Abstract:
The invention relates to an apparatus for examining documents, in particular documents of value, identification or security documents, having at least two detector units ( 1, 2, 3 ) for detecting light ( 16 ) emanating from a document ( 10 ) to be examined. 
     To increase reliability when examining luminescence, reflection and/or transmission properties of documents, a scattering element ( 5 ) is provided on which the light ( 16 ) emanating from the document ( 10 ) is scattered, the scattering element ( 5 ) and detector units ( 1, 2, 3 ) being disposed such that the scattered light can be detected by the detector units ( 1, 2, 3 ). 
     The scattering element ( 5 ) causes spatial mixture and homogenization of the light ( 16 ) emanating from the document ( 10 ) so as to greatly reduce any parallactic errors that occur in particular with detector units ( 1, 2, 3 ) disposed side by side.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an apparatus for examining documents, in particular, documents of value, identification or security documents, having at least two detector units for detecting light emanating from a document to be examined. 
     To increase forgery-proofness, identification documents, security documents and documents of value, such as bank notes, are provided with security features or printed with suitable security inks. 
     Security features or security inks can contain luminescent substances that can be excited to glow e.g. by light, electric fields, radiation or sound. To check authenticity, the documents are excited to glow and the luminescence light emitted by the luminescent substances of the document is detected. With reference to the intensity and/or spectral characteristic of the luminescence light it can then be ascertained whether the document is authentic or counterfeit. 
     Certain security features or security inks are also distinguished by characteristic reflection and/or transmission behavior in certain spectral regions. If a document of value is imitated with the aid of a color copier, for example, usually only the visible color effect of a printed area can be reproduced. Since customary color particles do not have the spectral behavior in certain, in particular invisible, spectral regions that is characteristic of security features or inks, however, counterfeit documents can generally be recognized by corresponding measurement of their reflection and/or transmission behavior in said spectral regions. 
     The reliability of statements about the authenticity of the checked documents is highly dependent here on the accuracy with which the spectral characteristic, i.e. color, of the light emanating from a document is analyzed. Such analysis can be effected for example by spectrometers, but these require relatively high technical effort and high production costs. 
     A simpler solution is therefore to use individual detector units, such as photodiodes or photomultipliers, each with different spectral sensitivity. Depending on the the spectral characteristic of the light emanating from the document, the detector units deliver different detector signals which can then be used for spectral analysis of the light. Apparatuses of this type have the disadvantage, however, that the light detected by the various detector units generally does not come from exactly the same partial area of the document due to parallactic errors. This makes it impossible to reliably assess the color properties of the light emanating from a certain partial area of the document. This is of disadvantage in particular when areas with small extensions, such as individual lines of a printed image, are to be examined for their spectral properties, since in this case even small parallactic errors can lead to especially great inaccuracies in the spectral analysis of the light emanating from the document. 
     SUMMARY OF THE INVENTION 
     It is the problem of the invention to state an apparatus allowing higher reliability when examining the luminescence, reflection and/or transmission properties of documents, in particular documents of value, identification and security documents. 
     The problem is solved by providing a scattering element on which the light emanating from the document to be examined is scattered, the scattering element and detector units being disposed such that the scattered light can be detected by the detector units. 
     The invention is based on the idea of scattering the light emanating from different partial areas of the document by means of a scattering element whereby the light components emanating from the individual partial areas are mixed. Individual detector units disposed side by side can thus detect light having components from the different partial areas of the document. The scattering element causes spatial mixture and homogenization of the light emanating from the document. 
     The invention permits the detector units to detect the light emanating from a common area of the document equally well. Any parallactic errors which would occur with a laterally shifted assembly of detector units are greatly mitigated by the inventively provided scattering element. From the spectral components of the light emanating from the document detected by the individual detector units, statements about the spectral characteristic of the luminescence, reflection and/or transmission behavior of the document can then be derived with high reliability. 
     In a preferred embodiment of the invention, the scattering element is formed for diffuse transmission and/or diffuse reflection of the light emanating from the document. Diffuse transmission or reflection is intended to refer here to any substantially nondirected transmission or reflection. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained in more detail in the following with reference to examples shown in figures, in which: 
     FIG. 1 shows a first embodiment of the invention; 
     FIG. 2 shows a second embodiment of the invention; 
     FIG. 3 shows an example of different spectral sensitivities of the detector units used in FIGS. 1 and 2; and 
     FIG. 4 shows an example of a preferred electric circuit of the detector units used in FIGS.  1  and  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a first embodiment of the invention. A document to be examined, bank note  10  in the example shown, is transported past sensor system  7  by means of a transport device indicated by transport rollers  40  and transport belts  41 . At the same time, bank note  10  is irradiated with light  15  from two light sources  12 . Light sources  12  are for example fluorescent tubes, incandescent lamps, lasers or light-emitting diodes (LEDs). 
     In an embodiment of the invention it is provided that excitation light  15  emitted by particular light sources  12  is in different wavelengths or wave ranges. This permits even more exact statements about the properties of light  16  emanating from bank note  10 . It may in particular be provided that light sources  12  illuminate bank note  10  either individually or in combination and light  16  detected when bank note  10  is illuminated individually or in combination is evaluated. If only one light source  12  is first used for illumination in the example of FIG. 1 shown, then detector units  1  to  3  detect three first intensity values. Upon subsequent illumination with other light source  12 , three second intensity values are generated. Upon simultaneous illumination with both light sources  12 , three third intensity values are finally obtained. Comparison and/or mathematical combination of the resulting, generally different, intensity values permits especially exact examination of the properties of light  16  emanating from examined bank note  10 . 
     In case luminescence light is to be excited in or on bank note  10 , light sources  12  emit light suitable for exciting luminescence light in or on bank note  10 . Preferably, this is ultraviolet (UV) light. To eliminate spectral components at higher wavelengths, for example in the visible or infrared spectral region, corresponding filters (not shown) can be disposed before light sources  12 . 
     For the case of application that the light diffusely reflected by bank note  10  in certain spectral regions is to be examined, light sources  12  are formed to emit light  15  with spectral components in said spectral regions. 
     In the shown example, the excitation of luminescence light  16  in or on bank note  10  is effected by light  15  from light sources  12 . A corresponding luminescence phenomenon is therefore called photoluminescence. Alternatively or additionally, electric fields, radiation or sound can be used to excite other types of luminescence phenomena, such as electron, radio- or sonoluminescence, in or on bank note  10 . Excitation is effected by corresponding excitation devices, such as electric contacts or field plates, radiation sources for cathode rays, ion beams or x-rays, ultrasonic sources or antennas. Depending on the decay time behavior, luminescence light can be distinguished as phosphorescence or fluorescence light. 
     Luminescence light  16  excited in or on bank note  10 , or the light reflected by bank note  10 , hits detector units  1  to  3  disposed side by side and is detected thereby. Detector units  1  to  3  have different spectral sensitivities and thus detect different spectral components of light  16  emanating from bank note  10 . Accordingly, detector units  1  to  3  generate different detector signals S which are supplied to evaluation device  9  for evaluation and analysis. 
     In the shown example, first device  13  is provided between bank note  10  and detector devices  1  to  3  for directing, in particular focusing, light  16  emanating from bank note  10  onto detector units  1  to  3 . This may be an imaging optic that images partial area  11  of bank note  10  onto detector devices  1  to  3 . For this purpose, self-focusing lenses, so-called Selfoc lenses, are preferably used. Self-focusing lenses are cylindrical optical elements made of material having a refractive index decreasing from the optical axis of the cylinder to the surface thereof. The use of Selfoc lenses obtains an adjustment-free 1:1 image transfer of partial area  11  of bank note  10  to be examined onto detector units  1  to  3  independently of the distance of bank note  10  and detector units  1  to  3 . 
     Alternatively or additionally, first device  13  can also have a light guide element, e.g. of one or more glass and/or plastic fibers. This has the advantage that detector units  1  to  3  can be disposed at any desired places, allowing especially compact integration of such apparatuses into bank note processing systems. 
     According to the invention, a scattering element formed as diffusing disk  5  on which light  16  emanating from bank note  10  is scattered is provided before individual detector units  1  to  3 . The scatter results in the shown example from diffuse transmission of light  16  through diffusing disk  5 . This process is indicated in the Figure by a plurality of small arrows  8 . 
     A second device is provided between bank note  10  and detector units  1  to  3  for limiting the aperture and thus the size of partial area  11  examined on bank note  10 . Aperture limitation is of advantage in particular when the spectral properties of small partial areas of bank note  19 , for example thin lines or details of a printed image, are to be examined. In the shown example, the second device has diaphragm  4 , in particular a pin or slit diaphragm. Together with first device  13  formed as a Selfoc lens, especially simple and precise aperture limitation is obtained. A plurality of alternative embodiments of aperture limitation are fundamentally possible, for example combining diaphragm  4  with a light guide element, e.g. based on glass and/or plastic fibers, or combining a light guide element with an imaging optic that images partial area  11  of bank note  10  to be examined onto the light guide element, in particular into a glass and/or plastic fiber. 
     FIG. 2 shows a second embodiment of the invention wherein, in contrast to the embodiment shown in FIG. 1, reflector  6  is used instead of diffusing disk  5  as a scattering element. Light  16  emanating from bank note  10  is diffusely reflected on reflector  6 , for example a matt or rough mirror, and then detected by individual detector units  1  to  3  disposed side by side. The functionality of all other components of the apparatus is analogous to the example described in FIG.  1 . 
     As an alternative to the scattering elements formed as diffusing disk  5  or reflector  6 , an Ulbricht sphere can also be used for scattering light  16  emanating from bank note  10 . This is a hollow sphere whose interior is provided with a diffusely reflecting coating, for example of magnesium oxide, barium sulfate or Teflon. Light  16  emanating from bank note  10  enters a first opening in the Ulbricht sphere, is diffusely reflected many times in its inside and exits through another opening. The passage of light directly from the entry to the exit openings is prevented by corresponding additional means inside the sphere, e.g. reflectors. The diffuse light leaving the Ulbricht sphere can then be detected by detector units  1  to  3 . 
     A further possibility for spatially mixing light  16  emanating from bank note  10  is offered by a scattering element formed as a hologram in which light beams emanating from bank note  10  are split into a plurality of light beams of different direction and thus mixed before hitting the detector units. 
     An optical filter (not shown) can be disposed before scattering element  5  or  6 , said filter being permeable e.g. only to those spectral components of light  16  emanating from bank note  10  which are to be detected by detector units  1  to  3  disposed behind scattering element  5  or  6 . 
     In a further alternative embodiment of the invention it is provided that the scattering element includes first device  13  and/or the second device, in particular diaphragm  4 . Preferably, the first and/or second devices contain light-scattering particles on which light  16  emanating from bank note  10  is scattered. In this embodiment, the scattering element can be formed by the first and/or second device, so that separate scattering element  5  or  6  can possibly be dispensed with. 
     Detector units  1  to  3  are preferably formed as photodiodes, which can be integrated on a common semiconductor substrate. This obtains an especially dense arrangement of detector units  1  to  3  side by side, so that any parallactic errors can be greatly reduced. Especially suitable and commercially available three-color sensors (e.g. types MCS3AT/BT or MCSi from the company MAZeT GmbH, D-07745 Jena) are constructed from three Si-PIN photodiodes integrated on a chip and executed as segments of a circle or hexagon with typical diameters between about 0.07 millimeters and 3 millimeters. To obtain low crosstalk between the photodiodes, the individual segments are separated from each other by additional structures. Each of the photodiodes is sensitized with a corresponding dielectric color filter to a different color range, in particular to the primary colors, red, green and blue. 
     Alternatively, detector units  1  to  3  can be disposed along a line or on one plane so as to form a one- or two-dimensional detector array, in particular a photodiode array (PDA). 
     Types of detectors other than photodiodes are also suitable for detecting light  16 , for example photomultipliers. 
     FIG. 3 shows an example of different spectral sensitivities E of detector units  1  to  3  used in FIGS. 1 and 2. Sensitivities E are plotted over wavelength λ. As indicated by the diagram, spectral sensitivities E 1 , E 2  and E 3  of the individual detector units are in substantially separate spectral regions. Depending on the type of analysis of the spectral characteristic of light emanating from a document, the spectral position and spectral course of individual sensitivities E 1  to E 3  can be accordingly selected. Spectral sensitivities E 1 , E 2  and E 3  are preferably in the blue, green and red spectral regions, respectively. Depending on the case of application, individual sensitivities E 1  to E 3  can also be in invisible spectral regions, such as the infrared or ultraviolet. Sensitivity curves EB to E 3  of individual detector units  1  to  3  can of course overlap at least partly, and output signals S 1  to S 3  of the detector units be used to determine color values of the document to be examined. 
     In a further embodiment of the invention, sensitivity curves E 1  to E 3  of individual detector units  1  to  3  overlap over a wide spectral region, in particular over the total spectral region examined, the maxima or mean values of particular sensitivities E 1  to E 3  being in different wavelengths or wave ranges. This can be realized in a simple realized in a simple way e.g. if detector units  1  to  3  have three photodiodes with preferably the same sensitivity curve and sensitive over the total spectral region examined, at least two of the photodiodes being provided with optical filters of different permeability in a wide spectral region. The individual photodiodes thus detect the intensity of light  16  emanating from bank note  10  at different wavelengths or wave ranges. From the detected intensities, statements can then be made about the spectral properties of detected light  16 . The spectral transmission curves of the filters are preferably selected such that in particular their ratio is a unique function of the wavelength in the relevant, i.e. examined, spectral region. 
     The spectral properties of detected visible or invisible light  16  refer in connection with the invention not only to its color but in particular also to the wavelength, such as the central wavelength, and/or the wave range. 
     FIG. 4 shows a preferred circuit of detector units  1  to  3  used in FIGS. 1 and 2, in particular when using one of the above-described commercial three-color sensors. Detector units  1  to  3  formed as photodiodes are switched here so that their cathode outputs are on common potential  18  and their anode outputs  19  are connected with evaluation device  9 . In evaluation device  9  statements about the spectral properties, in particular the wavelength, such as the central wavelength, and/or the wave range and/or the color, of detected light  16  can then be derived from detector signals S 1  to S 3  of the photodiodes.