Note: Descriptions are shown in the official language in which they were submitted.
1~ 2032~87
A data carrier having a llquld crystal security element
The presenl invention relates to data carriers. in particular papers of
~ alue. documents. identity cards or the lil;e. ha~ing ~n optically ~-ariahle
security element containing a liquid crystal material. For e~sier rea(i~bil-
ity the abbre~,-iation "LC" will be frequently used in the follou-ing for
"liquid crystal."
The increasing technical maturity of color (~or iers leads tO copies
that are ever more difficult to distinguish from the originals in color.
resolution and quality. To protect data carriers from being forged with
the aid of color copiers or scanners. optically ~-ariable elements are being
used more and more as security elements. Such elements ha~ e in common
that they exhibit different colors or brightnesses depending on the light-
ing and viewing conditions. The commonest opticall~ ~ariable elements
are diffraction grids. holograms! lnterference coatings. metameric inks and
polarizing coatings.
Holograms and grids are based on diffractive effects. Interference
coatings usually comprise several superjacent la~v-ers! the la!t er thicknesses
being within the size of the wavelength of the light.
Metameric inl;s customarily consist of mixtures of pigments with dif-
ferent reflectance bands. This composition causes the metameric inks to
change their visual color effect when the type of lighting changes.
Dichroic dyes have the property of absorbing white light in different
wave ranges depending on the polarizing direction. The result is a
polarization-dependent color effect.
The disadvantage of the known opticall~ variable authenticity fea-
tures is that they are t, ery expensive to produce. cannot be processed
using conventional production methods or are not fully compatible with
other authenticity features or card elements.
- ~ 2032587
The in~-ention is based on the problem of proposing a featur~ effec-
tive as copying protection that has effects dependent on the viewing
angle! can be produced inexpensk-ely and using con~-entional methods and
is compatible or combinable ~vith other features.
This problem is solved by the features stated in the characterizing
part of the main claim~ Developments are stated in the dependent and
independent claims.
The invention is based on che use of liquid crystal polymers as secu-
rity elements. ~fter suitable oriented production these polymers constitute
at room temperature a plastics-like solid ~h-ith a pronounced play of col-
ors. .~ suitable production method is, for example. ~o doctor the material
u~hile still liquid onto a base and subsequently harden it by UV irradia-
tion. Suitable liquid crystal polymers are in particular liquid crystal-
silicone polymers and cholesteric organopolysiloxanes. Their chemical
structure and production are described in the published patent applica-
tions EP-A 0 136 501. EP-A 0 060 335 and European patent no. 0 066 13,.
The disclosed content of these publications is expressly referred to.
The use of conventional liquid crystals as securit~ elements is already
~nown and is proposed. for example. in Australian patent no. ~88 652
(Commonwealth). This reference describes a laminated bank note with an
intermediate layer in which a security element is embedded in the form of
a liquid cry, stal material. The LC material is applied to an inlay by print-
ing technology. The liquid crystals are in a liquid state of aggregation.
are embedded in microcapsules closed on all sides and mixed into an ink.
The test for authenticity involves a change of color of the security ele-
ment due to a change of temperature.
Despite a structural anisotropy, liquid crystals customarily behave
lil;e liquids. which is why it is necessary to enclose these materials in
capsules or cavities. This results in complicated production engineering.
~ot onl~ is the encasing of the LC materials elaborate. it is also impos-
sible to embed the proposed security elements in films or identity docu-
ments in the con~, entional way under the action of pressure and heat
~classical laminating technique) due to the danger of injuring the cavities
or capsules. Encased liguid crystals are also unsuitable as security ele-
-
3 2032587
ments on ban~ notes ol papers of ~-alue riith steel intaglio printing since
the high pressllre stresses required in this production method lead to
destruction of the capsules and ca~-ities.
Howe~-er. after cortesDonding processin,~ uid cryslals can also be
present in a solid form and e.Yhibit a high-grade orientation of their
molecllles depen(iing on the processing method. making their opt~call~
~-ariable properties stand out to their full e.~tent and in full brillianc~. Jn
the inventive LC sy, stems the color p~lrity of the reflected light onl~-
rarely e.~iceeds a range of 100 nm. the color change effects upon a change
of ~ie~.ving angle ;are ~,-ery pronounced. and the reflected and transmitted
light has a pronounced circular polarization. These fully de~-eloped opti-
cally variable properties make such LC polymers particularly suitabiP for
use as security elements on data carriers. papers of t-alue and identity
cards. The plays of color are easily obser~ able e~;en for the la~-man. The
~-a~ elength-selective reflecti~ ity and the polariza~ion effects mal;e the
material highly suitable for automated testing. The multiplicity and pro-
nounced character of the optical effects mal;es it diffLcult to prepare
simulacrum counterfeits. in v irtually all embodiments the LC elements
can be additionally employed both as machine-readable authenticity fea-
tures and together w-ith other machine features. Due to the IR permeabil-
ity of LC polymers the other machine features might also be disposed
under the LC polymers.
The solid properties of LC polymers make it considerably easier to
produce security elements from them. Firstly. it is unnecessarv to enclose
the liquid crystals in a hollow body. Secondly, there is no danger of
bursting and of the liquid crystals leal;ing out during following pro-
cessing steps and during the lif'e of the data carrier. This makes the
production processes and application extremel~. uncomplicated.
The plastics-like properties of liquid crystal polymers allow for easy
processing into semifinished products or into the finished product. The
starting material is generally present as granular material and can be
shaped and processed with the methods and machines known from plastics
production. This makes it possible in the field of security technology to
produce completely different types of security elements on the basis of
L~ polymers and to co~rer different cases of application. ~-
4 2032587
Thlls. carriel Uel~S made of tear-resistarlt pLast,^s can l~e t oate(l -~ith
a la~-er of LC pol~mers. The resulting w,eb of material can then he cut
into narrow webs or threads that can be embedded ~n paper or other sub-
stances as safeguarding threads.
.'~lternativel-. multilayer wehs of fi]rn c,ln also be produced that con-
.ain an embedded layer made of an LC polymer. Such ~.4ehs an ~)e forrrled
as adhesive or transfer bands that are suitable for gluing or s.amping
transfer elements onto paper or plastics surfaces.
Finally LC polymers can also be produced as selr-supporting films.
These films can be used. for e.~;ample. as layers of film for m~lltilayer
identity cards.
Further advantages and features of the invention can be found in the
dependent and independent claims and the follo~ ing e~;emplary embodi-
ments and flgures. in which
Fig. 1 -shows the spectral transmission and reflection properties of
LC polymers from various viewing angles.
Fig. '' shou,-s a bank note with a window safeguarding thread having
one or more layers made of LC polymers!
Fig. 3 shows a safeguarding thread having a layer made of an LC
polymer.
Fig. 1 shows a symmetrically constructed safeguarding thread having
outside layers made of LC polymers.
Fig. 6 shows a symmetrically constructed safeguarding thread having
inside layers made of LC polymers,
Figs. 6a, b show a printed. symmetrical .vindo~- safeguarding thread
in cross section and from the top,
Figs. 7a, b show a printed safeguarding thread having kinetic effects
in cross section and from the top,
2032587
Figs ~a. b sho w an identity cal d ha~ ing a tr~r ;fer element -.~ ith an
LC la~ er from the from ~nd in cross secti()n.
F;~JS. ~a. b shou an identitv car d ha~ing a ~ isu~hy unreadable coding
co~,-ere(i b~ the se~urit-; element.
Fig. 1~) shows a cross section through a transfer ba-.d.
Fig. 11 shows the transfer of an LC securit v elemen- to a sllbstrate.
Fig. 1~ shows an identity card having a laminated-in layer made of
LC polymer.
Fig. 13 shows a test assembly for LC security elemeats.
Figs. ~ show detector assemblies ~or de~ecting LC security
elements .
To mal;e it easier to understand the applicat ons and effects of
liquid crystal polymers that are discussed in the Figures and e.~emplary
embodiments, some important properties of these subs;ances shall be e.~-
plained in advance.
LC polymers are a special variant of liquid c.-stals in ~hich the
liquid cr!.-stalline state is "frozen" in a pol~-mer rnatri~, so that the
optical properties stand out particul&rly clearly. Thus. liquid crystal pol~
mers normally absorb no light; their coloring arises by multiple interfer-
ence of light on the individual crvstal planes. The coior effect in incident
and transmitted light is accordingly different. The reflected color spec-
trum contains only a narrow frequency range about a central wavelength
and thus shows a high color saturation. The transmitted spectrum is com-
plementary to the reflected one and has a sag in the area about the
central wa~-elength.
When LC polymers are used on opaque substrates one obtains a par-
ticularly high degree of color purity for all viewing angles if the liquid
crystal layer is applied to a black background. The reflected spectrum is
then not disturbed by secondary reflections on-the background.
- ~ 2032~87
The lattice ronstants ~f in~ enti~;e oriente~ L-~ polymers can he set.
or defined during syn~hesis. to be in the range of 300 nm to l~000 nm. so
that the reflected central wa~,-elength when verticall~ incident is in the
near infr ared or the ~ isible range. .~s the viewing angle fla~;tens. the
central wa~,elength of the reflection banli shifts in t~ direction of short-
er wavelengths. For e.~ample, the perpendicular r eFlected l~avelength is
appro~. 20o greater than reflection at ~0 .
Fig. 1 shows spectral reflection R of an LC layer h-ith vertically inci-
dent illumination in curve 1 and with a lighting direction of 60 in
cur--e ''. For special LC polymers the color effect can accordinglv change
from green to ~i-iolet. -ellow to blue. light red to ~reen or. with an IR
!eflection band from black to red. The lattice constan~ and thus the basic
color of the liquid crystal polymer depends on the precise chemical struc-
ture of the liquid cr,~ stal and can be defined by the synthesis conditions
to be in the range between 300 and l.000 nm.
Fig. 2 shows an application of an LC polymel for a window safe-
guarding thread. In a banl; note 11 ~-ith a security print 12 a safeguard-
ing thread 13 has been embedded during the paper production in such a
~ay that it comes to lie in ~indows 11 on the surface of the paper and
is thus visuallv recognizable. Depending on the embodiment. the ~idth of
such safeguarding threads fluctuates between 0.5 and a few millimeters.
~ o give the bank note copying protection by optically variable ef-
fects, safeguarding thread 13 is designed in such a ~ay that it contains
one or more layers made of an LC polymer. ~;ariants for producing and
constructing safeguarding threads are shown in Figs. 3 tO ~.
Fig. 3 shows the cross section of a first variant for a safeguarding
thread 1 3a. It comprises a plastics carrier 20, for which a polyester film
~ith a typical thickness of 20 to 100 micrometers is preferably used.
Carrier 20 is coated on one side with a several micrometer thick layer 21
made of an LC polymer. To bring out optically the plays of color of the
liquid crystals, film 20 is preferably dyed blacl;. The thread is oriented
during the paper production in such a way that the liquid crystal layer is
present on the visible outer surface.
-
-- I 2032~87
Fig. I shows a cross section of a further variant of a safeguarding
rhread ].3b -Alith a c~mmetrical laver structure. Symmetric.llly constructed
safeguarding threads have the advantage that the orientation of the
thread need not be heeded when it is embedded in the paper, Thre:3d 13b
comprises two carrier films ~0 th.lt ~re bolh coated on one side ~ith
la~er 21 of LC polvmers. Carrier films 20 are interconnecte(i with ~l lami-
nating agent 2 ~ so as tO give rise to a svmmerrical lavel structur e wil h
outside LC la~ ers. T(- increase the wealth of coll)r one can optionally dye
carrier webs ~'0 and, or laminating agent ~ with transparent or pigrnent
inl;s. A simple solution in terms of production engineering is to d~-e onl,v
laminating agent "~, preferably using an opatllle black.
Fig. 5 shows a further v ariant of a svmmetricall~ (~onstructed safe-
guarding thread 1 3c in cross section. In contrast to Fig. 3, carrier films
_0 now lie on the outer sides of thread 1 3c, thereby protecting inside
LC la!,~ers 21 from being damaged. In this variant, preferablv only the
laminating agent is colored with a dye, Since outside carrier la,vers 20
must remain transparent the,v are colored either weakly or not at all.
Figs. 6a and 6b show a further variant of a safeguarding thread 13d
in cross section (Fig, 6a) and from the top (Fig. 6b). As in Fig. ~. thread
i 3d has a svmmetrical la,ver structure comprising two carrier films 20,
two LC layers ~'1 and an adhesive layer 2~'. In the course of a production
process the thread was joined together from two coated pairs of films 30,
31. Before joining, surface 33 of one of the pairs of films was furnished
~-ith a printed pattern 3~ of black ink, alphanumeric marks being applied
in microwriting to the surface of an LC polymer layer by a conventional
printing method. A transparent laminating agent 22 was additionally used.
In transmitted light the characters now appear black before the optically
~-ariable colored background of the polymer layer in the window areas of
the paper. In incident light, however, only the micro-characters show a
change of color.
In another variant of the safeguarding thread of Figs. 6a and 6b
characters 34 are applied in green microprint to one of the LC layers
~hile laminating agent 22 is dyed black, Simultaneously the LC material
is selected so as to appear green at a certain viewing angle, for example
perpendicular to the black background. When the safeguarding thread is
8 2032587
viewed at this angle the total surface then appears green. When the
viewing angle changes, the color of the LC polymer layer changes. While
in the writing the green color remains ~m;n~nt. The result is a safe-
guarding thread whose writing is visible only when the thread is tilted.
It is of course also possible to orientate the liquid crystal
material by doctoring. This could for example be done by doctoring or
rolling the liquid crystal polymer onto the surface of the roller. The
liquid crystal polymer is then transferred from the roller to the surface
of the data carrier by a printing process.
Figs. 7a and 7b show a further variant 13e in cross section (Fig.
7a) and from the top (Fig. 7b). The safeguarding thread comprises a
carrier film 20 and a layer 21 made of LC polymers. The polymer layer
was printed with a pattern of different-colored diagonal stripes 40 by a
conventional printing method. In the example shown, the special color
sequence selected for pattern 40 was red 41, yellow 42, green 43, blue
44, the pattern being repeated any number of times over the length of the
thread. When this safeguarding thread 13e is viewed, colored surface
areas 40 appear through the LC layer with different color effects each
time. The color spectrum of the individual areas is composed of the
reflection bands of the printed dyes. In addition, the colors of the
liquid crystal layer are admixed additively. Due to the angle-dependent
reflection characteristics of LC polymers, the colored stripes with the
arrangement shown can create the illusion of a colored stripe moving
along the thread when the thread is tilted, if the colors of the LC
polymer are suitably coordinated. As in Fig. 5, this variant can also be
expanded into a safeguarding thread having a symmetrical layer structure.
The variants shown in Figs. 3 to 7 can be varied in manifold ways
depending on the desired appearance. The optically variable effects of
3 ' `''
8a 2032S87
LC polymers can be combined by dyeing any desired layers with "classical"
inks, using et her transparent dyes or pigment dyes. The dyes themselves
can be introduced into any layer (including the LC layer, but then only
in a low concentration) of the safeguarding thread and/or applied as a
printed pattern to any layer of the thread.
The ways of dyeing stated in the description of the figures are only
intended by way of a proposal; the stated colors can be replaced at will
by any other dyes. These possibilities of combination result in an
enormous multiplicity of possible color variations, color illusions and
kinetic effects.
The variants of safeguarding threads shown in Figs. 3 to 7 can all be
A
-- 9 2032587
pro(luced oll the basis of one semifinished pr od~lct. The semifinished pJ od-
uct is prodl~c~cl bv coatinS~ a l,veb of film ~0 made of a carrier material
such as polyesteI plastics -~4ith a lay~ 1 of LC polymers. Depending on
the coh~r design Or the safeguar~ing thread~ orle llses printed. transp,lrent
or dved c~rrier filrns. The thicl;ness of the ~.~eh of film is preferably in
the range of less than one tenth of a millimeter: ~or the LC c()ating a
film thicl~ness of ~ppro.~i. 10 micrometers is usl1allv ,ufficient. For m~nu-
facturing reasorls the tvpical web widths of the semifinished prodllct are
in the range of one meter.
Printed safeguarding threads are prodllced by printing the desired
patterns or characters on the carrier web and or the LC laver bv a suit-
able production method on known printing machines. ~,Ililtilaver, especiallv
symmetricallv constructed safeguarding threads are produced bv placing
the coated and possibl-- printed webs of film one ~Ipon the other ~nd con-
necting them with a laminating agent.
Only when the webs have the desired layer structure are they cut
into the threads on know-n cutting apparatus. The flnal thread ~.4idth is
in the range of 0.5 to 5.0 rnm depending on the desired application. The
resulting threads are suitable in particular for being embedded in paper
but can also be embedded between the plastics lavers of an identity card.
Another class of security elements are transfer elements. which are
frequentlv applied to credit cards, identity cards! banl; notes, papers of
value and the like tO protect them from forgerv and in particular from
being multiplied by copving. For these purposes one can also use security
elements based on LC polymers due to their opticallv variable properties.
~he transfer elements are transferred to the surface of the objects to be
protected from carrier bands by the transfer method.
Figs. 8a and 8b show an identity card 70 with a symbolically repre-
sented data record ~9 and a transfer security element 51 in a front view
and a sectional view. Security element 51 contains a layer made of an LC
polymer, which is why it has the plays of color typical of these materials.
Transfer elements customarily comprise several layers. Fig. 8b shows
a section through the identity card along the line 1.1. In the Figure the
-- IO 2032S87
height of the elements is highl~ e~;aggerated. it is usually on]v a fe h
tenths of a micrometer. Substrate ~3 bears successivelv an adhesive laver
.~. a lavel of protf~ctive lacquer 5~. an LC layer .~6 and final outside
laver of l)rotecti~-e lacquer .~,. This securitv element. shown here in
ery simple embodiment. can be v~rie(i in manv different ~hays.
The ~ossibilities for the color desi~n of L.C elements al e analogou~; to
those for safeguardin~, threads. If one desires clearly (visuallv) r ecogniz-
able plavs of color one preferably dyes the bacl;ground black. To mi,Y a
color into the reflected spectrum one applied element .51 to a print.ed
bachground 60. as shown in Fig. 8a. The printed pat;ern can ~ aried in
manv wavs: a simple design is a monochrome background: a polvchrome
printed background with contrasting alphanumeric characters or patterns
such as brightly colored diagonal stripes. interpenetrating colored circles.
etc., has an impro~!ed optical effect. Particularly interesting effects are
obtained if bacl;ground 60 contains a black and white or colored photo-
graph, a signature and the lil~e.
Similar color effects to the printing of the background can be ob-
tained by dyeing, printing or writing on suitable optically effective lavers
of the transfer element. that do not change during transfer.
As ~-ill be e~;plained below. the transfer principle makes it possible tO
give the optical element any desired outer contour. The coat of arms
shape 61 shown in Fig. 8 is therefore representative of a stripe, a seal. a
company logo! an alphanumeric character, numbering, a guilloche pattern,
etc. The shape of outline 61 gives the optically variable element an
individual character.
Figs. 9a and 9b show a front view and a sectional view of an appli-
cation variant in which card data are both inconspicuousl,v camouflaged
and protected from falsification by an LC element. LC polymers with
readily visible plays of color are usually transparent in the infrared and
can thus be easily combined with codings readable in the infrared range.
In a first printing process a coding 72 was applied for this purpose to
the surface of a card 70 with an IR-absorbent ink 71. In the next step
this IR coding 72 was overprinted with an IR-transparent ink , 3 that is
~ opaque in the visible spectral range. In the last step an LC security ele-
Il 2032587
ment , ~ was then sealed onto opaque inl; , 3 in this al ea
For manufacturing r easons one prefers the tr;lnsfer principle forapplving security elements made of LC polvmers to the surface of a sllb-
strate. In a first method step a transfer band is ~rodllced. and in a ~c-
ond method step the security element is detached from the transfer band
and connected with the substrate.
Fig. 10 shows the structure of a transfer band 100 in Cl'OSS section.
as is suitable for applying security elements with an LC laver tO a sub-
strate surface. A carrier film 101 bears successi~relv a wax la-er 102, a
layer of prolective lacquer 103. a layer made of an LC polymer 101. a
color layer 105 and a heat-sealing layer 106. The carrier film is pref-
erably made of a tear-resistant plastics polvester with a thickness in the
range of less than one tenth of a millimeter. The other layers of a trans-
fer band customarily have a thickness of a few micrometers to a few
tenths of a micrometer. Layers 103 to 106 located on the wax layer form
the subsequent security element. To obtain color effects one can dye or
print the transfer band in various layers during its production.
To apply the security element to the substrate one places transfer
band 100 with heat-sealing layer 106 on substrate l l l, as shown in Fig.
11, and presses them. The pressing is performed with a heated transfer
die 11 2 or alternatively with a transfer roll. Under the action of pressure
and heat the heat-sealing layer bonds with the substrate. Simultaneously,
separation layer 102 melts. allowing for carrier material 101 to be -re-
moved. The security element is bonded with the substrate only in the
surface areas in which the separation layer has become liquid, i.e. only
in the surface areas heated by the transfer die. In the other surface
areas the layer structure and the carrier material remain firmly inter-
connected. When the carrier film is removed from the substrate the laver
structure tears along contour edges 11 3 of the transfer die, whereby
contour 11 3 of the transferred security element always corresponds to the
contour of the press die. In this way one can also realize complicated
contour structures such as company logos, block letters and the like. The
process of heat-sealing as such is known and is described, for e~ample, in
German laid-open print no. 33 08 831.
2032587
LC pol~mers can also be made into films. In this form thev are sllit-
able in partic~llar as large-sulface or all-over securit~ elements for multi-
lay~r identitv cards.
Figs. l')a an~ l''b sho~- b~v wa~, ot e~ample a larninated idenritv c ard
1~0 comprising a paper inlay 1"1 and two outside rhermoplastic cover
films 1 '''' and 1''3. The lavers aI e pressed under the action of ~ressur e
and heat into a compact identit~ card. The card information is custorTI-
arily printed on the inla~ that. in the e.~ample shown. has a picture of
the o~-ner 1~. card data 1~ and a companv logo 1~'6. ~he protection
from forgerv ~-as increased bv integrating a film made of l C pol~mer 1",
in~o the card structure bet~heen the inlay and the upper ~over film in the
left half of the card. The plays of color of the liquid crystal film can be
obser~ied through the transparent cover film. whereby color-printed com-
pany logo 1'~6 provides additional color effects.
Some LC compounds crosslink under the action of high-energy ~e g.
U~;) radiation and only then form a chemically stable film. T:ne.yposed, i.e.
unhardened. areas can be removed with solvents. In analogy to the kno ~n
photographic methods of semiconductor and printing plate production one
can thus expose a predefined surface of an LC film through a mask and
then remove the coating chemically in the unexposed areas, thereby cre-
ating patterns, letters. numbers, etc.
It is of course also possible to cover the entire card surface -~-ith the
liquid crystal polymer film. As an alternative to integrating a film into
the card structure one might prefer to transfer the liquid crystal element
to the inlay before lamination by the transfer principle. A further variant
is to replace one or both cover films 122, 123 as a whole by an LC film
in the usual structure of laminated cards.
Films made of LC materials are suitable as large-surface or all-over
security elements. Such films are preferably made from a liquid crystal
substance. To obtain a film suitable for security purposes one processes
the LC substance on a roller frame. The orientation of the liquid crystal
molecules necessary for the optical effects is effected by shearing forces
that occur during rolling. The resulting film material is suitable in
particular for the production of identity cards but can also be used for
_ 1~ 2032587
other authenticity marl;s. such as safeguarding threa~s.
For automatic testing of authenticit-- marl;s based on the inventive
liquid crvstal pol! mers. their polarization properties and their ~avelerlgth
selectivity are particularlv suitable. The reflected light is initially con-
stricted spectrally to a range about the central wavelength. and llnpolar-
ized light is also brol~en down in liquid crystal polymers into right- and
left-handed components. Depending on the chemical composition of the
polymer only one of the components is reflected, while the complementary
component is transmitted.
One possibilit~, of automatic testing shall be shown in the following
with reference to an LC polymer film located on a black. completely
absorbent carrier 128. As shown in Fig. 13, element 130 is illuminated at
a predetermined angle with an unpolarized light beam 131. for e~ample an
incandescent lamp 129 . After reflection. Iight beam t 32 hits detector
system 133 shown in Fig. 14 used for detecting the spectral filtering and
the circular polarization.
The structure of detector system 133 is shown in Fig. 1~L. Within
detecror system 133 reflected beam 132 initially passes through a color
filter 141, that onlv lets through light of the expected central wave-
length. The light beam then hits a lambda, 4 plate 11~ that converts the
circular polarization into a linear polarization. The light then hits a 1:1
beam splitter 113. from where two partial beams 144, 145 reach two
detectors 146, 147 with polarizing filters 148,- 149 disposed therebefore.
Planes of polarization 150! 151 of the two filters are perpendicular to
each other, while being aligned at 45 with the two optical axes of the
lambda/ 4 plate.
The automatic authenticity testing is based on an analysis of the two
detector signals. The mode of functioning of the detector system is shown
in the following with reference to several cases.
A) Authentic element
The reflected light passes the color filter without hindrance. In the
lambda/4 plate a linear, either horizontal or vertical, polarization is pro-
-- ,~ 2032587
duced from the circular polarization. The linear polarization callses one ofdetectors l 16, 1~7 receive the full intensity while the second detector
receives no light.
B~ Forged element with unpolarized reflection
The spectrally correct but unpolarized reflected light has no pre-
ferred polarizing direction even after passing the lambdai4 plate. Each
detector receives 50% of the reflected light.
C) Forged element with spectral error
The reflected light is absorbed in color filter 1~'' and accordingly
neither of the detectors receives a signal.
D) Forged element with linear polarization
The ~5' arrangement of the lambda/4 plate and the two polarizers
causes both detectors to receive the same signal regardless of the orig-
inal polarizing direction of the reflected light.
To increase the error significance one can also emplo!Y several detec-
tor systems for testing one element, the systems being disposed for ex-
ample at different angles and accordingly reacting to different central
wavelengths .
It is clear to the expert that the detector system can be realized in
many different ways. Fig. 15 shows, as an easy-to-service alternative, an
arrangement using optical fibers. The basis of the optical arrangement is
again Fig. 13. In detector system 133, reflected light beam 132 initially
passes through a color filter 161 to check the central wavelength. In
following lambda/4 plate 162 the circular polarization is converted into a
linear one. An input coupling optical system 153 couples light beam 132
into a waveguide system 154, known beam separators separate the beam
into equivalent partial bundles. At the end of each partial bundle there is
a polarizer-detector pair 155/156 and 157/158 for the two different polar-
izing directions.
~ 2032~87
If the light has the corr~ct wavelength and polarization one of the
two detectors 156/15~S receives (in the case of lossfree optics) 5(1,'o of the
input intensity. the second receives no light. In the case of a forged ele-
ment reflecting unpolarized light. each of the detectors receives 50% of
the input intensity. In this wav one can distingl]ish forgery from the
original .
