Abstract:
An apparatus for determining the fitness of a bank note by sensing the bank note transported along a transport path by a transport device. The apparatus includes a plurality of identical sensor and illumination units positioned along each side of the transport path whereby the sensor and illumination units are focused at a single predetermined section of the transport path.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an apparatus and method for checking bank notes for their state of use, in particular for dirt and stains which can impair the service life of the bank notes. 
     For checking bank notes for dirt and stains, one usually illuminates the bank notes by means of at least one light source and evaluates the diffusely reflected light by means of suitable optical sensors. 
     However the problem arises that particularly places on the bank note which contain security features like watermarks are evaluated as stains or soiled places. 
     Further problems result from the fact that when a bank note is judged for its degree of soiling great effort is necessary for evaluating the signals of the sensors used since soiling or stains are felt to be especially disturbing in certain areas of the bank note, e.g. in the area of a portrait. The evaluation effort must be adapted for the whole bank note according to the critical areas and is thus especially great. 
     The problem of the present invention is therefore to state an apparatus and method for checking bank notes for their state of use, in particular dirt and stains which can impair the service value of the bank notes, which allow accurate judgment of the bank notes. The effort for judging the degree of soiling of the bank notes should advantageously be reduced. 
     SUMMARY OF THE INVENTION 
     The invention starts out from the consideration that two identical illumination and sensor units are disposed on both sides of a transport path for bank notes to be checked. The illumination and sensor units of both sides are disposed in such a way as to be aligned with a given place. Illumination at the same time and of the same kind from both sides avoids misjudgments in areas, e.g. in the area of the watermark. Additionally it is possible to judge the front and back of bank notes to be checked simultaneously. 
     One advantageously reduces the effort for judging the degree of soiling of bank notes by defining areas for the bank notes to be investigated where judgment is performed with greater effort, e.g. higher resolution, than in other areas. It has proved to be especially advantageous to fix the areas of higher resolution in accordance with the currency and/or denomination for the bank notes to be investigated. 
     Further advantages of the present invention can be found in the dependent claims and the following description of embodiments with reference to figures. The figures show only the components relevant for understanding the present invention. Similar components of the figures have the same reference signs. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a section parallel to the transport direction of bank notes to be checked through an apparatus for checking bank notes for their state of use, 
     FIG. 2 shows an example for an evaluation of sensors of the apparatus for checking bank notes, and 
     FIG. 3 shows an example for the division of a bank note into areas critical and uncritical for soiling. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a section parallel to the transport direction of bank notes to be checked through an apparatus for checking bank notes for their state of use, in particular dirt and stains which can impair the service value of the bank notes. 
     The apparatus has illumination and sensor units of the same kind on both sides of a transport path for bank notes to be checked. The illumination and sensor units of both sides are disposed in such a way as to aligned with or focused on a given place. Illumination units  10 ,  11  and  20 ,  21  with two different wavelengths or wave ranges are provided on both sides. The illumination units can be formed by light-emitting diodes (LEDs). For example, LEDs  10  and  20  can emit white light, whereas LEDs  11  and  21  emit infrared light. In order to obtain uniform illumination, two or more LEDs can be disposed linearly for each of illumination units  10 ,  11  and  20 ,  21 . The white light can be used for example for recognizing stains, whereas the infrared light can be used for judging uniform soiling extending over the total bank note. LEDs  10 ,  20  and  11 ,  21  are operated alternatingly, i.e. the LEDs with different wavelengths are operated alternately. Sensors  12  and  22  on both sides serve to control LEDs  10 ,  11 ,  20 ,  21  and compensate influences like aging and temperature fluctuations which can falsify judgment. For this purpose one uses the signals of sensors  12  and  22  which receive light emitted by LEDs  10 ,  11 ,  20 ,  21  and diffusely reflected by reference surfaces  16 ,  26 . The properties of reference surfaces  16 ,  26  usually correspond to the properties of bank note paper. 
     Light diffusely reflected by bank note BN is received by sensors  13 ,  23  which can be formed by semiconductor sensors. Sensors  13  and  23  are formed by a row of semiconductor sensors or by a sensor array, the longitudinal extension of sensors  13  and  23  being greater than the extension of bank note BN to be investigated perpendicular to the transport direction. The sensors have a resolution of e.g. 1.0×1.0 mm. To improve the optical imaging one can provide lenses  14 ,  24  which are positioned by means of holding devices  15 ,  25 . Especially suitable for lenses  14 ,  24  are lens arrays, i.e. linearly disposed gradient lenses which produce a one-to-one image of the bank notes to be investigated on sensors  13 ,  23 . Such linearly disposed gradient lenses are known under the name SELFOC®. 
     Bank note BN to be investigated is transported by means of a transport unit (not shown) in the transport direction shown by an arrow through the apparatus. For protecting the sensors one can provide covers  18 ,  28 , such as windows, which are permeable to the wave ranges used and prevent mechanical damage or dust collection on the sensors. The transport speed of the bank notes in the transport unit is selected so as to permit all-over scanning of the bank notes for the two alternating wave ranges in accordance with the resolution of sensors  13 ,  23 . 
     Illumination at the same time and of the same kind from both sides with the same brightness (intensity) avoids misjudgments in areas, e.g. in the area of the watermark. Additionally it is possible to judge the front and back of bank notes to be checked simultaneously. Additionally the illumination&#39;s dependence on distance is compensated or reduced by the compensation effect of the opposite sensor and illumination units. A further improvement of judgment is possible if the soiling of windows  18 ,  28  is determined at times when no bank note is located in the detection area of sensors  13 ,  23 , in order to optionally stop the apparatus if a specified threshold is exceeded and issue a request for cleaning windows  18 ,  28  on a display of the apparatus. For judging soiling one evaluates both the light of LEDs  10 ,  11 ,  20 ,  21  scattered on the dirt particles and that reflected thereby. 
     FIG. 2 shows an example for an evaluation of the sensors of the apparatus for checking bank notes and has control and evaluation unit  30 , for example a micro-processor or signal processor, with associated memory  31 . Microprocessor  30  evaluates signals from sensors  12 ,  22  and controls LEDs  10 ,  11 ,  20 ,  21 , as described above, for controlling the illumination. Sensors  12 ,  22  can likewise be semiconductor sensors. Microprocessor  30  also evaluates the signals of sensors  13  and  23  for determining the soiling of the front and back of the bank note to be judged. A value for soiling can be derived from the brightness of all pixels:          S   i     =         3        P   i           P     i   -   1       +     P   i     +     P     i   +   1           -   1                            
     where values P i  correspond to the brightness or intensity of pixel i. Value S i  must be determined for all pixels, a value for soiling then resulting from the standard deviation of all S i . For reducing the computing effort it is possible to perform a simple evaluation by which one determines only values of consecutive pixels i in the transport direction, i.e. only one-dimensionally:          S   T     =         ∑   i                 P   i     -     P     i   +   1                    ∑   i          P   i                                
     The mean value of all tracks in transport direction S T  is then used as the value for soiling. 
     FIG. 3 shows bank note BN having different areas  40 ,  41 ,  42 . Area  40  corresponds to total bank note BN, area  41  corresponds to a central area containing for example a portrait, and area  42  corresponds for example to a bank note number. Such areas are advantageous because stains are especially disturbing in areas  41  and  42  for example. In areas  41  and  42  the search for stains can be effected at high resolution, e.g. at the abovementioned maximum resolution of 1.0×1.0 mm. In area  40  the evaluation can be effected at a lower resolution, e.g. 2.0×2.0 mm. For this purpose one combines the signals of two adjacent pixels of sensors  13 ,  23  for example. One thus obtains a resolution of 2.0×1.0 mm. Since the pixels result in the transport direction through the motion of the bank note, as described above, one ob-direction through the motion of the bank note, as described above, one obtains the resolution of 2.0×2.0 mm since two temporally successive sensor signals are combined. 
     Areas  40 ,  41 ,  42  can be fixed singly and stored in memory  31  of microprocessor  30  for later evaluation. They can be fixed in currency- and/or denomination-specific fashion in order to take account of the peculiarities of the particular bank notes. As shown, the areas of different resolution can overlap, e.g. areas  41 ,  42  are in area  40 . For each of the areas one can also fix individual limiting values as of which a bank note is classified as no longer fit for circulation. This may be for example a certain number of pixels within areas  40 ,  41 ,  42  which are recognized as stained. 
     As further shown in FIG. 3, bank note BN can be transported both in longitudinal direction L and in transverse direction Q. It is obvious that larger sensor arrays  13 ,  23  and larger illumination units  10 ,  11 ,  20 ,  21 , i.e. a larger number of linearly disposed LEDs, are required upon transport in transverse direction Q than upon transport in longitudinal direction L. At equal transport speed, a higher computing power of microprocessor  30  is in addition necessary upon transverse transport in order to permit evaluation of the sensor signals obtained.