Patent Publication Number: US-2011061988-A1

Title: Improvements relating to banknote validation

Description:
FIELD OF THE INVENTION 
     The present invention concerns improvements relating to banknote validation and in particular, though not exclusively, to an improved method of and apparatus for sensing optical characteristics of a banknote in order to determine its authenticity. It is to be appreciated that the term ‘banknote’ as used herein is to be considered refer to any manufactured item provided on a special paper-like substrate and having a value, such as a ticket, voucher or currency note, the substrate having fluorescence characteristics. 
     BACKGROUND OF THE INVENTION 
     Automatic recognition (validation) of banknotes is well known, (see for example European patent application EP 0 738 408 to Mars Inc.) and many of the techniques used to discriminate real and false banknotes are well documented. Historically, such automatic banknote validators require a set of several different types of sensors (and associated radiation sources) that each measure a different physical parameter of the banknote, e.g. optical, magnetic, density characteristic etc, to discriminate between an extensive range of real and false banknotes reliably. This is for example seen in international patent application WO 03/063098 (Eurosystems Limited). This produces a set of different results, which can be compared to that expected for a valid banknote to determine whether the banknote is considered to be valid in respect of the current testing location. 
     Typically, each banknote needs to be tested at several locations in order to be verified as an authentic banknote. Accordingly, in many cases the banknote is moved with respect to the sensors such that different locations on the surface of the banknote can be tested by the set of different sensors. Quite often two or more sets of different sensors, displaced with respect to the direction of the banknote path through the validator and each other, are provided. This ensures that different locations across the width of the banknote are also tested to avoid common fraud. A valid banknote is one, which passes these tests at all sampled locations. The results from all of the sets of different sensors represent a massive amount of data. 
     A further issue with the previous sensor arrangements including sources, radiation guides, sensors and signal processing means is the relatively high cost. Not only does providing these pluralities of different sensors increase the cost of the banknote validator, but also the processing and storage of the vast amounts of generated and stored comparison data also requires more processing power and memory in the banknote validator. 
     It is also known to irradiate banknotes with ultraviolet light and to measure a specific fluoresced wavelength of light, commonly known as a ‘blue’ fluorescence response. It is known that this fluorescence occurs in the majority of counterfeit currency; genuine banknotes don&#39;t fluoresce in this way. This blue fluorescence comes from dyes (such as OBA—Optical Brightener Agent) in plain paper designed to make it appear whiter by fluorescing blue, these dyes are not used in banknotes. The same dyes are used in washing powder to make clothes look whiter, which is why if a genuine banknote is accidentally washed, it will glow blue under UV and probably be mistaken as a fake by shopkeepers with UV light testers. 
     Typically, such banknote sensing arrangements, see for example European patent application EP 0 738 408 to Mars Inc., incorporate a narrow band-pass filter designed around this ‘blue’ fluorescence wavelength. All other visible wavelengths of light are filtered out to prevent potential stray light problems for example, and to maximise the signal to noise ratio (S/N) of the ‘blue’ fluorescence response. A further sensor is also provided to sense the reflected non-visible ultraviolet light too and this uses another band pass filter that filters out fluoresced light. The use of such specific wavelength narrow band-pass filters is considered necessary but is relatively expensive. Alternatively, a filter does not have to be used at all and sensors having a narrow band-pass sensitivity characteristic could be used also having a peak response based around the desired sensed wavelength. Such narrow band sensors are even more expensive than the narrow band-pass filters mentioned above. 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention seeks to overcome or reduce at least some of the above-described problems with the prior art. More particularly, the present invention desires to maintain and improve on the existing methods and validators&#39; reliability of discriminating between real and false banknotes, whilst at the same time seeking to reduce costs of such discriminating systems. 
     The present invention resides in the application of a discovery made whilst experimenting on optical characteristics of banknotes. The present inventor determined that by stimulating banknotes with non-visible wavelengths of light, looking at either reflected or transmitted fluoresced characteristics of the banknote, unique banknote characterising interactions may be observed by sensing visible wavelengths of received light whilst stimulating the banknote with non-visible wavelengths of light. 
     More specifically, it has been found that many real banknotes have a very different behaviour than false banknotes when examined in this way. In particular, when a banknote is stimulated with non-visible ultraviolet wavelengths of light, the wavelengths of light emitted (reflected or transmitted through the banknote) at visible light wavelengths are very different for authentic and false banknotes. This florescence effect is also achievable by using non-visible infrared wavelengths of light, which show very different results in certain authentic and counterfeit substrates for lower wavelength, visible fluoresced light transmitted or reflected from the substrate. 
     This phenomenon is new and differs from prior art tests in that the test is not based on merely a response being obtained at a specific frequency when a false banknote is irradiated with a given wavelength of light, such as the above-described effect of ‘blue’ florescence. 
     The present inventor has determined that by examining the relatively wide spectrum of emitted visible light, when a genuine banknote is stimulated with non-visible wavelengths of light, including ultraviolet or infrared wavelengths of light, other less apparent visible wavelength emissions occur that are useful in discriminating real banknotes from false ones. These visible wavelength emissions contain more useful and subtle information than the above-mentioned known ‘blue’ wavelength florescence for counterfeit banknotes. 
     According to one aspect of the present invention there is provided a banknote validator arranged to discriminate between real and false banknotes, the validator comprising: a light source arranged to emit light in the non-visible spectrum onto a banknote being validated, the emitted light including an ultraviolet (10 nm to 400 nm) or infrared (800 nm-100000 nm) wavelength of light; a broadband light sensor arranged to sense a relatively broad band of visible light wavelengths of light emitted by fluorescence from the banknote in response to the banknote being irradiated with the non-visible wavelengths of light; and an optical filter positioned between the banknote and the sensor and arranged to prevent illumination of the sensor with reflected or transmitted non-fluoresced light from the light source, the optical filter having a selected −3 db cut-off point which at least filters the non-visible light emitted from the light source, having a wavelength closest to the visible light spectrum wavelengths of light. 
     The whole point of the present invention is to apply this discovery at a low cost. Whilst it would be possible to use a spectrophotometer, this is clearly too expensive as it is several orders of magnitude greater in price that the banknote validator itself. Rather, the present invention is embodied in a simple, light source, sensor and optical filter arrangement, which minimises cost. It is to be appreciated that a high-pass or a low-pass filter, namely a filter having a simple step function, is typically far cheaper than a band-pass filter. Also the present invention can be implemented with a single sensor if needs be rather than two sensors as is seen in EP 0 738 408. Furthermore, the computational overhead of validating a genuine banknote is significantly reduced as compared with the prior art devices, because a single integrated reading for each different part of the banknote to be sensed is obtained rather than multiple different readings at each banknote sensing location. Furthermore, and very importantly, the use of a broadband sensor is key to keeping the cost of the present invention low. This is because a single broadband sensor is cheaper than a narrow band sensor or multiple sensors. 
     As an example, if the highest relative wavelength of light being emitted from an ultraviolet light source was 300 nm, the high-pass filter (in terms of wavelength) would have to have a −3 db cut-off point of 300 nm or greater. Preferably, the cut-off point would be at least 50 nm higher (in terms of wavelength which of course, is lower in terms of frequency) than the highest relative wavelength emitted from the light source. This is considered to be an ideal offset in wavelength, which ensures that no directly reflected or transmitted light gets to the sensor. 
     The present invention accords with the above-mentioned desire and specifically provides a method and validator that can discriminate a very large variety of different types of false banknotes from real banknotes at a very low relative cost with a minimum number of sensors and sensor arrangements. Sensors that have a broadband sensing characteristic are a far lower cost than narrow-band sensing characteristic sensors, which helps to reduce costs. Also by simplifying the sensor arrangement, the reliability of the validator increases and the amount of data generated decreases as does, very importantly, its costs. 
     The present invention can also be considered to be a banknote validation method of discriminating between real and false banknotes, the method comprising: irradiating light in the non-visible spectrum onto a banknote being validated, the emitted light including an ultraviolet (10 nm to 400 nm) or infrared (800 nm-100000 nm) wavelength of light; optically filtering the light received from the banknote to obtain fluoresced light from the banknote, the optical filtering having a selected −3 db cut-off point which at least filters the non-visible light emitted from the light source, having a wavelength closest to the visible light spectrum wavelengths of light; and measuring the filtered emitted light over a relatively wide range of visible light wavelengths in response to the banknote being irradiated with the non-visible wavelengths of light in use. 
     According to another aspect of the present invention there is provided a banknote validator arranged to discriminate between real and false banknotes, the banknote validator comprising: a light source arranged to emit light in the non-visible spectrum onto a banknote being validated, the emitted light including an ultraviolet (10 nm to 400 nm) or infrared (800 nm-100000 nm) wavelength of light; two broadband light sensors, one positioned in use with respect to the portion of the banknote to measure the filtered reflected light and the other sensor positioned in use with respect to the portion of the banknote to measure the filtered transmitted light, each sensor being arranged to be sensitive over a relatively broad band of visible wavelengths of light emitted by fluorescence from the banknote in response to the banknote being irradiated with the non-visible wavelengths of light and to generate an output signal representative of the measured light; two high-pass filters, each high-pass filter being arranged to have a −3 db point at least closer to the visible light spectrum than the closest wavelength of light radiated from the irradiating means, one of the filters being positioned to filter the light reflected from the portion of the banknote and the other filter being positioned to filter the light transmitted through the portion of the banknote; a comparator for comparing the value of the output signal of each sensor with predetermined values that represent a valid banknote; and a determining means for determining the validity of the portion of the banknote based on the results of the comparator. 
     It is possible in different embodiments of the present invention to measure the light reflected from the banknote, transmitted through the banknote or both the reflected and the transmitted light. 
     In one embodiment of the present invention, the banknote can be first excited (irradiated with non-visible light) at low cost by using an ultraviolet emitting LED (light-emitting diode). However, by using a broadband optical receiver with in conjunction with a high-pass wavelength (low-pass frequency) optical filter, higher wavelength emissions can be detected. This can be created using a general-purpose low-cost photodiode and a low-cost conventional plastics filter. The photodiode has a broad response but the high-pass filter cuts off at 500 nm or at a point at least greater than the highest wavelength emitted from the excitation light source. The filter separates out the light that is reflected back from the banknote that does not cause any excitation from the other low-level florescence wavelengths. 
     The use of a single sensor also makes the task of making the comparison between a valid and a false banknote easier, because the single sensor integrates the emitted responses over a wide range of wavelengths which in turn results in a single voltage output signal being produced representing a value for that banknote location. Once several such values have been obtained they can be compared against reference values representative of a valid banknote. In the actual validation of a banknote, a whole banknote would need to be read by taking a plurality of readings at different positions with the single sensor and comparing all the results with reference voltages. Using the present embodiment, for each point on the banknote, a single integrated multiple wavelength reading is taken rather than multiple readings with different optical wavelengths. A single value for a banknote location can be used for comparison instead of the prior art method of carrying out more complex analysis on multiple results for each banknote location. 
     The main benefit of this system is that it provides much better recognition of real banknotes compared to false banknotes than the previously known visible ‘blue’ emission based systems. Furthermore, the present invention is also significantly lower in cost to manufacture and is simpler than other systems that necessarily require more than one sensor for measuring each of the reflected and transmitted light interactions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings in which: 
         FIG. 1  is a perspective view of a two-part banknote housing of a validator incorporating a light sensing arrangement embodying the present invention; 
         FIG. 2  is an exploded perspective view of the upper and lower parts of the banknote validator housing shown in  FIG. 1 ; 
         FIG. 3   a  is an exploded perspective view of the lower part of the banknote validator housing of  FIG. 1  showing a lower light sensing arrangement; 
         FIG. 3   b  is an exploded perspective view of the upper part of the banknote validator housing of  FIG. 1  showing an upper light sensing arrangement; 
         FIG. 4  is a sectional view through the banknote validator housing of  FIG. 1  along the length of the validator in the direction of a banknote part; 
         FIG. 5  is a schematic circuit diagram showing how each sensing arrangement is positioned with respect to the banknote and the signal processing elements of the validator; 
         FIG. 6  is a graph of the emission characteristics of the light source used in the upper and lower sensing arrangements; 
         FIG. 7  is a graph showing the reception characteristics of the light sensor used in the upper and lower sensing arrangements; 
         FIG. 8  is a flow chart showing the operation of the sensing arrangement in determining the validity of a banknote; and 
         FIG. 9  is a graph of the emission characteristics of an Infrared light source used in a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Referring to  FIG. 1  there is shown a banknote validator housing  10  incorporating a sensing arrangement according to the first embodiment of the present invention. The banknote validator housing  10  comprises an upper part  12  and a lower part  14 , which are arranged to releasably interlock to form the validator housing  10 . When locked together, the upper and lower parts  12 ,  14  define a banknote validation pathway that commences at a note entrance  16 . For the purposes of this invention, the banknote housing  10  represents the banknote validator. Even though all of the validator&#39;s functioning parts are not shown, the essential ones to this invention are. 
       FIG. 2  shows the upper and lower parts  12 ,  14  of the banknote validator housing  10  in separated positions. Here the banknote validation pathway  18  can be seen on both parts  12 ,  14  starting at the note entrance  16 . 
     Whilst it is possible to simply have a single sensing arrangement provided on one side of the banknote path, in this embodiment each of the lower and upper parts comprises its own dual sensing arrangements as are described in further detail below. 
     Referring now to  FIG. 3   a  an exploded view of the lower part  14  of the banknote validator housing  10  is shown. The lower part  14  comprises a robust plastics lower case  20  and an interlocking lower lens  22  made from a transparent plastics material. Also provided is a first sensing arrangement  24  on a printed circuit board  26 . The first sensing arrangement  24  comprises a UV light emitting diode (LED)  28 , a photodiode  30 , a first opaque lens holder  32  a plastics high (wavelength) pass filter  34  and a second opaque lens holder  36 . The elements of the first sensing arrangement  24  are designed to fit together to form a compact unit, which in use, shines light onto a banknote and also senses fluoresced light, through the transparent lower lens  22 . 
     The filter  34  is shaped so that it only covers the photodiode  30  and not the UV light emitting diode  28 . The filter  34  can be made of any plastics material, as it is cheap and easy to manipulate into a desired shape. The actual type of plastics is unimportant so long as it exhibits the correct desired cut-off wavelength. In this embodiment, the plastic sheet filter is made from polyester. 
     In this embodiment, a second sensing arrangement  24   a  is provided adjacent the above-described first sensing arrangement  24 . The second sensing arrangement provides a second testing location across the width of the banknote being sensed which can be useful for protection against some types of banknote counterfeiting. 
     The second sensing arrangement  24   a  comprises a second UV light emitting diode  28   a  and a second broadband visible light sensor  30   a , which are identical to those of the first sensing arrangement  24 . The second sensing arrangement  24   a  shares the first opaque lens holder  32 , the plastics high-pass (wavelength) filter  34  and the second opaque lens holder  36  of the first sensing arrangement, these elements being physically wide enough to cover both light sources  28 ,  28   a  and sensors  30 ,  30   a  of the two arrangements  24 ,  24   a . However, the filter  34  only covers the sensors  30 ,  30   a  and does not cover the light sources  28  and  28   a.    
       FIG. 3   b  shows an exploded view of the upper part  12  of the banknote validator. The upper part houses a sensing arrangement  34 , which is identical to the sensing arrangement  24  of the above-described lower part  14  of the banknote validator housing  10 . The upper part  12  comprises a transparent plastics material upper lens  38  and an interlocking upper body  40 . Together, the lens  38  and the body  40  house the third and fourth sensing arrangements  42 ,  42   a . The third sensing arrangement  42  is provided on a printed circuit board  44  and comprises a UV light emitting diode  46 , a photodiode  48 , a first opaque lens holder  50 , a plastics high (wavelength) pass filter  52  and a second opaque lens holder  54 . It is to be appreciated that the UV (ultraviolet) light diode emitting  46  and the photodiode  48  are shown very close together in  FIG. 3   b  and they are not a readily distinguishable as compared to the photodiode  30  and UV LED  28  shown in  FIG. 3   a . The elements of the third sensing arrangement  42  are designed to fit together to form a compact unit, which in use, shines light onto the banknote and also senses fluoresced light, through the transparent upper lens  38 . 
     The fourth sensing arrangement  42   a  is provided adjacent the above described third sensing arrangement  42  and is also located on the printed circuit board  44 . The fourth sensing arrangement  42   a  provides a second testing location across the width of the banknote being sensed which can be useful for protection against some types of banknote counterfeiting. The fourth sensing arrangement  42   a  shares the first opaque lens holder  50 , the plastics high-pass (wavelength) filter  52  and the second opaque lens holder  54  of the third sensing arrangement, these elements being physically wide enough to cover both light sources  46 ,  46   a  and sensors  48 ,  48   a  of the two arrangements  42 ,  42   a . However, the filter  52  only covers the sensors  48 ,  48   a  and does not cover the light sources  46  and  46   a.    
     It is to be appreciated that the first and fourth sensing arrangements  24 ,  42   a  are positioned to face each other so that light generated in one sensing arrangement can be sensed as florescence transmitted through the banknote in the other sensing arrangement. Clearly, reflected fluoresced light can be detected by the sensor in the same sensing arrangement that generates the non-visible light. In this way, reflected/transmitted sensed light-readings can be taken in either direction from either one of the upper or lower parts  12 ,  14  of the validator housing  10 . The same is true of the second and third sensing arrangements  24   a ,  42  that are also aligned together. 
       FIG. 4  is a cross-section through the validator housing  10  with its upper and lower parts connected. This figure shows how the first and second sensor arrangements  24 ,  24   a  are aligned with the third and fourth sensor arrangements  42 ,  42   a  to enable reflected and transmitted fluorescence to be detected. 
     An electronic circuit which processes the output of the sensor arrangements  24 ,  24   a ,  42 ,  42   a  is now described with reference to  FIG. 5 . For the sake of brevity, only the processing in relation to the first and third aligned sensor arrangements  24 ,  42 , is described herein. However, the following also applies to the second and fourth aligned sensor arrangements  24   a ,  42   a , which are simply incorporated as extra inputs into the circuit described below. 
     The electronic circuit  60  whilst not shown in  FIGS. 3   a  and  3   b , is provided on the printed circuit boards  26 ,  44 . The LED  28 , filters  34 ,  52  and sensors  30 ,  48  are all directed to a banknote substrate  62  which is being analysed. Sensor  30  detects reflected fluoresced visible light whereas sensor  48  detects transmitted fluoresced visible light. The outputs of the sensors  30 ,  48  are sent to a microprocessor  64  via respective buffers  66 ,  68 , which act to stabilise the analogue signal being generated by the sensors. The microprocessor controller (microcontroller)  64  is arranged to receive and process signals from the sensors  30 ,  48 , to compare those signals with stored data and to determine whether the banknote  62  is acceptable. The controller  64  also controls the activation of the LED  28  when the banknote  62  is in the correct position for sensing. 
     In order to perform the above, the microcontroller  64  is programmed to carry out particular functions. The first function is an analogue to digital (A/D) conversion function  70 , which is used on incoming buffered signals from the sensors to generate a stream of digital values representing the analogue signal. The A/D function conversion results are stored in local memory  74 : microprocessor RAM in this embodiment. A comparator function  72  is also provided in the microcontroller  64  for comparing the digitised sensor signals with a set of comparison values taken from the stored memory  74 . This can conveniently be in the form of an algorithm that determines if the banknote is real or not and also what the value of the banknote  62  being sensed is. The algorithm is not described in the present application because those skilled in the art will be aware of many different algorithms to perform this function. However, the operation of the microcontroller  64  in controlling the LED  28  and the A/D function  70  to take samples from the banknote  62  is briefly described below and later with reference to  FIG. 8 . 
     Readings are taken at regular intervals and stored by the microprocessor  64 . The number of readings depends on the length of the banknote, however, in this embodiment around 30 to 60 readings are taken, but this is not critical. It would be possible to make the present embodiment work with a continuous stream of data or pretty much any number of samples. This series of samples is compared to those expected from a valid note to determine firstly if it is a fake and secondly what value of note it is, as each valid note will have a certain characteristic fluorescence profile along its length. More specifically, if the banknote is counterfeit, the fluorescence output profile along the whole banknote  62  is probably relatively constant due to the blue fluorescence phenomenon, which would occur at every location. However, if output profile has specific ‘pulses’ at known positions along the length of the banknote then the banknote is likely to be genuine. This is because there is fluorescence at different wavelengths in the visible spectrum for different parts of a valid banknote, which is detected to make up a unique profile for the valid banknote  62 . By using a broadband sensor, the generated signal integrates all of the sensed visible wavelengths of fluoresced light and so it is not known exactly what wavelength was being fluoresced. However, this is not important to the present embodiment as so long as both blue wavelength emissions (for counterfeit banknotes) and other visible wavelength emissions (for genuine banknotes) can be detected by the same sensor. The strength of the blue fluorescence for a counterfeit banknote is far greater than the more subtle fluorescence generated by a genuine banknote and this also helps to distinguish between false and genuine banknotes. 
     It is to be appreciated that readings cannot be taken from both sides of the banknote  62  at the same time with the same sensor  30 ,  30   a ,  48 ,  48   a . Rather, the microprocessor  64  has to control the circuitry such that only one LED  28 ,  28   a ,  46 ,  46   a  is on at a time so as not to confuse the sensed signals. Accordingly, the microprocessor  64  is arranged to quickly alternate illumination of each LED  28 ,  28   a ,  46 ,  46   a  and take the relevant readings from the sensors. 
     In this embodiment, the comparison function  72  is activated once all of the readings from the sensors have been taken in order to simplify the whole comparison procedure  72 . However, in alternative embodiments, processing on the stored readings can commence before all of the readings have been stored, which is more complex a process but a quicker one. 
     In order to appreciate how the present embodiment implements the present invention is it important to consider the characteristics of the LEDs  28 ,  28   a ,  46 ,  46   a  and the sensors  30 ,  30   a ,  48 ,  48   a .  FIG. 6  shows the output characteristics 60 of a Kingbright KP-2012-UVC LED device which is used for all of the LEDs  28 ,  28   a ,  46 ,  46   a . The irradiating non-visible light is selected to have a wavelength between 100 nm and 400 nm in this embodiment, as this is the UV band. It is important that there is no light emitted at a longer wavelength (higher wavelength) than the cut-off wavelength of the filter; otherwise this would get through to the sensor. This effectively cuts out any light that does not cause the banknote to fluoresce. Accordingly, as the wavelength of the LEDs peaks at 400 nm and has a relative intensity of zero at 450 nm, a filter material having a −3 dB cut-off point characteristic at 450 nm is selected for use. This is considered to be the closest frequency to the visible spectrum at which non-visible light could possibly be irradiated from the LED. In another embodiment a filter material having a −3 dB cut-off point characteristic at 500 nm can be selected for use as it is 50 nm closer to the visible light spectrum than the highest possible wavelength emitted from the LED. 
     It is to be appreciated that longer wavelength UV LEDs are the least expensive devices so UV LEDs having a peak output in the range 380 nm-400 nm (right at the top of the band) are used. This is purely for economy, using shorter wavelengths may reveal more security features in the future, but these emitters are not cost effective at the moment. 
     Referring now to  FIG. 7 , the relative spectral sensitivity characteristics 90 of the OSRAM SFH 2400 photodiode is shown. The photodiodes  30 ,  30   a ,  48 ,  48   a  of the present embodiment comprise this type of fairly common ordinary broadband photodiode. This type of diode is sensitive from 400 nm to 1100 nm in wavelength, which covers the whole of the visible spectrum (400 nm to 800 nm). It is only important that some of the light fluoresced from the banknote (not necessarily all of it) is sensed. It would be preferred that the sensor is sensitive to all visible light, but this is not essential. Accordingly, the broadband sensor can have a smaller range, which is big enough to pick up the ‘blue’ fluorescence of counterfeit banknotes as well as the fluorescence of genuine banknotes that occurs at other wavelengths in the visible spectrum. 
     Wide bandwidth devices are inexpensive, and a large bandwidth does not cause a problem in the present embodiment because a high-pass (wavelength) filter is used to limit the bandwidth. It is important to remember that the filters are selected and positioned such that none of the light from the LEDs  28 ,  28   a ,  46 ,  46   a  get to the sensors  30 ,  30   a ,  48 ,  48   a.    
     A general method  100  of operation of the sensing arrangement and circuit of  FIG. 5  is now described with reference to  FIG. 8 . The method starts at Step  102  with initialisation, of the banknote validator, namely loading of programs into the microprocessor memory for effecting the correct banknote validation. In this regard, different world currencies may have very different sensed characteristics and so the appropriate set of values has to be loaded into the validator. 
     The method  100  continues with the sensing of the insertion of a new banknote  62  at Step  104 . If no new banknote  62  has been inserted, the validator goes into a wait and retry-loop  106 . Otherwise, the first UV LED  28 ,  28   a  is illuminated at Step  108  to cause fluorescence in the banknote  62 . The fluoresced visible light is filtered and sensed to generate an analogue signal, which is then digitised at Step  110  at the microcontroller  64 . The digitised fluorescence value is then stored at Step  112  in memory  74 . Subsequently, a second UV LED  46 ,  46   a  is energised at Step  114  to generate fluoresced visible light from the banknote  62  being validated. An analogue signal generated by filtered and sensed light is then digitised at Step  116  by the A/D converter function  70  at the microprocessor  64  and stored at Step  118  as a value for that banknote location. 
     Having completed the testing of the current location, the method determines whether there are any more banknote locations to be tested at Step  120  and if there are, the microprocessor instructs the banknote validator to advance the banknote at Step  122  to the next sensing position (this is done by means of controlling the drive mechanism (not shown) which is an inherent part of the banknote validator). Then the steps  108  to  120  are repeated for the new position on the banknote surface. 
     Alternatively, if there are no more banknote locations to be sampled, then the data processing stage can commence. The data processing stage starts with retrieval of the at Step  124  stream of stored digitised values from the local memory  74 . The comparator function  72  of the microprocessor  64  then compares at Step  126  the measured values against prestored values from the memory  74 . These prestored values represent fluoresced visible light profiles along the length of valid banknotes. If the comparison results in each of the sensed values being constant and higher than the profiles as determined at Step  128 , then the banknote is likely to be counterfeit and it is rejected by being ejected from the validator. Alternatively, if the sensed values are not constant and all higher than the profile values, then a check is made at Step  132  to determine if any match the prestored profile values. If the stream of sensed values matches a profile (within tolerance limits) then the banknote is considered to be valid. The banknote is accepted at Step  134  and the matching profile determines the value of the banknote. 
     However, if the stream of sensed values do not match a stored profile, then the banknote is still valid, but is not recognised as being of a particular amount. In this case the banknote can either be accepted into the validator or rejected because the amount of the banknote, its value, cannot be ascertained. 
     Having described a first embodiment of the present invention, a second embodiment of the present invention is now described. The second embodiment is very similar to the first embodiment and accordingly, only the differences will be described hereinafter. 
     The main difference is that the LED light source is a near infrared light emitter rather than an ultraviolet light emitter. This change in irradiation wavelength employs the present invention at the other end of the visible spectrum to determine the validity of the banknote. Again this is based on the fact that the present inventor has determined that using infrared irradiation, real and false banknotes can have different characteristics of fluoresced light in the visible light spectrum. At present, the paper substrate used for known banknotes is not designed to have any special characteristics when illuminated under near infrared light. However, it is readily possible for banknote manufacturers to design or select a paper substrate for the banknote to have such a fluorescence characteristic and accordingly, the second embodiment could be used to distinguish such genuine banknotes from forgeries. The current second embodiment has been tested against certain paper substrates to prove the occurrence of the effect. 
     In the present embodiment, the near infrared LED has an output characteristic  140  shown in  FIG. 9 . As can be seen, the vast majority of energy of the emitted infrared radiation (non-visible light) is within the range 825 nm to 930 nm. 
     The banknote validator also comprises a low-wavelength pass (high-frequency pass) optical filter as opposed to a high-wavelength pass (low-frequency pass) optical filter. The filter has a −3 dB cut off point at 825 nm. This is considered to be the closest frequency to the visible spectrum at which non-visible light could possibly be irradiated from the LED. 
     In another embodiment a filter material having a −3 dB cut-off point characteristic at 775 nm can be selected for use as it is 50 nm closer to the visible light spectrum than the lowest possible wavelength emitted from the infrared LED. 
     It is to be appreciated that the present embodiments can be varied in many ways whilst still implementing the present invention. For example, even though the present embodiments describe measuring the banknote in a plurality of locations along its length and width and then comparing these readings to stored profiles, this amount of comparison is not necessary. Rather, the present invention can work with a single sensor arrangement and also with comparison at a single location if required. The principle works even at this crude level. More sensors and locations are provided to make the validator more robust against different types of fraud, but these are not essential. 
     Even though cost is a major issue in the present invention, having reduced the cost and complexity of verifying at a single location the validity of a banknote, the number of sensors can be increased for other applications. For example, if multiple receivers were provided that were physically separate, then these could be sampled all at the same time to provide quicker results albeit at a cost. 
     Having described a preferred embodiment of the present invention, it is to be appreciated that the embodiment in question is exemplary only, and that variations and modifications, such as those which will occur to those possessed of the appropriate knowledge and skills, may be made without departure from the spirit and scope of the invention as set forth in the appended claims. For example, it is also possible in an alternative embodiment, for the microcontroller functions of digitising the analogue sensor signals and comparing them to values stored within the RAM of the computer to be replaced by discrete components performing these functions. For example, discrete A/D converters could be used together with a comparator and a separate memory store. However, these discrete components would likely increase the cost of the circuitry, which is undesirable, as well as reduce its reliability and robustness. 
     Also in theory it would be possible to build a totally analogue validator (without A/D converters being required. However, this would be very expensive and not very flexible as it would not be able to be programmed differently.