3+T )
enabl<strong>in</strong>g excitation IR radiation to be cut <strong>of</strong>f and visible radiation
<strong>in</strong> the green range to be passed were used to take photographs <strong>of</strong>
f<strong>in</strong>gerpr<strong>in</strong>ts illum<strong>in</strong>ated with IR radiation.
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For comparison purposes, f<strong>in</strong>gerpr<strong>in</strong>ts were also recorded
with the use <strong>of</strong> white light and UV radiation. Photographs <strong>of</strong>
developed f<strong>in</strong>gerpr<strong>in</strong>ts excited with MIC and MultiKolor 10Xe
illum<strong>in</strong>ation sources were taken with a Canon EOS 5D camera,
100 mm f/2.6 macro USM lens, <strong>in</strong> .jpg and .raw formats. An
electro-optical analog-to-digital imag<strong>in</strong>g system for acquisition
<strong>of</strong> f<strong>in</strong>gerpr<strong>in</strong>ts by means <strong>of</strong> time-resolved lum<strong>in</strong>escence
(TRL) [6] was used to record the results with the use <strong>of</strong> the IR
pulse illum<strong>in</strong>ation source. Recorded graphic files were viewed
<strong>in</strong> Adobe Photoshop CS3.
Figure 1
Illum<strong>in</strong>ation source for acquisition <strong>of</strong> f<strong>in</strong>gerpr<strong>in</strong>ts <strong>in</strong> IR. MIC device
consists <strong>of</strong> chamber with illum<strong>in</strong>ation source (<strong>in</strong> the middle), power
supply, reduction r<strong>in</strong>gs, filter, and magnify<strong>in</strong>g glass.
Results and Discussion
F<strong>in</strong>gerpr<strong>in</strong>ts were developed only with the use <strong>of</strong> UP54
pigment and its mixture with two BVDA powders. Additionally,
both the pigment and its mixtures were used for contrast<strong>in</strong>g
f<strong>in</strong>gerpr<strong>in</strong>ts developed with cyanoacrylate.
Reference material consisted <strong>of</strong> f<strong>in</strong>gerpr<strong>in</strong>ts developed with
cyanoacrylate and contrasted with fluorescent dyes (ardrox,
basic yellow, safran<strong>in</strong>e).
The best results were obta<strong>in</strong>ed while us<strong>in</strong>g UP54 pigment
alone on “fresh” f<strong>in</strong>gerpr<strong>in</strong>ts. The pigment poorly deposited on
older (one-week-old) f<strong>in</strong>gerpr<strong>in</strong>ts. This probably stems from the
fact that the pigment lacks the adhesion properties <strong>of</strong> typical
f<strong>in</strong>gerpr<strong>in</strong>t powder and demonstrated poorer adhesion when
Journal <strong>of</strong> Forensic Identification
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applied to older f<strong>in</strong>gerpr<strong>in</strong>ts. Slightly better results <strong>in</strong> develop<strong>in</strong>g
older f<strong>in</strong>gerpr<strong>in</strong>ts were achieved with use <strong>of</strong> the UP54 pigment
mixture with BVDA white f<strong>in</strong>gerpr<strong>in</strong>t powders. However, these
mixtures demonstrated remarkably lower IR lum<strong>in</strong>escence when
compared to the pigment alone. Attempts to contrast cyanoacrylate-developed
f<strong>in</strong>gerpr<strong>in</strong>ts with UP54 alone or its mixtures with
powders did not yield positive results. Figures 2a through 2c
present f<strong>in</strong>gerpr<strong>in</strong>ts developed on the same background by means
<strong>of</strong> the cyanoacrylate method followed by contrast<strong>in</strong>g with dyes
(ardrox, basic yellow, safran<strong>in</strong>e) and illum<strong>in</strong>ated with appropriately
suited excitation radiation (350–505 nm), whereas Figure
2d shows a f<strong>in</strong>gerpr<strong>in</strong>t developed with UP54 pigment alone (with
no prior cyanoacrylate treatment) and illum<strong>in</strong>ated with IR radiation.
In these figures, one can easily observe that the background
lum<strong>in</strong>escence <strong>in</strong> IR is significantly lower than <strong>in</strong> the case <strong>of</strong> the
application <strong>of</strong> UV and visible light, which contributes to the fact
that a developed f<strong>in</strong>gerpr<strong>in</strong>t is better when viewed <strong>in</strong> the visibile
range and illum<strong>in</strong>tated with IR radiation.
(a)
(b)
(c)
Figure 2
F<strong>in</strong>gerpr<strong>in</strong>ts developed with cyanoacrylate and contrasted with the
follow<strong>in</strong>g dyes: (a) ardrox; (b) basic yellow; (c) safran<strong>in</strong>e.
(d) F<strong>in</strong>gerpr<strong>in</strong>t developed with UP54 pigment alone.
(d)
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Additional attempts to record f<strong>in</strong>gerpr<strong>in</strong>ts developed with the
UP54 pigment on surfaces exhibit<strong>in</strong>g strong fluorescence were
carried out. For this purpose, boxes with f luorescent powder
were used as backgrounds, and they demonstrated emission <strong>in</strong>
red and green colors. F<strong>in</strong>gerpr<strong>in</strong>ts deposited on these boxes were
developed with the UP54 pigment and photographed with the use
<strong>of</strong> UV radiation, white light, and IR radiation. The best results
were obta<strong>in</strong>ed while apply<strong>in</strong>g IR illum<strong>in</strong>ation, which caused the
excitation <strong>of</strong> a f<strong>in</strong>gerpr<strong>in</strong>t alone and not the background fluorescence
(Figures 3 and 4). Figures 3a and 3b show a f<strong>in</strong>gerpr<strong>in</strong>t
developed with the UP54 pigment on a box with red fluorescent
powder and illum<strong>in</strong>ated with visible and IR radiation; Figures
4a and 4b exhibit a f<strong>in</strong>gerpr<strong>in</strong>t developed with the UP54 powder
on a box with green fluorescent powder and illum<strong>in</strong>ated with
UV and IR radiation.
(a)
Figure 3
(b)
F<strong>in</strong>gerpr<strong>in</strong>ts developed with UP 54 on red background:
(a) white light; (b) IR light.
(a)
Figure 4
(b)
F<strong>in</strong>gerpr<strong>in</strong>ts developed with UP 54 on green background:
(a) UV light; (b) IR light.
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F<strong>in</strong>gerpr<strong>in</strong>ts developed with the UP54 pigment were also
successfully recorded by means <strong>of</strong> the optoelectronic device
TRL, which allows short-lived background f luorescence to
be chopped <strong>of</strong>f from a longer-lived f<strong>in</strong>gerpr<strong>in</strong>t lum<strong>in</strong>escence,
consequently lead<strong>in</strong>g to the acquisition <strong>of</strong> nondisturbed latent
pr<strong>in</strong>t images.
Conclusion
These exam<strong>in</strong>ations demonstrated that upconversion pigments
may f<strong>in</strong>d application <strong>in</strong> the development <strong>of</strong> latent pr<strong>in</strong>ts on
surfaces characterized by strong lum<strong>in</strong>escence. Their advantage
lies <strong>in</strong> their potential to excite lum<strong>in</strong>escence with IR radiation,
whose energy is sufficiently low to avoid excitation <strong>of</strong> other
dyes. Because <strong>of</strong> this feature, <strong>in</strong> many <strong>in</strong>stances, a surface where
a latent pr<strong>in</strong>t was detected rema<strong>in</strong>ed black.
Another advantage <strong>of</strong> upconversion pigments is their ability
to demonstrate f luorescence emission <strong>in</strong> the range visible for
the human eye, which allows the expert to assess the quality <strong>of</strong>
a developed pr<strong>in</strong>t visually, with no need to apply specialized
equipment.
Upconversion pigments may also be employed for the
visualization <strong>of</strong> latent pr<strong>in</strong>ts with the use <strong>of</strong> time-resolved
lum<strong>in</strong>escence.
UP54 pigment and its mixtures with white f<strong>in</strong>gerpr<strong>in</strong>t
powders did not demonstrate characteristics <strong>of</strong> a good f<strong>in</strong>gerpr<strong>in</strong>t
powder because <strong>of</strong> their low adhesive property when applied to
older f<strong>in</strong>gerpr<strong>in</strong>ts and the ones developed with cyanoacrylate.
Hence, there is a need for a cont<strong>in</strong>uation <strong>of</strong> research aimed at the
production <strong>of</strong> good-quality powders or suspensions that conta<strong>in</strong>
mixtures <strong>of</strong> pigments with upconversion properties.
For more <strong>in</strong>formation, please contact:
Jaroslaw Moszczynski
03-126 Warszawa
ul. Picassa 9 m l9
Poland
jaroslaw_moszczynski@go2.pl
Journal <strong>of</strong> Forensic Identification
34 / 62 (1), 2012

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