Detector methods and apparatus

A method and apparatus for detecting radiation in a relatively narrow wavelength band within radiation having a wide range of wavelengths. The method comprises detecting radiation in a first wavelength band including the narrow wavelength band and other wavelengths; detecting radiation in a second wavelength band which comprises substantially only the other wavelengths; and comparing the levels of detected radiation to determine the presence of radiation in the narrow wavelength band.

FIELD OF THE INVENTION 
The invention relates in one aspect to a method and apparatus for detecting 
radiation in a relatively narrow wavelength band within radiation having a 
wider range of wavelengths. 
DESCRIPTION OF THE PRIOR ART 
There are several applications where it is necessary to detect a particular 
narrow wavelength band within a wide range of wavelengths. These include 
elemental analysis techniques and flame detection. In the field of 
security, use is made of compounds having particular spectral 
characteristics to authenticate documents and other articles. Examples 
include documents of value such as banknotes which may carry an ink or 
have inclusions in the paper or security thread which include a component 
having a special spectral response. A particularly difficult spectral 
response to detect and reproduce is one which results in the generation of 
radiation in a very narrow wavelength band upon stimulation. In this 
context, by "narrow" we mean typically wavelength bands of 100 nm or less, 
the wavelengths being typically in the UV, visible or infrared part of the 
spectrum. 
One approach to detecting such a spectral response would be to utilise a 
detector which can only detect radiation within the specified wavelength 
band. Typically, this would be by choosing a filter with a pass band 
corresponding to the spectral response wave band. Suitable glass filters 
may not exist, however, and so it is necessary to use specially designed 
interference filters. These are difficult to obtain and are expensive, 
especially in small quantities, and cannot easily be cut to the shapes 
required for small detectors. 
SUMMARY OF THE INVENTION 
In accordance with one aspect of the present invention, a method of 
detecting radiation in a relatively narrow wavelength band within 
radiation having a wider range of wavelengths comprises detecting 
radiation in a first wavelength band including the narrow wavelength band 
and other wavelengths; detecting radiation in a second wavelength band 
which comprises substantially only the other wavelengths; and comparing 
the levels of detected radiation to determine the presence of radiation in 
the said narrow wavelength band. 
In accordance with a second aspect of the present invention, apparatus for 
detecting radiation in a relatively narrow wavelength band within 
radiation having a wider range of wavelengths comprises a first detector 
for detecting radiation in a first wavelength band including the narrow 
wavelength band and other wavelengths; a second detector for detecting 
radiation in a second wavelength band which comprises substantially only 
the other wavelengths; and a processor for comparing the levels of 
detected radiation to determine the presence of radiation in the said 
narrow wavelength band. 
With this invention, instead of attempting to detect just the narrow 
wavelength band, radiation is detected in a pair of overlapping wavelength 
bands, the narrow wavelength band corresponding to those wavelengths which 
are detected in the first wavelength band but are not detected in the 
second wavelength band. This enables much cheaper and more readily 
obtainable filters to be employed. 
Conveniently, the upper limit of the first wavelength band substantially 
corresponds to the upper limit of the second wavelength band. However, 
other arrangements are possible and, for example, the lower limit of the 
two wavelength bands may substantially coincide. 
The radiation may lie in the visible wavelength range but typically lies 
outside the visible wavelength range, for example in the infrared or 
ultraviolet ranges. 
Typically, a first signal is generated corresponding to radiation detected 
within the first wavelength band and a second signal is generated 
corresponding to radiation generated within the second wavelength band and 
the comparison step comprises subtracting the second signal from the first 
signal. Of course, this could be done using either analog or digital 
processing and the term "signal" should be construed accordingly. 
In some cases, it may be necessary to modify one or both of the signals 
prior to the subtraction step in order to take account of noise and other 
well known variables. Furthermore, since the detection steps are likely to 
be spaced apart in time, it will be necessary to temporally offset one 
signal relative to the other before carrying out the subtraction. This 
ensures that the two signals which are subtracted relate to the same area 
of material. This is particularly important when the method is used to 
inspect material on moving articles or documents. 
In some cases, the detectors may only be responsive to wavelengths within 
the specified wavelength bands. Typically, however, the apparatus includes 
a number of filters, the filters having pass bands such that each detector 
receives only radiation in a respective one of the first and second 
wavelength bands. The detectors will typically comprise photodiodes or the 
like. 
The radiation may be generated from a variety of sources. One such source 
is a material which generates radiation in response to external 
stimulation. Typically, the material will be stimulated by irradiating the 
material with radiation in a continuous manner. This may comprise 
irradiation in either the visible or invisible wavelength bands. The 
result of continuous radiation results in a measure of the luminescence of 
the material but cannot distinguish between fluorescence and 
phosphorescence and in some cases it is necessary to make that 
distinction. 
The ability to distinguish between fluorescent and phosphorescent radiation 
is useful not only in the present context but also more generally. We 
have, therefore, devised a new method in accordance with a third aspect of 
the present invention for distinguishing between fluorescent and 
phosphorescent radiation emitted from a material in response to an 
external stimulation, the method comprising stimulating the material using 
a modulated stimulation to cause the material to emit fluorescent and/or 
phosphorescent radiation; sensing radiation emitted by the stimulated 
material; generating a first signal representing total radiation emitted 
by the stimulated material; generating a second signal representing 
radiation emitted by the material with substantially the same modulation 
as the stimulation; and determining the difference between the first and 
second signals whereby the second signal represents the presence of 
fluorescent radiation and the difference between the two signals 
represents the presence of phosphorescent radiation. 
In accordance with a fourth aspect of the present invention, we provide 
apparatus for detecting radiation in a relatively narrow wavelength band 
within radiation having a wider range of wavelengths, the apparatus 
comprising a first detector for detecting radiation in a first wavelength 
band including the narrow wavelength band and other wavelengths; a second 
detector for detecting radiation in a second wavelength band which 
comprises substantially only the other wavelengths; and a processor for 
comparing the levels of detected radiation to determine the presence of 
radiation in the said narrow wavelength band. 
The invention makes use of the fact that fluorescent radiation responds 
substantially immediately to the stimulation whereas phosphorescent 
radiation once initiated decays over a relatively long period and so will 
not exhibit a modulation similar to that of the stimulation. The first 
signal therefore represents a measure of the luminescence (fluorescence 
and phosphorescence); the second signal represents fluorescence alone; and 
the difference between the two signals represents phosphorescence alone. 
This method and apparatus is particularly suitable for use with methods and 
apparatus according to the first and second aspects of the invention where 
the material is phosphorescent and has an emission wavelength closely 
similar to other non-phosphorescent materials. 
The material will typically be stimulated by exposing it to radiation 
having a wavelength in the visible or near-visible region. However, other 
forms of stimulation such as heat could be used with suitable materials. 
Where exposure to radiation in the visible and near-visible region occurs, 
preferably any reflected radiation is removed prior to generating the 
first and second signals. This simplifies the construction of the various 
filters needed for use with the sensor. 
It will appreciated that when the two sets of apparatus are used together, 
the first and second detectors may each constitute also a sensor. 
Conveniently, the first signal is generated by averaging the received 
radiation over a sampling interval; while the second signal may be 
generated by detecting radiation from the material which has been 
modulated with substantially the same frequency as the external 
stimulation, and determining the amplitude of the detected radiation to 
constitute the second signal. 
It should be understood, however, that although the invention is 
particularly suitable for analogue processing, the generation and use of 
the first and second signals may be handled digitally using a suitably 
programmed computer.

DETAILED DESCRIPTION OF THE EMBODIMENTS 
The apparatus shown in FIGS. 1 and 2 is for sorting banknotes between those 
which are genuine and those which are fraudulent or suspected of being 
fraudulent. This distinction is made by inspecting each note as it passes 
under a detector head 1 to see whether an ink emits radiation within a 
specific narrow wavelength band 42 (FIG. 3) upon illumination. The sorting 
apparatus includes a hopper 2 in which a stack of banknotes 3 is provided, 
the hopper 2 having an outlet opening 4 through which individual banknotes 
are fed upon rotation of a roller 5. The banknotes 4 are fed in any 
conventional manner, for example via friction belts, vacuum feed devices 
or the like along a path 6 beneath the detector head 1. The detector head 
inspects each banknote as will be explained in more detail below and a 
processor 7 connected with the detector head 1 determines whether or not 
the narrow band radiation has been detected. The processor 7 then controls 
a diverter 8 either to allow the banknotes to pass on for further 
processing along a path 9 when a genuine banknote is detected or to divert 
a banknote into a reject hopper 10 when a suspected fraudulent banknote is 
detected. 
The construction of the detector head 1 is shown in more detail in FIG. 1 
and comprises a housing 20 which is mounted via a support bracket 21 to a 
casing of the sorting apparatus (not shown). The housing 20 includes a 
lens housing 22 in which is mounted a pair of focusing lenses 23,24. The 
lens 23 is sealed to the housing 22 via an O-ring 25 and is held in 
position by an annular spacer 26. A gelatin filter 27 is provided above 
the lens 24 to absorb any reflected UV light. Above the gelatin filter 27 
is located an IR interference filter 28 which absorbs all wavelengths 
above a wavelength .lambda..sub.1. Light which has passed through the 
filters 27,28 then impinges on a laterally spaced pair of glass filters 
29,30 which are aligned with respective photodiode detectors 31,32. The 
filter 29 absorbs radiation below about 12 and the filter 30 absorbs 
radiation below about .lambda..sub.3. The effect of these filters in 
combination is that radiation impinging on the photodiode 31 falls within 
a first wavelength band 40 (FIG. 3) extending from .lambda..sub.2 to 
.lambda..sub.1 while radiation impinging on the photodiode 32 falls within 
a second wavelength band ranging from .lambda..sub.3 to .lambda..sub.1. It 
will be understood that the filters 29,30 have sharp lower cut-offs, the 
narrow wavelength band 42 of the material to be detected falling between 
these cut-offs. 
A pair of mercury discharge lamps 33,34 are provided, one on each side of 
the lens housing 22 for illuminating banknotes as they pass beneath the 
head 1. The illumination wavelength generated by the lamps is chosen to 
correspond to that which will stimulate the ink, if present, to generate 
radiation within the narrow wavelength band 42. This may cause the ink to 
fluoresce or phosphoresce, or both. 
In order to determine the presence of the special ink, the photodiodes 
31,32 are connected to a subtractor 35 (FIG. 4) mounted on a printed 
circuit board 36. If the signal supplied by the photodiode 31 is labelled 
A and that from the photodiode 32 is labelled B, the output from the 
subtractor 35 is A-B. Since the signal A relates to the intensity of 
radiation received within the wave band 40 and the signal B represents the 
intensity of radiation within the wavelength 41, the output from the 
subtractor 35 will represent the intensity of radiation received in the 
wave band 40 but not in the wave band 41. This difference signal will then 
be compared with a threshold by the processor 7, after A/D conversion, and 
if greater than the threshold will indicate the presence of the special 
ink. If the ink is not present then the processor 7 will cause the 
diverter 8 to move to the position shown in dashed lines in FIG. 2 and the 
note will be rejected. 
In practice, a delay 36 is built into one of the lines from the photodiodes 
31,32 to the subtractor 35 to compensate for movement of the banknote 
beneath the photodiodes 31,32. 
In the example described, the photodiodes 31,32 measure the total 
luminescence generated by the ink and the lamps 33,34 will be continuously 
illuminated. In some cases, the ink may phosphoresce within the wavelength 
band 42. In order to detect this phosphorescence, the radiation due to 
phosphorescence must be distinguished from that due to fluorescence. FIG. 
5 illustrates a circuit-which can make that distinction. In this example, 
the lamps 33,34 are modulated on and off by a lamp control circuit 50 
which responds to a clock input from a source 51. The output from each 
diode 31,32 is fed in parallel to a respective averaging filter 52,54 and 
to a respective bandpass filter 53,55. The outputs of the bandpass filters 
53,55 are fed to respective amplitude detectors 56,57. The outputs of the 
averaging filter 52 and the amplitude detector 56 are fed to a subtractor 
58 while the outputs of the averaging filter 54 and the amplitude detector 
57 are fed to a subtractor 59. The characteristics of the averaging 
filters 52,54 are such that they only pass frequencies below the 
modulation frequency of the lamp 50, and so the signals fed to the 
non-inverting inputs of the subtractors 58,59 represent the received 
luminescence (phosphorescence plus fluorescence). The characteristics of 
the bandpass filters 53,55 are such that they pass only frequencies 
including and close to the modulation frequency of the lamp. Any 
phosphorescence signal will decay much more slowly and will be 
substantially eliminated by the filters 53,55 and so the signals fed from 
the amplitude detectors to the inverting inputs of the subtractors 58,59 
represent the received fluorescence. 
The amplitude detectors 56,57 are necessary to determine the amplitude of 
the output generated by the bandpass filters 53,55. 
Other processing techniques well known to those skilled in the art may be 
used. For example, a synchronous detector driven by the clock signal may 
replace bandpass filter 53 and the amplitude detector 56 or bandpass 
filter 55 and amplitude detector 57. 
Each subtractor 58,59 then subtracts the fluorescence signal from the 
fluorescent/phosphorescent signal to generate output signals C,D which 
represent phosphorescence. These signals C,D are then processed in the 
same way as the signals A,B in FIG. 4 by passing them to a subtractor 61 
which generates an output signal representing the intensity of radiation 
received in the wavelength band 40 but not the wavelength band 41. The 
signal C is passed to the subtractor 61 via a delay 60 to compensate for 
movement of the banknotes beneath the photodiodes 31,32. 
FIG. 6 illustrates a specific example of the characteristics of the filters 
28, 29 and 30. FIG. 6A illustrates the individual characteristics of those 
filters while FIG. 6D illustrates the effect of superposing the filters 
resulting in wavebands 40,41 described earlier. FIG. 6C illustrates the 
narrow band 42 resulting from subtracting wavelength band 41 from 
wavelength band 40. It will be noted that the response of the difference 
extends up into waveband 41. This is inevitable unless the characteristic 
of filter 30 is infinitely steep--a physical impossibility. The example as 
shown is suitable for detecting a narrow spike in the region 540 to 570 
nm, i.e. between the lower cut-offs of wavebands 40 and 41, which are the 
same as the lower cut-offs of filters 29 and 30, respectively.