Security document

In order to increase the protection against forgery of a security paper, a security inlay made of polymer material having piezoelectric and/or pyroelectric properties is embedded in the security paper. The presence of such an inlay can be tested by means of contacting and non-contacting measuring methods.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a security document, in particular a 
security paper, bank-note, identification card, etc., having a feature 
which is embedded, for example in the form of a safety thread, in the 
interior of the document and capable of being tested mechanically in order 
to recognize the authenticity of the document. 
2. Description of Related Art 
It is known to protect security papers and other moneyvalue products made 
of paper or synthetic material against unauthorized imitation by embedding 
so-called safety threads therein. These safety threads generally consist 
of narrow strips of a synthetic or metal film, textile threads, wires and 
similar materials. 
It is also known (e.g. German Offenlegungsschrift No. 14 46 851) to equip 
safety threads additionally with special properties by printing very fine 
letters (so-called micro-letters which are approx. 0.4 mm high) on them, 
for example, which make it especially difficult for a forger to imitate 
them. Coatings made of colored materials across the surface are also known 
in this connection. 
In the course of the increasing automation, there have also been efforts to 
design the safety threads in such a way that they are suitable for 
mechanical testing. Thus, safety threads are provided, for example, with 
substances which fluoresce under UV light or have magnetic or special 
properties detectable using X-rays (see German Offenlegungsschrift No. 27 
54 267, for example). The embedding of such threads in bank-notes or other 
security documents then offers the possibility of ascertaining the 
authenticity or the value of a document using appropriate testing devices. 
The mechanical testing of security documents such as identification cards, 
bank-notes, etc., is desirable nowadays above all for money-issuing 
machines, so-called POS terminals or at bank counters. Rapid and 
mechanical testing of documents, for example with respect to their 
authenticity or their value, is also particularly useful and helpful, 
however, for the automatic sorting or processing of bank-notes or similar 
money-value products. 
BRIEF SUMMARY OF THE INVENTION 
The invention is based on the problem of proposing a feature suitable for 
security documents, for example in the form of a safety thread, which is 
capable of being tested mechanically in many different ways. 
The films used according to the invention which are known per se and have 
piezoelectric and/or pyroelectric properties are provided on each side 
with an electrically conductive coating. Due to their piezoelectric 
property, a potential difference comes about between the conductive 
coatings in the case of mechanical deformation. If electrical voltage is 
applied to the conductive coatings, on the other hand, this causes the 
film to be deformed. Many of the known piezofilms have not only 
piezoelectric properties but also pyroelectric properties, i.e. a 
potential difference can also be measured between the conductive coatings 
when the film is heated. 
The films are flexible and can be manufactured in thicknesses of 10 to 50 
.mu.m. They can be cut into threads of the desired width which are then 
excellently suited, due to these good mechanical properties, for being 
embedded in bank-notes, identification cards or other security documents. 
The idea on which the invention is based, i.e. to embed films having 
piezoelectric and/or pyroelectric properties in the form of safety threads 
in documents such as banknotes, identification cards or other money-value 
products, offers a particularly wide assortment of testing possibilities. 
The great variety of these possibilities means that a kind of testing can 
be used which is particularly well-suited to the area automated in any 
specific case in order to detect the special properties of the inventive 
feature. 
Thus, it is possible to measure and evaluate piezoelectric voltage which 
comes about due to mechanical deformation of the safety thread. For this 
purpose, the security document may be bent in the direction of the safety 
thread or subjected to brief mechanical jolts. The resulting d.c. or a.c. 
voltage can then be picked up in a contacting or non-contacting (e.g. 
capacitive) manner via the conductive coatings located on the film which 
serve as electrodes. 
Further, the pyroelectric properties can also be used for mechanical 
testing. In this case it is necessary to first heat the zone of the 
document containing the safety thread. This is done expediently by 
irradiating infrared light of a suitable wavelength. Microwaves are also a 
suitable means for heating the threads since they are absorbed well by the 
film material. 
These methods, which are based on mechano-electric or thermoelectric 
transformation, are suitable, for example, for testing identification 
cards or bank-notes in money-issuing machines or vending machines since 
the testing is stationary in this case, i.e. the security documents do not 
move during the testing. The mechanical and electrical components 
necessary for testing can be housed in relatively small housings so that 
the testing can also be carried out using simple hand-operated devices 
which may then be used, for example, at bank counters or the places of 
sale in department stores, etc. 
Rapid non-contacting testing is advantageous in the case of automatic 
sorting or voucher-processing equipment since the documents are processed 
at high speed. In this case it is possible, for example, to utilize the 
sensitivity of the piezofilm to an ultrasonic field in order to test the 
safety thread as to its piezoelectric properties. The deformation of the 
piezofilm caused by an ultrasonic field creates a.c. voltage which can be 
picked up in a noncontacting manner from the conductive coatings of the 
film via appropriately arranged capacitor plates. It is also possible to 
transmit a.c. voltage to the conductive coatings of the piezofilm via the 
capacitor plates. The piezofilm is thereby set oscillating and emits sound 
waves of equal frequency which are detected via appropriate detectors, 
e.g. microphones. 
In addition to the stated methods, which utilize for testing the effects 
resulting from piezoelectric and pyroelectric properties following 
mechanical, thermal or electrical excitation, it is also possible to test 
further physical properties of the inventive feature, such as the 
conductivity of its conductive coating or its optical transparency. If a 
plurality of properties are included in the authenticity testing and the 
individual results weighted at the same time, the reliability of 
recognition can be increased, on the one hand, and the efforts required by 
a forger to imitate the documents can even be increased many times over, 
on the other hand. 
Further advantages and developments of the invention can be found in the 
subclaims and in the embodiments which are explained in more detail in the 
following with reference to the drawing.

FIG. 1 shows a security paper 1 having a printed picture 2. A safety thread 
3 is embedded in the interior of the security paper. This safety thread 
consists of a polymer film cut into strips and having piezoelectric and/or 
pyroelectric properties. The embedding of the safety thread in the 
security paper, for example a bank-note, takes place during the 
manufacture of the bank-note according to methods known in security paper 
technology. In the case of identification cards, which are generally 
composed of a plurality of synthetic layers, the safety thread is inserted 
between two of these layers which are then joined together across their 
entire surface by a cold or hot laminating process. Care must be taken in 
the case of the hot laminating process that the physical properties of the 
inserted safety thread are not impaired by temperatures which are too 
high. The safety thread embedded in a security paper or an identification 
card in this manner does not offer an observer any special features, when 
checked visually, since threads having the same or a similar appearance 
are well-known in bank-notes. 
FIG. 2 shows a cross-section of a security pape 1 having. a safety thread 3 
embedded between paper layers 5 and 6. The thickness relations are not 
shown true to scale in the interests of clear illustration. 
The safety thread has a thickness of 10 to 50 .mu.m, preferably 15 to 30 
.mu.m. The sheet thickness of the security paper is approx. 100 .mu.m. The 
width of such safety threads is usually 0.4 to 1.5 mm, values ranging from 
0.6 to 1.0 mm being used preferably. The film material is provided on each 
side with conductive coatings (metallization) 4 in order to apply 
electrical charges to the material or remove them therefrom. Conductive 
coatings 4 may be applied to the film so as to cover its entire surface or 
in an interrupted form by vacuum deposit, lamination or other appropriate 
methods. It is also possible to vacuum deposit transparent semiconductive 
metal oxides. Safety threads produced in this manner then remain 
transparent and can only be detected with difficulty, or in some cases not 
at all, in the paper or any other moneyvalue means of payment. 
A film material having piezoelectric and pyroelectric properties suitable 
for the inventive application is, for example, polyvinylidene fluoride 
(PVDF). It exhibits the greatest piezoelectric and pyroelectric 
coefficients of all substances known up to now. PVDF crystallizes in three 
different phases .alpha., .beta. and .gamma.. The general .alpha. phase, 
in which the substance crystallizes out of the melt during cooling, has a 
non-polar character so that no piezoelectric properties arise here. Via 
mechanical deformation of the film, during which it is orientated in one 
or two directions, thereby orientating the polymer chains, the substance 
is transformed into the .beta. phase. For polarization, the film material 
is coated on each side with electrically conductive material. The film 
thus provided with conductive coatings is then subjected to an electrical 
field of approx. 1 MV/cm, heated to 80.degree.-120.degree. C. and cooled 
while maintaining the field. Polarization is "frozen" by this process and 
the film then exhibits the desired piezoelectric and pyroelectric 
properties. 
The pieces of film mentioned above, which are provided in security 
documents, for example, in the form of threads, can be tested using a 
great variety of methods. Thus, it is possible to convert mechanical 
energy acting on the thread in the form of mechanical tension, jolts or 
vibrations into electrical energy inside the safety thread. This can be 
picked up from the thread as d.c. voltage in the case of bending, as an 
electrical voltage pulse in the case of jolts, as a.c. voltage of equal 
frequency in the case of vibrations. A contacting method can be selected 
to pick up the voltage. 
In order to facilitate direct contacting, the safety thread may be embedded 
in such a way that an area sufficient for contacting is freely accessible 
on the safety thread. The safety thread should be embedded for this 
purpose according to known methods in such a way that contacting on each 
side is possible for each individual copy produced from a paper web. 
Multilayer identification cards offer the possibility of leading the ends 
of the safety thread out onto the surface of the laminate. A further 
possibility is, for example, to provide the cover layers overlying the 
safety thread with appropriate windows which either remain free or are 
filled with electrically conductive material. In the latter case the card 
shows an uninterrupted smooth surface in order to prevent the windows from 
possibly being soiled. 
FIG. 3 shows schematically a testing device for measuring a piezoelectric 
voltage caused by mechanical bending of the thread. For this purpose, 
electrodes 12, 13 are placed on conductive coatings 4 of safety thread 3. 
The electrodes are connected to a measuring device 15, for example via a 
charge amplifier 14. Via a bending device not shown in the figure, the 
strip is bent once or several times, for example in the direction of arrow 
7. The piezoelectric voltage caused by the mechanical deformation of the 
thread can be displayed by aid of the measuring device. 
Relatively high voltages can be obtained using so-called bimorphous films. 
These films are composed of two superjacent, uniaxially orientated 
piezofilms whose directions of polarization are opposite. The voltages V 
obtainable can be calculated according to the following formula: 
##EQU1## 
V=voltage g.sub.31 =piezoelectric elongation constant 
Y=Young's modulus 
t=thickness of the film 
L=length of the film 
S=deflection of the free end 
Using a PVDF film (polyvinylidene fluoride) for which g.sub.31 =0.17 Vm/N; 
y=2.7 10 N/m.sup.2, the thickness is 30 .mu.m and the length is 70 mm, 
voltages of approx. 4 V result when the free end is deflected by 3 cm. 
The measuring arrangement shown in FIG. 3 can also be used to evaluate the 
pyroelectric effect. In this case the film is heated instead of being 
mechanically deformed. The heat sources used may be IR or microwave 
emitters, for example. The pyroelectric voltages obtainable are 
proportional to the temperature change of the film strip and may be 
calculated according to the following formula: 
##EQU2## 
v=voltage p=pyroelectric coefficient 
t=thickness 
.DELTA.T=temperature change 
.epsilon.=dielectric constant 
Using PVDF films (.epsilon.=15, p=-40 .mu.C/m K) having a thickness of 10 
.mu.m, a temperature change of 1.degree. C. results in a pyroelectric 
voltage of approx. 3.5 V. The metal coating of the film may be designed 
accordingly to increase the absorption of the incident radiation. 
Appropriate coating materials are, for example, blackened gold or bismuth 
layers, which are characterized by a high wide-band absorptive response 
and by their good electrical conductivity. 
On the other hand, it is also possible to provide the piezofilm on one side 
with a thin transparent or semitransparent conductive coating. In this 
case the incident radiation penetrates the polymer material, a quick 
change of temperature being obtainable exploiting the strong absorption of 
the polymer material (PVDF) in the wavelength range of 8 to 11 .mu.m. The 
metal coating on the opposite side of the film should then be a layer 
which reflects well so that the transmitted portion of the radiation is 
reflected into the material again. 
In addition to the contacting measurement of a piezoelectric or 
pyroelectric voltage between the conductive coatings of polymer films, it 
is also possible to evaluate the charges or voltages in a non-contacting, 
e.g. capacitive, manner. 
A further possibility consists in having sound energy act on the thread, as 
shown in FIG. 4, by aid of a transmitter 19 controlled by a generator 9. 
In this case an a.c. voltage analogous to the sound frequency comes about 
on the safety thread, which may be used for identification. It proves to 
be particularly advantageous to have sound frequencies in the ultrasonic 
range, i.e. having a frequency of approx. 40 to 400 kHz, act upon the 
safety thread. The resulting high-frequency a.c. voltages can be coupled 
out of the thread material effectively in a non-contacting manner. For 
this purpose, two or more series-connected capacitor plates 17, 18 are 
placed in an appropriate manner along safety thread 3 so that the latter 
is subjected to the high-frequency a.c. voltage due to the resulting 
electrical influence. It appears to be expedient to interconnect the 
capacitors with an inductance coil or an ohmic resistor in order to obtain 
a resonant circuit which may be tuned to the frequency of the transmitter. 
The transmitter and receiver signals may be processed via a correlation 
element 10 in such a way as to eliminate or suppress external or spurious 
portions when the received measuring signals are being evaluated. Since a 
large portion of the sound waves are reflected by the surface of the 
paper, it is possibly to place the ultrasonic source directly on the 
surface of the paper, thereby avoiding the lossy air-paper transition. 
In contrast to the method just described, it is also possible to have 
electrical voltages or electrical a.c. fields act upon the thread 
material. The necessary electrical voltages are generated by means of a 
generator 20 and directly applied to or coupled into the conductive 
coatings of the piezofilm. In the latter case capacitor plates 8 are 
arranged along the safety thread or the security paper is directed between 
two capacitor plates 8. When the safety thread passes through the 
electrical a.c. field built up between the capacitor plates, the safety 
thread is deformed periodically and stimulated to emit sound frequencies. 
It is useful to have an electrical a.c. voltage of 50 kHz, for example, 
act upon the safety thread made of piezoelectric material which then emits 
a sound wave of equal frequency. Detectors 11 constructed according to the 
principle of a microphone are then suitable for discovering the emitted 
sound frequencies. Depending on the dimensions of the safety thread, 
higher harmonics of the irradiated frequency will also come about which 
are thus dependent on the dimensions of the thread and allow for it to be 
identified. 
Analogously to the testing method described above, the excitation signal 
and the measuring signal may be correlated in an appropriate correlation 
element 10 for evaluation, excluding external or spurious influences. 
The piezovoltage generated by an ultrasonic field may further also be used 
for generating a magnetic a.c. field. The electrodes of the piezofilm are 
connected to an armature loop. The armature loop, which may also consist, 
for example, of several concentrically arranged loops, is provided, for 
example, on one side of the piezofilm by selectively coating the film with 
electrically conductive material. The electromagnetic field which comes 
about under the effect of an ultrasonic field can then be detected, for 
example when passing through appropriately arranged Helmholtz coils. This 
testing method thus also allows for quick and non-contacting testing. 
The measuring results of the piezoelectric or pyroelectric properties may 
be combined during evaluation with other measuring results, such as the 
result of measuring the conductivity of the film coating or with results 
of testing the optical properties of the film (transparency in various 
spectral ranges). 
Linking different individual results, possibly weighting them at the same 
time, increases the efforts required to imitate the feature since the film 
must in this case fulfill several conditions simultaneously.