Abstract:
Methods and apparatus for screening the unknown contents of containers using Raman spectroscopy are disclosed, especially for security screening applications such as in airports. A probe light beam is directed through the wall of a container to a sample region within the container contents. Light scattered out of the beam within the sample region is collected along a path which passes through a separate part of the container wall, for Raman spectral analysis.

Description:
FIELD OF THE INVENTION 
     The present invention relates to methods and apparatus for determining characteristics of the contents of containers using Raman spectral techniques, especially in circumstances when the contents of the container are unknown such as in security screening applications. 
     BACKGROUND OF THE INVENTION 
     Recent events and a perceived increased threat from terrorist activities have resulted in an increased need to be able to screen the contents of bottles and similar containers which passengers may wish to carry onto an aeroplane or into a similar security sensitive environment. Such screening should be able to identify whether the contents of a container pose a threat to security, for example containing substances which could be ingredients for explosives, toxic, or otherwise unauthorised. Moreover, such screening needs to be rapid, avoiding taking samples out of relevant containers while still providing accurate results and a very low incidence of false alarms. 
     Raman spectroscopy is a known technique for determining characteristics or composition of materials, and has excellent chemical sensitivity, although is largely restricted to measurements of surface materials. Raman scattering occurs when a photon is inelastically scattered within a medium, changing the frequency of the photon by typically a few hundred to a few thousand cm−1 according to a spectrum characteristic of the scattering material. Although chemically specific, Raman scattering is very weak, and the spectral signal is easily swamped by fluorescence. In the case of the contents of a glass, or especially a plastic container, the relatively weak Raman signature of the typically liquid contents is often likely to be badly swamped by the much stronger signature of the container. 
     There are many other applications in which Raman spectral determination of characteristics within a container would be beneficial, but are at present hampered by the above problems, including for example the determination of beverage content, especially alcohol content, either on a production line or in a production, distribution or retail context. 
     The invention seeks to address the problems of the related prior art. 
     SUMMARY OF THE INVENTION 
     Aspects of the invention provide an effective solution for non-invasive security screening of liquids or gels for explosives and other harmful substances at airports and other sensitive facilities and installations. Raman spectroscopy gives highly specific chemical information in a manner similar to infra red spectroscopy, but at optical frequencies, rapidly and without absorption problems with water. The invention permits Raman spectroscopy measurements of the contents of unopened plastic containers to be made. However, although bottles are described herein as an example of the invention being applied, the technique is widely applicable, for example being useful for detecting compositional information of gels and other forms of material in transparent and semi-transparent containers. 
     Generally, the invention provides a method of probing the contents, especially liquid or gel contents which are transparent or semi transparent such as beverages and cosmetics, of a container, such as a plastic bottle or tube, by directing probe light through the container wall into the contents. Light scattered out of the beam within the contents is collected, and Raman spectral features in the scattered light are detected to determine characteristics of the contents. The wall of the container may frequently be a much stronger source of Raman scattering than the contents of the container, as well as a strong source of fluorescence and other light which may reduce the sensitivity of the technique to the characteristics of the contents. The light scattered out of the beam is therefore collected along an optical pathway or beam which avoids intersection with the container wall in the same region as the probe light. 
     The probe light beam is preferably one or more beams of laser light, and the scattered light may be collected using a variety of optical arrangements, for example using optics having one or more focal points or regions coincident with or containing a portion of the probe light beam or beams within the contents. In particular, the collection optics may act to image the sample region, for example onto the end of a fibre optic bundle for delivery to a spectrograph. 
     Concentric probe and collection beams, and fragmented beams in various configurations may be used, for example to assist in constructing a single delivery/collection probe, which could be hand-held. Such a probe, connected by fibre optic links to a laser light source and to a spectral detector/analyser would be particularly convenient for rapid screening of bottles or other containers, for example in a security screening operation such as an airport, or in checking for counterfeit or otherwise sought beverage bottles or other containers in a manufacture, distribution or retail environment. 
     In particular, the invention provides a method of screening unknown contents within a container, and especially within a plurality of such containers, to determine one or more characteristics of the contents, comprising: 
     directing a probe light beam (or beams) through a wall of said container and through said contents to a sample region (or regions) within said contents, the intersection of said probe light beam and the wall defining a delivery region (or regions); 
     collecting light scattered out of said beam within said sample region along a collection path not containing said delivery region; 
     detecting one or more Raman spectral features of said collected light; and 
     analysing said one or more Raman spectral features to determine said characteristics. 
     The delivery region of said container wall is preferably at least semi-transparent, or sufficiently transparent to said light beam, to permit continuation of a substantial part of said probe light beam to said sample region within the contents. Similarly, the contents are preferably at least semi-transparent, or sufficiently transparent, to said light beam, to permit continuation of a substantial part of said probe light beam from said delivery region to said sample region. 
     In particular, the method is advantageous when used with containers having walls which exhibit strong Raman scattering and/or fluorescence, such as silica glass or a polymer plastic. The method is suitable, for example, for with contents materials such as liquids and gels, for example liquid or gel cosmetics, medicaments and beverages. Typically, the method may be used to detect characteristics of explosives, ingredients for explosives, alcohols and toxins within container contents. 
     The invention also provides apparatus corresponding to the above methods, for example a screening probe for determining characteristics of the unknown contents of a container, comprising optics arranged to deliver a probe light beam into said contents, through a first region of the container wall, and to collect light scattered out of the beam within the contents, through a second region of the container wall substantially separate, spaced from or not intersecting with the first region. 
     In particular, the invention provides apparatus for screening the contents of containers for specified characteristics comprising: 
     delivery optics arranged to direct a probe light beam through a wall of a said container and through said contents to a sample region within said contents, the intersection of said probe light beam and the wall defining a delivery region; 
     collection optics arranged to collect light scattered out of said beam within said sample region along a collection path or beam substantially not containing or intersecting said delivery region; and 
     a spectral analyser adapted to detect one or more Raman spectral features of said collected light. 
     The apparatus may typically further comprising a data processing unit adapted to determine said specified characteristics from said one or more detected Raman spectral features, such as those mentioned above. 
     Conveniently, the optics to direct a probe light beam into the container and to collect light scattered out of the beam within the sample region may be provided as a common delivery/collection probe, for example a hand held probe suitable for use in security and other screening operations. 
     The invention is relevant to containers and contents in which the probe light beam, or a substantial part of the beam propagates through the container wall and the contents to the sample region. For this to happen, the path length of the beam through the container wall, and the path lengths thereafter within the contents to the sample region should both be less, and preferably much less than (such as less than 20% of) the relevant photon propagation distance, t, over which a photon direction is fully randomised. This distance is termed the transport length of the scattering medium (l t ), with the scattering medium either being the container wall or the contents. The transport length is typically an order of magnitude longer than the mean free scattering length l s  of photons in the medium. The mathematical relationship is l s =(l−g)l t , where g is the anisotropy for an individual scattering event. Imaging within a medium is possible on distances shorter than the transport length l t . Clearly, the collection path of the photons scattered out of the beam within the sample region and which are thereafter collected by the collection optics is subject to the same constraints. 
     Transport lengths within scattering media are discussed in Brenan C. J. H and Hunter I. W., J. Raman Spectroscopy 27, p 561 (1996), and in Das B. B., Liu F. and Alfano R. R., Rep. Prog. Phys. 60, p 227 (1997). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described, by way of example only, with reference to the drawings, of which: 
         FIG. 1  illustrates the transmission of a probe light beam into the contents of a container, and detection of Raman spectral features in light scattered from the beam into a collection optical path, according to an embodiment of the invention; 
         FIG. 2  illustrates a first scheme for putting the arrangement of  FIG. 1  into effect; 
         FIG. 3  illustrates a second scheme for putting the arrangement of  FIG. 1  into effect; 
         FIGS. 4   a  to  4   e  illustrate various other configurations of the probe light beam or beams (stippled) and collection path or paths (shaded) at the wall of the container further to the basic point laser beam delivery system shown in  FIG. 1 ; 
         FIGS. 5   a  and  5   b  show optical arrangements suitable for providing a concentric configuration of the probe light beam or beams and collection path or paths using a lens ( FIG. 5   a ) and a double axicon arrangement ( FIG. 5   b ); 
         FIG. 6   a  is a plot of Raman spectral intensity measured in an arrangement in which the collection beam or optical path from a region of scattering intersects the probe light beam at wall of a Lilt® plastic soft drink bottle: curve  100  is for the soft drink alone, curve  104  is from an empty bottle, and curve  102  is from a bottle containing the drink; 
         FIG. 6   b  corresponds to  FIG. 6   a , but using methanol instead of the Lilt® soft drink; 
         FIG. 7   a  is a plot of Raman spectral intensity measured in an arrangement in which the collection beam does not intersect the probe light beam at the container wall of the Lilt® plastic soft drink bottle of  FIGS. 6   a  and  6   b : curve  120  is for the soft drink alone, and curve  122  is for the soft drink within the bottle; 
         FIG. 7   b  corresponds to  FIG. 7   a , but using methanol in place of the Lilt® soft drink; 
         FIGS. 8   a - 8   c  relate to similar experiments using a plastic Highland Spring® mineral water bottle containing either the mineral water product or methanol. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to  FIG. 1  there is shown, schematically, a container  10  having a container wall  12  which encloses container contents  14 . Probe optics, shown here as separate delivery optics  20  and collection optics  22 , serve to direct a laser light beam  24  through the container wall and into the contents  14 . A small proportion of the photons of the laser beam are Raman scattered throughout the volume and length of the laser beam as it passes through the container wall  12 , through the contents  12 , and typically through the container wall again. Two regions of Raman scattering of particular note are a delivery region  26  of container wall illuminated directly by the laser beam as it enters the container  10 , and a sample region  28  within the contents, from which the collection optics  22  collects scattered light along a collection optical path  29 . 
     A spectral analyser  30  coupled to the collection optics is used to measure one or more Raman spectral features of the scattered light collected from the sample region  28 . Measurements of Raman spectral features made by the spectral analyser  30  may be communicated to a data processor  32  which uses the measurements to provide indications of the nature of the contents  14  within the container. 
     The probe optics may take a variety of forms, for example as discussed in particular examples below. In particular, the laser beam and/or the collection optical path may be divided into plural separate beams or paths, and concentric ring, line and other geometries may be used. 
     The laser beam may typically be provided by a quasi-monochromatic laser such as a single-line continuous wave diode laser operating at 830 nm, with a power of around 100 mW directed to the delivery region  26 . The collection optics may use a focal arrangement to optimise collection of scattered light from the sample region  28 , for example having one or more focal points or focal regions coincident with the sample region  28 . The spectral analyser may take a variety of forms for example using one or more filters, or a conventional or Fourier transform spectrograph to isolate Raman features of interest. 
     The container wall  12  may typically exhibit much stronger Raman scattering per unit volume than the contents, and typically also considerable fluorescence. As illustrated therefore, the probe optics are arranged such that the delivery region  26  of the container wall does not lie within the direct optical path between the sample region  28  and the collection optics  22 . This configuration dramatically reduces contamination of the desired Raman signal of the contents  14  by undesired Raman signal and/or fluorescence of the container wall, thereby increasing sensitivity to the Raman signal of the contents  14 . 
     The described arrangement may be used to determine indications of the nature of the content of containers in a variety of contexts, such as the detection of: counterfeit alcoholic beverages; beverage content in a production environment; unauthorised contents in beverage, medicament, cosmetic or toiletries containers, such as potential explosives ingredients or other hazardous or poisonous materials, in an airport or other security environment. 
     The container wall  12  may therefore typically be made of a conventional silica glass or a plastic such as polyethylene or polypropylene. For the laser beam  24  to pass through the container wall  12  and into the contents  14  as a beam well enough formed to deliver sufficient photon intensity to the sample region  28 , the container wall should be transparent or semi-transparent at the wavelength of the laser light, at least over an area suitable to be used as the delivery region, bearing in mind that there may be paper labels, printing and regions of different colours and opacities on the container wall. For the laser beam to pass through the contents as a beam again well enough formed to deliver sufficient photon intensity to the sample region  28 , the contents should also be transparent or semi-transparent at the wavelength of the laser light. A strongly scattering contents  14  would also have the adverse effect of scattering the laser light to all parts of the container wall  12 , increasing the Raman signal of the container wall in the light received by the collection optics and thereby reducing the sensitivity of the arrangement to the Raman signal of the contents  14 . 
     The degree to which the probe beam may be scattered within the container wall and the contents will vary depending on the nature of the materials involved. For example, the unscattered probe beam may retain close to 100% of its intensity when reaching the sample region in a clear plastic bottle of water, while retaining only perhaps 30% when passing through a coloured bottle containing a moderately turbid fruit juice. If the beam loses more intensity into scattering, especially elastic scattering within the container wall and contents, the intensity of unscattered beam photons and photons scattered elastically from the beam, in combination within the sample region, is likely to be less. However, a larger volume of the container contents will be illuminated by the incident light, contributing to Raman scattering over a larger volume. Raman photons will themselves be elastically scattered within the content and some proportion of these will consequently pass back through the wall of the container and be received by the collector optics. The large volume of the content, and the spacing between the delivery region on the container wall and the optical collection path ensure that the Raman signature continues to be heavily weighted toward the content, with little contribution from the container wall. Embodiments of the present invention therefore accommodate increasing scattering or turbidity within samples without serious loss of function. 
     Specific Arrangements 
       FIGS. 2 and 3  illustrate some ways in which the arrangement of  FIG. 1  may be out into effect, in slightly more detail and using like reference numerals where appropriate. Referring to  FIG. 2 , the container  10  is provided by a glass or plastic drinks bottle. The delivery optics  20  to deliver the laser light into the container at a delivery region  26  is provided by a system of one or more mirrors  41 , and a laser unit  40  providing the laser beam is also shown. The Raman signal scattered within the bottle contents is collected by collection optics  20  provided as imaging optics  42 , which receive scattered light from the sample region  28  within the contents of the container, along an optical path which does not include the delivery region. The imaging optics feed into a collection fibre optic bundle  44  for delivery to spectrometer  46 . The spectrometer presents a spectrum onto a CCD pixel array  48 , and data from the imaged spectrum is delivered to a data processor  32  provided by a personal computer or similar. 
     In the arrangement of  FIG. 3  the laser beam is delivered from the laser unit  40  using a delivery fibre optic bundle  50  to a combined delivery/collection probe head which both delivers the laser light into the container  10  through one or more delivery regions  26 , and collects scattered light from the sample region within the container along one or more optical paths which do not include the one or more delivery regions. As for  FIG. 2 , the collected light is delivered to a spectrometer  46  by a delivery fibre optic bundle  44 . 
     Delivery and Collection Geometries 
       FIGS. 4   a  to  4   e  illustrate a variety of other ways, in addition to the basic point delivery system shown in  FIG. 1 , in which the laser light may be delivered into the container  10  and collected through the container, especially when using a combined delivery/collection probe head as shown in  FIG. 3 . The delivery region or regions  26  of the container wall  12  when the probe is brought close to the container wall are in each case shown stippled, and the intersection of the collection optical path from the sample region  28  to the collection optics  22  is shown shaded. 
     In  FIG. 4   a  a single ring beam of laser light is delivered into the container, and the single collection optical path is concentric within this ring. To avoid the laser beam focussing to a point, which could burn some targets, a phase aberrator could be inserted into the laser beam to distort the laser beam profile slightly. 
     In the similar arrangement of  FIG. 4   b  the ring beam of  FIG. 4   a  is broken into two arcuate ring segments. Of course, different numbers of segments with different shapes could be used, and in  FIG. 4   c  six circular delivery regions  26  are shown equally spaced around a central circular collection optical path  29 . Finally, in  FIG. 4   d  there are provided two parallel rectilinear delivery regions of the same length, with a circular collection optical path spaced centrally between. 
       FIG. 4   e  shows an arrangement in which a laser beam is delivered through a central delivery region  26 , and the collection optical path  29  at the container wall  12  forms a ring (or alternatively segments of a ring or other multiple regions similar to those of  FIGS. 4   a  to  4   d ). 
     A combined delivery/collection probe head may be realised using optics such as those illustrated in  FIGS. 5   a  and  5   b . In  FIG. 5   a  laser light is directed towards the sample region  28  by refraction towards the periphery of a lens  60  forming part of a combined delivery/collection probe head  52 . Light scattered from the sample region  28  is collected by a central region of the lens. 
     In  FIG. 5   b  a broad laser beam  24  passes through two coaxial conical axicon lenses  54 ,  56 , of different cone angles, to form a ring beam convergent on the sample region  28 . A small prism or mirror in front of the centre of the second, larger axicon collects scattered light from the sample region  28  and redirects this light laterally away from the axicon axis. To form line segment delivery regions as shown in  FIG. 4   d , a similar arrangement using triangular prisms may be used. 
     Experiments 
     An embodiment of the invention was used to obtain Raman intensity spectra on a variety of samples, including commercial plastic beverage bottles sold containing the Lilt® soft drink, and as an alternative containing methanol, and on a bottle sold containing Highland Spring® mineral water, and again as an alternative containing methanol. The experimental set up was as follows. The spectra were obtained using a 55 mW continuous wave laser beam generated from a temperature stabilised diode laser operating at 827 nm. The beam was spectrally purified by removing any residual amplified spontaneous emission components from its spectrum using two 830 nm bandpass filters. The beam was weakly focused onto the bottle surface to about 1 mm diameter spotsize. The point of incidence of the laser beam on the bottle surface was displaced from the Raman collection pathway intersection with the bottle surface by about 5-7 mm. 
     The Raman light was collected in backscattering mode using a 1.2 f-number lens from a depth of several millimetres within the bottle. The scattered light was imaged, with magnification 1:1, onto the front face of a fibre probe. A combination of notch and edge filters was used to suppress the elastically scattered component of light. The fibre probe consisted of 22 fibres collecting Raman signal. The Raman light was then propagated through the fibre system of a length of about 2 m to a linear fibre end oriented vertically and placed in the input image plane of a Kaiser Optical Technologies Holospec spectrograph (f#=1.8i). 
     The Raman spectra were collected using a NIR back-illuminated deep depletion TE cooled CCD camera (Andor Technology, DU420A-BR-DD, 1024×256 pixels) by binning all the fibres into one Raman spectrum. The Raman spectra measured in the conventional geometry in which the laser delivery and Raman collection regions intersect on the bottle surface was collected by reducing the displacement between the collection and deposition points on the surface of the bottle from 7 mm to zero. This was accomplished merely by redirecting the laser beam to the new position. The other parameters of the system remained unchanged. The overall acquisition time for all the spectra was 10 s. 
     The Raman intensity spectra from using the conventional arrangement, in which the collection pathway contains the delivery region, for Lilt® plastic drinks bottle containing Lilt soft drink are shown in  FIG. 6   a . The vertical scale is adjusted separately for each spectrum to bring the curves into similar scales. The relatively smooth top curve  100  shows the spectrum for the Lilt drink without a container wall present. The spectrum of the container wall alone is shown as the lowest curve  104 , and the container wall with the Lilt drink present in the container is shown as the curve which is very similar, but slightly above the container curve. Clearly, drink characteristics of curve  102  are largely swamped by characteristics of the plastic bottle itself. 
       FIG. 6   b  corresponds to  FIG. 6   a , but with the Lilt soft drink substituted by methanol. The pure methanol spectrum is shown as relatively smooth curve  110 . The spectrum for methanol in the Lilt bottle is shown as curve  114  which is only slightly different to the spectrum  112  for the bottle alone. 
     The corresponding Raman intensity spectra from using the arrangement according to the invention, in which the collection pathway is spaced from and does not contain the delivery region, are shown in  FIGS. 7   a  and  7   b . The slightly smoother lower curve  120  of  FIG. 7   a  shows the spectrum for the Lilt drink without a container wall present. The spectrum of the container wall with the Lilt drink present in the container is shown as the curve  122  which is very similar, but slightly above the pure drink curve. Clearly, using the optical geometry of the invention the Raman spectrum of the drink behind the container wall is very close to that of the drink with no intervening wall. In  FIG. 7   b , the pure methanol spectrum  130  and the spectrum of methanol within the plastic Lilt bottle  132  are almost indistinguishable, with spectral features of the plastic bottle itself being almost eliminated. 
     Returning to the conventional Raman technique in which the delivery region interferes with the collection path, the lower, relatively featureless curve  140  of  FIG. 8   a  is a Raman spectrum of water. Water exhibits a more complex Raman signature only at rather higher frequencies. The more complex upper curves are the spectrum of a Highland Spring® plastic mineral water bottle  144 , and that of the bottle including its water contents. It is clear that using the conventional technique the water signature is overwhelmed by the signature of the plastic bottle. 
     To obtain the spectra of  FIG. 8   b  the same mineral water bottle was filled with methanol, and was again tested using the conventional arrangement. The signature of methanol on its own is shown as the smoother curve  150  with the highest peak. The main methanol peak is apparent in the curve  152  of methanol behind the container wall, but generally this curve and the curve  154  of the container alone are very similar. To obtain the spectra of  FIG. 8   c  the same mineral water bottle filled with methanol was tested using the arrangement embodying the invention. Clearly, in this case, the curve for pure methanol  160  and the curve for methanol behind the plastic container wall are much more similar. 
     Although particular embodiments of the invention have been described in detail, it will be apparent to the skilled person that a variety of changes and modifications may be made to such embodiments without departing from the scope of the claims.