Note: Descriptions are shown in the official language in which they were submitted.
CA 02879428 2015-01-19
Security element for security papers, value documents or the like
[0001] This invention relates to a security element for an object to be
protected,
such as e.g. a security paper, value document or the like, which has a
plurality of
microreflectors disposed in a pattern and a plurality of microstructures which
together
with the microreflectors produce an image perceptible to a viewer.
[0002] The invention relates further to a security paper or value document.
[0003] The invention finally also relates to a method for manufacturing a
security
element for an object to be protected, such as e.g. a security paper, value
document or
the like, wherein there are formed on a substrate a plurality of
microreflectors disposed
in a pattern and a plurality of microstructures which together with the
microreflectors
produce an image perceptible to a viewer.
[0004] Objects to be protected are frequently equipped with a security
element
which allows the authenticity of the object to be checked and thus serves as
protection
from unauthorized reproduction. Such objects are for example security papers,
identity
documents or value documents (such as e.g. bank notes, chip cards, passports,
identification cards, badge cards, shares, bonds, deeds, vouchers, checks,
admission
tickets, credit cards, health cards) as well as product authentication
elements, such as
e.g. labels, seals and packages. They may also be products themselves, such as
for
example capsules of a drug for which forgeries are to be feared.
[0005] For security elements the prior art describes in detail so-called
moire
magnification arrangements, for example in WO 2005/106601 A2, EP 1979768 Al,
EP
1182054 Bl, WO 2011/029602 A2, WO 2002/101669 A2 and EP 1893074 A2. Such
magnification arrangements combine focusing elements with microimages which
are
located in the image plane of the focusing elements. The microimages are
aligned with
the focusing elements such that a synthetic image results upon viewing of the
security
element due to the so-called moire effect. Said synthetic image has properties
(for
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CA 02879428 2015-01-19
example an orthoparallactic effect) that are not reproducible by simply
copying the
images.
[0006] The focusing elements can be configured as microlenses or
microreflectors.
The latter design corresponds to the generic type stated at the outset and is
the subject
matter of WO 2010/136339 A2 and WO 2011/012460 A2.
[0007] Known moire magnification arrangements have in common that the
microimages are a greatly reduced form of at least a partial detail of the
synthetic
image. They are formed by relief surfaces corresponding to the image content
which
are filled with color or which otherwise have light-absorbing properties. For
the light
absorption it is also known from the stated prints to employ regular or
irregular
subwavelength structures which act as light traps and are therefore also
designated as
moth-eye structures.
[0008] Known security elements require a distance between microimages and
focusing elements that corresponds approximately to the focal length of the
focusing
elements. The prior art normally satisfies this requirement by microimages and
microfocusing elements being disposed on opposing sides of a foil whose
thickness
corresponds approximately to the focal length of the focusing elements. This
procedure
requires the foil to be embossed on both sides in very exact mutual register.
This is
elaborate and therefore disadvantageous.
[0009] The invention is based on the object of remedying this disadvantage
in a
security element, security paper or value document and in a manufacturing
method of
the type stated at the outset, i.e. of doing without the strict registration
demands
required in the prior art.
[0010] This object is achieved according to the invention by a security
element for
an object to be protected, such as e.g. a security paper, value document or
the like,
which has a plurality of microreflectors disposed in a pattern and a plurality
of
microstructures which together with the microreflectors produce an image
perceptible
2
CA 02879428 2015-01-19
to a viewer, wherein each microstructure is configured as a reflective grating
and
associated with one of the microreflectors, thereby forming grating reflectors
each
consisting of one microreflector and at least one grating, each grating is so
configured
that it diffracts visible radiation incident from a half-space into a first
diffraction order
and toward the associated microreflector, in each grating reflector the
grating and the
microreflector are matched to each other such that the microreflector reflects
radiation
diffracted by the grating into the first diffraction order back into the half-
space, and
within the pattern at least one of the following properties of the grating
reflectors
varies to produce the image: diffracting property of the gratings, position of
the
gratings relative to the respectively associated microreflector, reflecting
property of the
microreflectors.
[0011] The object is further achieved with a security paper or value
document
having such a security element.
[0012] The object is finally likewise achieved with a method for
manufacturing a
security element for an object to be protected, such as e.g. security papers,
value
documents or the like, wherein there are formed on a substrate a plurality of
microreflectors disposed in a pattern and a plurality of microstructures which
together
with the microreflectors produce an image perceptible to a viewer, wherein
each
microstructure is configured as a reflective grating and associated with one
of the
microreflectors, thereby forming grating reflectors each consisting of one
microreflector and at least one grating, wherein each grating is so configured
that it
diffracts visible radiation incident from a half-space into a first
diffraction order and
toward the associated microreflector, in each grating reflector the grating
and the
microreflector are matched to each other such that the microreflector reflects
radiation
diffracted by the grating into the first diffraction order back into the half-
space, and
within the pattern at least one of the following properties of the grating
reflectors is
varied to produce the image: diffracting property of the gratings, position of
the
gratings relative to the respectively associated microreflector, reflecting
property of the
microreflectors.
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CA 02879428 2015-01-19
[0013] According to the invention, the microstructures are thus no longer
configured as micro picture elements having a separate microreflector.
Instead,
microstructures and microreflectors are combined into grating reflectors
having at least
one reflective grating and an associated microreflector, wherein the grating
configuration and the geometry of the microreflector are so chosen that the
first
diffraction order is reflected on the grating and thrown back into the half-
space by the
microreflector, preferably being concentrated. The return radiation depends on
the
configuration of the grating reflector with regard to direction in which it is
emitted,
angle of radiation and color. Through the arrangement of differently
configured grating
reflectors in the pattern there can be produced colored symbols or images.
[0014] From a viewing direction a viewer perceives an individual grating
reflector
differently in intensity and/or color, but without picture information.
Departing from
the prior art, the picture information does not come from the microstructuring
itself,
since a reflective grating per se supplies no picture information. The picture
information is instead obtained by the interaction of a plurality of grating
reflectors of
different configuration (combination of microreflector and microstructure in
the
respective grating reflector), so that between the grating reflectors there is
effectuated
a color difference or intensity difference which as a whole produces the
perceptible
image.
[0015] Since the reflective gratings in combination with the
microreflectors
normally possess strongly angular-dependent properties, there can also be
produced
parallax images as with a moire magnification arrangement.
[0016] By a suitable configuration it is also possible to produce a
stereoscopic
effect, by adjusting the direction in which microreflectors reflect back, and
preferably
concentrate, radiation diffracted by the grating into the first diffraction
order such that
some grating reflectors supply picture information for the left eye and others
for the
right eye (in accordance with the visual-angle difference).
4
CA 02879428 2015-01-19
[0017] The picture information is normally produced by the security element
by
each grating reflector consisting of microreflector and grating(s) having the
function of
a pixel for image production.
[0018] The security element can be manufactured in a single molding
process. No
molding steps to be performed in mutual register on different sides of a foil
are
required. It is instead possible to design a substrate, e.g. a foil, with a
single embossing
process such that the structures of both the microreflectors and the gratings
are
produced. The relative position (and shape) of grating and associated
microreflector is
predetermined by the corresponding embossing tool, so that it is no longer
necessary to
perform a series of processing steps involving mutual register on the
substrate.
Manufacture is thus drastically simplified compared with the stated generic
security
elements.
[0019] Further, the minimum thickness of the security element is no longer
predetermined by a focal length of focusing elements. The security element can
be
configured considerably thinner than was possible in the prior art. The
thickness is
restricted solely by the depth of the microreflectors. Said depth corresponds
approximately to the height that microlenses of known moire magnification
arrangements have, which leads to the result that the minimum thickness of the
security element amounts to only a fraction of that of conventional security
elements
with moire magnification arrangements. Nevertheless, a moire effect can be
realized
equally well.
[0020] The gratings that are associated with the microreflectors and lie
for example
on the bottom of a concave mirror or of a mirror trough are reflective
gratings. They
reflect the first diffraction order toward the associated microreflector. This
is a
fundamental difference over the absorbent structures known in the prior art
which, as
moth-eye structures or also as regular subwavelength structures, are to
produce
reflexes as small as possible to effectuate a good contrast in a microimage.
CA 02879428 2015-01-19
[0021] Preferably, the gratings are so configured here that diffraction
orders higher
than the + 1st diffraction order are, if possible, not reflected toward the
microreflector
or reflected at angles so flat that the microreflector only directs radiation
intensity of
the higher diffraction orders into the half-space to a smaller extent than
radiation that
was diffracted into the + 1st diffraction order. Further, it is preferable
that the intensity
of the radiation diffracted into the 1st diffraction order lies above the
intensity of the
radiation diffracted into the 0th diffraction order.
[0022] For certain arrangements it can be advantageous if one of the two +
1st
diffraction orders is preferred. This can be obtained by a so-called blazed
grating,
which supplies an asymmetry between ¨1st and +1st order.
[0023] The microreflectors can be configured as trough-shaped reflectors.
They are
then concave troughs which preferably have a plane bottom. The plane bottom
need
not necessarily be reflective. On the plane bottom there is disposed the
grating, which
is then likewise configured as a linear grating. In particular with trough-
shaped
reflectors it is also possible to dispose more than one grating along the
concave troughs
of the reflector. Optionally, the microreflectors can be designed as concave
concave
mirrors, which preferably have a plane bottom on which the grating lies. Here,
too, the
bottom need not be reflective. The microreflectors can of course also be
realized by
non-concave concave mirrors or troughs. The plane bottom of the
microreflectors can
in particular be configured obliquely, so that the grating is inclined toward
the
microreflector.
[0024] Alternatively, there can also be used microreflectors whose bottom
is not
plane, but for example of bulged configuration.
[0025] Using the individual grating reflectors as pixels is especially
simple when
the microreflectors are configured as concave concave mirrors which are
rotationally
symmetric, having for example the form of the surface area of a spherical
segment or
ellipsoid segment. Such a surface area is obtained when a segment is cut out
from an
ellipsoid or sphere through two parallel planes truly intersecting the sphere
or ellipsoid.
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CA 02879428 2015-01-19
The smaller one of the two parallel circular areas or ellipses arising from
the section
then constitutes the bottom of the concave concave mirror. It does not have to
be
mirror-coated.
[0026] For conventional optical embodiments it is preferred that the
microreflectors have a depth of 2 to 30 Rm, preferably 5 to 20 Rm. Further, it
is
preferred that each grating has a grating period between 0.3 Rm and 1 Rm. Such
structure dimensions yield good results at the same time as simple
manufacturability.
[0027] If the security element is designed such that each grating reflector
acts as a
pixel, it is preferred to configure the microreflector as a rotationally
symmetric
concave mirror, as mentioned hereinabove. The grating can then be a circular
grating.
The perceptible properties of each pixel are then adjusted e.g. by the
position of the
circular grating on the bottom of the concave mirror.
[0028] Depending on the lateral position of the grating in the respective
microreflector, the individual pixel to which the grating reflector then
corresponds has
a different intensity in the image.
[0029] Grating reflectors with circular apertures are preferably disposed
in a
hexagonal pattern, since there can then be achieved an area fill as high as
possible.
[0030] The individual grating reflectors can be configured with bars
surrounding
them, i.e. at least some neighboring grating reflectors then do not abut each
other
directly, but are separated by a bar. Consequently, there are bar areas
between the
grating reflectors. Through the structuring of the coating of the bar areas
and/or the
design of the thickness of the substrate in the region of the bar areas one
can ensure
that the bar areas reflect and/or transmit incident light differently. The
effect is
preferably designed to be laterally variable in order to additionally encode
symbols and
make them visible in transmission. Thus, additional anti-forgery security is
obtained.
[0031] A laterally varying coating of the bar areas is preferably realized
by the bar
areas first being furnished with a coating, for example with a metallization,
and said
7
coating being removed again regionally, e.g. by an etching method.
Alternatively, it is
possible to regionally transfer the coating present in the bar areas by
laminating with
an acceptor foil, and thus regionally remove it from the bar areas. Further
details on
such a transfer method can be found in the print WO 2011/138039 Al.
[0032] An especially good efficiency of the 1st diffraction order is
obtained when
microreflectors and gratings are coated metallically on their side facing the
half-space,
preferably with aluminum, silver, gold, copper, chromium or an alloy
containing one or
more of said metals. The microreflectors and gratings can also be realized by
high-
refractive dielectrics, such as e.g. TiO2 or ZnS, or semimetals such as
silicon and
germanium. In particular, the gratings can be configured as such by said
materials.
Further, said materials can serve as a reflective coating of the
microreflectors.
[0033] The microreflectors can have in principle any arbitrary shape in
their
apertures (openings), for example square, circular or rectangular apertures.
[0034] The security element can be configured in particular as a security
thread,
pull thread, security band, security strip, patch or as a label. In
particular, the security
element can cover transparent regions or recesses of an object to be
protected.
[0035] The security element can in particular be part of a precursor, not
yet fit for
circulation, of a value document which can for example also have additional
authentication features (such as e.g. luminescent substances provided in the
volume,
etc.). Value documents are understood here to be documents having the security
element, on the one hand, but value documents can also be other documents or
objects
that can be furnished with the security element according to the invention in
order to
have uncopiable authentication features, on the other hand. Chip cards or
security
cards, such as e.g. bank cards or credit cards, are further examples of a
value
document.
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CA 2879428 2020-02-25
= CA 02879428 2015-01-19
[0036] For the manufacturing method according to the invention there come
into
consideration in particular direct exposure techniques, e.g. using a laser
writer.
Manufacture can be effected analogously to known manufacturing methods for
microlenses. The original of the structure is written via direct exposure
using a laser
writer into a substrate coated with photoresist, and subsequently the exposed
portion of
the photoresist removed. An exposed original can subsequently be electroformed
and
thus an embossing stamp produced. Finally, the structure is replicated for
example in
UV lacquer on foil via an embossing process. Alternatively, a nanoimprinting
process
can be used. More elaborate methods for manufacture of originals such as
electron-
beam or focused-ion-beam exposure methods allow an even finer configuration of
the
geometry.
[0037] The manufacturing method according to the invention can be
configured
such that the described preferred configurations and embodiments of the
security
element are manufactured.
[0038] It will be appreciated that the features mentioned hereinabove and
those to
be explained hereinafter are usable not only in the stated combinations but
also in other
combinations or in isolation without going beyond the scope of the present
invention.
[0039] Hereinafter the invention will be explained more closely by way of
example
with reference to the attached drawings, which also disclose features
essential to the
invention. For better illustration, the representation in the figures is not
true to scale or
to proportion. There are shown:
Fig. 1 a schematic representation of a security element,
Figs. 2a and b illustrations of the reflections that can occur with the
security element of
Fig. 1,
Figs. 3a and 3b schematic representations similar to Fig. 1 for illustrating
different
grating structures that can be used with the security element of Fig. 1,
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CA 02879428 2015-01-19
Figs. 4, 5 and 6 diffraction efficiencies for the different grating structures
that can be
employed in the security element of Fig. 1,
Figs. 7a-i schematic illustrations of the reflections that occur with a first
embodiment
of the security element,
Figs. 8, 9 and 10 representations similar to Fig. 7 for further embodiments of
the
security element,
Fig. 11 a plan view of a security element similar to that of Fig. 1 with
modifications
with regard to forms of reflectors and grating structures,
Fig. 12 a plan view similar to Fig. 11, but with reflectors and grating
structures having
a circular aperture,
Fig. 13 a plan view of a security element with an arrangement of linear
gratings and
trough-shaped reflectors that are respectively configured similar to the
design
of Fig. 11 and produce an image stereoscopically,
Fig. 14 an arrangement of linear gratings with trough-shaped reflectors for
representing a motif likewise for stereoscopic image production,
Fig. 15 a representation similar to Fig. 1 for a development of the security
element
shown there, and
Fig. 16 a plan view of the security element of Fig. 15.
100401 Fig. 1
shows schematically in the upper part a sectional representation of a
security element 1 and in the lower part the appurtenant plan view thereof.
The security
element 1 is made of a foil 2 having an embossed structure 3 formed on its
upper side
(this term being intended to be purely exemplary and not to state any
preferential
direction). The embossed structure 3 comprises a multiplicity of reflectors 4,
which are
designed as elliptical reflectors with a plane bottom 5 in this exemplary
embodiment.
On the bottom 5 of each reflector 4 there is located a reflective grating 6.
The total
CA 02879428 2015-01-19
embossed structure 3 is overlaid with a metal layer. The metal layer thus
covers both
the concave area defining the reflecting properties of the reflector 4, and
the grating 6.
[0041] One grating 6 forms with the associated reflector 4 a grating
reflector 7 in
each case. Each grating reflector 7 throws radiation incident on the aperture
8 of the
grating reflector 7 back from the upper side in concentrated form into the
half-space
from which the radiation came. The direction and intensity depend on the
configuration of the grating reflector 7.
[0042] Fig. 1 makes clear that a multiplicity of reflectors and gratings
are present in
the security element, with one grating 6 being associated with one reflector 4
in each
case in this exemplary embodiment. This is obtained by the grating 6 being
disposed
on the bottom 5 of the corresponding reflector 4. The grating reflectors 7 are
disposed
in a pattern which is configured as a side-by-side array of the grating
reflectors 7 in
simplified form in the plan view which is to be seen in the lower part of Fig.
1. Within
the pattern the design of the grating reflectors 7 varies, so that each
grating reflector 7
forms a pixel of a motif characterizing the security element 1. For the
variation of the
grating reflectors 7 there are different possibilities which will be explained
hereinafter.
In the embodiment of Fig. 1, the position of the grating 6 on the bottom 5
varies by
way of example.
[0043] Between the grating reflectors 7 there is a distance, in the
representation of
Fig. 1, which leads to bar areas 11 being formed on the substrate which lie
between the
edges of neighboring grating reflectors 7. Said bar areas can be used for
encoding a
further optical effect, as to be explained hereinafter with reference to the
design of
Figures 15 and 16. In the embodiment of Fig. 1, however, the bar areas are
without any
further importance.
[0044] For the reflection of illumination radiation that is incident from
above
relative to the sectional representation in the upper part of Fig. 1, what is
essential is
the reflection of the radiation that the grating 6 backscatters, as will be
explained. Said
reflection is effected on the respective reflector 4. The term "reflector"
thus does not
11
CA 02879428 2015-01-19
relate to a direct reflection of the incident illumination radiation, as would
be the case
e.g. with a retroreflector, but rather to the reflective property that the
reflector 4 has for
radiation diffracted back by the grating 6.
[0045] This effect can be readily recognized in Fig. 2, which shows in its
partial
Figures 2a and 2b how incident radiation 9 is diffracted by the grating 6
toward the
reflector 4 and reflected there, preferably concentrated, so that it is thrown
back again
as return radiation 10 into the half-space from which the incident radiation 9
came.
Fig. 2a shows the relations for the 2nd diffraction orders, and Fig. 2b
represents the
corresponding 1st diffraction order. The ¨ 2nd and ¨ 1st orders would only be
shifted
relative to the axis of symmetry of the reflector and of the grating if the
grating 6 were
disposed symmetrically on the bottom 5, as is the case in the simplified
representation
of Fig. 2. For the sake of simpler explanation, Fig. 2 assumes a rotationally
symmetric
reflector 4, and the geometry of the reflector 4 is so chosen that the 1st
diffraction
order of the visible spectrum is reflected and concentrated.
[0046] In the example, the reflector 4 possesses an elliptical geometry
with an
aperture of 11.4 1.1m, an apex of 12.8 vim and a grating disposed in the
center at a depth
of 11.4 pm. The grating period amounts to 800 nm.
[0047] For a wavelength of 520 nm the above-mentioned 2nd and 1st
diffraction orders are present, for which Figs. 2a and 2b show the marginal
rays of the
incident radiation 9 for which return radiation 10 arises, and of the return
radiation 10.
Due to the rotational symmetry it is possible to replace the statement of a
solid angle
by a plane angle for the incident radiation 9 and the return radiation 10. In
the case of
the 2nd diffraction order, the aperture angle of the incident radiation
(also called
acceptance angle) amounts to 9 . This light is converted as return radiation
10 to an
exit angle range of 3.7 . For the 1st order, the aperture range of the
incident radiation
amounts to 31.5 , and the exit angle range 17.30.
[0048] For the security element 1 of Fig. 1 it is desirable to maximize the
efficiency
of the 1st diffraction order while, on the other hand, suppressing other
diffraction
12
CA 02879428 2015-01-19
orders if possible, i.e. not converting them to return radiation 10. This can
be obtained
e.g. by a suitable choice of the grating 6. If there is chosen for example,
for the
mentioned grating with a period of 800 nm, aluminum as the material with a one-
to-
one bar-to-gap ratio as well as a bar height of 150 nm, the efficiency, i.e.
backscatter
intensity, of the 1st diffraction order will be considerably greater than
that of other
diffraction orders.
[0049] With a grating having a rectangular bar profile, negative and
positive
diffraction orders are the same, since the grating is symmetric (always based
on the
case that the grating is disposed symmetrically on the bottom 5). The
preference of a
direction can be obtained by an asymmetric grating profile, as is known in the
prior art
as a blazed grating. The corresponding relations are represented by way of
example in
Figs. 3a and 3b. Here, Fig. 3a shows a rectangular grating in which ¨ 1st and
+ 1st
orders are configured symmetrically, i.e. both equally strong. If a blazed
grating is
employed, one order is preferred and the other suppressed. This is obtained
e.g. by the
asymmetric triangular form of the grating bars. Other possibilities for
producing an
asymmetric preference of negative or positive diffraction orders are so-called
Dammann gratings or slanted gratings.
[0050] In principle, with the reflective grating 6 the intensity of the
1st order in
the visible spectral region can lie above the intensity of the 0th diffraction
order, i.e. of
the mirroring return radiation. Figs. 4a and 4b show the diffraction
efficiencies of the -
1st and 0th diffraction orders of a grating with a period of 550 nm in the
visible
spectral region for different angles of incidence; Fig. 4a the efficiency of
the 1st
diffraction order, Fig. 4b that of the 0th diffraction order. The reflective
grating here is
one with a rectangular profile consisting of aluminum bars which have a width
of 275
nm and a height of 120 nm and are disposed in a grating period of 550 nm.
[0051] Figs. 5a and 5b show an analogous representation for a grating with
a
grating period of 650 nm and aluminum bars with a width of 325 nm and a height
of
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CA 02879428 2015-01-19
120 nm. These efficiencies obtained here already suffice for use as a
reflective grating
in a security element 1 according to Fig. 1.
[0052] Further improved diffraction efficiencies are shown by a blazed
grating
according to Fig. 3b, as evidenced by the plottings in Figs. 6a and 6b, which
correspond to those of Figs. 4a and 4b. The grating period again amounts to
550 nm
and the maximum height of the triangular profile of the aluminum bars 340 nm.
Comparison with Fig. 4 shows that the efficiency of the ¨ 1st order is raised
further.
100531 Figs. 7a-c, d-f and g-i show the reflection of the 1st diffraction
order for
different wavelengths of the incident radiation. Figs. 7a, 7d and 7g shows the
relations
with blue light (wavelength 450 nm), Figs. 7b, 7e and 7h with green light
(wavelength
520 nm), and Figs. 7c, 7f and 7i the relations with red light (wavelength 700
nm). The
relations between Figs. 7a-c, 7d-f and 7g-i differ with regard to the depth of
the
reflectors. All reflectors have an aperture of a =12 pm. In Figs. 7a-c the
depth amounts
to 9 pm, in Figs. 7d-7f to 12 gm and in Figs. 7g-7i to 15 pm. The apex of the
reflectors
amounts to 10.5 11111 (Figs. 7a-c), 13.5 gm (Figs. 7d-7f) and 16.5 pm (Figs.
7g-7i). The
grating parameters correspond to those of Fig. 4. The grating period
accordingly
amounts to 550 nm. The entry angles for incident radiation 9 and exit angles
for
emergent radiation 10 for Figs. 7a-7i are summarized in the following table:
14
CA 02879428 2015-01-19
Figure Acceptance angle Exit angle
a 25.0 28.1
26.0 29.6
17.5 18.5
31.5 17.2
29.5 14.9
10.5 4.1
31.5 6.4
25.4 2.8
5.5 1.1
[0054] The stated numbers are in degrees, and show that with increasing
structure
depths, i.e. depths of the reflector 4, the divergence of the return radiation
10
decreases, i.e. return radiation 10 is concentrated increasingly toward the
viewer, since
the exit angle describes the concentration. Greater wavelengths moreover
restrict the
acceptance angle of the incident radiation 9.
[0055] The variation of the reflecting properties of the reflectors 4 as
performed in
Fig. 7, explained by way of example with reference to the reflector depth, is
a first
possibility for changing the reflecting property of the grating reflectors 7,
for thereby
modulating the pattern of grating reflectors 7 and for finally producing
picture
information.
[0056] A variation of the reflecting properties can also be obtained by the
variation
of other geometrical parameters of the grating reflectors 7 consisting of
grating 6 and
reflector 4. Fig. 8 shows representations similar to those of Fig. 7, whereby
not the
CA 02879428 2015-01-19
depth of the reflectors is varied as in Fig. 7, but rather the size of the
aperture. It
amounts to 9 gm in Figs. 8a-c, 12 gm in Figs. 8d-f, and 15 gm in Figs. 8g-i.
The
wavelengths that are represented correspond to those of Fig. 7. The reflector
depth is
constant at 12 gm in all arrangements, the apex amounts to 13.5 gm. The
grating
period amounts to 550 nm. The corresponding acceptance angles for incident
radiation
9 and exit angles for return radiation 10 are as follows:
Figure Acceptance angle Exit angle
A 30.5 7.5
23.5 8.4
4.5 4.7
31.5 17.2
29.5 14.9
10.5 4.1
26.5 9.4
27.5 9.5
1 16.0 2.3
[0057] It is apparent that greater apertures do not substantially worsen
the
concentration of the return radiation 10. On the other hand, the area of the
bottom 5
and thus the area available for the grating 6 grows with increasing aperture.
As in Fig.
7 as well, the exit angle of the return radiation 10 decreases with growing
wavelength.
[0058] The variation of the aperture is a second possibility for modulating
the
pattern of grating reflectors 7.
[0059] In Figs. 7 and 8 the reflecting properties of the reflectors 4 were
varied.
However, it is also possible to vary the diffracting properties of the grating
in order to
16
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vary the acceptance angle of the incident radiation 9 and/or the exit angle of
the return
radiation 10. Fig. 9 shows a representation similar to Fig. 7, with the
grating period
now being varied between Figs. 9a-c, 9d-f, and 9g-i. The reflector geometry,
on the
other hand, is constant, at least for the cases shown in Figs. 9d-f and Figs.
9g-i, with an
aperture of 12 gm, a depth of 12 gm and an apex of 13.5 1AM (Figs. 9a-c), 15
gm (Figs.
9d-f) and 15 gm (Figs. 9g-i). In Figs. 9a-c the grating period amounts to 400
nm, in
Figs. 9d-f 600 nm and in Figs. 9g-i 800 nm. The represented colors in Figs. 9a-
i also
correspond to those of Figs. 7a-i and 8a-i. The corresponding values for the
acceptance
angle and the exit angle arc the following:
Figure Acceptance angle Exit angle
A 19.0 7.7
9.0 3.7
0.0 0.0
31.5 8.8
32.0 9.7
16.5 8.9
32.0 8.9
31.5 8.7
32.0 19.6
[0060] The variant of Figs. 9a-c shows no reflection of red light. Blue or
green
light, on the other hand, is directed toward the viewer in concentrated form
by the
reflector 4, since the exit angle is considerably smaller than the acceptance
angle. The
two other cases (Figs. 9d-f and 9g-i) show no significant difference for blue
and green
light. The acceptance angle of red light increases considerably for the
greater grating
period.
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= CA 02879428 2015-01-19
100611 The variation of the grating parameters is a third possibility for
modulating
the pattern of grating reflectors 7.
100621 The way the return radiation 10 is produced from the incident
radiation 9
may be due not only to the reflecting properties of the microreflectors or the
diffracting
properties of the grating 6, but also to the position of the grating 6
relative to the
respectively associated reflector 4. Fig. 10 shows a variation in this regard,
with the
grating 6 being shifted to the left on the bottom 5 relative to the axis of
symmetry of
the reflector 4 in Figs. 10a-c, the grating lying symmetrically to the axis of
symmetry
but not covering the total bottom 5 in Figs. d-f, and the grating 6 being
shifted to the
right on the bottom 5 in Figs. 10g-i. In this exemplary embodiment the shift
amounts to
¨ 1.6 gm (Figs. 10a-c), 0 gm (Figs. 10d-f) and 1.6 gm (Figs. 10g-i). The
reflector
geometry is constant in all cases with an aperture of 12 gm, an apex of 13.5
gm and a
depth of 12 gm. The grating period amounts to 550 nm. The colors on which the
representation of Figs. 10a-i is based correspond to those of Figs. 7, 8 and
9. For the
individual colors and arrangements there thus results the following:
Figure Acceptance angle Exit angle
A 37.5 15.0
35.0 12.4
16.0 6.0
31.5 17.2
29.5 14.9
10.5 4.1
26.5 12.9
23.5 10.7
4.5 2.0
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[0063] The representations of Fig. 10 are based on a microreflector with an
aperture of 12 um and a depth of 12
100641 Fig. 10 shows that a link shift of the grating increases the
acceptance angle
for incident radiation 9 and thus the reflected light intensity of the ¨ 1st
order. The
divergence of the return radiation 10, on the other hand, is influenced by the
different
position of the grating considerably less.
100651 The variation of the position of the grating 6 relative to the
reflector 4 is a
fourth possibility for modulating the pattern of grating reflectors 7.
100661 In the above examples, reflectors with elliptical geometries have
been
examined and represented. It is of course possible to optimize the light
concentration
of a certain color toward the viewer by a different geometry, so that a viewer
perceives
this color dominantly.
100671 Further, parameters other than those represented are also suitable
and
employable in embodiments for influencing the diffracting properties of the
gratings or
the reflecting properties of the microreflectors. Besides the curvature of the
reflectors 4
being varied, the geometry of the reflectors 4 can also be designed
asymmetrically.
Further, besides the period being varied, the profile of the grating 6 can
also be varied
to achieve a locally varying total reflection behavior of the respective
grating reflectors
consisting of reflector and grating. Moreover, it is possible to configure the
bottom 5
obliquely, i.e. to incline the grating toward the reflector 4 in a location-
dependent
manner.
100681 The stated possibilities can of course also be combined at will.
100691 Thus, a multiplicity of parameters come into consideration for
designing the
return radiation 10 differently in a location-dependent manner within the
pattern and
thus modulating the radiation backscattered by the grating reflectors 7 within
the
pattern in the security element 1 according to the invention.
19
CA 02879428 2015-01-19
[0070] As a grating there can not only be employed one-dimensionally
periodic
gratings, and they need also not be combined with reflectors that are curved
in the
same spatial direction. Gratings 6 are also possible that are rotated around
an axis of
symmetry of the reflector 4, and the rotation is varied within the pattern for
modulation. For two-dimensional periodic gratings 6 cross gratings are
particularly
suitable, the periodicity preferably extending perpendicular to the curvatures
of a
reflector with a rectangular aperture. Circular gratings are particularly
suitable for
reflectors with circular apertures, on the other hand. Mixed forms or
elliptical gratings
in reflectors with elliptical apertures are of course also possible.
[0071] For producing the motifs, grating reflectors 7 respectively formed
from
grating and reflector are disposed side by side, with their configuration
varying
laterally to form an image for example as a colored symbol. The variation of
the
diffracting properties of the gratings 6, or of the position of the gratings 6
relative to
the respective associated reflector 4 or of the reflecting property of the
reflectors 4 or
of a combination therefrom, effectuates a color contrast or intensity contrast
in the
motif. Since the gratings 6 moreover possess strongly angular-dependent
properties,
the motifs can be employed for producing parallactic images, as mentioned
hereinabove. It is thereby possible to implement both parallactic motions and
spatial
effects.
100721 Fig. 11 shows a simple example of a plurality of grating reflectors
7 in plan
view, whose gratings 6 are configured as linear gratings and are combined with
trough-
shaped reflectors 4. The trough-shaped reflectors 4 lie side by side
periodically. The
gratings 6 are shifted toward the reflector laterally, the shift being
continuous and
coming about in the exemplary embodiment through an about 15% smaller
frequency
of the individual gratings relative to the frequency of the reflectors 4. A
viewer will
perceive the individual grating reflectors consisting of reflector and grating
with
different brightness, resulting in an intensity modulation for the individual
strips.
= CA 02879428 2015-01-19
[0073] Outside the optically active grating, the bottom can be plane or
also be
furnished with a (light-absorbing) moth-eye structure or a subwavelength
structure.
The latter variants improve the contrast, since the specular reflection is
suppressed as a
result.
[0074] Further, in a typical viewing situation the perception will be
different for the
left eye and the right eye due to the different viewing angles, which can be
used for a
stereoscopic effect, as to be explained.
[0075] A lateral variation can of course also be achieved by two-
dimensional
grating reflectors 7. Fig. 12 shows a representation that is similar to the
lower part of
Fig. 1. Here, rotationally symmetric reflectors 4 are combined with circular
gratings 6,
with the individual gratings 6 being shifted differently within the associated
reflector 4.
For a viewer, the individual grating reflectors 7 will differ in brightness,
since they
concentrate the incident radiation toward the viewer to different extents at a
given
viewing angle. An especially high area coverage is obtained with the grating
reflectors
7 when they are disposed in a hexagonal pattern. Each individual grating
reflector 7
realizes a pixel, i.e. an image point, so that the variation of the stated
property, in this
case of the position of the grating 6 relative to the respectively associated
reflector 4,
modulates the image.
[0076] A stereoscopic perception requires two images; one image for the
left eye
and another for the right one. Such stereoscopic images can likewise be
produced with
the security element 1 according to Fig. 1. The light is then so directed in
different
directions through the variation of the properties that a different modulation
is obtained
for the left eye and the right eye. Fig. 13 shows an exemplary arrangement of
linear
gratings 6 in trough-shaped reflectors 4, with the gratings 6 being tilted
relative to each
other and having different periods. The different grating periods effectuate a
color
effect of the structure. The grating reflectors 7 located on the left side
preferably direct
the incident radiation in the direction of the left eye of a viewer, while the
grating
reflectors on the right subject the right eye to return radiation 10. The
color impression
21
= CA 02879428 2015-01-19
is varied by the choice of different grating periods or reflector geometries.
The
arrangement of the grating reflectors 7 according to Fig. 13 thus allows both
spatial
effects and kinetic effects to be represented.
[0077] A simple embodiment exemplifying this principle is shown in Fig. 14.
The
area of the two letters "AB" is filled with a multiplicity of grating
reflectors 7. The
grating reflectors are so oriented in the letter "A" that they preferably
direct the light
toward the left eye. The light from the letter "B", on the other hand, will be
perceived
primarily by the right eye. If the grating reflectors 7 have an accordingly
small design,
i.e. the pixelation is accordingly small, the two symbols can also be present
in mutually
interlaced form.
[0078] Fig. 15 shows an exemplary embodiment similar to that of Fig. 1.
Accordingly, the reference signs for functionally or structurally identical
elements are
also taken from Fig. 1. In the security element 1 of Fig. 15, a pattern of
grating
reflectors 7 is likewise employed. Between neighboring grating reflectors 7
there
remain bars in the substrate, which result in bar areas 11 lying between the
individual
grating reflectors 7. The bar areas are structured laterally in the design of
Fig. 15 such
that some bar areas 11 are furnished with a metallization 12 while others are
not.
Fig. 16 shows this state in a plan view similar to the view of Fig. 12.
[0079] The only regional metallization of the bar areas 11 creates regions
I and II
in which the bar areas with a metal layer 12 (regions I) and without a metal
layer
(regions II) are provided. The metallization has the consequence that incident
radiation
9 falling on a metallized bar area 11 is thrown back as a reflex 13. In the
regions II
where the bar areas 11 are not metallized, on the other hand, the incident
radiation is
not reflected. The security element 1 is thus subdivided into regions I and II
which
differ in their reflection behavior and also in their transmission behavior.
By lateral
structuring of the metallization it is thus possible to encode additional
information in
the security element 1. Instead of a metallization there can also be employed
a different
kind of reflective layer.
22
CA 02879428 2015-01-19
[0080] Instead of a metallization or reflective layer there can be formed
an
absorbent layer; the lateral structuring of the bar areas 11 then does not
affect the
reflection behavior, but the transmission behavior.
[0081] A further possibility of influence is likewise indicated in Fig. 15.
What is
involved here is the thickness of the bars under the bar areas 11, i.e. the
thickness of
the substrate 2 in the region of the bars 11. In Fig. 15 the bars are less
thick in the
region of the bar areas in the regions I than in the regions II by way of
example. If the
substrate is suitably chosen, this can likewise influence the transmission
behavior. The
different height of the plateaus on which the bar areas 11 lie results
automatically, in
the design of Fig. 15, when grating reflectors 7 with different depth are
formed, and
the bottoms 5 lie at a level. The different depth of the grating reflectors 7,
for example
in the regions I and II, then leads automatically to a different thickness of
the substrate
in the region of the bar areas 11. As to be explained more closely
hereinafter, a
different height of the plateaus on which the bar areas 11 lie can also be
used
advantageously in the production of the laterally varying coating.
[0082] The bar areas 11 always arise when grating reflectors do not abut
each other
seamlessly. The total area of the bar areas 11 thus depends on the pattern in
which the
grating reflectors 7 are disposed. If they are disposed in the form of a
hexagonal
pattern with a high area coverage, as in Fig. 12 for example, the total area
of the bar
areas 11 is very small. With rectangular apertures of the grating reflectors,
the total
area of the bar areas 11 can be made arbitrarily small. If a distance is
deliberately left
between the grating reflectors, as in Fig. 16, the total area of the bar areas
11 increases.
This measure makes it possible to adjust in a targeted manner the transmission
effect or
reflective effect that is effectuated by the bar areas 11. In other words, the
clarity with
which a symbol or image is recognizable through the lateral structuring of the
bar areas
11 can be adjusted by accordingly choosing the total area of the bar areas
through the
distance of the grating reflectors 7.
23
CA 02879428 2015-01-19
[0083] The principle employed in the security element 1 of Fig. 1 supplies
a large
construction kit for designing patterns or symbols by grating reflectors 7.
The variation
of the grating reflectors 7 in the pattern can be realized by many parameters.
[0084] The structure of the security element 1 can be manufactured very
simply by
a single embossing process. For this purpose one requires only a corresponding
embossing tool that has a corresponding negative form for each grating
reflector 7, i.e.
produces both the reflectors 4 and the gratings 6. To obtain a high yield of
the grating
reflectors 7, the gratings 6 must be precisely formed in the embossing tool.
The
manufacture of the embossing tool can be obtained using electron-beam writing
systems or interferometric methods.
[0085] The reflectors 4 typically have a depth of 2 to 30 pm, a
particularly
preferred range lying between 5 and 20 m. Smaller depths are advantageous
with
regard to both the manufacture of the embossing tool and later duplication.
However,
the grating 6 produces its optical effect only when at least 4 to 10 grating
periods can
be accommodated on the bottom 5. This gives a lower limit for the size of the
reflectors 4. An upper limit results when the grating reflectors 7 are
intended to be
used as pixels, which should naturally be as small as possible and in
particular should
no longer be resolved with the unarmed eye. On the other hand, as flat an
embossing as
possible is advantageous for duplication.
[0086] For producing the embossing tool, the reflectors 4 are preferably
first
produced, for example photolithographically by direct laser writing.
Independently
thereof, the structure of the gratings 6 is produced. These two operations are
preferably
performed in an accurately fitting manner in one and the same photoresist
which, as a
template, is the basis for the embossing tool. Alternatively, two different
photoresist
coating operations are also possible.
[0087] Also, it is possible in a variant to first produce a homogeneous
grating, coat
it with photoresist and then produce laterally differently configured
microreflectors, for
example by laser writing. For this purpose, a grating is first manufactured
for example
24
CA 02879428 2015-01-19
using an electron-beam exposure method, and molded. Subsequently, said grating
substrate is coated with a photoresist in sufficient thickness, e.g. 10 p.m.
Then,
reflectors 4 are exposed with a laser writer by the direct exposure method, so
that the
grating 6 lies open in the middle of the individual reflectors after
developing.
[0088] The employed laser writers can operate with two-photon absorption
processes. It is then possible to produce reflectors 4 and gratings 6 in a
single process.
[0089] The template manufactured for the embossing tool is now copied
galvanically or by a nanoimprinting process.
[0090] Since in the embossing of a foil a multiplicity of security elements
are
usually to be produced in one embossing step, it is preferable to provide a
plurality of
mutually adjacent stamp elements in the embossing tool which have respectively
been
produced from a template in the previously described manner.
[0091] The embossed foil is preferably overlaid with an opaque metal layer.
This
may be done by sputtering, electron-beam vapor deposition or thermal
evaporation.
Particularly suitable metals are aluminum, silver, gold, nickel or chromium or
alloys of
said materials. The thicknesses of the metal layer lie between 20 and 100 nm.
The
metal surface is then finally preferably overlaid with a protective layer or
laminated
with a cover foil.
[0092] Here, a further advantage of the security element over known moire
arrangements becomes apparent, since the microlenses usually employed there
cannot
be overlaid with a cover layer. Their optical imaging property would thereby
be lost.
[0093] For producing a laterally structured coating of the bar areas 11, a
corresponding coating is preferably first applied to all bar areas 11 and then
removed
from some bar areas again. In the case of a metallization, removal is effected
by a
suitable demetallization method. In particular an etching process or a
transfer method
according to WO 2011/138039 Al can be used here. It is also possible to remove
the
CA 02879428 2015-01-19
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coating, for example a demetallization, by means of ultrashort-pulse lasers
and the use
of a writing laser beam.
[00941 Upon the
removal of the coating, the latter can be prevented in a targeted
manner from also being removed in the region of the grating reflectors 7 or in
the
regions 1. Thus, it is possible for example to fill the metallized embossed
structure 3
completely with a photoresist, i.e. to flatten it out. Then one can etch the
photoresist
for example up to the levels of the higher bar areas 11 (regions II) and
remove the thus
laid-open coating, for example etch a metal layer. A subsequent removal of the
photoresist then lays open the grating reflectors 7 as well as the bar areas
11 with the
metallization 12 lying at a lower level (regions I) again, which in this way
are not
affected by the intervention on the bar areas 11 in the regions II.
26
CA 02879428 2015-01-19
0
List of reference signs
1 Security element
2 Foil
3 Embossed structure
4 Reflector
Bottom
6 Grating
7 Grating reflector
8 Aperture
9 Incident radiation
Return radiation
11 Bar area
12 Metallization
13 Reflex
I, II Region
27
