HK1177809B

HK1177809B - Spectral sensor for inspecting value documents

Info

Publication number: HK1177809B
Application number: HK13104881.9A
Authority: HK
Inventors: Jörg Frankenberger; Wolfgang Deckenbach; Wolfgang Rauscher
Original assignee: Giesecke+Devrient Currency Technology Gmbh
Priority date: 2009-12-18
Filing date: 2010-12-16
Publication date: 2016-04-01

Description

Spectral sensor for checking value documents

Technical Field

The invention relates to a spectral sensor for inspecting value documents and to a method for inspecting value documents by means of the spectral sensor.

Background

In order to check the value documents, sensors are generally used, by means of which the type of the value documents is determined and/or by means of which the value documents are checked to determine the authenticity and/or their status. Such sensors are used to inspect documents of value such as banknotes, cheques, identity documents, credit cards, check cards, tickets, receipts and the like. The examination of the value document is carried out in an apparatus for processing the value document, in which apparatus one or several different sensors are contained, depending on the nature of the value document to be examined. During the examination, the value document is usually scanned by a sensor, wherein the sensor and the value document are moved relative to one another.

By means of a plurality of sensors, the value document is illuminated by light sources of certain colors, for which the visually perceptible color of the value document is determined from the reflection (transmission) of the value document. These sensors have only three color channels realized by, for example, red, green and blue light emitting diodes (RGB sensors), corresponding to three different color receptors of the human eye. However, for such an optical sensor with only three color channels, the spectral intensity distribution of the light emitted from the value document cannot be recorded.

For recording the spectral intensity distribution, there are known spectral sensors which illuminate the value document with white light and detect the light reflected by the value document in a spectrally resolved manner. For such a spectral sensor, a diffraction grating is used to spectrally separate the light reflected by the value document. However, spectral separation requires a relatively long optical path from the diffraction grating to the detector line, and thus such a spectral sensor requires a large installation space. Furthermore, the spectral range that can be captured by such a spectral sensor is relatively narrow and therefore spectral intensity distributions in a wide spectral range cannot be recorded by it. Because the diffraction grating is optimized for a certain wavelength, the reflection coefficient of the grating for light of that wavelength is as large as possible. However, for wavelengths deviating from this wavelength, a significant drop in the reflection coefficient of the diffraction grating occurs, so that of the light of these wavelengths only a very low intensity is available for detection.

Disclosure of Invention

It is therefore an object of the present invention to provide an improved spectral sensor for inspecting a value document, which is capable of recording spectral intensity distributions in the visual visible spectral range and/or in the near infrared spectral range from one or more regions of the value document.

This object is achieved by the subject matter of the independent claims. The dependent claims set forth advantageous developments and embodiments of the invention.

The spectral sensor has an illumination device for illuminating the value document examined by the spectral sensor, imaging optics and a detection device. The lighting device has a plurality of light sources whose emission spectra are different from each other. The emission spectrum of these light sources lies in the visual visible spectral range and/or in the near infrared spectral range. The imaging optics image the light emitted by the illumination device onto an area of the value document to be examined. By means of the imaging optics, it is possible to illuminate a clearly defined and spatially limited region of the value document to be examined. The detection device is configured for detecting light emitted from the illumination area upon operation of the spectral sensor when the value document is illuminated by the illumination device.

The lighting device of the spectral sensor has a plurality of different light sources, the emission spectra of which differ from one another. Within the lighting device, the light sources may be arranged side by side, for example in a two-dimensional grid, in particular on a light source receiving device common to the light sources. The light source may also be arranged annularly, for example around the detection device. The imaging optics are configured to image the emitted light of the respective light source onto the value document to be examined. The light emitted by the illumination device is imaged by the imaging optics via a predetermined light path onto the illuminated area of the value document. The imaging optics have for this purpose, for example, one or several refractive optical elements (e.g. lenses) and/or diffractive and/or specular optical elements, which image the light emitted by the light source onto the document of value. Preferably, the imaging optics are configured as an imaging lens. By realizing the imaging of the illumination light onto the value document, the illuminated area of the value document is clearly defined and spatially limited. This represents an advantage compared to direct illumination of the document of value by a light source without any optics therebetween, and compared to simple light guide optics (without imaging optics) which do not image the light, but rather carry it from the light guide onto the document of value without defining a light path.

In order to image the light of the various light sources emitted by the illumination device largely onto the same illuminated region of the document of value, the imaging optics are preferably configured such that the illuminated region of the document of value is located exactly or approximately in the focal point of the imaging optics. Hereby it is achieved that, despite the irradiation of the value document by different light sources arranged side by side, substantially the same area of the value document to be examined can be irradiated and can be detected by the detection device. The imaging optics can be configured to illuminate a patched (patch) area, in particular a circular illumination patch, on the value document. It may also be configured to illuminate a band-shaped area on the value document. As the imaging optics, it is possible to employ, for example, radially symmetrical imaging optics in the first case and cylindrical optics in the second case.

The light emitted by the light source can be concentrated by means of a concentrating optic which directs the concentrated light in a suitable manner onto the imaging optics and which can be a component of the illumination device. The light source, the collection optics and the imaging optics are in this case configured relative to one another such that the emitted light of the respective light source can be imaged by the collection optics and the imaging optics onto a value document which is examined by the spectral sensor upon operation of the spectral sensor. The collection optics are configured between the light source and the imaging optics to collect light emitted by the light source. The collection optics may be implemented by a plurality of, for example, refractive or diffractive, side-by-side configured lenses, each of which collects the emitted light of one of the light sources. The lenses of the collecting optics and of the imaging optics are here arranged and constructed such that the light source is preferably imaged onto the illuminated region of the value document in a blurred manner. It is furthermore preferred that each light source of the lighting device is smaller than the focal length of the lens separate from the lens assigned to it. The lenses of the collection optics may be configured as a single lens or as a plurality of microlenses of a microlens array.

In other embodiments, the collection optics are formed by one or several light guides arranged between the light source and the imaging optics. Here, a common light guide may be provided for all light sources, or a separate light guide may be provided for each light source. The emission light of the light source is coupled into one or more light guides, and the light guides guide the emission light of the light source to the imaging optics. The light leaving the light guide is imaged by the imaging optics onto the value document. As the light guide, for example, a light guide body having a circular or belt-shaped light exit region or a glass fiber may be employed.

The lighting device has a plurality of different light sources, the emission spectra of which lie in the visible spectral range and/or in the near infrared spectral range and are different from one another. That is, the plurality of light sources provide a plurality of different emission spectra having intensity maxima at different wavelengths. For example, each light source of the lighting device is configured to emit an emission line of a certain wavelength, whose spectral position is different from the emission lines of all other light sources of the lighting device. Alternatively, however, the illumination device can also have several identical light sources, in order to likewise achieve sufficient illumination intensity with low-brightness light sources, for example in the spectral range. Preferably, the plurality of light sources cover a part of the near infrared spectral range, so that the spectral sensor, through detection of measured values, is able to record a spectral intensity distribution in said part of the near infrared spectral range. The light source of the illumination device is for example selected such that the spectral sensor is capable of recording a spectral intensity distribution in the near infrared spectral range, which extends from the visually visible spectral range up to in the near infrared spectral range, for example from the visually visible spectral range up to a wavelength of at least 1000nm, preferably up to a wavelength of at least 1200 nm. Alternatively or additionally, the plurality of light sources also cover a portion of the visually visible spectral range, such that the spectral sensor is capable of recording the spectral intensity distribution of the detected light in said portion of the visually visible spectral range. The lighting device may also have one or several light sources with an emission spectrum in the ultraviolet spectral range. As light source, preferably light-emitting diodes are used, such as light-emitting diodes (LEDs), in particular semiconductor light-emitting diodes or organic light-emitting diodes (OLEDs); and/or laser diodes, in particular Vertical Cavity Surface Emitting Lasers (VCSELs).

During operation of the spectral sensor, the light source is switched on and off continuously to illuminate regions of the value document by an illumination sequence of light pulses with different emission spectra. The detection device is configured for detecting light emanating from the region of the value document illuminated by the illumination sequence upon operation of the spectral sensor. As such, for each of the light pulses of the illumination sequence, a measurement value is detected to record the spectral intensity distribution of the detected light. The detected measured values correspond to the light intensities detected during illumination with one of the light pulses of the illumination sequence, respectively. The spectral intensity distribution of the detected light is derived from the detected measurement values.

To check the value document, the illumination sequence is repeated periodically: the value document is illuminated by the same illumination sequence at least over a partial region of the value document to be examined. In other local regions, the value document can be illuminated by different illumination sequences. The illumination sequence can be selected here as a function of the value document to be checked. From the measured values detected during one single illumination sequence, it is already possible to determine the spectral intensity distribution of the light emitted from the value document. Alternatively, however, it is also possible to combine the measured values of different illumination sequences, preferably of at least two consecutive illumination sequences. For example, at least two measurements detected when illuminated with the same light source in successive illumination sequences are combined into one resulting measurement.

During operation of the spectral sensor, the value document to be examined is conveyed past the spectral sensor at a conveying speed. Preferably, the duration of the illumination sequence is adjusted relative to the transport speed of the value documents to be examined such that all light pulses emitted by the light source during the illumination sequence illuminate almost the same region of the value documents. In particular, the area of the document of value illuminated by the first light pulse of the illumination sequence and the area of the document of value illuminated by the last light pulse of the same illumination sequence have an overlap of at least 75%. This means that for all light pulses of the same illumination sequence at least 75% of the area of the illuminated area consecutively illuminated by these light pulses is the same, although the value document is moving during the illumination sequence.

Preferably, the spectral sensor is not configured for full-range inspection of the value document, but for inspection of the value document in one or several tracks on the value document. When the inspection is performed in a plurality of tracks, regions of the value document that are not to be inspected by the spectrum sensor are arranged between the tracks. The regions illuminated for the inspection of the value documents form tracks which extend parallel to one another and which extend in the transport direction of the value documents. These tracks are distributed discretely over the value document. For each of the tracks, at least one illumination device, one imaging optic and one detection device are provided in accordance with the above description. The illumination sequences preferably succeed one another so quickly that the value document is inspected in a quasi-continuous manner along each track.

The part of the near infrared spectral range covered by the light source comprises, for example, wavelengths of at least 750nm-1000nm and/or wavelengths of 1000nm-1600nm, optionally also wavelengths above 1600 nm. Preferably, the spectral sensor is equipped with a light source covering a spectral range above 1000 nm. Conventional spectral sensors using silicon-based detectors have not been suitable for this purpose since they advantageously are able to record also the spectral intensity distribution in this long-wave spectral range. The part of the visually visible spectral range covered by the light source may be, for example, a spectral range belonging to a certain color, for example a spectral range perceived by the human eye as red. However, the light source may also cover two or several colors, so that the spectral intensity distribution extends across two or several colors, for example across the green and red spectral range. The emission spectrum of the light source of the lighting device comprises, for example, at least 5 different emission spectra in the visible spectral range. However, the part of the visually visible spectral range covered by the light source may also be the entire visually visible spectral range.

The spectral sensitivity of the eye is based on three color channels only. There are thus colors which are different from one another but which nevertheless trigger the same color impression in the human eye. These colors, which have different spectral properties but appear the same to humans under the same lighting conditions, are called metameric colors (metameric colors). Prior sensors, such as RGB sensors, that have only three color channels like the human eye, cannot distinguish metameric colors from each other. However, the spectral sensor of the present invention is configured to distinguish metameric colors. In the spectral sensor, the emission spectra of the light sources are selected such that metameric colors can be distinguished from each other based on the spectral intensity distribution recorded by the spectral sensor. For example, the spectral sensor may record a spectral intensity distribution for two metameric colors, respectively, contained on the same or different value documents, so that they can be compared with each other and their differences can be determined.

In the spectral sensor, the emission spectrum of the light source is preferably spectrally positioned such that metameric colors that may be contained in the illuminated area of the value document can be distinguished from each other based on the respective spectral intensity distribution that the spectral sensor may record upon detecting light emitted from metameric colors. For example, the plurality of light sources cover a red spectral range and/or a green spectral range and/or a blue spectral range and/or a near infrared spectral range of 750nm to 1000nm, so that metameric colors that may be contained in the illuminated region can be distinguished from each other by the spectral sensors based on a spectral intensity distribution recorded by the spectral sensors when detecting light emitted from metameric colors. In order to distinguish between metameric colors whose optical properties differ within a certain color channel (e.g. red), it is advantageous to select the light source such that within the spectral range of this color channel there are at least two different emission spectra of the light source. In order to enable the spectral sensor to distinguish a number of different metameric colors from one another, it is also preferred to cover the other color channels (e.g. green, blue) by at least two different emission spectra, respectively. The same applies to the distinction of colors of different optical properties in the near infrared spectral range. It is therefore preferred that the plurality of light sources cover the red spectral range and/or the green spectral range and/or the blue spectral range such that in each spectral range there are at least two different emission spectra of the light sources. For the near infrared spectral range, it is preferred that the plurality of light sources cover a near infrared spectral range of 750nm to 1000nm and/or a near infrared spectral range of 1000nm to 1600nm, such that in each spectral range there are at least three, preferably at least five, different emission spectra of the light sources.

In addition, it is preferred that at least three, in particular at least five, emission spectra of the spectrally adjacent light sources overlap spectrally and/or have emission maxima which differ from one another in each case and whose spectral distance is not more than 60 nm. For example, each of the emission spectra of the light sources of the lighting device spectrally overlaps with at least one of the emission spectra of one of the light sources adjacent to the other spectrum of the lighting device.

The detection means preferably has a spectral sensitivity whose spectrum is so broad that the emitted light of each of the light sources of the illumination means is detectable by the detection means. In particular, the detection device is configured at least for detecting light in the visible spectral range and for detecting light adjacent thereto in the near infrared spectral range up to at least 1000 nm. Silicon-based detection devices are typically employed that are suitable for the visible spectral range, but are not suitable for spectral ranges above 1000 nm. It is therefore particularly advantageous to equip the spectral sensor with a detection device which is designed both for detecting light in the visible spectral range and for detecting light in the near infrared spectral range up to 1000nm and above. In particular, the spectral sensor has an InGaAs photodetector as a detection device for this purpose, which is designed both for detecting light in the near infrared spectral range, in particular for detecting wavelengths above 1000nm, and for detecting light in the visible spectral range.

For detecting the reflected light, the spectral sensor may also comprise several identical detection means, for example to capture reflected light over a larger angular range. The spectral sensor may also have several different detection means in order to, for example, widen the spectral range that can be captured by the spectral sensor. The different detection devices can be arranged side by side or one behind the other, for example in the form of a sandwich structure.

The measured values recorded by the detection device are evaluated by an evaluation device, which may be a component of the spectral sensor or formed by an external evaluation device. Preferably, at least the preprocessing of the measured values is already effected by the spectroscopic sensor, in particular by an internal evaluation device of the spectroscopic sensor. Further evaluation can also be effected by an internal evaluation device or alternatively by a central evaluation device of the apparatus in which the spectral sensor is installed.

In front of the detection device, detection optics can be provided, by means of which light emanating from the value document is collected and guided onto a photosensitive area of the detection device.

The detection optics comprise, for example, one or more refractive or diffractive optical elements or mirrors. The detection optics and the detection device are constructed and arranged such that, in operation of the spectral sensor, only light of a detection region of the value document which is completely arranged within the illuminated region is detected among the light emitted from the illuminated region. By arranging the detection region completely within the illuminated region, the result is achieved that the detected light intensity is insensitive to fluttering of the value document that may occur during transport of the value document. The spectral sensor thus also becomes tolerant to any positional fluctuations of the illumination means, of the imaging optics, of the detection means or of the detection optics that may occur during manufacture or during assembly of the spectral sensor. Preferably, the detection area is arranged entirely within the uniformly illuminated portion of the illuminated area. In this uniformly illuminated portion, the intensity of the illumination is preferably uniformly distributed for all light pulses of the illumination sequence.

Preferably, a control device is provided for the spectral sensor, which control device is adapted to switch the light sources of the illumination device on and off continuously in order to illuminate the value document continuously by means of different emission spectra of the different light sources. The control device can be designed as a component of the spectral sensor, but it can also be designed as an external control device, for example as a component of a device for processing documents of value, in which the spectral sensor is installed. The control means are adapted to drive the lighting means, in particular the light source, of the spectral sensor and the detection means of the spectral sensor. During operation of the spectral sensor, the control device continuously switches the light sources on and off, so that, for example, exactly one of the light sources is switched on at any point in time. However, it is also possible to switch on several light sources, for example several light sources with the same emission spectrum, simultaneously at one or several points in time. Furthermore, the control device triggers the detection device to capture, in the on phase of the light source, a measured value corresponding to the light intensity emitted from the value document, respectively. Since the detection means each record a measured value which is synchronized with the illumination of the light source, the light intensity emitted from the value document is detected for those wavelengths which are predetermined by the emission spectrum of the respective light source.

In the construction of the spectral sensor, an illumination sequence is specified which is used to check the value document, in particular the light source is switched on and off to illuminate the value document. The control means provided for the spectral sensor may already be constructed when the spectral sensor is manufactured. However, the control device may be configured only after the completion of the spectrum sensor. It may also be provided that the configuration of the control device is variable even after the spectral sensor has been put into operation. Such a reconfiguration after commissioning can be carried out, for example, by the manufacturer of the spectral sensor, or by an operator of the spectral sensor or of the device in which the spectral sensor is installed. When reconfiguring, it may also be necessary to adjust the drive of the detection device to the drive of the illumination device, for example when the number of light sources that are switched on and off for measurement changes. The evaluation device for evaluating the detected measured values will also be adjusted to the changed configuration of the control device when reconstructing, for example when using other light sources for checking value documents.

Preferably, the spectral sensor also has a housing in which the illumination device, the imaging optics and the detection device, and optionally also the control device and the detection optics, are arranged.

A further aspect of the invention is a method for checking value documents, which can be carried out with the aid of the abovementioned spectral sensor. In order to inspect the value document, the value document is conveyed past the spectral sensor at a conveying speed. The value document is illuminated by an illumination device having a plurality of light sources whose emission spectra differ from one another. The plurality of light sources are successively switched on and off when illuminating the value document to illuminate an area of the value document by an illumination sequence of light pulses having different emission spectra. The light emitted by the illumination device is imaged onto the illuminated area of the document of value by means of the imaging optics. The light emitted by the light source is preferably collected by means of a collecting optics arranged between the light source and the imaging optics. Light emanating from the illuminated area of the value document is detected. In this way, for each of the light pulses of the illumination sequence, one measurement value is detected to record the spectral intensity distribution of the light emanating from the illuminated area. The plurality of light sources covers at least a part of the visual visible spectral range and/or a part of the near infrared spectral range such that upon detection of the measurement values a spectral intensity distribution in said part of the visual visible spectral range and/or said part of the near infrared spectral range is recorded.

In one embodiment, the lighting device has a light source receiving means on which a plurality of light source locations are arranged, each light source location being configured for receiving a light source. The light source positions are arranged side by side on the light source receiving device and are defined by a plurality of individual recesses, through which one chip-shaped light source can be received each. However, the light source location may also be defined by a bump and/or by an electrical contact area, which may be present by the light source receiving device and which is configured for receiving the chip-like light source.

The illumination device of the spectral sensor may have collection optics. The collection optics are configured, for example, as a microlens array comprising a plurality of microlenses. Here, the microlens array and the light source receiving device are arranged relative to each other such that the light sources arranged on the light source receiving device are each assigned to exactly one microlens. In operation of the spectral sensor, the emitted light of each light source is therefore collected by exactly one microlens of the microlens array. By assigning the micro-lenses to the respective light sources, the emitted light of the light sources can be concentrated with high efficiency. In order to obtain a one-to-one assignment between the microlenses and the light sources, the configuration of the microlenses in the microlens array is identical to the configuration of the light sources on the light source receiving device. For example, the microlenses and light sources are configured in the same two-dimensional grid. Preferably, the microlens array is constructed in one piece with the fastening means as an integral component. The light source receiving device has a counterpart that mates with the fastening device of the microlens array.

The use of a microlens array provides a great advantage over illumination devices that use a single lens for each light source. In this case, a separate mount must be provided for each einzel lens, and the exact positioning with respect to the respective light source is ensured when the einzel lens is fastened. As such, it may be necessary to subsequently adjust the exact position and/or orientation of the singlet lens. In contrast, when a microlens array is employed which has exactly one microlens for each light source, a single exact positioning is sufficient. This positioning may be achieved by fastening means of the microlens array, which are connected to corresponding counterparts of the light source receiving means. The production of the spectral sensor can thus be realized more simply and without adjustments. The microlens array furthermore contains no or very few voids between the individual microlenses, compared to the case where the respective illumination means are realized with single lenses, which have to be mounted separately and whose configuration always leaves voids. Because the microlens array is constructed as a unitary piece, the individual microlenses can enter one another directly. Quasi-area coverage light concentration can thus be obtained by the microlens array. By means of the microlens array, a very compact illumination device with high light collection efficiency is formed.

Drawings

The invention will now be described, by way of example, with reference to the following drawings. In the drawings:

fig. 1 shows a spectral sensor, which examines a value document conveyed past the spectral sensor,

figure 2a shows an example of the emission spectrum (with normalized intensity) of a light source of a lighting device,

figure 2b shows a time sequence of illumination by several illumination sequences respectively from a plurality of light pulses,

fig. 3a is a detailed view of a value document, on which the illuminated area and the detection area are shown,

fig. 3b-3c are detailed views of the value document at a first point in time (fig. 3b) and at the last light pulse of the illumination sequence (fig. 3c), wherein the shift of the illuminated area during the illumination sequence is shown,

fig. 4a-4b show examples of spectral intensity distributions of two metameric colors different from each other in the red spectral range and measurements of a spectral sensor.

Detailed Description

The spectral sensor for checking value documents will be explained below by way of example of a reflection sensor (reflection sensor). However, the spectral sensor according to the invention may also be configured as a transmission sensor. For this purpose, the detection device is arranged opposite to the illumination device, so that the illumination light transmitted through the value document is detected.

Fig. 1 shows an example of a spectral sensor 100, which spectral sensor 100 is configured for examining a value document 1 conveyed past the spectral sensor 100. For illuminating the value document 1, the spectral sensor 100 has an illumination device 50, which illumination device 50 is equipped with a plurality of light sources 15 having a plurality of different emission spectra. The illumination light emitted by the illumination device 50 is imaged by the collection optics and the imaging lens 25 onto the document of value 1. The collection optics 20 are configured in this example as a microlens array 20. However, for imaging the light emitted by the illumination device 50 onto the document of value 1, other optical components, such as a lens system, one or several diffractive optical components, such as fresnel lenses or imaging mirrors, can also be used as imaging optics, instead of the imaging lens 25. Through the value document 1, a portion of the illumination light is reflected, depending on the optical properties of the value document 1. The light reflected by the value document 1 is detected with the aid of a detection device 30 having a photosensitive area 31. The detection device 30 may be formed, for example, from an InGaAs photodiode or an InGaAs phototransistor. In front of the detection device 30, a detection optics 35 is arranged, by means of which light reflected by the value document 1 is collected and guided onto the photosensitive area 31. In the illustrated example, the illumination light is imaged perpendicularly onto the value document 1 and the detection device 30 captures light reflected at an oblique angle. Alternatively, the illumination may also be implemented at an oblique angle, and the detection device 30 may capture light reflected in a vertical direction or in an oblique direction.

In the example of fig. 1, the lighting device 50 comprises a light source receiving means 10 on which a plurality of light source locations 11 are arranged, each configured for receiving a light source 15. The light source receiving device 10 is configured as, for example, a circuit board, and has an electric wiring structure (not shown) required for operating the light sources 15, which allows selective driving of each individual light source 15. The light source locations 11 are formed in the present example by recesses in the light source receiving device 10, in which one light source 15 is respectively fastened. To form the lighting device 50, some or all of the light source locations 11 are provided with light sources 15, respectively. As light source 15, for example, LEDs and/or OLEDs and/or VCSELs are used.

The microlens array 20 of the illumination device has a plurality of microlenses 21. The light source receiving device 10 and the microlens array are adjusted relative to each other such that the light source positions 11 are each assigned exactly one microlens 21. For this purpose, the microlenses 21 are arranged within the microlens array 20 in the same grid as the light source positions 11 are arranged on the light source receiving device 10. The light emitted by the individual light sources 15 is collected by microlenses 21 arranged above the respective light sources 15. The microlens array 20 is constructed in one piece and is formed, for example, from a glass body or from a transparent plastic body. The diameter of the individual microlenses is in the micrometer range or in the millimeter range, for example. To fasten the microlens array 20, the body of the microlens array 20 is equipped with fastening pins 22, which are inserted into holes matching the holes in the light source receiving device 10. By fastening the microlens array 20 by means of the fastening pins 22, an optimal positioning of the microlens array 20 relative to the light source 15 is automatically achieved. Therefore, the illumination device 50 does not have to be adjusted when manufacturing the spectral sensor 100.

The spectrum sensor 100 includes a case 90, and a transparent window 101 is disposed below the case 90. The light emitted by the illumination device 50 is directed through the window 101 onto the value document 1 to be examined, which value document 1 is conveyed past the spectral sensor 100 in the conveying direction T. The illumination device 50 (in particular the light source 15) and the detection device 30 are driven by a control device 60, which control device 60 is arranged in the present example within a housing 90. The control device 60 switches the light sources 15 on and off continuously, for example such that exactly one light source 15 is switched on at any one point in time. During the on-phase of the light source, the detection means 30 respectively capture measured values corresponding to the light intensity reflected by the value document 1. The value document 1 is illuminated successively by different emission spectra of different light sources 15. Since the detection means 30 each detect one measured value in synchronism with the illumination of the light source 15, the intensity of the light reflected by the value document 1 is detected for different emission spectra of the light source 15.

The light source 15 has a plurality of different emission spectra. Fig. 2a shows, for an example of a lighting device with twelve light sources 15, the emission spectra of the light sources E1-E12, which lie partly in the visible 1 spectral range and partly in the near infrared spectral range. In the present example, the emission maxima E1-E12 of all twelve light sources 15 are located at different wavelengths λ 1- λ 12. The spectral distances between the individual emission maxima of λ 4- λ 8 are in the present example each less than 60 nm. Emission spectra E10, E11, and E12 of light sources spectrally adjacent to each other of λ 10, λ 11, and λ 12 overlap with each other spectrally.

The control device 60 drives the light source 15 such that the illumination sequence B1 (by which the light source 15 is switched on and off) is periodically repeated. Fig. 2B shows by way of example an illumination sequence B1, which consists of twelve light pulses P1-P12 and is repeated periodically (B2, B3.). For example, the control device 60 may be programmed such that each light source 15 of the lighting device 50 is switched on and off exactly once during the respective lighting sequence B1, B2, B3. Alternatively, the same light source 15 may also be driven several times per illumination sequence, for example in order to counteract the low intensity of the weak-intensity light source 15 by means of multiple measurements. The illumination sequence may comprise driving all light sources 15 present in the illumination device 50, or driving only a subset of the light sources 15 present. After the illumination sequence B1, i.e. after the measured values have been recorded under illumination of each emission spectrum E1-E12 measured by the provision, the next illumination sequence B2 is started, wherein again the measured values are recorded under illumination of each emission spectrum E1-E12 measured by the provision, etc. Between the lighting sequences B1, B2, B3, there may be a lighting interruption. The measured values obtained during the illumination sequence convey the spectral dependence of the reflection of the detection region of the corresponding value document. Alternatively, several measurements detected in a sequence of successive illuminations when illuminated by the same light source may be combined into one resulting measurement. Thus, for example, a measurement value detected upon illumination by the first light pulse P1 of the first illumination sequence B1 and a measurement value detected upon illumination by the first light pulse P1 of the second illumination sequence B2 may be combined into one resulting measurement value.

Fig. 3a shows a partial region of the document of value 1, on which the region 2 illuminated by the illumination device 50 is shown. By means of the light pulses P1-P12 of the illumination sequence B1, one portion 4 of the illumination area 2 is illuminated with a uniform light intensity, respectively. There is further shown a detection area 3 arranged entirely within the uniformly illuminated portion 4 of the illumination area 2.

The duration Δ t of the illumination sequences B1, B2, B3. For illustration, fig. 3b and 3c show two different points in time t_P1And t_P12Local regions of value document 1. The uniformly irradiated portions 4 are not shown in fig. 3b, 3 c. At a point in time t_P1At this point, the value document 1 is illuminated by a first light pulse P1 of an illumination sequence B1, whichWherein the area illuminated by it is referred to as 2_P1And the associated detection area is referred to as 3_P1See fig. 3 b. By transmitting the value document, the value document 1 comes from the point in time t_P1To a point of time t_P12Moved by a distance d in the transport direction T. At a point in time t_P12Here, the value document 1 is illuminated by the last light pulse P12 of the illumination sequence B1, the region illuminated by which is referred to as 2_P12And the associated detection area is referred to as 3_P12See fig. 3 c. In addition, in fig. 3c, an area 2 of the value document 1 illuminated by the first light pulse P1 is again outlined_P1Associated with the illumination area 2_P12Offset by a distance d. However, the distance d is very short compared to the length L of the irradiated area. Illumination region 2 on a document of value_P12And detection area 3_P12Is thus in contact with the illuminated area 2 on the document of value 1_P1And a detection area 3_P1Is only slightly shifted from the other. The distance d traveled by the value document 1 from the beginning of the run until the end of the same illumination sequence is so short compared to the length L of the illumination areas that the two illumination areas 2 are_P1And 2_P12Overlap by at least 75% in terms of surface area.

Fig. 4a shows an example of the reflection spectrum (dashed line) of the first color C1. The symbol x denotes the measured value detected by the spectral sensor when recording the spectral intensity distribution of the first color C1. To record the spectral intensity distribution, the spectral sensor employs ten light sources of different wavelengths λ 1- λ 10, five of which are located in the RED spectral Range (RED) (λ 4- λ 8). In fig. 4b, in addition to the reflection spectrum of the first color C1, the reflection spectrum of the second color C2 (solid line) is shown, as well as the measured values, represented by the symbol o, which were detected by the spectral sensor when recording the spectral intensity distribution of the second color C2. The first color C1 and the second color C2 are metameric colors with respect to one another, wherein their reflection spectra differ from one another only in the red spectral range, and are otherwise equally extended.

Prior RGB sensors are capable of detecting reflected light in the RED spectral range, but they detect the entire RED channel RED in an integrated manner (integral fast). This means that the total intensity of the reflected light in the red spectral range is detected independently of its spectral distribution in the red spectral range. The RGB sensor is able to distinguish two colors from each other only if their total intensities, which the RGB sensor detects from the respective color in one of its color channels, differ. Since the two reflection spectra of the colors C1 and C2 have the same area when viewed over the red spectral range (see fig. 4b), an RGB sensor that measures the red spectral range in one piece will detect the same total intensity from both colors in red. Therefore, the RGB sensor cannot distinguish the two metameric colors C1 and C2.

However, the spectral sensor of the present invention is capable of distinguishing metameric colors from each other based on the spectral intensity distribution that the spectral sensor records from these colors within one color channel. In the example of fig. 4a, 4b, the spectral sensor is able to distinguish between the two colors C1 and C2 by comparing the spectral intensity distribution in the red spectral range, in particular by comparing the five measurements (x or o) it detects at wavelengths λ 4- λ 8.

Claims

1. A spectral sensor (100) for inspecting value documents (1), the value documents (1) being transported past the spectral sensor (100) at a transport speed upon operation of the spectral sensor (100), the spectral sensor (100) comprising:

-an illumination device (50) having a plurality of light sources (15) with emission spectra (E1-E12) that differ from each other, wherein the plurality of light sources (15) are continuously switched on and off upon operation of the spectral sensor (100) so as to illuminate one region (2) of the value document (1) with an illumination sequence (B1) of light pulses (P1-P12) with different emission spectra (E1-E12), and

-imaging optics (25) by means of which the light emitted by the illumination device (50) is imaged onto the illuminated region (2) of the value document (1) upon operation of the spectral sensor (100), and

-detection means (30) for detecting light emitted from the region (2) illuminated by the light pulses (P1-P12) of the illumination sequence (B1) upon operation of the spectral sensor (100), wherein for each of the light pulses (P1-P12) of the illumination sequence (B1) a measurement value corresponding to the intensity of the detected light is detected,

wherein the plurality of light sources (15) cover a part of the near infrared spectral range and/or a part of the visual visible spectral range such that the spectral sensor (100) is capable of recording a spectral intensity distribution in the part of the near infrared spectral range and/or in the part of the visual visible spectral range by detection of the measurement values,

characterized in that the plurality of light sources (15) cover a red spectral range and/or a green spectral range and/or a blue spectral range and/or a near infrared spectral range of 750nm-1000nm, such that metameric colors (C1, C2) can be distinguished from each other based on a spectral intensity distribution recorded by the spectral sensor (100) when detecting light emitted from the metameric colors (C1, C2), which metameric colors (C1, C2) can be contained in the illuminated region (2).

2. The spectral sensor (100) according to claim 1, characterized in that the light source (15) is selected such that the spectral sensor (100) is capable of recording a spectral intensity distribution extending from the visually visible spectral range up to the near infrared spectral range.

3. The spectral sensor (100) according to claim 1 or 2, characterized in that the plurality of light sources (15) covers a red spectral range and/or a green spectral range and/or a blue spectral range such that in each spectral range there are at least two different emission spectra (E1-E12) of the light sources (15).

4. The spectroscopic sensor (100) according to claim 1 or 2, wherein the plurality of light sources (15) covers a near infrared spectral range of 750nm-1000nm and/or a near infrared spectral range of 1000nm-1600nm, such that in each spectral range there is a different emission spectrum of at least three of the light sources (15).

5. The spectroscopic sensor (100) according to claim 4, wherein the emission spectra are at least five.

6. The spectroscopic sensor (100) according to claim 1 or 2 wherein the emission spectra (E1-E12) of the plurality of light sources (15) comprise at least five different emission spectra in the visually visible spectral range.

7. The spectroscopic sensor (100) according to claim 1 or 2, wherein at least three emission spectra of the light sources (15) spectrally adjacent to each other spectrally overlap and/or have respectively mutually different emission maxima, the spectral distance of which is not more than 60 nm.

8. The spectroscopic sensor (100) of claim 7 wherein said emission spectra are at least five.

9. The spectral sensor (100) of claim 1 or 2, wherein the illumination device (50) has a collection optics arranged between the light source (15) and the imaging optics (25) to collect light emitted by the light source (15).

10. The spectroscopic sensor (100) according to claim 9, wherein the collection optics have a plurality of lenses arranged side by side, by means of which lenses light emitted by one of the light sources (15) respectively can be collected.

11. The spectroscopic sensor (100) according to claim 1 or 2, wherein the spectroscopic sensor (100) has a detection optics (35), wherein the detection optics (35) and the detection device (30) are constructed and arranged such that, in operation of the spectroscopic sensor (100), only light of a detection region (3) of the document of value (1) which is completely arranged within the illuminated region (2) is detected out of the light emanating from the illuminated region (2).

12. The spectroscopic sensor (100) according to claim 11, wherein the detection region (3) is arranged entirely within the uniformly illuminated portion (4) of the illuminated region (2).

13. The spectroscopic sensor (100) according to claim 1 or 2, characterized in that the duration (Δ Τ) of the illumination sequence (B1) is adjusted relative to the transport speed of the value document (1) such that all light pulses (P1-P12) emitted by the light source (15) during the illumination sequence (B1) illuminate substantially the same region (2) of the value document (1).

14. The spectroscopic sensor (100) according to claim 1 or 2, wherein the region (2) of the value document (1) illuminated by the first light pulse (P1) of the illumination sequence (B1)_P1) And an area (2) on the value document (1) illuminated by the last light pulse (P12) of the same illumination sequence (B1)_P12) With at least 75% overlap.

15. The spectroscopic sensor (100) according to claim 1 or 2, wherein the detection means (30) is an InGaAs photodetector configured for detecting light in both the visually visible spectral range and the near infrared spectral range.

16. Method for inspecting a value document, with the aid of a spectral sensor (100) according to one of claims 1 to 10, with the following steps:

-conveying a value document (1) to be inspected at a conveying speed past a spectral sensor (100) configured for inspecting the value document (1),

-illuminating the value document (1) by means of an illumination device (50), the illumination device (50) having a plurality of light sources (15) with emission spectra (E1-E12) that differ from one another, wherein the plurality of light sources (15) are switched on and off successively while illuminating the value document (1) so as to illuminate a region (2) of the value document (1) with an illumination sequence (B1) of light pulses (P1-P12) with different emission spectra,

-imaging the light emitted by the illumination device (50) onto an illuminated area (2) of the document of value (1) by means of imaging optics,

-detecting light emanating from the illuminated area (2) of the value document (1), wherein for each of the light pulses (P1-P12) of the illumination sequence (B1) a measurement value corresponding to the intensity of the detected light is detected,

wherein the plurality of light sources (15) cover a part of the near infrared spectral range and/or a part of the visual visible spectral range such that the spectral sensor (100) is capable of recording a spectral intensity distribution in the part of the near infrared spectral range and/or in the part of the visual visible spectral range by detection of the measurement values.

17. The method according to claim 16, characterized in that the illumination sequence (B1) by which the region (2) is illuminated is repeated periodically.

18. Method according to claim 16 or 17, characterized in that at least two measurement values detected when illuminated by a respective one of the light pulses of the same light source (15) in different illumination sequences (B1, B2) are combined into one resulting measurement value.

19. The method according to claim 16 or 17, characterized in that the illumination sequence (B1, B2) is continuous.