WO2010018353A1

WO2010018353A1 - Monitoring phosphorescence

Info

Publication number: WO2010018353A1
Application number: PCT/GB2009/000876
Authority: WO
Inventors: Matthew James Hayes
Original assignee: Talaris Limited
Priority date: 2008-08-14
Filing date: 2009-04-01
Publication date: 2010-02-18
Also published as: GB0814909D0

Abstract

A method of monitoring phosphorescence emitted by an article comprises exposing the article (2) to stimulating radiation (4), to generate a phosphorescent response from the article (2). The amplitude of the stimulating radiation oscillates between higher and lower magnitudes. The emitted phosphorescence is detected. The stimulating radiation is oscillated at a frequency with a period less than the decay time of the phosphorescence.

Description

MONITORING PHOSPHORESCENCE

The invention relates to a method of monitoring phosphorescence emitted by an article and apparatus for monitoring such phosphorescence. In order to prevent counterfeiting of articles, particularly documents of value such as banknotes, special inks that both fluoresce and phosphoresce are used. The reason is that attempts to forge luminescent features on banknotes are based typically on inks that only respond to stimulating radiation by fluorescing and not phosphorescing. The detection of phosphorescence can therefore indicate a genuine banknote or other article.

The principle on which the known method and apparatus operate is that fluorescence turns on and off simultaneously with exposure to the source of stimulating radiation, while phosphorescence has a significantly longer decay time which means that when not exposed to the stimulating radiation, the phosphorescent signal remains on and gradually decays. Thus, when a feature which generates both fluorescence and phosphorescence is activated, it will generate a signal with both fluorescent and phosphorescent components but when the activating radiation is not present, it will only emit phosphorescence. Suitable processing electronics can then be used to detect the phosphorescent signal (in the absence of the fluorescent signal) in order to determine authenticity of the article.

Most known apparatus is based on the use of UV stimulation using a UV fluorescent tube which is maintained on continuously. Banknotes are caused to move past the source so that they are initially exposed to the stimulating radiation and then no longer exposed, a sensor being placed downstream from the source to detect the emitted radiation from the banknote.

US-A-6024202 describes a detector in which a short wavelength UV (246nm) light source was modulated typically at 1.4 to 6kHz but at this frequency, the UV emission from the source was never practically extinguished. In the case of modulated sources, the frequency of switching is therefore slow and comparable to the decay time of the phosphor, and is limited by the time taken for the UV tubes to energise and de-energise. The use of tubes is problematic, not least because they are bulky and expensive, but also because their slow response time limits the information which can be retrieved from the banknotes to simply indicate the presence or absence of luminescent features. The power supply and control circuitry required is also expensive and complicated. A further disadvantage of the slow modulation technique is that the illumination must be focussed to a high intensity within the detection area (using expensive lenses) since otherwise the demodulation approach would become prohibitively complex. The slow modulation technique also imposes an artificial limitation on the speed with which an article can be transported past the source when imaging features of a given size, because it is important that the feature does not pass by the detector when the source is off.

In accordance with the first aspect of the present invention, a method of monitoring phosphorescence emitted by an article comprises exposing the article to stimulating radiation, to generate a phosphorescent response from the article, the amplitude of the stimulating radiation oscillating between higher and lower magnitudes; and detecting the emitted phosphorescence characterized in that the stimulating radiation is oscillated at a frequency with a period less than the decay time of the phosphorescence.

In accordance with a second aspect of the present invention, apparatus for monitoring phosphorescence emitted from an article comprises a source of stimulating radiation for generating a phosphorescence response from the article, the stimulating radiation regularly oscillating between higher and lower amplitudes; a detector for detecting the phosphorescent response and generating a suitable output signal; and a processing system for determining the phosphorescence from the output signal from the detector characterized in that the source is adapted to cause the stimulating radiation to oscillate at a frequency with a period less than the decay time of the phosphorescence.

We have realised that it is possible to monitor phosphorescence, particularly emitted together with fluorescence, even when the stimulating radiation is oscillated at a frequency with a period less than the decay time of the phosphorescence. Because the source flashes so quickly by comparison to the time in which the phosphor can respond, the component of the received signal due to the phosphorescence behaves as if the source were always on; this creates a slow offset (baseline) for the received signal. Although the phosphor responds as if the source were always on, the faster-responding fluorescence does respond almost immediately to the fast modulation, and so the received signal also has a fast component that changes between two time periods; when the source is on the signal is due to both fluorescence and phosphorescence, but when the source is off the signal returns to the baseline and is therefore only due to the phosphorescence. It is therefore possible to isolate the phosphorescence signal very simply, for example by averaging the received signal during the periods when the source is off. The new invention also enables LED sources to be used because of the inherent switching speed and stability of LEDs and the ability precisely to control the "on" time. LEDs have not been used in the past because their output power is limited. However, the advantages of using high switching speeds and precise "on" time control provided by the LEDs to implement the inventive method outweighs this disadvantage.

As an alternative to LEDs, a UV tube or the like could be used, the tube being maintained on at all times with a shutter being provided sequentially to pass or not pass the stimulating UV radiation.

Although a single LED could be used, preferably two or more LEDs are used. The LEDs effectively "pre-charge" the luminescent feature on the article in a similar way to a continuous light source and particularly where the article is transported past the source, the motion of the article is used to enable multiple LEDs to act as if they were one bright LED. Even though there may be a small amount of decay in the phosphorescent signal, this avoids the difficult and inefficient focussing of LED sources to create a high-intensity spot.

The fast modulation approach also supports resolution of small features at higher transport speeds.

An additional advantage of the arrangement is that it reduces the dynamic range requirement of the optical detector (the ratio of the maximum signal to the minimum signal) because the large fluorescence signal only depends upon the intensity within the detection area. The detector can be tuned to look purely for the signal given off by the phosphor (when the source is switched off) rather than trying to deal with the much brighter signal given off by the fluorescence as well.

Typically, the lower magnitude of the amplitude is zero although in some cases it could be non-zero providing fluorescence is not stimulated at that level. Typically, the stimulating radiation will have a square wave amplitude variation although other forms such as sinusoidal are also possible.

Typically, the phosphorescence and luminescence signals will be at the same wavelength.

The stimulating radiation typically comprises UV light although other stimulating wavelengths could be used depending upon the nature of the feature to be stimulated. Examples include a wavelength centred on 365nm or on 254nm.

In the preferred approach, the step of detecting the phosphorescent response comprises detecting radiation emitted from the article; and averaging the radiation detected when the stimulating radiation is at its lower or zero amplitude. This method operates on the basis that when the stimulating radiation is at its lower or zero amplitude, no fluorescence will be issued and detected, meaning that only the phosphorescent response is detected.

It is, however, also possible to detect fluorescence. Thus, where a feature on the article generates fluorescence only, the method further comprises detecting fluorescence emitted from the article by detecting radiation emitted from the article; and averaging the radiation detected when the stimulated radiation is at its higher amplitude.

In another alternative to detect fluorescence, where both fluorescence and phosphorescence are emitted from a feature on the article, the method comprises detecting radiation emitted from the article; and averaging the sum of i) the detected radiation when the stimulated radiation is at the higher amplitude, and ii) the inverse of the detected radiation when the stimulated radiation is at is lower or zero amplitude. Preferably, the method is carried out at a plurality of locations on an article. This can be achieved by causing relative motion between the source and the article or by using respective pairs of sources and detectors. A particular advantage of the invention is that different types of luminescence can be detected from different locations on the article and these could be predetermined or indeed determined on-the-fly.

An example of a method and apparatus according to the invention will now be described with reference to the accompanying drawings, in which:-

Figures 1 and 2 are schematic side and plan views of the apparatus;

Figure 3 is a schematic block diagram of the components of the processor shown in Figure 1 ;

Figure 4 illustrates schematically the arrangement of phosphorescent and fluorescent features on a banknote; and

Figures 5A-5C illustrate the signal detected by the detector, and the output from the processor when operating in phosphorescent and fluorescent modes respectively.

The apparatus shown in Figures 1 and 2 comprises a transport system, in this case formed by a pair of laterally spaced conveyor belts 1 driven by a motor (not shown). The conveyor belts 1 transport a banknote 2 in the direction of an arrow 3.

Extending across the transport system is an array of UV sources 4A-4F (Figure 2), the source 4F being seen also in Figure 1. Laterally spaced along the transport system (as can be seen in Figure 1) is a corresponding array of detectors of which the detector 5F can be seen in Figure 1. Each detector 5A-5F is aligned with a respective source 4A-4F in the transport direction.

Each source 4A-4F comprises a pair of LEDs 6A.6B, aligned in the transport direction 3, which are turned on and off simultaneously in response to a control signal delivered along a line 7.

Typically, the sources 4A-4F are turned off and on together although it would also be possible to supply different control signals to each source or different groups of sources. The control signal is generated by an oscillator (not shown) within a processing system 10 which issues a voltage or current signal on the line 7 whose amplitude oscillates in a square wave manner, typically between zero amplitude and a maximum. A typical oscillation frequency is 1OkHz. The mark:space ratio of the signal is typically 1 :1.

By providing an array of sources and detectors 4,5 (defining respective channels) extending across the banknote 2 it is possible to inspect a corresponding plurality of tracks extending along the banknote in the transport direction by regularly sampling radiation detected as the banknote is transported.

Figure 3 illustrates some of the primary electronic components within the processing system 10. Each detector 5A-5F comprises a photodiode 20 and this is coupled to a respective circuit of the form shown in Figure 3. This circuit comprises a preamplifier 22 which amplifies the received radiation (which may be due to fluorescence, phosphorescence or a combination of fluorescence and phosphorescence) and outputs a signal \Λ with a corresponding amplitude. The signal Vi is supplied to a mixer circuit 24 where the signal is mixed with a signal from the oscillator in the manner to be described below and dependent upon the mode of operation of the processing system for the channel or detector concerned. The output signal from the mixer circuit 24 V₂ is fed to a low pass filter 26 which generates an output signal V₃ which is amplified by an amplifier 28 and then fed to a comparator 30. In the comparator 30, the output signal from the amplifier 28 is compared with a predetermined reference signal. A suitable output signal depending upon the result of the comparison is fed out of the processing system on a line 32. This signal can be used to operate a visual and/or audible output for example to indicate the passage of a non-authentic banknote and could also be used to operate a suitable diverter in the transport so as to feed the banknote to a particular output location. To illustrate operation of the circuit, processing of radiation detected by one detector from a banknote provided with various fluorescent and phosphorescent features will be described. The arrangement of the features is indicated in Figure 4. Thus, the banknote 2 has a background region 40. This is followed in the transport direction 3 by a region of fluorescence 42 and then by a region of fluorescent and phosphorescence 44. There then follows a region generating phosphorescence alone 46 and thereafter a further background region 48. Since the LEDs 6A,6B are oscillated together in response to a square wave control signal, they will successively turn on and off. As a result, the photodiode 20 will detect some radiation due to background fluorescence in the background region 40 when the LEDs are on (indicated at 50 in Figure 5A) but in the intermediate regions 52 will detect no radiation from the banknote.

When the region 42 of the banknote passes beneath the source, a fluorescent signal will be emitted from the banknote due to the presence of a fluorescent feature, when the LEDs 6A,6B are on. Thus, the photodiode 20 will detect a relatively high amplitude signal in the periods 54 (when the LEDs are on) but will detect no signal in the periods 56 when the LEDs are off).

When the next region 44 of the banknote receives radiation from the LEDs 6A,6B, a higher intensity signal will be detected by the photodiode 20 due to the presence of phosphorescence as well as fluorescence. When the LEDs 6A.6B are on, the detected signal 58 will have an amplitude equal to the sum of the amplitudes of the fluorescent and phosphorescent radiation. When the LEDs are off, the received signal 60 will have an amplitude corresponding to phosphorescence only because the phosphorescence will not have time to decay between successive activations of the source.

Next the region 46 of the banknote passes by the source. In this case, the only fluorescence is due to the background and thus the radiation received by the photodiode 20 will oscillate between a lower level (corresponding to phosphorescence only) 62 and a higher level corresponding to phosphorescence plus background fluorescence 64. The level 62 remains high because the phosphorescence does not have time to decay between successive activations of the source.

Finally, the region 48 of the banknote passes beneath the source and background fluorescence only is detected as with the region 40.

The processing system 10 is operable in two modes so as to detect either phosphorescence or fluorescence. In the phosphorescence mode, the mixer circuit 24 is operated so as to pass the amplified received radiation signal V₁ when the LEDs are off corresponding to regions 60,62 in Figure 5A, the signal from the mixer then being low pass filtered by the filter 26 to generate a filtered and averaged signal V₃ shown in Figure 5B. It will be seen that the amplitude of this signal is 50% of the amplitude of the signal due to phosphorescence alone.

That signal is then compared with the reference in the comparator 30 following amplification and if the reference is exceeded indicating that phosphorescence has been detected, the banknote is considered genuine at least in respect of the channel concerned.

In a second mode, the system can be used to detect fluorescence. In this mode, the mixer circuit 24 is operated to pass the signal V₁ when the LEDs are on and to pass an inverted version of the signal V₁ when the LEDs are off. The resultant signal V₂ is then averaged by passing it through the low pass filter 26 to generate the signal V₃ and this signal is shown in Figure 5C. By inverting the signal V₁ when the LEDs are off, the phosphorescence signal is inverted and thus removed enabling a fluorescence signal to be generated representative of the presence of fluorescence in the regions of the note under test. The signal V₃ is then compared with a fluorescence reference by the comparator 30.

The switching between modes can be controlled in a variety of ways. In the case of banknote inspection, when it is known in advance what type of banknote is to be inspected, it will be known in which regions phosphorescence is expected to be present and in which regions fluorescence. The system can therefore be preprogrammed to control each mixer circuit 24 accordingly.

It will be noted, however, that it would also be possible to switch between modes within a particular channel as well as between channels (where a channel corresponds to a single source 4A-4F respectively) and indeed switching between modes could be carried out in a non-preprogrammed manner dependent upon information obtained during earlier processing of the banknote. Thus, a denomination detection could be carried out first before the note reaches the authenticity detector and the mode switching carried out according to the results of the denomination detection.

It will be realised that the detailed description of the processing system given above relates to a single channel and there would therefore be six such circuits to support the six channels shown in Figures 1 and 2. In practice, it is envisaged that more than six channels, for example 12 or even 18 will be used. In the example described, the output of the amplifier 28 is compared with a reference value. In an alternative approach, the output could be sampled over time and compared with a corresponding profile of a known genuine note to see whether the variation in phosphorescence (and/or fluorescence) corresponds to the known note.

It will be noted that Figure 3 illustrates an analogue circuit for processing the signals. Although in theory a digital circuit could be used, since the signal levels involved are small it is preferred to use an analogue circuit since the process of digitizing of the preamplifier output would require extremely high- resolution ADCs to cope with the inherently wide dynamic range. In addition, the analogue approach allows the use of high-gain AC-coupled amplification that can be used to extend sensitivity beyond the offset performance on the PiN diode.

Claims

1. A method of monitoring phosphorescence emitted by an article, the method comprising exposing the article to stimulating radiation, to generate a phosphorescent response from the article, the amplitude of the stimulating radiation oscillating between higher and lower magnitudes; and detecting the emitted phosphorescence characterized in that the stimulating radiation is oscillated at a frequency with a period less than the decay time of the phosphorescence.

2. A method according to claim 1, wherein the stimulating radiation is oscillated between zero and a non-zero amplitude.

3. A method according to claim 1 or claim 2, wherein the stimulating radiation comprises UV light.

4. A method according to any of the preceding claims, wherein the step of detecting the phosphorescent response comprises detecting radiation emitted from the article; and averaging the radiation detected when the stimulating radiation is at its lower or zero amplitude.

5. A method according to any of the preceding claims, further comprising detecting fluorescence emitted from the article by detecting radiation emitted from the article; and averaging the radiation detected when the stimulated radiation is at its higher amplitude.

6. A method according to any of the preceding claims, the method further comprising detecting fluorescence emitted from the article in a mixture of phosphorescence and fluorescence, the method comprising detecting radiation emitted from the article; and averaging the sum of i) the detected radiation when the stimulated radiation is at the higher amplitude, and ii) the inverse of the detected radiation when the stimulated radiation is at is lower or zero amplitude.

7. A method according to any of the preceding claims, wherein the article comprises a document of value such as a banknote.

8. A method according to any of the preceding claims, comprising carrying out the method at a plurality of locations on an article.

9. A method according to claim 8, further comprising causing relative movement between the stimulating radiation and the article so as to expose the plurality of locations to the stimulating radiation.

10. A method according to claim 8 or claim 9, comprising carrying out the method at a plurality of locations on the article simultaneously using respective pairs of sources and detectors.

11. A method according to any of claims 8 to 10, when dependent on claim 5 or claim 6, wherein phosphorescence is detected from some locations and fluorescence from other locations.

12. A method according to claim 11 , wherein the locations from which phosphorescence is monitored and the locations from which fluorescence is monitored are predetermined.

13. Apparatus for monitoring phosphorescence emitted from an article, the apparatus comprising a source of stimulating radiation for generating a phosphorescence response from the article, the stimulating radiation regularly oscillating between higher and lower amplitudes; a detector for detecting the phosphorescent response and generating a suitable output signal; and a processing system for determining the phosphorescence from the output signal from the detector characterized in that the source is adapted to cause the stimulating radiation to oscillate at a frequency with a period less than the decay time of the phosphorescence.

14. Apparatus according to claim 13, adapted to carry out a method according to any of the preceding claims.

15. Apparatus according to claim 13 or claim 14, wherein the source of stimulating radiation comprises one or more light emitting diodes (LEDs).

16. Apparatus according to any of claims 13 to 15, further comprising an article transport for transporting articles relative to the stimulating radiation.

17. Apparatus according to claim 15 and claim 16, wherein the source comprises at least two LEDs disposed along the transport direction.

18. Apparatus according to claim 17, wherein the LEDs are disposed linearly along and parallel with the transport direction.