[0001] The present invention concerns an engraved plate for intaglio printing of sheets
of security papers, comprising a plate portion generating on each said security paper
a control element upon intaglio printing. The invention relates also to a process
for manufacturing an aforesaid plate and to a security paper imprinted by means of
an aforesaid plate.
[0002] The term "security paper" primarily designates here banknotes, but also designates
documents of any kind having a financial value, such as cheques, lottery tickets,
title deeds and the like, or identity documents such as passports, ID cards, driving
licences and the like.
[0003] The term "control element" designates any sign readable either by human beings or
by a specific machine. Such control elements comprise individualised identity markings,
such as serial numbers or code bars. They also comprise markings that are difficult
to manufacture and to reproduce, but which may be checked as far as quality parameters
like colorshade, thickness, consistency and the like are concerned. Among these, optically
variable markings which may be easily checked by the human eye upon merely handling
an illuminated security paper, that is to say by lay users without necessitating a
specific detection equipment, are particularly valuable in so far as such control
elements cannot be reproduced by means of commonly available equipments, such as photocopiers,
scanners and similar copying machines.
[0004] Among various processes used in the security printing industry, intaglio printing
is a preferred technique, because it provides prints of high quality. Additionally,
intaglio printing provides an embossing of the printed sheet which may be felt by
the human hand. Its feel is different from the feel of a sheet imprinted by another
technique. Thus, it provides a quite simple control element to the touch against coarse
counterfeits like photocopies.
[0005] It was also already proposed in the past to make use of the embossings produced at
the surface of a security paper sheet upon intaglio printing to generate an optical
variable effect.
[0006] When an intaglio imprint formed of inked lines co-operatively defining a visible
pattern is seen from an acute angle of view in a plane perpendicular to the lines,
the inked lines forming ribs above the substrate surface occlude more or less the
spaces between them, whereas they do not occlude these spaces when seen from an angle
of view normal to the substrate surface. Documents US 4,033,059 (R.G. Hutton et al.)
and US 4,124,947 (A. Kuhl et al.) disclose several embodiments of a technique based
on this effect, using intaglio printing elements printed in an ink which contrasts
with the colour of the underlying paper. The intaglio pattern elements comprise at
least two arrays, one of which constitutes an image of readily recognisable form,
and the other array constitutes a background. The two arrays differ in orientation
or in overall sizes, that is to say in heights and spacings of the ribs. Varying orientation
and heights of the pattern elements, in combination with inking, results in an image
portion which blends with the background portion if viewed from one range of angles
of view but which appears as a readily recognisable image from another range of angles
of view. Such an image appearing or disappearing when the angle of view is changed
is called a "latent image" or a "transient image".
[0007] If an observer tilts a security document bearing a control element of this type,
he may see an image distinguishable from the background when he sees the surface of
the security paper from an acute angle of view. The image disappears at approximately
normal angles of view, and the same image appears again at acute angles of view opposite
the first acute angles of view. With other variants of this technique, the observer
may not see such image at acute angles of view, the image appearing only at approximately
normal angles of view.
[0008] Although the technique described in the two documents mentioned above has certain
advantages over other methods of detecting counterfeits, several problems remain.
First, in order to become visible, the image must be inked with the colour of the
raised portion of the image being necessarily different from the colour of the underlying
document substrate. If a colour photocopy is made, the colours of the copy are the
same as the ones of the original, and this make detection of a counterfeit more difficult,
although of course the latent image effect is lost.
[0009] A second problem is that great care must be taken to create an image that is clearly
visible from one angle of view, and essentially invisible from a second angle of view.
Unless great care is taken in creating the image, it will be more or less visible
from any angle of view.
[0010] Further, besides the drawbacks of the above-described techniques, it is an aim of
the present invention to offer a control element exhibiting a variable optical effect
differing from those of the latent images of the state of art. It is a further aim
to offer such a control element, whose implementation necessitates a sophisticated
equipment which is hardly accessible to forgers.
[0011] Now, the present inventors have found that not only the overall size and the angles
from which said embossings are illuminated and viewed, but also the shape of the embossings
may determine the visual aspect of the security paper surface: this aspect varies
depending upon the slope of the flanks of the embossings. The inventors found in particular
that a sheet area bearing a set of asymmetric embossings, one flank of which has a
high average slope and an other one has a low average slope, if viewed sideways, may
appear dark if viewed from one side of the embossings and appear bright if viewed
from the other side.
[0012] Thus, according to one aspect, the invention is directed to a security paper bearing
a control element consisting of an area portion obtained by intaglio printing or embossing,
wherein said portion is provided with at least one first set of asymmetrical embossing
elements, each said embossing element having a first flank and an opposite second
flank, the average slope of said first flank being different from the average slope
of said second flank, wherein the slopes of said first flanks of said set of embossing
elements are substantially equal amongst themselves and the slopes of said second
flanks of said set of embossing elements are substantially equal amongst themselves.
[0013] By virtue of the control element according to the invention, when the angle of view
of an observer examining the security paper varies from a first acute angle of view
to opposite acute angles of view, the observer will observe the following sequence:
- the image defined by the first set of embossing elements in a dark shade, ⇒ more or
less complete disappearance of the image, ⇒ reappearance of the image in a bright
shade, or vice versa;
whereas, with a control element bearing a latent image of the prior art, he would
observe a sequence:
- image in dark shade, ⇒ disappearance of the image, ⇒ reappearance of the image in
the same dark shade.
[0014] The shape of the embossings of a security paper surface are directly determined by
the shape of the cuts of the intaglio printing plate, namely the furrow elements engraved
in the intaglio plate, by means of which the security paper may be imprinted.
[0015] Thus, according to a further aspect, an object of the invention is an engraved plate
for intaglio printing or embossing of sheets of security papers, comprising a plate
portion generating on each said security paper a control element upon intaglio printing
and/or embossing, wherein said plate portion is provided with at least one first set
of asymmetrical furrow elements, each said furrow element having a first side wall
and an opposite second side wall, the slope of said first side wall being different
from the slope of said second side wall, wherein the slopes of said first side walls
of said set of furrow elements are substantially equal amongst themselves and the
slopes of said second side walls of said set of furrow elements are substantially
equal amongst themselves.
[0016] Conveniently, the average slope of said first walls has a value of between 60° and
90° and the average slope of said second side walls has an average value of between
1° and 50°, preferably between 10° and 25°.
[0017] Preferably, said asymmetrical furrow elements of said first set are contiguous and
define at least one first area, said first area defining an alphanumerical or an other
easily recognisable graphical sign.
[0018] The furrow elements providing the optical effect according to the invention may present
various configurations. They may consist of elongated straight segments, elongated
curved segments, in particular portions of circles, short segments or pinpoint asymmetrical
recesses, which may be considered and termed as dots. Furrow segments generate ribs
with a steep flank and a gentle flank; furrow dots generate asymmetrical pyramidal
toothlike protrusions on the security paper surface.
[0019] In the areas recognisable as alphanumerical signs or graphical signs by the human
eye, the furrow elements are preferably contiguous. If the furrow elements are segments,
the latter are preferably parallel and contiguous over their whole length or at least
a part thereof. If the furrow elements are in form of dots, they may be spread over
an area, preferably almost in point contact the ones with the others.
[0020] In embodiments where the furrow elements of the first set are segments, the plate
portion generating a control element comprises preferably a second area with a second
set of asymmetrical furrow segments having first and second opposite side walls with
unequal slopes, the furrow segments of said second set being parallel to the furrow
segments of said first set, the side walls with the higher slope of the furrow segments
of said first set and the side walls with the lower slope of the furrow segments of
said second set having the same orientation.
[0021] According to a preferred embodiment, for enhancing the contrast effect, said second
area may surround said first area, thereby constituting a background for the image
defined by the first area. A user, upon merely tilting a security paper provided with
this control element, will observe the following sequence:
- bright image on dark background, ⇒ melting of the image with the background, ⇒ reappearance
of the image in dark shade on a bright background.
Thus, the invention permits to realise a sort of flip-flop effect. Both the image
area and the background area may be uniformly inked by means of the same colour prior
to embossing the sheet with a dry plate. This will not prevent the flip-flop effect,
but the image will be impossible to copy by means of a photocopy machine.
[0022] If the furrow elements of the first set are dots, then the plate portion generating
the control element may comprise a second set of asymmetrical furrow elements having
first and opposite second side walls with unequal slopes, the side walls with the
lower slope of the furrow elements of said first set and the side walls with the lower
slope of the furrow elements of said second set having different orientations.
[0023] In a preferred embodiment, the asymmetrical furrow elements of said second set are
dots and their side walls with the lower slope are oriented crosswise relatively to
the corresponding side walls of said first set of dots.
[0024] In a particularly preferred embodiment of a plate portion consisting of asymmetrical
dots, said plate portion comprises a first, a second, a third and a fourth set of
asymmetrical furrow elements in form of dots, each set with their respective side
walls with the lower slope orientated crosswise to the corresponding side walls of
two other sets. Said first, second, third and fourth set define a first, second, third
and fourth area. Said areas may overlap at least partially; in the overlapping portions
of said areas, the dots of said first, second, third and/or fourth set are arranged
in alternating relationships.
[0025] Traditionally, the manufacture of intaglio plates is a long and complex process,
which begins with the hand engraving of a steel or copper plate, making a copy of
this first plate, adding by chemical engraving other elements, making several plastic
imprints of this final original plate, welding them together, and going through an
important number of galvanic bathes to obtain the final product, namely the intaglio
printing plate to be mounted on the machine. Some improved processes permit to shorten
the manufacturing process.
EP 0 322 301 proposes an electro-erosion step for engraving intaglio plates. WO 96/26466
describes a method of manufacturing polymeric precursor plates of intaglio printing
plates by photo-ablation. The aforesaid methods provide cuts of sufficient quality
as far as the macroscopic optical aspect of the printed image is concerned, but do
not permit a precise control of the microscopic shape of the cuts.
[0026] Thus, a further object of the invention is a process of manufacturing an engraved
plate as defined above, wherein an non-engraved plate is submitted to a programmed
engraving process by a computer controlled engraving tool, the slope of the side walls
of the furrow elements engraved by said tool being programmed by said computer.
[0027] The Applicant's co-pending Application WO 03/103962, the content of which is incorporated
herein by reference, discloses a method of manufacturing an engraved plate for intaglio
printing of sheets of security papers, wherein a non-engraved plate is submitted to
a programmed engraving process by a computer controlled engraving tool, wherein said
programmed engraving process engraves said non-engraved plate according to the three-dimensional
guiding pixel data (X, Y, Z) of a master depth-map of one said sheet, wherein said
master depth-map is generated by at least one computer stored original depth-map,
said original depth-map consisting of a three-dimensional raster image of at least
a portion of one said security paper, and wherein an elementary engraving step is
associated to each three-dimensional pixel data. The engraving tool is a laser-engraving
machine.
[0028] The plate engraved by this process may be an intaglio printing plate, which is generally
a metallic plate. The engraved plate may also be a precursor of an intaglio printing
plate, and may be made of or comprise an upper layer of polymer. The intaglio printing
plate is thereafter derived from the precursors by techniques known in the art. The
plate may be flat or cylindrical.
[0029] Since one pixel is much smaller than the width of the furrow elements embodied within
the frame work of the present invention, the engraving of one furrow element profile
necessitates a plurality of adjustable elementary engraving steps and therefore the
process permits to engrave stepwise the side walls of the furrow elements, the respective
depths (Z) of adjacent steps determining the average slope of a said side wall.
[0030] Other particulars and advantages of the invention will further appear to those skilled
in the art from the following description of preferred embodiments, referring to the
drawings, in which:
- Fig. 1a, 1b and 1c illustrate schematically the profile of three cuts obtained by
state of the art methods of production of intaglio plates;
- Fig. 2a, 2b and 2c illustrate schematically three profiles of cuts obtained by methods
embodying the present invention;
- Fig. 3a, 3b and 3c illustrate schematically embossings obtained by means of cuts of
Fig. 2a, 2b and 2c;
- Fig. 4 illustrates schematically the optical effect on the viewer's eye inspecting
a control element according to the invention from opposite sides;
- Fig. 5 illustrates schematically, at an enlarged scale, a control element composed
of two adjacent sets of furrow segments;
- Fig. 6a, 6b, 6c and 6d show a first example of the optical effect exhibited by asymmetric
embossings, obtained by means of furrow segments;
- Fig. 7a, 7b, 7c and 7d show a second example of the optical effect exhibited by asymmetric
embossings, obtained by means of furrow segments;
- Fig. 8 illustrates schematically, at an enlarged scale, a control element composed
of four groups of dots;
- Fig. 9a, 9b, 9c and 9d show a third example of the optical effect exhibited by asymmetric
embossings, obtained by means of furrow dots;
- Fig. 10a, 10b, 10c and 10d show a fourth example of the optical effect exhibited by
asymmetric embossings, obtained by means of furrow dots.
[0031] Fig. 1a, 1b and 1c show profiles of grooves engraved in an intaglio plate by traditional
methods:
- Fig. 1a represents a groove obtained by chemical etching. The width is well-defined
by the portion of the surface from which a protective layer was removed before etching.
The overall depth is defined by the etching time. However, this chemical etching process
does not permit to select a precise geometrical shape of the side walls of the groove.
- Fig. 1b represents the profile of a groove obtained by engraving a plate by means
of a cylindrical drill tool.
- Fig. 1c shows the profile of a groove engraved by a conical drill tool.
[0032] As well-known to those skilled in the art of printing, usually in an intaglio printing
process, grooves such as those shown in Fig. 1a, 1b and 1c are filled with printing
ink, and the paper sheet and the plate are pressed together under high pressure. The
paper penetrates into the grooves and fills them more or less. After separation of
the sheet from the plate, the surface of the paper bears an embossed rib reproducing
approximately the shape of the groove. The reproduction of the shape of the groove
is approximate, in as more as relaxation occurs to a certain extent upon pressure
release. The profile of the remaining rib is therefore an intermediate shape between
a U and a V. Anyway, this profile is substantially symmetric, like the engraved grooves
of Fig. 1a, 1b and 1c.
[0033] Fig. 2a, 2b and 2c illustrate profiles of furrow elements embodying the present invention:
- Fig. 2a shows a furrow profile obtained by a mechanical drill tool. This drill tool
may be the same as the drill tool used for engraving groove of Fig. 1c. For producing
the groove of Fig. 2a, the rotation axis of the tool is not hold vertically to the
surface of the plate, but in an inclined relationship, and thus the resulting profile
is asymmetric.
- Fig. 2b is a schematic representation of a furrow profile made by a process of the
type disclosed in the Applicant's Patent Application WO 03/103962. The engraving laser
tool can be selected for example among the excimer lasers and the YAG lasers, engraving
the plate pixel by pixel. The area of one pixel is much smaller than the size of the
groove: the size of a pixel is typically of about 3 µm, whereas the depth of the groove is typically of between 5 and 100 µm, and the width of the groove is typically of between 10 and 300 µm. Several elementary engraving steps at varying programmed depth define a side wall
profile and its average slope: Fig. 2b is obviously schematic, for explanatory purposes.
The squares and steps of the drawing represent the digital pixel data (X, Y, Z) provided
to the engraving machine. The real laser engraving process does not produce a right
angled flight of steps, but a more or less smoothed curve profile having the desired
average slope.
- Fig. 2c represents as above, pixel data for producing a furrow element in form of
a dot covering a substantially rectangular surface of about 50 to 400 µm edge length;
the maximal depth may be set at 5 to 100 µm. On three sides of the recess, the side
walls are steep. The fourth side wall has a generally smooth slope, obtained by engraving
steps at progressively, pixel by pixel varying depths.
[0034] The cuts of Fig. 2b and 2c may be obtained by processing respectively three-dimensional
line patterns, and three-dimensional raster patterns. In addition, the depth of the
individual pixels are determined by computer calculation so as to generate differences
in the slopes of the side walls, and thereby the asymmetrical character of the furrow
elements. After generation of an original depth-map pertaining to one security paper,
including the inventive control area and other parts of the security paper that shall
be imprinted by the intaglio printing process, this original depth-map is repeated
in rows and columns so as to cover all the prints of the plate. As disclosed by WO
03/103962, the resulting master depth-map may contain corrected pixel data to compensate
for the sheet distortion during intaglio printing.
[0035] Fig. 3a, 3b and 3c show the profiles of embossing elements generated on a security
paper sheet after contact under pressure by the herein before described cuts.
- A symmetric embossing profile like Fig. 3a may be obtained by means of the cuts of
Fig. 1a, 1b and 1c.
- An asymmetric embossing profile like Fig. 3b may be generated either by furrow segment
of Fig. 2a or furrow segment of Fig. 2b. Indeed, the steps engraved pixel by pixel
in groove of Fig. 2b are so small that the fibrous structure of a paper sheet does
not follow them exactly and, upon relaxing, the surface of a paper does not retain
these microscopic steps. And thus, the rib of Fig. 3b presents a steep average left
flank profile and a smoother average right flank profile.
- Fig. 3c illustrates the toothlike profile remaining after pressing the security paper
sheet into dot of Fig. 2c and removing it. The tooth of Fig. 3c represents a blurred
reproduction of the dot of Fig. 2c. The essential feature is that three flanks of
the tooth of Fig. 3c are steep and the fourth flank is smooth.
[0036] Fig. 4 illustrates the optical effect of the invention on the user's eye. Fig. 4
represents schematically two contiguous parallel ribs under illumination by a light
beam perpendicular to the security paper sheet. The light source is schematically
represented by a little sun at the top of the figure. A human eye is schematically
represented both on the left side and on the right side of Fig. 4. It may be easily
seen from the figure that much more light is reflected and sent to the eye on the
left side than to the eye on the right side, and thus when the security paper is viewed
from the side to which the smooth flank of the ribs are oriented, it gives an impression
of brightness, whereas, when viewed from the opposite side, it gives an impression
of darkness. The difference in the optical aspects between the dark and the bright
sides is enhanced if the portion of the security paper sheet bearing the control element
according to the invention is imprinted with a dark ink, black, dark brown and the
like, selected among brilliant and satin-finished inks; the effect appears less striking
when dull ink is used.
[0037] Those skilled in the art, upon considering the reversed light path, that is to say
upon inverting the light source and the user's eye in Fig. 4, will easily recognise
that if the viewer inspects the paper sheet from a normal angle and if he illuminates
the paper sheet from an acute angle, respectively from the left side and from the
right side, he will respectively get an impression of brightness and an impression
of darkness.
[0038] Fig. 5 represents schematically an enlarged view of a part of a control element comprising
an area A1 where the smooth flanks are oriented to the left, and an area A2 where
the smooth flanks are oriented to the right of the figure. Area A1 occupies the upper
left side of the figure, and area A2 occupies the right side and the lower side of
the figure. When viewed from the left, A1 appears bright, and A2 appears dark, and
vice versa from the right side; since both the ribs of areas A1 and A2 are asymmetrical,
the contrast bright/dark is enhanced.
[0039] Fig. 6a, 6b, 6c and 6d show a first example of a control element obtained by means
of an area of a plate engraved with two sets of parallel furrow segments. The first
set forms the letters CTIP and the second set surrounds the area occupied by the letters,
thus forming the background. The slopes of the side walls are arranged so that in
the embossed control element, the portions of the ribs corresponding to the letters
CTIP present their smooth flank to the left side of the figure, whereas the rib portions
forming the background present their smooth flank to the right side of the figures.
The control element is always illuminated from a normal angle to the sheet. In the
four positions shown by Fig. 6a, 6b, 6c and 6d, schematically illustrated by the four
little eyes, the control element is observed at acute angles respectively from the
left, the right, the top and the bottom side. The left/right flip-flop effect is clearly
shown by Fig. 6a and 6b. It is worthwhile to notice that if viewed and illuminated
in a plane parallel to the ribs as shown by Fig. 6c and 6d - and this would happen
in a photocopy machine -, no substantial contrast appears.
[0040] Fig. 7a, 7b, 7c and 7d show a further example of embossings obtained by means of
furrow segments according to the invention. The four positions of the observer and
the position of the light source are the same as in Fig. 6a, 6b, 6c and 6d. A first
set of furrow segments generates ribs with a smooth flank portion oriented to the
left, so that, if they are viewed from the left side, bright letters ONE appear. A
second set of furrow elements, in offset position versus the first set of furrow elements,
produces embossed rib segments with their smooth flank oriented to the right, so that,
viewed from the right side, the letters CTIP appear. The remaining surface of the
rectangular control element bears mere symmetric embossings with steep flanks. Fig.
7c and 7d show that viewing from the top or the bottom side do not provide any contrasted
image.
[0041] Fig. 8 shows schematically a portion of a security paper sheet on which rectangular
pyramidal embossings of the dot type are distributed. The pale portion of each rectangle
represents schematically the smooth flank, and the black portion represents schematically
the opposite steep flank. Obviously, the two other flanks are also steep. As may be
seen in Fig. 8, the rectangular dots are arranged in repeating groups of four dots,
each dot being disposed crosswise to the neighbours, providing a periodically alternating
relationship. One may observe that, by way of an example, the orientation of the dots
at the left lower and upper right corners of Fig. 8 differs from the rest of the figure.
The dots, whose smooth flanks are oriented in the same direction, generate altogether
an image which may represent a letter, a figure, or any other graphical sign. Those
skilled in the art will easily understand that by this arrangement in groups of four
dots, four different signs may be generated at the same location on the security paper
sheet, each of the four signs appearing as a bright sign, surrounded by a darker background,
only when viewed in one specific direction.
[0042] Fig. 9a, 9b, 9c and 9d present an example of a control element combining several
overlapping sets of dots. The position of the light source and the four positions
of the observer are the same as in the preceding examples. A first set of dots is
spread over an area subdivided in four subareas, so as to correspond to the four letters
CTIP; the smooth flanks of the first set are oriented to the left. A second set of
dots is spread over the remaining area of the control element, and their smooth flanks
are oriented into the opposite direction. As shown in Fig. 9a and 9b, the letters
CTIP appear respectively bright on a dark background and dark on a brighter background
when viewed respectively from the left and from the right. Thereby, these two sets
of dots produce a flip-flop effect similar to the effect described concerning Fig.
6a, 6b, 6c and 6d. A third set of dots is spread over an area subdivided into three
subareas, so as to form the letters ONE. The smooth flanks of the third set of dots
are oriented to the bottom of Fig. 9a, 9b, 9c and 9d, that is to say at 90° vs. the
first and second set. A fourth set of dots is spread over the remaining surface of
the control element, their smooth flanks being oriented in the opposite direction
of those of the third set. Thereby, when viewed respectively from the top and the
bottom, as shown in Fig. 9c and 9d, the third and fourth sets let appear ONE respectively
as bright on dark background or dark on bright background with a flip-flop effect
crosswise to the first flip-flop effect.
[0043] Finally, a more sophisticated example of this kind of arrangement is shown by Fig.
10a, 10b, 10c and 10d. The position of the light source and the four positions of
the viewer are the same as above. The first set of dot-like embossings present their
smooth flank to the bottom, so as to exhibit the word ONE in bright shade when observed
from that direction. The second set of dot-like embossings present their smooth flank
to the left, so that the word TWO is distinguishable when the paper is observed at
an acute angle from that direction. The third set of dot-like embossings present their
smooth flank to the top, permitting to distinguish the word THREE. The fourth set
of dots present their smooth flank to the right, so that the word FOUR appears upon
viewing from said direction. The embossings that do not participate to any of the
signs ONE, TWO, THREE, FOUR, are mere symmetric embossings with steep flanks.
[0044] The high density of information per surface unit, that is to say the small size and
precise shaping of the individual dots required for forming such a superposition of
images, necessitates the use of a computer controlled, pixel by pixel engraving process
with a laser tool. Counterfeits by hand-held mechanical tools are not feasible.
[0045] Those skilled in the art will recognise that several other arrangements are feasible
without departing from the scope of the present invention:
[0046] The dots of a dot assembly forming a control element are not limited to dots having
a rectangular base. For example round, triangular or hexagonal bases with corresponding
conical or pyramidal asymmetric protrusions can be made, so that the most striking
differences in optical aspects are not viewed from opposite directions or directions
being crosswise, but at different angles.
[0047] The above examples of control elements are made by merely embossing a homogeneously
pre-inked flat surface by means of a dry plate. Polychrome effects may be obtained
by simultaneously printing and embossing the surface of a security paper sheet by
means of an inked intaglio plate bearing asymmetric cuts according to the invention.
1. An engraved plate for intaglio printing or embossing of sheets of security papers,
comprising a plate portion generating on each said security paper a control element,
characterised in that said portion is provided with at least one first set of asymmetrical furrow elements,
each said furrow element having a first side wall and an opposite second side wall,
the average slope of said first side wall being different from the average slope of
said second side wall, and in that the slopes of said first side walls of said set of furrow elements are substantially
equal amongst themselves and the slopes of said second side walls of said set of furrow
elements are substantially equal amongst themselves.
2. A plate as claimed in claim 1, wherein the slope of said first walls has an average
value of between 60° and 90° and the slope of said second side walls has an average
value of between 1° and 50°, in particular 10° and 25°.
3. A plate as claimed in claim 1 or claim 2, wherein said asymmetrical furrow elements
of said first set are contiguous and define at least one first area, said fist area
defining an alphanumerical or a graphical sign.
4. A plate as claimed in claim 3, wherein said asymmetrical furrow elements are parallel
segments extending over said first area.
5. A plate as claimed in claim 4, comprising in a second area, a second set of asymmetrical
furrow segments having first and second opposite side walls with unequal average slopes,
the furrow segments of said second set being parallel to the furrow segments of said
first set, the side walls with the highest slope of the furrow segments of said first
set and the side walls with the lower slope of the furrow segments of said second
set having the same orientation.
6. A plate as claimed in claim 5, wherein said second area surrounds said first area.
7. A plate as claimed in claim 5, wherein said second set of furrow segments is offset
versus said first set and occupies a second area defining a second sign.
8. A plate as claimed in claim 3, wherein said asymmetrical furrow elements of said first
set are dots distributed over said first area.
9. A plate as claimed in claim 8, comprising a second set of asymmetrical furrow elements
having first and opposite second side walls with unequal average slopes, the side
walls with the lower average slope of the furrow elements of said first set and the
side walls with the lower average slope of the furrow elements of said second set
having different orientations.
10. A plate as claimed in claim 9, wherein the asymmetrical furrow elements of said second
set are dots surrounding said first set and wherein their side walls with the lower
average slope are oriented in opposite directions relatively to the corresponding
side walls of said first set.
11. A plate as claimed in claim 10, comprising a third set of asymmetrical dots and a
fourth set of asymmetrical dots surrounding said third set, said third and fourth
sets having their side walls with the lower average slope in opposite directions and
crosswise to said first and second sets.
12. A plate as claimed in claim 9, comprising a first, a second, a third and a fourth
set of asymmetrical furrow elements in form of dots, with their respective side walls
with the lower average slope orientated crosswise.
13. A plate as claimed in claim 12, wherein said first, second, third and fourth set define
a first, second, third and fourth area, each said area defining a sign wherein said
areas overlap at least partially and wherein in the overlapping portions, the dots
of said first, second, third and/or fourth set are arranged in alternating relationships.
14. A process of manufacturing an engraved plate as claimed in anyone of the preceding
claims, wherein an non engraved plate is submitted to a programmed engraving process
by a computer controlled engraving tool, the slope of the side walls of the furrow
elements engraved by said tool being programmed by said computer.
15. A process as claimed in claim 14, characterised in that said programmed engraving process engraves said non-engraved plate according to the
three dimensional guiding pixel data (X, Y, Z) of a master depth-map of one said sheet,
wherein said master depth-map is generated by at least one computer stored original
depth-map, said original depth-map consisting of a three-dimensional raster image
of at least said portion generating said controlled element on said security paper.
16. A security paper bearing a control element obtained by intaglio printing of a sheet
by means of an engraved intaglio plate as claimed in anyone of claims 1 to 13.
17. A security paper bearing a control element obtained by embossing a sheet by means
of an engraved intaglio plate as claimed in anyone of claims 1 to 13.