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APPARATUS AND METHOD FOR
DETECTING THE AUTHENTICITY OF
This invention relates to a method and apparatus for 5 detecting the authenticity of secured documents.
BACKGROUND OF THE INVENTION
There are innumerable different types of documents and things which are subject to counterfeiting or forgery, and many different techniques and devices have been developed for determining the authenticity of a document or a thing. By way of example only, documents which are particularly in 15 need of authentication include bank notes, identification papers, passports, packagings, labels and stickers, driver's licenses, admission tickets and other tickets, tax stamps, pawn stamps, and stock certificates. As used herein, the term "secured document" includes any document or thing which 20 is provided with a distinguishing device (whether printed or not) which can be used to authenticate, identify or classify the document.
Furthermore, in addition to determining the authenticity of a secured document, it is sometimes useful to also 25 determine the nominal value of the document or the nature of the document. For example, in a postal system, it is not only necessary to establish the authenticity of the postal stamps and/or release stamps, it may also be beneficial to determine the value of the postage stamps as they are passed 30 through a postal sorting machine.
Accordingly, as used herein, the term "authentication element" is intended to refer to any "device" which may be printed on, or otherwise attached to, a secured document for 35 the purpose of authenticating the document or for the purpose of determining its value and/or type or any other characteristic. Likewise "authenticity" is meant to encompass value, type or other characteristic of a secured document, as well as the genuineness of a document or thing. 4Q
It is known to provide secured documents such as bank notes with an authentication element in the form of a distinctive luminescent ink which, when excited by a light of a predetermined wavelength, will emit a distinctive low intensity radiation that can be detected and analyzed as a 45 means for authenticating a secured document. German Patent No. DE 411 7911 Al discloses such a system which includes a conically expanding fiber optical waveguide and an optical processing system. The radiation from the object to be tested can be collected over a large spatial angle with 50 the narrow cross-sectional end of the fiber optical waveguide. Because of the cross sectional transformation, the radiation emerges from the fiber at a significantly smaller angle, which is coordinated with the cone angle of the optical processing system. 55
With such a system it is possible to detect relatively low intensity distinguishing luminescent authenticity elements. However, the magnitude of the distinguishing luminescent elements must exceed a certain threshold. The system is therefore still relatively insensitive. Because of the use of a 60 conical fiber, there is also the disadvantage that only a small region of the document can be monitored and checked. Moreover, the system may fail if the authenticity element is disposed at certain places in the document. Further, documents such as postage stamps cannot be identified with this 65 arrangement at the high speeds customary in sorting, distributing and/or counting machines. In the case of laser
excitation, characteristic pulse responses, which are of decisive importance for identifying authenticity, also may not be recognized and evaluated.
It is a principal object of this invention to provide an improved method and apparatus for determining the authenticity of a secured document.
Another object of the invention is to provide an improved method and apparatus for determining the authenticity of secured documents while they are moving at high speeds.
A more specific object of the invention is to provide an improved method and apparatus for determining the authenticity of secured documents which contain an authentication element of the type which emits radiation when excited by radiation of a predetermined excitation wavelength, and which include none of the above-mentioned drawbacks.
A still further object of the invention is to provide an improved method and apparatus for distinguishing between different types of secured documents.
SUMMARY OF THE INVENTION
The invention is intended to be used with secured documents containing an authentication element which, when excited with radiation of a predetermined excitation wavelength, emits radiation. According to the invention, the intensity profile of the emitted radiation is determined in a specified wavelength range over a predetermined measuring time interval after excitation. The intensity profile is then analyzed in a number of different ways to determine the authenticity of the secured document.
In one embodiment, the intensity profile is analyzed by determining the length of the time period during which the intensity of the emitted radiation is equal to or greater than a specified threshold value. The secured document is regarded as "authentic" when the determined time period is greater than or equal to a specified nominal value.
In another embodiment, the intensity profile is compared with one or more intensity profile patterns stored in a database. In this case, a secured document is regarded as authentic when the difference between the determined intensity profile and at least one of the stored intensity profile patterns is less than or equal to a specified summation threshold value.
In a third embodiment of the invention, the rise time of the intensity over a specified period of time is measured and compared with at least one nominal rise time. If the difference between the measured rise time and at least one of the nominal rise time values is less than or equal to a predetermined value, the secured document is regarded as authentic.
FIG. 1 is a diagrammatic top view of the inventive sensor;
FIG. 2 is a diagrammatic representation of the radiation bundle, emitted by the radiation source;
FIG. 3 diagrammatically shows an emission response of the fluorescent material in the spectral diagram;
FIGS. 4, 5 and 6 are diagrammatic representations of the time dependence of the signals recorded; and
FIG. 7 is a diagrammatic representation of a further example of the sensor.
The sensor in accordance with the invention is suitable for installation in a high speed transporting device and can also be constructed as a scanner. It is capable of detecting a
distinguishing authenticity element, predominantly on flat objects. In the preferred embodiment, the authenticity element comprises a printing ink with which up-conversion pigments (also known as anti-Stokes fluorescent materials) are mixed. If need be, these pigments can be mixed directly 5 with an applied solution, an applied lacquer, an adhesive or a carrier, such as paper. Advantageously, pigments with a rapid rise time and a rapid decay time (such as 0.01-1 ms) are used in order to permit detection at the high speeds desired. Of course, the electronic evaluation is adapted to the 10 characteristic times of the pigments.
The distinguishing authenticity element is preferably an anti-Stokes fluorescent material (also known as an antiStokes pigment, anti-Stokes phosphor, or up-conversion material), which is a gandolinium oxysulfide activated with 15 thulium and co-doped with ytterbium, having the composition
or also (different notation) 20
Examples of chemical compositions for the authentication element of the present invention can be found in U.S. application Ser. Nos. 10/101524 and 10/101520, filed Mar. 25 15, 2002, which are incorporated herein by reference.
Yttrium and/or lanthanum can also be used proportionately as the basic lattice (host lattice material, matrix material) instead of gadolinium. The fluorescent material is able 3Q to convert comparatively low energy infrared (IR) excitation radiation into higher energy radiation (up-conversion or anti-Stokes effect).
A high level of security is obtained by analyzing the time dependence of the intensity signal at a particular wave- 35 length. The time dependence of the signal is highly dependent on the rise time and decay time behavior, especially on the rise time behavior, of the emitted signal. This signal, in turn, can easily be varied, for example, by doping the fluorescent material with Yb and Tm. The invention offers a 4Q forgery-proof capability of identifying the nominal value of the secured document or the nature of the document.
Alternatively, other pigments can also be used, in which case the build-up and decay behaviors, especially the buildup behavior of the emitted radiation of the pigment must 45 permit a rapid detection of the emitted radiation. For example, photoluminophores, cathodoluminophores or electroluminophores may be used.
The build-up and decay characteristics of the anti-Stokes fluorescent material and, in particular, the matching of the 50 excitation and evaluation unit to the build-up and decay characteristics of the corresponding fluorescent material largely determine the attainable detection reliability and the possible reading speed of the distinguishing luminescence feature. Moreover, the build-up can be characterized, for 55 example, by the time required to reach 90% (t90) of the saturation intensity or by the so-called build-up constant (the time required to reach 1/eth of the steady state luminescence intensity).
For a given high reading speed, the build-up time of the 60 anti-Stokes luminescence may not exceed a specific value if an effective luminescence intensity is to be assured above the sensitivity threshold of the detector. This effective value of the intensity is determined by the relationship between the steady state intensity and the build-up time. 65
Moreover, because of their particular build-up and decay behavior, the signals emitted by the fluorescent material
show a characteristic intensity profile as a function of time. The realization that anti-Stokes intensities and other luminescence intensities can be used not only in relation to their spectral distribution, but also in relation to their temporal dependence for the analytical verification of authenticity (which includes identifying value or other characteristic) is a feature of this invention.
In the case of the (Gdx Yb^Tm^O^S fluorescent material, the relationships between the saturation intensity and the build-up and decay times can be varied. In particular, it is possible to assure the low build-up times required for high-speed detection. For this purpose, the ytterbium and/or thulium concentrations are varied within certain limits. The selective incorporation of defects in the lattice of cations or anions of the luminescent material represents a further possibility for influencing the build-up and decay characteristics.
The distinguishing authentication element may be of small dimensions (for example, 5 mmx5 mm). When the authentication element is applied by a printing method, the imprint can be applied within relatively wide limits. The required measuring range of the sensor should therefore include the entire region of the possible printed field, although the imprinted distinguishing authenticity element may appear anywhere in the printed region and the printed region may be several times larger than the authenticity element. The measuring region (scanning region, transverse to the transporting direction) may, for example, be up to 70 mm in size.
Preferably, the detection is locally resolved in the transporting direction. The speed in the transporting direction may vary from 0 to 12 m/s.
When a synchronization input is used, to which a switching signal proportional to the speed is supplied, a certain, specified, partial section of the test object alone can also be investigated, even if the speed varies.
For the sake of simplicity, it is assumed in the following description that a laser is used as the source of the light beam although other light sources such as LED's may be used. The use of a laser has the advantage that the scanning line is imaged with a relatively high radiation intensity on the surface of the document. This does not happen to the same extent when other light sources are used. The brightness is correspondingly lower when other beam sources, such as LEDs, are used.
A laser wavelength, for example, above 900 nm may be used. Other laser wavelengths are also possible. In addition, the spectral width of the laser line may vary. Several relatively close parallel beams may be used in order to recognize the small, labeled, distinguishing authenticity element. Moreover, it is possible to use a broadband source of electromagnetic radiation.
The laser line may be produced with normal cylindrical lenses and produces an illumination density which is greatest in the center of the line. The laser line preferably is produced with an aspherical cylindrical lens or, alternatively, with an array of cylindrical lenses or with a sinusoidal lens surface. The radiation intensity is either distributed uniformly over the whole length of the laser line or is slightly greater at the edge (or in the center) in order to compensate for sensitivity variations of the detector over the measuring range.
The focusing in the plane of the object is such that, if need be, when used without a laser line, there is a slight defocusing, in order to achieve an optimum radiation intensity for the pigments. The luminescence efficiency varies with the intensity of the radiation and generally has an optimum
at a radiation intensity which is high, but not too high. If the radiation intensity is too high, the signal strength of the light received drops.
Advantageously, a strongly diverging laser beam is used, in order to be able to use less expensive lasers for the 5 production of the sensor.
The undesired wavelengths of the light source in the spectral detection region are filtered out optically. A suppression to <10~7 is preferred. The distinguishing authentication element must be recognized during at least two 10 periods; otherwise, it is discarded as unsatisfactory.
In FIG. 1, a laser 2 is arranged in a housing 1. A focusing lens 4 is disposed in a manner, the details of which are not shown, in the interior of the housing (see also FIG. 2). A cylindrical lens 5, which expands the beam 6, is disposed in 15 front of the beam opening 3 so that the beam 6 is radiated in the direction of the arrow shown onto the measuring window 10 to form an approximately line-shaped scanning line 9. The beam 6 passes through a window 8 in a diaphragm 7 which has several windows. 20
The measuring window 10 is closed by a glass pane. The secured document 11 which is to be authenticated passes in the direction of arrow 12 as close as possible to the measuring window, practically in contact with the glass pane. A distinguishing authenticity element 13 is disposed in a 25 predetermined region on the secured document. The authenticity element 13 maybe placed at different sites, for example, also at sites 13' or 13". The length of the scanning line 9 advantageously is selected so that, at most, it corresponds to the width of the secured document so that the 30 scanning line 9 always encounters an authenticity element 13, even when the latter is disposed at an unexpected site on the secured document 11.
The authenticity element 13 functions as described above and, after excitation by the laser light, radiates an emitted 35 beam 14 along the scanning line 9 back through the measuring window 10 and through the window 16 in the direction of arrow 15.
This beam is then processed further in an optical head 17 and finally supplied to an evaluating unit 18. This evaluating 40 unit preferably consists of a photomultiplier (secondary electron multiplier). Instead of a photomultiplier, other evaluating units can also be used, such as photodiodes or a matrix camera, which works with a CCD chip or a CMOS chip. 45
In order to achieve synchronous amplification, the evaluation is conducted over an analog circuit with sample and hold elements. Synchronous amplification enables evaluation of light signals which are received in phase with the repetition frequency of the emitted laser light. The signal 50 evaluation may also involve further functions such as sampling of a signal at a leading edge at a first time after the start of the pulse and comparing this signal with the signal at a second time after the start of the pulse. For this purpose, the selected time windows must be adapted to the bandpass 55 frequency of the electronics and, in particular, to the buildup and decay times of the pigment. These signals and time signals are controlled advantageously by a microprocessor. The same principle can be employed in the pulse pause and the decay behavior of the signal can be investigated. 60
Alternatively, the evaluation can be carried out using a microprocessor with an integrated or external A/D converter.
For greater clarity, the document 11 in FIG. 1 is shown spaced from the measuring window 10. This is not actually the case since document 11 should be moved past the 65 measuring window 10 as close as possible to it, if not even in contact with it, in the direction of arrow 12.
Optionally, a document sensor 19, which preferably is constructed like a light barrier, is included in the housing 1 to determine whether a document to be authenticated is present. A measuring beam 21 is projected onto the secured document to be authenticated and reflected from the document in the direction of arrow 22 through the window 20. In a further example, the excitation by the measuring beam 21 can also cause an emission of radiation in the direction of arrow 22, which then passes through the measuring window. After sensing the presence of a document, the optical system of the laser is cleared and then the scanning line 9 is generated on the surface 11 of the secured document which is to be authenticated. In this case, the distinguishing authenticity element is evaluated only during the time in which the document sensor 19 has noted the presence of a document.
In FIG. 2 the configuration of the beam 6 produced by the laser 2 is shown in greater detail. The beam initially passes from the laser through a focusing lens 4 and then is expanded linearly by a downstream cylindrical lens 5 and bounded by one or more consecutive diaphragms 8, 8' in such a manner that in the region of the measuring window 10, it produces the linear scanning line 9 on the document 11.
If a laser 2 is used, the scanning line, for example, may be about 0.1 to 0.3 mm wide and 70 mm long. The wavelength of the laser may, for example, be in the infrared, visible or ultraviolet wavelength range.
The optical head 17 contains at least one filter (not shown) and limits the wavelength region evaluated by the evaluating unit 18. For example, at least one filter is provided which selects the wavelength which is to be transmitted. Such wavelengths may be in the infrared, as well as in the visible or ultraviolet wavelength range and are independent of the radiation emitted by the distinguishing authenticity element 13. In a further example, an additional filter may be provided to prevent the visible light from reaching the evaluating unit. In a further example, mirrors and/or lattices may be provided in the optical head 17 in addition to and/or instead of the filters. The mirrors and/or lattices, which maybe located in the beam path, select a particular wavelength range.
To compensate for different heights of the secured documents, the optical head 17 may contain a hollow mirror (not shown) which bundles the radiation emitted by the distinguishing authenticity element 13, and realizes this bundling independently of the height of the document, which is to be examined.
Moreover, the optical head 17 may contain a reflecting cone (not shown) on which the entire ray bundle is bundled. This reflecting cone is a metal-coated hollow body which is constructed in the form of a funnel and has internally reflecting surfaces. This ensures not only that the beams which are imaged directly on the receiving element pass through the reflection cone, but also that those beams which strike the internal surface of the reflection cone are reflected and combined with the main beam. The reflecting cone thus amplifies the light beam received significantly, because not only the direct beams, but also the lateral beams which strike the interior walls of the reflection cone at an angle, are used for the evaluation.
As mentioned above, different elements can be used for the evaluating element 18; a photomultiplier is the starting point for the following description. The photomultiplier may include an 8 mm active zone disposed directly in contact with the outlet surface of the reflecting cone, the dimensions corresponding approximately to the dimensions of the outlet surface.