Method and sensor for testing documents

Abstract

A method and a sensor for checking documents are provided in which the same detector is used for a remission measurement and a luminescence measurement of the value document. The remission measurement value being detected during the illumination of the value document with an excitation light used for luminescence excitation, and the luminescence measurement value after switching off the illumination. In order to reduce a distortion of the remission measurement value by the luminescence, a spectral detection filter is incorporated into the detection ray path, which has a transmission of at least 0.5% in the spectral region of the excitation light. The increased transmission of the spectral detection filter achieves that the excitation intensity impinging on the detector far exceeds the luminescence intensity occurring simultaneously with the excitation and thus reduces the mentioned distortion.

Claims

1. A sensor for checking the authenticity of documents, comprising: an illumination device for illuminating a document with one or several excitation light pulses of an excitation light which is suitable to excite the document to emit luminescent light; and a detector for detecting at least one remission measurement value of the document and at least one luminescence measurement value of the document; and a detection filter which is located in a detection ray path formed between the document and the detector; and a control device for controlling the illumination device and the detector, wherein the control device is arranged to drive the detector such that the detector detects at least one remission measurement value of the document at at least one point in time at which the document is illuminated with an excitation light pulse of the excitation light, and detects at least one luminescence measurement value of the document at at least one point in time after the end of the respective excitation light pulse; an evaluation device for checking the document on the basis of the at least one remission measurement value detected by the detector and on the basis of the at least one luminescence measurement value detected by the detector; wherein the detection filter is a spectral detection filter whose spectral transmission is selected such that both the luminescent light of the document impinging on the spectral detection filter and at least 0.5% of the excitation light impinging on the spectral detection filter are transmitted through the spectral detection filter; wherein the remission of the excitation light irradiated for the luminescence measurement is detected for the at least one remission measurement value.

2. The sensor according to claim 1, wherein the spectral transmission of the spectral detection filter is selected such that at least 80% of the luminescent light of the document impinging on the spectral detection filter is transmitted through the spectral detection filter.

3. The sensor according to claim 1, wherein a maximum transmission which the spectral detection filter has in the spectral region of the luminescent light is greater by at least a factor of 4 than a maximum transmission which the spectral detection filter has in the spectral region of the excitation light.

4. The sensor according to claim 1, wherein the spectral detection filter has a transmission spectrum which has a spectral luminescence transmission band in the spectral region of the luminescent light of the document and at least one additional spectral transmission band in the spectral region of the excitation light.

5. The sensor according to claim 4, wherein the at least one additional transmission band spectrally overlaps with the excitation light or spectrally completely encloses the excitation light.

6. The sensor according to claim 4, wherein the spectral detection filter has a greater transmission in its luminescence transmission band than in its at least one additional transmission band.

7. The sensor according to claim 4, wherein the additional transmission band has a spectral distance from the luminescence transmission band of at least 10 nm.

8. The sensor according to claim 4, wherein the excitation light has a spectral excitation band with an upper spectral flank and a lower spectral flank, and the spectral detection filter has a first additional spectral transmission band, which lies spectrally in the lower spectral flank of the excitation band and has a second additional spectral transmission band which lies spectrally in the upper spectral flank of the excitation band.

9. The sensor according to claim 1, wherein the control device is arranged to drive the detector, or an electronic circuit connected therewith such that the respective remission measurement value is measured with lower sensitivity than the respective luminescence measurement value.

10. The sensor according to claim 9, wherein the control device is arranged to switch over a sensitivity setting of the detector or of an amplifier connected with the detector or of a current-voltage converter connected with the detector in the time period between the detection of the respective remission measurement value and the respective luminescence measurement value such that the remission measurement value is measured with lower sensitivity than the luminescence measurement value.

11. The sensor according to claim 1, wherein the detector is a semiconductor-based detector with a charge carrier lifetime of at most 20 μs.

12. An apparatus for checking a document with a sensor according to claim 1.

13. The apparatus according to claim 12 having a transport device which is arranged to transport the document and the detector relative to each other during the detection of the remission and luminescence measurement value, wherein the control device of the sensor is arranged to drive the detector such that the respective remission measurement value and the respective luminescence measurement value are detected with such a short time interval between each other that the detection regions on the document, from which the respective remission measurement value and the respective luminescence measurement value are detected, overlap by at least 50%.

14. A method for checking the authenticity of the documents, comprising the steps of: illuminating a document with one or several excitation light pulses of an excitation light which is suitable to excite the document to emit luminescent light; detecting at least one remission measurement value of the document at at least one point in time at which the document is illuminated with an excitation light pulse of the excitation light, by means of a detector; detecting at least one luminescence measurement value of the document at at least one point in time after the end of the respective excitation light pulse by means of the detector; checking the document on the basis of the at least one remission measurement value detected by the detector and on the basis of the at least one luminescence measurement value detected by the detector; wherein in a detection ray path formed between the document and the detector there is located a spectral detection filter whose spectral transmission is selected such that both the luminescent light of the document impinging on the spectral detection filter and at least 0.5% of the excitation light impinging on the spectral detection filter, which has been remitted by the document, is transmitted through the spectral detection filter; wherein the remission of the excitation light irradiated for the luminescence measurement is detected for the at least one remission measurement value.

15. The method according to claim 14, wherein the document and detector are transported relative to each other during detection and that the remission measurement value and the luminescence measurement value are detected with such a small-time interval between each other that the detection regions on the document, from which the respective remission measurement value and the respective luminescence measurement value are detected, overlap by at least 50%.

16. A sensor for checking the authenticity of documents, comprising: an illumination device for illuminating a document with one or several excitation light pulses of an excitation light which is suitable to excite the document to emit luminescent light; and a detector for detecting at least one remission measurement value of the document and at least one luminescence measurement value of the document; and a detection filter which is located in a detection ray path formed between the document and the detector; and a control device for controlling the illumination device and the detector, wherein the control device is arranged to drive the detector such that the detector detects at least one remission measurement value of the document at at least one point in time at which the document is illuminated with an excitation light pulse of the excitation light, and detects at least one luminescence measurement value of the document at at least one point in time after the end of the respective excitation light pulse; an evaluation device for checking the document on the basis of the at least one remission measurement value detected by the detector and on the basis of the at least one luminescence measurement value detected by the detector; wherein the detection filter is a spectral detection filter whose spectral transmission is selected such that both the luminescent light of the document impinging on the spectral detection filter and at least 0.5% of the excitation light impinging on the spectral detection filter are transmitted through the spectral detection filter; wherein the control device is arranged to drive the detector, or an electronic circuit connected therewith such that the respective remission measurement value is measured with lower sensitivity than the respective luminescence measurement value; and wherein the control device is arranged to switch over a sensitivity setting of the detector or of an amplifier connected with the detector or of a current-voltage converter connected with the detector in the time period between the detection of the respective remission measurement value and the respective luminescence measurement value such that the remission measurement value is measured with lower sensitivity than the luminescence measurement value.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Hereinafter the invention will be described by way of example with reference to the accompanying drawings. There are shown:

(2) FIG. 1 schematic structure of a sensor according to the invention,

(3) FIG. 2a example of a bank note with fluorescent printing ink,

(4) FIG. 2b the course of the remission intensity R and fluorescence intensity F emanating from the bank note of FIG. 2a as a function of the position x along the bank note,

(5) FIG. 3a-d the time course of the excitation intensity (FIG. 3a), the luminescence intensity of the bank note (FIG. 3b), the superimposition of luminescence intensity and detected (with high suppression) excitation intensity (FIG. 3c), the superimposition of luminescence intensity and detected (with low suppression) excitation intensity (FIG. 3d),

(6) FIG. 4a-e five examples of transmission spectra of the spectral detection filter compared to the spectral location of the excitation light and the luminescent light,

(7) FIG. 5 two-dimensional location of the first and second detection region on the bank note,

(8) FIG. 6 electrical circuit for switching over the sensitivity during detection.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

(9) The invention is hereinafter explained using the example of the authenticity check of a bank note 3, in whose substrate a luminescent substance is incorporated over the full area, the luminescence of which is evaluated for the authenticity check. The bank note of FIG. 2a viewed in this example has—in addition to the luminescent substance—an imprint of fluorescent printing ink 11. In addition, the denomination 13 is printed on the bank note and furnished with a region with non-fluorescent printing ink 12.

(10) FIG. 1 shows a sensor 10 which is arranged for capturing both remission measurement values and luminescence measurement values of a value document, such as e.g. the bank note 3 of FIG. 2a. The bank note 3 is transported along a direction (e.g. in FIG. 1 from right to left) past the sensor 10 with the aid of a transport device, so that the detector 6 can detect several measurement values one after the other as a function of the position x along the bank note 3. For the measurement of the remission and luminescence of the bank note the same detector 6 is used.

(11) In one embodiment, the sensor 10 has an illumination device with two light emitting diodes 1a and 1b which illuminate the bank note 3 from an oblique direction. The spectral region of the illumination device is selected such that the light emitted by the illumination device is configured for optically exciting the luminescent substance present over the full area of the bank note. The illumination device is switched on and off periodically to excite the bank note 3 to luminescence at a multiplicity of positions x along the bank note with excitation light pulses. In the detection ray path 8 of sensor 10, the light emanating from the bank note 3 passes through a front glass 2, then a lens 4, a spectral detection filter 5 and another lens 4, which directs the light to the detector 6. The spectral detection filter 5 is used for attenuating the excitation light A. The sensor 10 further has a control device 7 which ensures that the illumination device is periodically switched on and off, triggers the detection of the remission and luminescence measurement values at certain points in time and passes on the remission and luminescence measurement values detected by the detector to the evaluation device 9 which performs an authenticity check on the basis of the remission and luminescence measurement values.

(12) The excitation light A of the illumination device is used both for exciting the luminescence of the luminescent substance present over the full area and as illumination light for the remission measurement. During the illumination with an excitation light pulse used for the luminescence excitation, see FIG. 3a, the detector 6 detects a remission measurement value. After the end of the respective excitation light pulse, the detector 6 detects a luminescence measurement value. In order to achieve a measurement of the remission and luminescence of the bank note at the same value-document position as possible, the remission measurement value and the luminescence value are detected with the as small as possible time interval between each other. In this way, remission and luminescence measurement can be performed at nearly the same value document position x. Preferably, the detection region of the remission measurement (first detection region D1) and the detection region of the luminescence measurement (second detection region D2) overlap by at least 80% in terms of area, see FIG. 5.

(13) Since the remission measurement is performed during illumination with excitation light A, the remission measurement value may, however, be distorted by a luminescence occurring simultaneously with the remission. Thus, a quick rising luminescence, as shown in FIG. 3b, leads to an erroneous increase in the remission measurement value. During the illumination with excitation light, in such cases a superimposition of remission and luminescence is detected, see FIG. 3c. The remission measurement value detected during illumination with excitation light in such a case does not result from the remission intensity alone, but also contains a portion of luminescence intensity. The remission measurement value used for checking the authenticity can therefore be distorted by a luminescence occurring simultaneously with the illumination.

(14) In addition, the remission measurement value may also be distorted by the fact that a quick rising additional fluorescence is detected, such as that of the fluorescent ink 11, which the bank note emits only in the region of the fluorescent ink 11 in response to the excitation light pulse of the excitation light A, see FIGS. 2a and 2b. In FIG. 2b there is outlined the remission intensity R emanating from the bank note 3 along a line S as a function of the position x along the bank note. In the region of the nominal value 13 and the non-fluorescent printing ink 12 there results a lower remission intensity than outside the printed regions. In the region of the fluorescent printing ink 11 the remission of the bank note is also suppressed. However, in this region from the bank note 3 there emanates—in addition to the remission—the fluorescence F of the fluorescent printing ink 11, which significantly increases the measurement value detected in this region. At the x-position of the fluorescent printing ink 11, it thus additionally comes to an erroneous increase in the remission measurement value detected during illumination with excitation light.

(15) The remission measurement values MR detected during illumination with excitation light can thus be distorted both in the case of a quickly rising luminescent substance applied over the full area and by an additional fluorescence F of other locally applied inks or fluorescent substances.

(16) For checking the authenticity of the bank note 3, for example, the luminescence measurement values of a luminescent substance incorporated over the full area of the substrate are examined and in doing so compared with the remission measurement values of the bank note. If the distorted remission measurement values are now used for this comparison, this can lead to an erroneous judgement of the authenticity of the respective bank note.

(17) In a luminescence sensor, a blocking filter is usually installed in the detection ray path of the detector, which suppresses the excitation light as much as possible, e.g. to a factor of T*=10.sup.−5, so that as little excitation light as possible reaches the detector. However, since a complete suppression of the excitation light is not achieved despite the blocking filter, but a considerable intensity is used for the excitation light, a part of the excitation light A usually still advances to the detector. The excitation light advancing to the detector—despite the blocking filter—can have an intensity comparable to the luminescence to be detected, as is shown in the case of FIG. 3c.

(18) It has been found that the problem of distortion of the remission measurement values MR (due to the luminescence occurring simultaneously) can be solved in that no blocking filter is used in the detection ray path 8 for the excitation light A, but a larger portion of the excitation light A is allowed to pass through to detector 6. In the detection ray path 8 of the sensor 10 there is installed a spectral detection filter 5—instead of the blocking filter—which suppresses the excitation light only partially, e.g. only to a factor of T=10.sup.−2, and not—as otherwise usual—as strongly as possible. The low attenuation of the excitation light A in the detection ray path 8 leads to the fact that the portion of detected excitation intensity is significantly increased, while the contribution of luminescence (which leads to distortion) remains the same—due to the unchanged excitation intensity of the bank note—(the excitation intensity impinging on the bank note is not influenced by the changed attenuation in the detection ray path). Since the excitation intensity passed through to the detector is then much greater—due to the lower attenuation—than the (distorting) contribution which the luminescence intensity makes to the remission measurement value, the luminescence then only leads to a negligible distortion of the remission measurement value.

(19) FIG. 3c is the time course of the intensity impinging on the detector 6 in the hitherto usual case of an as strong an attenuation as possible of the excitation light (transmission of the spectral detection filter 5 of T*=10.sup.−5).

(20) And in FIG. 3d the time course of the intensity impinging on detector 6 is shown in the case of a lower attenuation of the excitation light (transmission of the spectral detection filter 5 of T=10.sup.−2). When comparing FIGS. 3c and 3d, it can be recognized that in the case of the strong attenuation, the remission measurement value MR detected at the point in time t1 is clearly distorted by the luminescence L. In the case of the lower attenuation, however, the remission measurement value MR detected at the point in time t1 remains nearly undistorted by the luminescence L. At the point in time t2 the luminescence measurement value ML is detected. The descending branch of the luminescence curve in FIG. 3d corresponds to that in FIG. 3c, but the larger y-scaling in FIG. 3d leads to the fact that the descending branch of the luminescence curve and thus also the luminescence measurement value ML are further down on the y-axis. In the larger y-scaling in FIG. 3d one can also recognize that the remission measurement value MR detected at the point in time t1 is strongly increased compared to the case in FIG. 3c.

(21) If the luminescent substance of the bank note to be detected rises slowly over time (i.e. does not excessively distort the remission measurement value), the transmission of the spectral detection filter for the excitation light need not be increased as much. Then both the increased remission measurement value MR and the significantly lower luminescence measurement value ML can be detected with sufficient accuracy with the same detector 6. Where applicable, a special detector 6 can be used, which has a particularly large dynamic region.

(22) If the luminescent substance of the bank note to be detected rises quickly over time (i.e. strongly distorts the remission measurement value), a significantly increased transmission of the spectral detection filter for the excitation light is necessary. To avoid overdriving the measurement in this case, a dynamic sensitivity switchover can be performed during the measurement. For example, for this purpose a current-voltage converter with switchable amplification is used, see the electronic circuit shown in FIG. 6. The control device 7 of the sensor 10 ensures a switchover of the amplification of the current-voltage converter with the aid of a semiconductor switch S1, which is selectively set to either the open or the closed state via a control signal Us of the control device 7. During the illumination with an excitation light pulse S1 is closed, so that the low-resistance resistor R2 is connected in parallel to the high-resistance resistor R1. For the detection of the remission measurement value MR, the current-voltage converter then has a low amplification. After the detection of the remission measurement value MR, the control device 7 opens the semiconductor switch S1 with the aid of the control signal Us, so that the current-voltage converter—for the detection of the low luminescence measurement value ML—has a large amplification. To avoid overdriving states, the timing of the control signal Us is preferably laid such that the semiconductor switch S1 is already closed before the start of the excitation light pulse and is only opened again after the end of the excitation light pulse.

(23) For an increased stability of the electronic circuit, capacitors can be used which are connected in parallel to the resistors. By a corresponding selection of the capacitors additionally the amplification bandwidth can be set. The capacitance values C1 and C2 of the capacitors can be selected, for example, in accordance with the following formula:

(24) $C_x=\frac{1}{4\pi R_x{f}_c}\left(1+\sqrt{1+8\pi R_x{f}_c{C}_i}\right)$
with R.sub.x=R1 or R2 and C.sub.x=C1 or C2
fc=amplification bandwidth product of the operational amplifier OP
Ci=sum of photodiode capacity and OP input capacity.

(25) In order to detect a low luminescence measurement value very shortly after the illumination with the intense excitation light pulse, a semiconductor detector with a highly doped substrate is preferably used as a detector 6, for example a silicon photodiode with a highly doped Si substrate. In particular, a semiconductor detector is used whose substrate has a charge carrier lifetime that is significantly shorter than the time interval between the excitation light pulse and the detection of the luminescence measurement value ML. Preferably, the charge carrier lifetime in the substrate of the semiconductor detector is at most 20 μs, particularly preferably at most 10 μs. This achieves that the luminescence measurement value ML can be detected in a very short time interval after the end of the excitation light pulse, e.g. already 50 μs-200 μs after the end of the excitation light pulse. This makes possible, even at high transport speeds of the bank note, that the detection region of the remission measurement (first detection region D1) and the detection region of the luminescence measurement (second detection region D2) overlap strongly in terms of area, e.g. by at least 80%, see FIG. 5.

(26) In FIG. 4a there is shown an example of the spectral course of the excitation light A used for exciting the bank note and the luminescent light L emitted by the bank note. In addition, in FIG. 4a there is shown by way of example a transmission spectrum T of a spectral detection filter 5 which is located in the detection ray path 8 of the sensor 10. The transmission spectrum T in FIG. 4a has a spectral luminescence transmission band BL in the spectral region of the luminescence light L and an additional spectral transmission band BA in the spectral region of the excitation light A, which spectrally completely encloses the spectral excitation band of the excitation light A. The transmission band BL likewise can completely enclose the luminescent light, but alternatively allows only a spectral portion of the luminescent light L to pass through.

(27) The spectral detection filter 5 allows for example 20% of the excitation light to pass through in the additional spectral transmission band BA, and in the spectral luminescence transmission band BL 95%. The spectral distance Δλ.sub.F of the two transmission bands BA and BL, measured at the half-value points of the respective transmission bands BA and BL, is preferably at least 10 nm, see FIG. 4a. For example, as a spectral detection filter 5 there is used an interference filter, in which the transmission bands BL and BA are selected according to the spectral location of the luminescent light L and of the excitation light A.

(28) The transmission spectrum T of the spectral detection filter 5 can have different shapes. For example, the additional spectral transmission band BA can be positioned symmetrically or asymmetrically around the spectral curve of the excitation light A. In FIG. 4b-e there are shown four examples of the additional spectral transmission band BA, which only partially overlap with the spectral excitation band of excitation light A. The additional spectral transmission band BA can lie e.g. in the upper spectral flank of the excitation light A (cf. FIG. 4b) or in the lower spectral flank of the excitation light A (cf. FIG. 4c).

(29) The spectral shape of the additional spectral transmission bands of FIGS. 4d and 4e is selected such that the spectral detection filter 5 in both spectral flanks of the excitation light A respectively has an additional spectral transmission band, namely a first additional transmission band BA.sub.u which lies spectrally in the lower spectral flank of the excitation light A and a second additional transmission band BA.sub.o which lies spectrally in the upper spectral flank of the excitation light A. This achieves that the intensity of the excitation light A transmitted through the spectral filter 5 is not changed even in the event of any spectral drift of the excitation light A (which may occur, e.g. due to a change in temperature). Because, for example, a spectral shift of the spectral excitation band to longer wavelengths would lead to an increased intensity in the transmission band BA.sub.o of the long-wave flank and to a reduced intensity in the transmission band BA.sub.u of the short-wave flank. This means, both changes are opposite to each other and at least partially compensate one another. In contrast, one single additional transmission band in only one of the two flanks would be less favourable, since no such compensation would be effected. Optionally, also a third additional transmission band BA.sub.m may be present in the spectral center of the excitation light.