Title: Device and method for verifying the authenticity of banknotes
Abstract: An apparatus and method for testing the authenticity of bank notes are proposed. The apparatus includes a light source for emitting light suitable for exciting fluorescent light in a bank note to be checked, and a fluorescence detector for detecting the fluorescent light emitted by the bank note to be checked. According to the invention it is provided that the light source is a light-emitting diode and the light-emitting diode is formed to emit light containing ultraviolet light at least in a partial spectral region. This makes it possible in particular to obtain a compact structure of the apparatus. The method involves exciting and detecting fluorescent light in the bank note to be checked and comparing the detected fluorescent light with a predefined threshold value. According to the invention it is provided that the detected fluorescent light is reduced by the ambient light detected with the light source switched off. This permits the elimination of measuring inaccuracies at the same time as simple operation.
Patent Number: 6,918,482 Issued on 07/19/2005 to Thierauf
| Inventors:
|
Thierauf; Klaus (Munich, DE)
|
| Assignee:
|
Giesecke & Devrient GmbH (Munich, DE)
|
| Appl. No.:
|
148176 |
| Filed:
|
November 30, 2000 |
| PCT Filed:
|
November 30, 2000
|
| PCT NO:
|
PCT/EP00/12055
|
| 371 Date:
|
August 20, 2002
|
| 102(e) Date:
|
August 20, 2002
|
| PCT PUB.NO.:
|
WO01/41079 |
| PCT PUB. Date:
|
June 7, 2001 |
Foreign Application Priority Data
| Dec 03, 1999[DE] | 199 58 048 |
| Current U.S. Class: |
194/207; 209/534; 382/135 |
| Intern'l Class: |
G07K 007/01.5; G07K009/00 |
| Field of Search: |
194/207,206,205,302
382/135
209/534
|
References Cited [Referenced By]
U.S. Patent Documents
| 3946203 | Mar., 1976 | Hecht et al.
| |
| 4202491 | May., 1980 | Suzuki.
| |
| 4592090 | May., 1986 | Curl et al.
| |
| 4722607 | Feb., 1988 | Anselment et al.
| |
| 5476169 | Dec., 1995 | Takarada et al.
| |
| 5574790 | Nov., 1996 | Liang et al.
| |
| 5666417 | Sep., 1997 | Liang et al.
| |
| 5912982 | Jun., 1999 | Munro et al.
| |
| 5915518 | Jun., 1999 | Hopwood et al.
| |
| 5960103 | Sep., 1999 | Graves et al.
| |
| 6101266 | Aug., 2000 | Laskowski et al.
| |
| 6297509 | Oct., 2001 | Lipkowitsch et al.
| |
| Foreign Patent Documents |
| 2018483 | Oct., 1979 | GB.
| |
| WO 94/1641/2 | Jul., 1994 | WO.
| |
| WO 95/1960/5 | Jul., 1995 | WO.
| |
Primary Examiner: Walsh; Donald P.
Assistant Examiner: Beauchaine; Mark J.
Attorney, Agent or Firm: Bacon & Thomas, PLLC
Claims
1. An apparatus for testing the authenticity of bank notes comprising:
at least one light source for emitting light suitable for exciting fluorescent
light in a bank note to be checked wherein the light source is a light-emitting
diode arranged to emit light containing ultraviolet light at least in a partial
spectral region;
at least one fluorescence detector for detecting fluorescent light emitted by
a bank note to be checked;
at least one amplifying device arranged to amplify output signals from the at
least one fluorescent detector and/or a monitor detector; the amplifying device
being formed as a lock-in amplifier; and
a transport plate disposed between the light-emitting diode and the fluorescence
detector, and a bank note to be checked, the transport plate having at least one-three
dimensional partial area permeable at least to a partial spectral region of the
light emitted by the light-emitting diode and at least to a partial spectral region
of the fluorescent light emitted by a bank note to be checked.
2. Apparatus according to claim 1, wherein the light-emitting diode has an emission
spectrum with a peak in the ultraviolet spectral region.
3. Apparatus according to claim 1, wherein
the light-emitting diode is arranged to emit light suitable for exciting fluorescent
light in an extended partial area of a bank note to be checked, and
the fluorescence detector is arranged for integral detection of the fluorescent
light emitted by a bank note.
4. Apparatus according to claim 1, including a first filter arranged to filter
light emitted by the light-emitting diode, the filter being permeable to a partial
spectral region of the light emitted by the light-emitting diode and suitable for
exciting fluorescent light in a bank note to be checked.
5. Apparatus according to claim 4, including a second filter arranged to filter
the fluorescent light emitted by a bank note to be checked and to be detected by
the fluorescence detector.
6. Apparatus according to claim 1, wherein the monitor detector is arranged to
detect at least part of the light emitted by the light-emitting diode.
7. Apparatus according to claim 6, wherein the light-emitting diode, monitor
detector and transport plate or first filter are disposed such that light emitted
by the light-emitting diode is partly reflected on the transport plate or the first
filter and reaches the monitor detector.
8. Apparatus according to claim 6, including a second screen arranged to collimate
light emitted by the light-emitting di-diode and partly reflected on the transport
plate onto the monitor detector.
9. Apparatus according to claim 6, including at least one amplifying device arranged
to amplify output signals from the fluorescence detector and/or the monitor detector,
the amplifying device being formed as a lock-in amplifier.
10. Apparatus according to claim 1, including a filter arranged to filter the
light emitted by the light-emitting diode and partly reflected on the transport
plate and to be detected by a monitor detector.
11. Apparatus according to claim 1, including a first screen arranged to collimate
fluorescent light emitted by a bank note to be checked onto the fluorescence detector.
12. Apparatus according to claim 1, including a system of optical lenses arranged
to collimate fluorescent light emitted by a bank note to be checked onto the fluorescence detector.
13. Apparatus according to claim 1, including a voltage source arranged to generate
a voltage with periodic time behavior for supplying power to the light-emitting diode.
14. Use of the apparatus for testing the authenticity of bank notes according
to claim 1, characterized by the steps of
measuring fluorescent light emitted by the bank note to be checked with the fluorescence
detector with the light-emitting diode switched on,
measuring phosphorescent light emitted by the bank note to be checked with the
fluorescence detector with the light-emitting diode switched off, and
using the measured fluorescent and phosphorescent light of the bank note to be
checked for authenticity testing.
15. Apparatus according to claim 1, including a filter arranged to filter the
fluorescent light emitted by a bank note to be checked and to be detected by the
fluorescence detector.
16. Apparatus according to claim 1, wherein the monitor detector is arranged
to detect at least part of the light emitted by the light-emitting diode.
17. A method for testing the authenticity of bank notes comprising the steps
of illuminating a bank note to be checked with ultraviolet light from a light source,
measuring visible fluorescent light emitted by the bank note to be checked with
a fluorescence detector, and comparing the measured fluorescent light with a predefined
threshold value, comprising the following steps:
a) illuminating at least a partial area of the bank note to be checked with light
emitted by a light-emitting diode, said light containing ultraviolet light at least
in a partial spectral region and being suitable for exciting fluorescent light
in the bank note to be checked, measuring with a fluorescence detector and generating
a first measured value,
b) measuring with the fluorescence detector with the illumination switched off
and generating a second measured value,
c) correcting the first measured value generated with the illumination switched
on with the second measured value generated with the illumination switched off,
and
d) comparing the corrected measured value with a predefined threshold value.
18. The method according to claim 17, wherein the light-emitting diode is switched
on and off periodically.
19. The method according to claim 17, wherein the first measured value generated
with the illumination switched on is corrected with the second measured value generated
with the illumination switched off by subtracting the second measured value from
the first measured value.
20. The method according to claim 17, wherein the first measured value generated
with the illumination switched on is corrected with the second measured value generated
with the illumination switched off by lock-in amplification of an output signal
generated by the fluorescence detector.
21. The method according to claim 17, wherein the corrected measured value or
the threshold value is corrected with correction value K which takes account of
the change of intensity in the light emitted by the light-emitting diode in the
course of the operating lifetime of the light-emitting diode.
22. The method according to claim 21, including the steps:
measuring at least part of the light emitted by the light-emitting diode with
a monitor detector at a first and a later second time with the light-emitting diode
switched on and without a bank note to be checked in each case, thereby generating
first and second monitor values associated with the measurements at the first and
second times in each case, and
determining correction value K from the first and second monitor values.
23. The method according to claim 17, wherein the method is performed at several
places on the bank note to be checked.
24. The method according to claim 23, wherein the measured values measured at
several places on the bank note to be checked and corrected are averaged, thereby
generating a mean value that is compared with the threshold value.
25. The method according to claim 17, including the steps of
measuring fluorescent light emitted by the bank note to be checked with the light-emitting
diode switched on,
measuring phosphorescent light emitted by the bank note to be checked with the
light-emitting diode switched off, and
using the measured fluorescent and phosphorescent light of the bank note to be
checked for authenticity testing.
Description
This invention relates to an apparatus and method for testing the authenticity
of bank notes according to the preambles of claims
1 and
15.
Counterfeit bank notes are usually printed on customary paper containing
optical brighteners. In contrast, the paper of authentic bank notes is generally
free from optical brighteners. Detection of optical brighteners in a bank note
can thus serve as a sign of the presence of a forgery. Optical brighteners are
normally detected by irradiating the bank note to be checked with ultraviolet (UV)
light. A counterfeit bank note printed on brightener-containing paper can be recognized
since the bank note shows visible fluorescence in the irradiated area.
In the apparatuses and methods for testing the authenticity of bank notes described
in the patent U.S. Pat. No. 5,640,463 the bank notes to be checked are illuminated
with light from a IN light source and both the UV light reflected by the bank note
to be checked and the fluorescent light emitted thereby are measured and compared
with the values measured on an authentic bank note. The UV light source used is
a fluorescent lamp.
However, the use of fluorescent lamps has various disadvantages. In particular,
a compact structure of such an apparatus can hardly be realized since the shape
and size of customary fluorescent lamps, e.g. a rod shape with a typical length
of about 5 cm, prescribe a minimum size of the total apparatus. In addition, the
light emitted by fluorescent lamps has not only UV components but also components
in the visible and infrared spectral regions. This can lead to undesirable heating
of the apparatus. Therefore, suitable cooling of the apparatus is necessary in
many cases. Last but not least, high voltage is required for operating UV fluorescent
lamps, which requires additional effort compared to the low voltage supply typically
used in many transport or checking devices in bank note processing machines, and
also favors the occurrence of disturbing voltages.
In known authenticity testing methods according to the prior art, measuring inaccuracies
due e.g. to aging, soiling or ambient light are compensated by repeated reference
measurements on reference documents, e.g. authentic and possibly new bank notes.
This is awkward and time-consuming, however, since to take a reference measurement
the actual checking operation must be interrupted each time for the corresponding
reference documents to be inputted. Due to this fact, reference measurements can
only be taken at relatively long time intervals, e.g. after a relatively large
number of tested bank notes, since otherwise authenticity testing as a whole would
be too time-intensive. Time-variant influences, e.g. due to ambient light and dark
current in the detectors, can thus not be fully taken into account.
It is the problem of the present invention to state an apparatus for authenticity
testing which is of compact construction and allows simple and reliable operation.
This problem is solved according to the invention by the apparatus and method
according to claims
1 and
15.
The inventive apparatus develops the prior art in such a way that the light source
used is a light-emitting diode and the light-emitting diode is formed to emit light
containing ultraviolet light at least in a partial spectral region. Since light-emitting
diodes generally have much smaller component sizes than the UV fluorescent lamps
hitherto used, a compact structure of the inventive apparatus is possible. This
permits a space-saving arrangement of a plurality of different sensors within a
very small space to be realized, as is required in particular in bank note processing
machines of small overall size. In addition, light-emitting diodes are more cost-effective
than the UV fluorescent lamps hitherto used. The maximum operating lifetime of
the provided light-emitting diodes is furthermore generally much longer than with
UV fluorescent lamps. Thus, UV fluorescent lamps must be replaced after a maximum
operating lifetime of typically less than 3000 hours, whereas the inventively provided
light-emitting diodes have substantially longer maximum operating lifetimes. Unlike
UV fluorescent lamps, light-emitting diodes can furthermore be switched on and
off fast without any additional special circuit complexity. When used for testing
the authenticity of bank notes, light-emitting diodes can therefore be operated
in clocked fashion, or switched off for fashion, or switched off for a short time
if for example the checking operation of one bank note is completed and the next
bank note to be checked is not yet available or the bank note processing machine
is stopped for a short time, e.g. to remove a jam. Temporarily switching off the
light-emitting diode during operation further increases its maximum operating lifetime
over UV fluorescent lamps. Further, the emission spectrum of the provided light-emitting
diodes has very slight or no infrared components compared to the TV fluorescent
lamps hitherto used. This avoids undesirable heating of the apparatus. This is
the case in particular when a light-emitting diode is used whose emission spectrum
has a peak in the ultraviolet spectral region.
According to a preferred development of the invention, a monitor detector
is provided in addition to the fluorescence detector for detecting at least part
of the light emitted by the light-emitting diode. The monitor detector measures
the light intensity of the light-emitting diode in the course of its operating
lifetime and delivers measured values that are used for correcting aging effects,
in particular the decrease in intensity to be observed in the provided light-emitting diodes.
In a further preferred embodiment of the inventive apparatus, a lock-in amplifier
is provided for amplifying output signals from the fluorescence detector and/or
monitor detector. Lock-in amplifiers are used in the measurement and processing
of very weak analog signals, whereby the signal background is very greatly suppressed.
With a lock-in amplifier, the modulated output signal from a detector is amplified
and demodulated in a synchronous demodulator with a standardized reference signal
of the same modulation frequency. In a low-pass filter the high-frequency components
are then filtered out. The resulting signal obtained is proportional to the amplitude
of the amplified emitted fluorescent light or the reflected excitation light. Since
the use of a lock-in amplifier is suitable in particular for amplifying very weak
output signals from the fluorescence or monitor detector, it permits very weak
fluorescent light or light emitted by the light source to be measured with high
accuracy. This is of advantage especially when the intensity of the light source
decreases in the course of its operating lifetime and the fluorescent light excited
in the bank note to be checked consequently becomes weaker. The property of amplifying
weak output signals also plays an impor-important role especially when the light-emitting
diode used has an emission spectrum with only slight spectral components in the
ultraviolet spectral region. This can be the case for example when a light-emitting
diode emitting mainly in the visible spectral region and emitting little light
in the ultraviolet spectral region is used instead of a UV light-emitting diode.
The inventive method is characterized substantially as follows. A partial area
of the bank note to be checked is illuminated with the light of the light-emitting
diode. The fluorescence detector is used to measure the fluorescent light emitted
by the partial area and generate a first measured value. Additionally the fluorescence
detector is used to measure with the light-emitting diode switched off and generate
a second measured value. The first measured value is then corrected with the second
measured value, e.g. by subtraction, and the thus corrected measured value is compared
with a predefined threshold value. Measurement with the light-emitting diode switched
off can generally also be done prior to measurement with the light-emitting diode
switched on.
This method makes it possible to eliminate in a simple way any falsification
by ambient light of the fluorescent light to be detected, since no measurement
on reference documents is required but only a "dark measurement" to be simply realized
by switching off the light-emitting diode. In particular this makes it possible
to take a "dark measurement" before or after each actual fluorescence measurement
without increasing the operative effort. Ambient light refers here to artificial
or natural room light and light from other measuring devices in the immediate surroundings
of the apparatus. Ambient light can pass into the detection area of the fluorescence
detector directly or indirectly, e.g. by reflection on optical components or the
bank note. The inventive method furthermore eliminates falsification of the measurement
due to dark currents typically occurring with photodiodes, which flow through the
detector even when no light is detected.
In a preferred embodiment of the inventive method, it is provided that the first
measured value generated with the illumination switched on is corrected with the
second measured value generated with the illumination switched off by lock-in amplifica-amplification
of an output signal generated by the fluorescence detector. The light-emitting
diode is switched on and off periodically by a suitable voltage source, e.g. a
frequency generator. This permits detection of even low intensities of the fluorescent
light emitted by the bank note and detected by the fluorescence detector. This
is of importance in particular when a counterfeit bank note to be checked contains
only small traces of optical brighteners and the fluorescent light emitted thereby
consequently has only a low intensity. Lock-in amplification of the output signal
is also of advantage when the intensity of the light emitted by the light source
becomes weaker in the course of the operating lifetime and the excited fluorescence
in the bank note is accordingly weak.
In a development of the method, it is provided that the corrected measured value
or the threshold value is corrected with correction value K which takes account
of the change of intensity of the light emitted by the light-emitting diode in
the course of the operating lifetime of the light-emitting diode. This permits
the influence of intensity fluctuations of the light-emitting diode to be eliminated,
so that an intensity of the fluorescence of the bank note that is substantially
independent of intensity fluctuations of the light exciting the fluorescence can
be determined at any time within the operating lifetime.
A further aspect of the invention is that fluorescent light emitted by the bank
note is measured with the light-emitting diode switched on and phosphorescent light
emitted by the bank note with the light-emitting diode switched off. The measured
fluorescent and phosphorescent light of the bank note is then used for authenticity
testing. Measurement is preferably done with the fluorescence detector in each
case. The light-emitting diode is operated in pulsed fashion, in particular by
periodic pulses. This method variant is suitable for testing the authenticity of
bank notes that show not only fluorescence but also phosphorescence. If the fluorescence
and phosphorescence properties of the bank note are known, the presence of certain
substances in the bank note can then be inferred, for example. Such separate measurement
of the fluorescent and phosphorescent light emitted by the bank note can be realized
very simply using a light-emitting diode as a light source, since light-emitting
diodes are particularly suitable for generating, in particular short, pulses.
In the following, the invention will be explained in more detail with reference
to examples shown in figures, in which:
FIG. 1 shows the schematic structure of a first example with two light-emitting
diodes and a fluorescence detector;
FIG. 2 shows the schematic structure of a further example with a light-emitting
diode, a fluorescence detector and a monitor detector;
FIG. 3 shows a schematic circuit diagram of the inventively provided lock-in amplifier.
FIG. 1 shows the schematic structure of an example with two light-emitting diodes
10 and fluorescence detector
12. The light emitted by light-emitting
diodes
10 has ultraviolet components that are used for exciting fluorescent
light in bank note to be checked
11. These may be for example light-emitting
diodes having an emission spectrum with a peak in the ultraviolet spectral region.
The emission peak of typical light-emitting diodes is about 370 nanometers and
has a half-value width of about 12 nanometers. Alternatively it is also possible
for the light-emitting diodes to have an emission peak in the visible spectral
region, e.g. at about 430 nanometers, but also emit sufficiently high components
of ultraviolet light. In both cases, particularly the latter, it may be necessary
to select a certain spectral region from the emission spectrum of the light-emitting
diodes for exciting fluorescent light in bank note to be checked
11. This
is done by first filter
15 disposed before light-emitting diodes
10.
Suitable filters typically have an edge at 400 nanometers and are impermeable to
light with greater wave-lengths.
In order to guarantee that fluorescence detector
12 detects substantially
the visible fluorescent light emitted by bank note
11, second filter
16
with suitable spectral transmission characteristics is mounted before fluorescence
detector
12. A suitable filter typically has an edge at about 400 nanometers
and is impermeable to light with smaller wavelengths.
Lens system
18 serves to collimate the fluorescent light emitted by bank
note to be checked
11 onto fluorescence detector
12. Lens system
18 can furthermore serve to parallelize the fluorescent light passing through
second filter
16. This is of importance when an interference filter is used
as second filter
16, since a substantially parallel beam path prevents a
shift of the filter edge on the interference filter. A screen (not shown) can fundamentally
also be used instead of lens system
18 to minimize costs.
In the shown example, window
13 that is permeable at least to parts of
the excitation light and the fluorescent light is provided between bank note to
be checked
11, on the one hand, and light-emitting diodes
10 and
fluorescence detector
12 including filters
15 and
16 and lens
system
18, on the other hand. During the authenticity testing process, bank
notes
11 can be transported with a suitable transport device parallel to
window
13, window
13 also serving to protect the individual components
of the apparatus from dust and other soiling.
As can be seen by schematically shown beam path
14, the light of light-emitting
diodes
10 illuminates an extended partial area of bank note to be checked
11, i.e. the light of light-emitting diodes
10 is not focused on
bank note
11. However, it is fundamentally of advantage here to concentrate
the light emitted by the light-emitting diodes, for example by convergent beams
(not shown), since this obtains higher intensity of the excitation light on the
illuminated partial area of bank note
11. At the same time, the fluorescent
light excited in the illuminated partial area of bank note
11 is detected
by fluorescence detector
12 integrally, i.e. the light emitted by a finite
area is measured. Nonfocusing illumination of bank note
11 and integral
detection of the fluorescent light over a certain surface area cause the fluorescence
properties of bank note
11 to be averaged, so that local irregularities
such as soil particles or folds have a less disturbing effect than is the case
with focused illumination. Typical sizes of the surface areas on bank note
11
are about 0.4 square centimeters for the illuminated partial area and about 0.4
square centimeters for the area over which the fluorescent light is detected integrally.
FIG. 2 shows the schematic structure of a further example. Light-emitting diode
10 provided with first filter
15 is used for illuminating bank note
to be checked
1111 which is transported parallel to transport plate
20. Transport plate
20 is opaque in this example and has three-dimensional
partial area
21 which is permeable at least to spectral components of both
the excitation light and the fluorescent light emitted by bank note
11.
Transport plate
20 can be part of a housing in which the total apparatus
is integrated, and thus serves to protect the individual components of the apparatus
from dust and other soiling. As can be seen by schematically indicated beam path
14, the light of light-emitting diode
10 is also unfocused when hitting
bank note
11 in this example. The typical size of the surface area of said
illuminated partial area on bank note
11 is about 0.4 square centimeters.
When passing three-dimensional partial area
21 of transport plate
20,
bank note
11 is illuminated by the ultraviolet components of the excitation
light emitted by light-emitting diode
10 including first filter
15.
If optical brighteners are present in bank note
11, this excites fluorescence
in the visible spectral region. The excited fluorescent light passes through three-dimensional
partial area
21 of transport plate
20 and hits fluorescence detector
12 provided with second filter
16. For collimating the fluorescent
light emitted by bank note
11 there is first screen
22 before detector
12. In this arrangement, fluorescent light is detected integrally over surface
areas with typical sizes of under 1 square centimeter.
In this example, monitor detector
25 is provided for measuring the brightness
of light-emitting diode
10. For this purpose, the light emitted by light-emitting
diode
10 and spectrally filtered is reflected at least partly on transport
plate
20 and/or on three-dimensional partial area
21 of transport
plate
20 and detected by monitor detector
25. Part of the light emitted
by light-emitting diode
10 can fundamentally also be coupled out by other
suitable means, such as mirrors or optical fibers, and supplied to monitor detector
25.
In a further variant of this structure, it is provided that light-emitting diode
10, monitor detector
25 and first filter
15 are disposed such
that the light emitted by light-emitting diode
10 is partly reflected on
first filter
15 and reaches monitor detector
25. First filter
15
is inclined relative to the light beams coming from light-emitting diode
10
such that the light beams obliquely hitting first filter
15 can be partly
reflected there and reach monitor detector
25. Monitor detector
25
is preferably disposed between light-emitting diode
10 and fluorescence
detector
12 (not shown). This variant has the advantage that the light reflected
by first filter
15 and hitting monitor detector
25 is not overlaid
and thus falsified by scattered light components from bank note
11 or possibly
soiled partial area
21 of transport plate
20. Coating the side of
first filter
15 facing light-emitting diode
10 in addition prevents
reflections on said side of the filter, so that only the side of first filter
15
facing away from light-emitting diode
10 reflects. This additionally takes
account of changes in the transmission behavior of first filter
15. If e.g.
the transmission of the filter decreases, the intensity of the UV light hitting
bank note
11 decreases and the component of reflected light hitting monitor
detector
25 is accordingly lower.
For beam collimation the shown example has second screen
23, and spectral
filtering is effected by third filter
17 disposed before monitor detector
25. The intensity of light-emitting diode
10 is preferably measured
here without a bank note, i.e. there is no bank note before transparent three-dimensional
partial area
21. This prevents any light diffusely reflected by the bank
note from hitting monitor detector
25 and falsifying the measurement of
the instantaneous brightness of the light-emitting diode.
The detectors used are preferably photodiodes. Three-dimensional partial area
21 of transport plate
20, which is at least partly transparent to
both excitation light and fluorescent light, is generally made of suitable glass
material. Absorption edges of typical glass materials are under 350 nanometers
so that the latter are permeable to light with greater wavelengths. Second filter
16 and third filter
17 can alternatively be firmly connected, e.g.
vapor-deposited or glued, to particular detector
12 or
25.
The measurement of ambient light and fluorescent light can be performed several
times on each bank note to be checked
11. For example, bank note
11
can be tested in a track measurement by transporting the bank note past partial
area
21 of the transport plate and measuring it at certain distances. Typical
track widths are about 6 millimeters, typical distances between measurements about
2 millimeters. The thus obtained values can then be averaged so as to generate
a mean value that is a measure of the average fluorescence behavior of bank note
11. The mean value can be determined e.g. determined e.g. by arithmetic
averaging of the values. Alternatively, the mean value can be determined from the
individual values by using a so-called median filter that eliminates peak values
from the series of individual values and thus "smoothes" the series.
The mean value can then be compared with the predefined threshold value. If the
mean value is greater than the threshold value, this indicates a forgery.
With a periodic or pulsed voltage supply of light-emitting diode
10 and
amplification of the output signal from fluorescence detector
12 with a
lock-in amplifier, the fluorescent light is reduced by the components of the ambient
light automatically, i.e. by the lock-in method itself, so that separate measurement
of the ambient light is unnecessary.
FIG. 3 shows a schematic circuit diagram of the inventively provided lock-in
amplifier in connection with light-emitting diode
10 and fluorescence detector
12. Frequency generator
30 is connected with light-emitting diode
10. The fluorescent light excited in bank note
11 (not shown) by
light from light-emitting diode
10 is detected by fluorescence detector
12 formed as a photodiode in this example. The output signal from fluorescence
detector
12 is then amplified in transimpedance amplifier
31 and
supplied to synchronous demodulator
33 together with a reference signal
generated by frequency generator
30 and passed through phase shifter
32.
The output signal from the synchronous demodulator is filtered in low-pass filter
34 and can be supplied to analog-to-digital converter
35 for further
processing. The schematic circuit explained can also be used for determining the
brightness of light-emitting diode
10 with the aid of monitor detector
25.
In this case, monitor detector
12 is simply connected or switched to transimpedance
amplifier
31, instead of fluorescence detector
25.
The inventive method or its developments are preferably repeated at several places
on the bank note. This makes the authenticity testing of bank notes safer and more
reliable than with measurement at only one place on the bank note to be checked.
The corrected measured values characterizing the fluorescence behavior of the bank
note at individual places can then be averaged, for example to reduce any soiling
effects at individual places on the bank note to be checked.
*
