Abstract:
A method for performing security screening of a liquid product at a security checkpoint is provided. The liquid product is placed in a tray having at least two reference areas manifesting respective X-ray signatures when exposed to X-rays, the X-ray signatures being distinguishable from one another. An X-ray inspection of the tray is performed while the tray holds the liquid product, the X-ray inspection including deriving the X-ray signatures. A determination is made as to whether the liquid product is a security threat at least in part based on the X-ray signatures. A tray having at least two reference areas manifesting respective X-ray signatures when exposed to X-rays is also provided.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is a continuation claiming the benefit of priority under 35 USC §120 based on PCT International Patent Application No. PCT/CA2007/001749 filed Oct. 1, 2007 and designating the United States; which for the purposes of the United States claims the benefit of priority under 35 USC §119 e) based on U.S. provisional patent application Ser. No. 60/827,784 filed on Oct. 2, 2006 by Aidan Doyle et al. 
     The contents of the above-referenced document are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to technologies for assessing the threat status of materials by means of penetrating radiation such as X-rays. More specifically, the invention relates to reference device, in particular a tray that supports the material while the material is being subjected to penetrating radiation, with one or more areas that can be used as a reference during the identification process of the material. The invention has numerous applications, in particular it can be used for scanning hand carried baggage at airport security check points. 
     BACKGROUND OF THE INVENTION 
     Some liquid or combinations of liquid and other compounds may cause enough damage to bring down an aircraft. As no reliable technology-based solution currently exists to adequately address this threat, authorities have implemented a ban of most liquid, gels and aerosols in cabin baggage. 
     As a result, there have been disruptions in operations (e.g., a longer screening process; changed the focus for screeners; additional line-ups), major inconveniences for passengers (as well as potential health hazards for some) and economic concerns (e.g., increased screening costs; lost revenues for airlines and duty free shops; large quantities of confiscated—including hazardous—merchandise to dispose of), and so on. 
     Clearly, there is a need to provide a technology-based solution to address the threat of fluids that are flammable, explosive or commonly used as ingredients in explosive or incendiary devices. 
     SUMMARY OF THE INVENTION 
     As embodied and broadly described herein, the invention provides a tray for holding a material while the material is being subjected to penetrating radiation. The tray has a surface on which the material rests while subjected to penetrating radiation. The surface has at least two areas, namely a first area and a second area, the first and second areas characterized by first and second signatures, respectively when exposed to penetrating radiation, wherein the first signature is different from the second signature. The second area constitutes a reference. This reference allows identifying the material based at least in part on a comparison between the signature of the material to penetrating radiation and the second signature. 
     In a specific and non limiting example of implementation, the tray can be used during security screening of liquid products at security checkpoints. The screening process includes requesting passengers with hand-carried baggage to remove from the baggage liquid products and place the liquid products in the tray. The tray with the liquid products is then inserted in an X-ray imaging system to perform an X-ray inspection. The image data generated during the X-ray inspection is processed by a computer. The computer compares the X-ray signature of the reference area to the X-ray signature of the liquid product. If the X-ray signature of the reference area is known to correspond to the X-ray signature of a liquid product that does not present a security threat, such as a bottle of plain water, and the X-ray signature of the liquid product that is being screened matches the X-ray signature of the reference, then in all likelihood the liquid product that is being screened is a safe product. 
     Generally speaking, X-rays are typically defined as electromagnetic radiation having wavelengths that lie within a range of 0.001 to 10 nm (nanometers) corresponding to photon energies of 120 eV to 1.2 MeV. Although the electromagnetic radiation referred to primarily throughout this description are X-rays, those skilled in the art will appreciate that the present invention is also applicable to electromagnetic radiation having wavelengths (and corresponding photon energies) outside this range. 
     For the purpose of this specification “liquid” refers to a state of matter that is neither gas nor solid and that generally takes the shape of the container  102  in which it is put. This definition would, therefore encompass substances that are pastes or gels, in addition to substances having a characteristic readiness to flow. For instance, toothpaste, and other materials having the consistency of toothpaste would be considered to fall in the definition of “liquid”. 
     As embodied and broadly described herein, the invention also provides a tray for holding a liquid product during an X-ray inspection of the liquid product performed to determine if the liquid product presents a security threat. The tray has a surface on which the liquid product rests while being subjected to X-rays. The surface has at least two areas, namely a first area and a second area, the first area characterized by a first X-ray signature and the second area characterized by a second X-ray signature that is different from the first X-ray signature. The second X-ray signature matches the X-ray signature of a liquid product, wherein the liquid product includes a container  102  holding a liquid material, the liquid material being selected from the group consisting of water, carbonated beverage, fruit juice, toothpaste and a cosmetic liquid. 
     As embodied and broadly described herein, the invention also relates to a method for performing security screening at a security checkpoint. The method includes the steps of placing a liquid product in a tray which has a reference area and performing an X-ray inspection of the tray holding the liquid product. The method further comprises comparing the X-ray signature of the liquid product to the X-ray signature of the reference area and determining if the liquid product is a security threat based at least in part on results obtained by comparing the X-ray signature of the liquid product to the X-ray signature of the reference area. 
     As embodied and broadly described herein, the invention also includes a security screening system to determine if an article presents a security threat. The screening system comprises an input for receiving image data conveying an image of the article and of a reference area produced when the article and the reference area are subjected to penetrating radiation. The screening system further has a logic module for:
         a) processing the image data to compare a signature to penetrating radiation of the article to a signature to penetrating radiation of the reference area;   b) processing the image data at least in part based on results obtained by processing the image data in step a) to assess if the article poses a security threat.       

     As embodied and broadly described herein the invention also provides a tray for holding an article while the article is being subjected to penetrating radiation. The tray having a surface on which the article rests while subjected to penetrating radiation, that surface including at least one area which when exposed to penetrating radiation produces a predetermined signature. The tray also has a machine readable indicia conveying information associated with the predetermined signature. 
     As embodied and broadly described herein, the invention further provides a security screening system to determine if an article presents a security threat. The screening system having an input for receiving image data conveying an image of the article and of a reference area produced when the article and the reference area are subjected to penetrating radiation. The screening system further has a logic module for:
         a. deriving a nominal signature of the reference area to penetrating radiation;   b. processing the image data to derive an actual signature to penetrating radiation of the reference area;   c. comparing the nominal signature to penetrating radiation to the actual signature to penetrating radiation;   d. processing the image data at least in part based on results obtained in step c) to assess if the article poses a security threat.       

     As embodied and broadly described herein, the invention further provides a security system for implementation at a check point to screen hand carried baggage for articles that potentially pose a security threat. The security system including a set of trays for receiving the hand carried baggage of passengers as the passengers arrive at the check point, and an X-ray apparatus. The X-ray apparatus has:
         a. a screening area;   b. a conveyor belt on which the trays with hand carried baggage are placed, the conveyor belt being movable to advance the trays with hand carried baggage through the screening area where the trays and the hand carried baggage are subjected to X-rays.       

     At least one of the trays in the set of trays having a reference area characterized by a nominal X-ray signature and the X-ray apparatus further including a logic module for:
         i) processing image data of the at least one tray to derive from the image data an actual X-ray signature of the reference area;   ii) performing a comparison between the actual X-ray signature and the nominal X-ray signature and processing the image data of the at least one tray at least in part based on results of the comparison to assess if hand carried baggage contains articles that pose a security threat.       

     As embodied and broadly described herein, the invention also includes a security screening system to determine if an article presents a security threat. The screening system having:
         a) an X-ray imaging system;   b) a reference device including at least one reference area characterized by a nominal X-ray signature;   c) the X-ray imaging system having a logic module for:
           i) processing X-ray image data generated when the reference device is subjected to X-rays to determine if the nominal X-ray signature of the reference area matches an actual X-ray signature of the reference area;   ii) processing the X-ray image data at least in part based on results obtained when processing the X-ray image data to determine if the nominal X-ray signature of the reference area matches the actual X-ray signature of the reference area to assess if an article X-rayed by the X-ray imaging system at the same time as the reference device or thereafter poses a security threat.   
               

     As embodied and broadly described herein, the invention also includes a method for performing security screening at a checkpoint. The method includes the steps of placing a liquid product in a tray having at least two reference areas manifesting respective X-ray signatures when exposed to X-rays, the X-ray signatures being distinguishable from one another. The method also includes performing an X-ray inspection of the tray while the tray holds the liquid product, the X-ray inspection including deriving the X-ray signatures. The method further includes the step of determining if the liquid product is a security threat based at least in part on the X-ray signatures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A detailed description of examples of implementation of the present invention is provided hereinbelow with reference to the following drawings, in which: 
         FIG. 1  is a block diagram of an apparatus using X-rays to scan hand carried baggage at a security check point, according to a non-limiting example of implementation of the invention; 
         FIG. 2  is a plan view of a tray for carrying materials as they undergo security screening, according to a non-limiting example of implementation of the invention; 
         FIG. 3  is a cross-sectional view taken along lines  3 - 3  in  FIG. 2 ; 
         FIG. 4  is an X-ray image of a liquid container  102  shown on a graphical user interface; 
         FIG. 5  is a more detailed block diagram of the processing module of the apparatus shown in  FIG. 1 ; 
         FIG. 6  is a flowchart illustrating the process implemented by the apparatus of  FIG. 1  to perform security screening; and 
         FIG. 7  is a plan view of the tray according to a variant. 
     
    
    
     In the drawings, embodiments of the invention are illustrated by way of example. It is to be expressly understood that the description and drawings are only for purposes of illustration and as an aid to understanding, and are not intended to be a definition of the limits of the invention. 
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , there is shown a specific non-limiting example of a system  10  for use in screening containers with liquids, in accordance with a non-limiting embodiment of the present invention. The system  10  comprises an X-ray apparatus  100  that applies an X-ray screening process to a material such as a liquid  104  contained in a container  102  that is located within a screening area of the X-ray apparatus  100 . In an airport setting, a passenger may place the container  102  in a tray which is then placed onto a conveyor  114  that causes the container  102  to enter the screening area of the X-ray apparatus  100 . The X-ray apparatus  100  outputs an image signal  116  to a processing module  500 . The processing module then processes the image data conveyed by the signal  116 . 
     The processing module  500  may be co-located with the X-ray apparatus  100  or it may be remote from the X-ray apparatus  100  and connected thereto by a communication link, which may be wireless, wired, optical, etc. The processing module  500  processes the image data and executes a method to produce a threat assessment  118 . The processing module  500  may be implemented using software, hardware, control logic or a combination thereof. 
     The threat assessment  118  is provided to a console  128  and/or to a security station  132 , where the threat assessment  118  can be conveyed to an operator  130  or other security personnel. The console  128  can be embodied as a piece of equipment that is in proximity to the X-ray apparatus  100 , while the security station  132  can be embodied as a piece of equipment that is remote from the X-ray apparatus  100 . The console  128  may be connected to the security station  132  via a communication link  124  that may traverse a data network (not shown). 
     The console  128  and/or the security station  132  may comprise suitable software and/or hardware and/or control logic to implement a graphical user interface (GUI) for permitting interaction with the operator  130 . Consequently, the console  128  and/or the security station  132  may provide a control link  122  to the X-ray apparatus  100 , thereby allowing the operator  130  to control motion (e.g., forward/backward and speed) of the conveyor  114  and, as a result, to control the position of the container  102  within the screening area of the X-ray apparatus  100 . 
     In accordance with a specific non-limiting embodiment the X-ray apparatus  100  is a dual-energy X-ray apparatus  100 . However, persons skilled in the art will appreciate that the present invention is not limited to such an embodiment. Such dual-energy X-ray apparatus  100  has a source which emits X-rays at two distinct photon energy levels, either simultaneously or in sequence. Example energy levels include 50 keV (50 thousand electron-volts) and 150 keV, although persons skilled in the art will appreciate that other energy levels are possible. 
     The processing module  500  receives the image signal  116  and processes the signal to determine if the liquid  104  in the container  102  poses a security threat. The determination can include an explicit assessment as to weather the liquid  104  is a threat or not a threat. Alternatively, the determination can be an identification of the liquid  104  or the class of materials to which the liquid  104  belongs, without explicitly saying whether the liquid  104  is threatening or not threatening. For example, the processing module can determine that the liquid  104  is “water” hence the operator  130  would conclude that it is safe. In a different example, the processing module  500  determines that the liquid  104  belongs to a class of flammable materials, in which case the operator  130  would conclude that it would be a security threat. Also, the determination can be such as to provide an explicit threat assessment and at the same time also provide an identification of the liquid  104  in terms of general class of materials or in terms of a specific material. The results of the determination are conveyed in the threat assessment signal  118  which is communicated to the console  128  and/or the security station  132  where it is conveyed to the operator  130 . 
       FIG. 5  is a high level block diagram of the processing module  500 . The processing module  500  has a Central Processing Unit (CPU)  508  that communicates with a memory  502  over a data bus  504 . The memory  502  stores the software that is executed by the CPU  508  and which defines the functionality of the processing module  500 . The CPU  508  exchanges data with external devices through an Input/Output (I/O) interface  506 . Specifically, the image signal  116  is received at the I/O interface  506  and the data contained in the signal is processed by the CPU  508 . The threat assessment signal  118  that is generated by the CPU  508  is output to the console  128  and/or the security station  132  via the I/O interface  506 . 
     In a specific example of implementation, the system  10  is used in conjunction with a tray  200  shown in  FIG. 2  to perform security screening of liquid products. The tray  200  is used as a receptacle in which objects to be screened, such as liquid products or other materials or articles, are placed and put on the conveyor belt of the X-ray imaging system  10 . To facilitate the identification of the liquid product or any other article placed in the tray  200  and/or to perform threat assessment of the liquid product or of any other article, the tray  200  is provided with one or more distinct areas that have X-ray signatures which can be used as references against which the X-ray signatures of the liquid product or any other article can be compared. The comparison can be made in order to perform an identification of the liquid product, for instance determine what its specific composition is. The comparing can also be made simply to find out if the liquid product poses a security threat, without necessarily determining its precise identity. 
     The tray  200  defines a surface  202  which is generally flat and on which the liquid product that is being screened rests. In the example shown in the drawings, the surface is shaped as a rectangle with rounded corners. Evidently, different shapes or configurations can be used without departing from the spirit of the invention. 
     The surface  202  is provided with raised edges or rim  204  that extend in a continuous fashion around the periphery of the surface  202 . The raised edges  204  prevent articles placed in the tray  200  to fall outside during the screening operation. The height of the raised edges  204  can vary without departing from the spirit of the invention. 
     The surface  202  defines five distinct areas. The first area  206  is the base material from which the tray  200  is made. That material may be any synthetic material that has the required strength and durability characteristics for the intended application. The four additional distinct areas  208 ,  210 ,  212  and  214  are in the form of inserts that are placed in respective receptacles in the base material  206 . The areas  208 ,  210 ,  212  and  214  are in the shape of rectangles placed near respective corners of the tray  200 . It is to be expressly noted that the shape, placement in the tray  200  and the number of the areas  208 ,  210 ,  212  and  214  can vary without departing from the spirit of the invention. 
     The areas  206 ,  208 ,  210 ,  212  and  214  are distinct in that they have different X-ray signatures. Accordingly, when an X-ray image is taken of the tray  200  alone, the areas  206 ,  208 ,  210 ,  212  and  214  will show up differently in the image. Preferably, the area  206  is made of material that is selected to provide a weak X-ray signature such as to limit its effect in the image and thus make the other articles that are put on the tray  200  more visible. 
     The areas  208 ,  210 ,  212  and  214  are made from substances that have X-ray signatures that are similar to the X-ray signatures of liquid products or other articles that are likely to be placed in the tray  200  during a security screening operation. In this fashion, the areas  208 ,  210 ,  212  and  214  constitute references against which the X-ray signatures of the articles placed in the tray  200  can be compared for identification purposes and/or to determine their threat status. 
     For example, in the context of a screening operation performed at an airport passengers are likely to bring, in hand carried baggage, liquid products. Those liquid products are typically for human consumption or toiletries for personal grooming or dressing. Examples of liquid products for human consumption include water, carbonated beverages and fruit juices, among others. Examples of toiletries include cosmetic liquids such as toothpaste, liquid soap (shampoo for instance), creams, deodorants, sun care products and hair care products, among others. 
     Reference areas  208 ,  210   212  and  214  in the tray  200  may be provided for some of those articles that are the most common such as to facilitate their identification and/or the assessment of their threat status. For instance if the tray  200  is designed in such a way as to be able to practically use only four reference areas, such as the areas  208 ,  210 ,  212  and  214  the materials from which the areas  208 ,  210 ,  212  and  214  are selected such as to mimic the X-ray responses of the four most common liquid products that passengers are likely to bring in their hand carried baggage. Consider for the purpose of this example that among all the liquid products that are brought the following ones are observed the most often:
         1. Water bottle;   2. Carbonated beverage sold under the trademark “Coke”;   3. Toothpaste commercialized under the trademark “Crest”;   4. Skin-care cream made by the company “RoC”.       

     Accordingly, the areas  208 ,  210 ,  212  and  214  are made of materials that have X-ray signatures that are similar or identical to the X-ray signatures to the respective liquid products above. In this fashion, if a water bottle is placed on the tray and scanned by the apparatus  100 , the X-ray image will show that the X-ray signatures of the water bottle and of the area  208  are the same. Since the area  208  is known to have an X-ray signature that is equivalent to water, the match between both X-ray signatures can be used to establish that the liquid  104  in the container  102  is in fact water. 
     More specifically, each area  208 ,  210 ,  212  and  214  can be made from a material whose X-ray signature is the same or very similar to the X-ray signature of the liquid product associated therewith. This solution can be implemented by providing an insert made from the selected material that is placed in the base material  206  of the tray  200 . This feature is best shown in  FIG. 3  which is a cross-sectional view of the tray  200  taken at the level of the area  214 . Specifically, the base material of the tray is provided with a receptacle  300  in which is placed an insert  302  defining the area  214 . To ensure a snug fit for the insert  302  is manufactured to be of about the same size as the receptacle  300 . In this fashion, the insert  302  is held in the receptacle  300  as a result of friction fit. Evidently, other mounting methods can be provided without departing from the spirit of the invention. One possible variant is to use a fastening mechanism that would allow the insert  302  to be removed. In this fashion, the insert  302  can be replaced with another insert, if the original insert is damaged or if it is deemed appropriate to change the X-ray response of the area  214 . 
     The main driver in selecting the material from which the insert  302  is to be made is to provide an X-ray signature that matches the X-ray signature of the liquid product associated with the area  214 . In a specific and non-limiting example of implementation the X-ray signature of an object that appears in an X-ray image can be expressed as the gray level intensity of the pixels in the portion of the image that depicts the object. This case assumes that the gray level intensity, which represents the degree of attenuation of the X-rays as they pass through the object, is relatively uniform across the object. This is the case when the object is made of material that is homogenous and thus attenuates the X-rays uniformly. Most liquid products would fall into that category. Another example is a situation when the object is not homogeneous and thus creates a certain gray level profile or pattern. The pattern may be regular or irregular. 
     Generally speaking, the X-ray signature of a material or object is the response produced by the material when the material interacts with X-rays. There are a number of interactions possible, such as:
         The Rayleigh scattering (coherent scattering)   The photoelectric absorption (incoherent scattering)   The Compton scattering (incoherent scattering)   The pair production   Diffraction       

     The photoelectric absorption of X-rays occurs when an X-ray photon is absorbed, resulting in the ejection of electrons from the shells of the atom, and hence the ionization of the atom. Subsequently, the ionized atom returns to the neutral state with the emission of whether an Auger electron or an X-ray characteristic of the atom. This subsequent X-ray emission of lower energy photons is however generally absorbed and does not contribute to (or hinder) the X-ray image making process. This type of X-ray interaction is dependent on the effective atomic number of the material or atom and is dominant for atoms of high atomic numbers. Photoelectron absorption is the dominant process for X-ray absorption up to energies of about 25 keV. Nevertheless, in the energy range of interest for security applications (for today&#39;s state-of-the-art security screening systems, the energy levels commonly utilized lie between 50 keV and 150 keV), the photoelectric effect plays a smaller role with respect to the Compton scattering, which becomes dominant. 
     Compton scattering occurs when the incident X-ray photon is deflected from its original path by an interaction with an electron. The electron gains energy and is ejected from its orbital position. The X-ray photon looses energy due to the interaction but continues to travel through the material along an altered path. Since the scattered X-ray photon has less energy, consequently it has a longer wavelength than the incident photon. The event is also known as incoherent scattering because the photon energy change resulting from an interaction is not always orderly and consistent. The energy shift depends on the angle of scattering and not on the nature of the scattering medium. Compton scattering is proportional to material density and the probability of it occurring increases as the incident photon energy increases. 
     The diffraction phenomenon of the X-rays by a material with which they interact is related to the scattering effect described earlier. When the X-rays are scattered by the individual atoms of the material, the scattered X-rays may then interact and produce diffraction patterns that depend upon the internal structure of the material that is being examined. 
     As to the pair production interaction, it refers to the creation of an elementary particle and its antiparticle from an X-ray photon. 
     That response produced by a material as it interacts with X-rays can be expressed in terms of gray level value, gray level patterns seen in the X-ray image or other physical manifestation. 
     The selection of the proper material for making the inserts  302  for the various areas  208 ,  210 ,  212  and  214  can be made by in a number of ways. One possibility is to pick materials that have a composition that is likely to provide a similar X-ray signature than the material associated with the area  208 ,  210 ,  212  and  214 . If adjustments are necessary, the thickness of the insert  302  can be varied so as to adjust its signature accordingly. 
     The insert may or may not be made from a homogenous material. An example of a non-homogeneous structure is an assembly of layers made from different materials that in combination would provide the desired X-ray signature. Another example is a mixture of different materials intended to create a pattern in the X-ray image. The person skilled in the art will recognize that an almost infinite number of different X-ray signatures can be developed by selecting the proper material or materials and by mixing or assembling them in the appropriate manner. 
     Examples of materials that can be used include plastics such as polyethylene, polypropylene or others. Their density or composition can be varied to obtain the desired X-ray signature. 
     The process for performing a security screening operation on the apparatus of  FIG. 1  and involving the tray  200  will now be described in greater detail, in connection with  FIG. 6 . 
       FIG. 6  is a flowchart of the method that is implemented at a security checkpoint at an airport or any other suitable location to screen hand carried baggage that relies on one example of implementation of the liquid screening process described earlier. The security checkpoint where this method is implemented would use an X-ray imaging system of the type shown in  FIG. 1  for example. At step  600  the passenger approaching the checkpoint is requested by security personnel or shown directives appearing on a board or any suitable display to remove any containers holding liquids that may be present in the hand carried baggage. The containers are placed on tray  200  and put on the conveyor belt of the X-ray imaging system. At step  602  an X-ray image is taken of the liquid product as it is carried on the tray  200 . The X-ray image is depicted on a monitor allowing the operator of the X-ray imaging system to examine X-ray image. An example of the X-ray image is shown in  FIG. 4  (note that for clarity  FIG. 4  shows the image purely in black and white without any shades of gray). The image shows the liquid product, in particular the container  102  and one of the areas, say the area  212 . 
     Assume that the liquid product is a water bottle and the area  212  is designed as a reference for water, in other words it X-ray signature matches the X-ray signature of a water bottle. By performing a comparison between the two X-ray signatures it is possible to determine the identity of the product in the container  102 . Specifically, if the X-ray signatures match it is highly probable that the liquid product is in fact water. The comparison process, which is shown at step  604  in  FIG. 6 , can be done in two ways. The first is by the human operator alone which observes both X-ray signatures visually and determines if they match. This is likely to be fairly imprecise; however it could work when the X-ray signatures are fairly distinctive such as when they are unique and easily recognizable patterns. 
     The other possibility to perform the X-ray signature comparison is to do it automatically by performing an image analysis. The image analysis can be done via image analysis software executed by the processing module  500 . The image analysis software processes the image portions that contain the area  212  and the container  102  to determine the likelihood of X-ray signature match between them. 
     More specifically, the software executed by the processing module  500  starts by identifying where are the edges of the container  102  in circumscribing the relevant image portion that is to be compared to the area  212 . The edge detection process includes the following steps:
         1. The first step is to locate a portion of the edge. The software searches for detectable gray level transition that occurs in the image as a result of the container  102  wall. Specifically, due to the structure/material of the container  102  wall a well defined gray level transition will show in the image. To facilitate the edge detection process it is possible to provide the operator console  128  with user interface tools that will allow the operator to designate in the X-ray image the general area where the container  102  is located. In this fashion, the software will start the image analysis in an area of the image that is known to contain the image of a container  102 . Once the X-ray image is shown to the operator  130 , he or she uses a tool to indicate where a container  102  lies. The operator  130  first identifies visually the container  102  to be processed. The operator  130  then uses a user interface tool to designate the container  102  to the software. The tool may be any suitable user interface tool such as pointer device such as a mouse or a touch sensitive feature allowing the operator  130  to touch the screen at the area of interest. When the pointer device is activated at the location  402 , which by convention is deemed to correspond generally to the center of the container  102 , the activation will produce location data. The location data identifies an area in the image where the container  102  resides. The software uses the location data to select the portion of the image data to which the location data points to and starts the image analysis in that area. The software operates with the assumption that the container&#39;s  102  features that will be identified should have some degree of symmetry about that location. The software scans the image data by moving further away from the location  402  until a sharp gray level gradient is located that corresponds to a container  102  edge. In principle, since the location  402  is in the center of the container  102  then a container  102  edge should be detected in the image at two places equally spaced from the location  402 .
           Another possibility is for the operator to designate with the pointing device specifically the edge of the container  102  that is to be analyzed. For instance, the operator  130  “clicks” the mouse or touches the screen with his/her finger at the location  404  that corresponds to the edge of the container  102 .   Yet another possibility is for the operator to perform the designation by “drawing” on the image a zone curtailing the area where the container  102  is located. For instance, the operator  130  can use the pointing device to draw the line  406  around the container  102 .   With any one of the methods described earlier, the edge detection software receives operator guidance to perform an image analysis and extract from the image one or more characterizing features of the container  102 .   
           2. The next step of the process is to track the outline of the container  102 . As the software has identified a portion of the container&#39;s edge, the software logic then starts tracking that edge. The tracking logic tracks the sharp gray level gradient in the image to follow the container&#39;s  102  edge.   3. When the container  102  detection process has been completed, the outline of the container  102  can be emphasized to the operator  130 , as a final “sanity check”. Specifically, the processing module  500  issues commands to the display such that the display visually enhances a portion of the image where the container  102  is located. This makes the container  102  more visible with relation to other objects in the X-ray image. Examples image enhancements include:
           a. Colouring or otherwise highlighting the areas of the image that correspond to the portions where the edge has been identified;   b. Coloring or otherwise highlighting the container  102  in its entirety.   c. De-emphasizing the image except the areas where the container  102  lies. This technique does not change the pixels of the X-ray image in the region of the container  102  but changes all the pixels that surround the container&#39;s  102  image such as to make the container  102  more visible.   The highlighting process uses the edge detection data obtained by the edge detection software as a result of the X-ray image analysis. The edge detection data defines in the X-ray image the areas where an edge has been identified. The highlighting process then uses this information to manipulate the X-ray image pixels such that the container  102  stands out with relation to its surroundings.   If the edge identification has been done correctly the operator  130  would see the container  102  highlighted. The operator  130  can then apply human judgment on the results. If the edge tracking operation is correct then the results can be accepted and the processing allowed continuing. Otherwise, if the operator  130  sees on the screen a highlighted shape that does not correspond to a container  102  then he/she aborts the operation.   
               

     Before the image processing can be initiated it is desirable to designate to the software the image portions to be compared. The designation of the area  212  can be done automatically since that area has a unique and known shape. The software can, therefore, perform an image analysis and search for that particular shape in the image. When several areas exist in the image, the software can identify them all and generate location information for each area  208 ,  210 ,  212  and  214 . 
     Alternatively, the operator  130  can designate in the image the areas  208 ,  210 ,  212  and  214  by using an appropriate graphical user interface tool, in a similar way to the designation of the container  102 . 
     Once the reference area  212  and the container  102  have been identified in the image, they are compared to determine if there is a match. Generally, this is a two step process. During the first step the X-ray signatures of the reference area  212  and of the container  102  are read. During the second step the X-ray signatures are compared to determine if there is a match. 
     The X-ray signature is read by performing an image processing operation. The operation is the same for the area  212  and for the container  102  and for the purpose of simplicity only one will be described. The software will process the image data to determine the gray level values at different positions in the area  212 . If they are all the same an assumption is then made that the area  212  has an X-ray signature that mimics a homogeneous material. Accordingly, that X-rays signature can then be expressed by the average gray level value of the pixels within the area  212 . On the other hand if the gray level values reflect a pattern, then the pattern itself represents the X-ray signature. For instance, a pattern will be produced if the X-ray apparatus  100  can sense X-ray scattering/diffraction. 
     When the X-ray signatures of the area  212  and of the container  102  have been determined, they are compared to find out if there is a match. In the case of homogeneous materials, the gray level values are compared and if they match within a predetermined tolerance, the X-ray signatures are considered to be matching. The predetermined tolerance can be varied according to the intended application. In instances where a high sensitivity is required, the tolerance will be small and conversely for situations that require less sensitivity a larger tolerance can be used. 
     When the X-ray signatures are expressed as gray level patterns, the degree of match can be established by using any suitable pattern matching algorithms. Also neural networks can be used to perform pattern matching operations. 
     An important advantage of performing a comparison between X-ray signatures extracted from the same image data is the elimination of X-ray induced variations in the system response. In this fashion, the system is self-referencing. 
     In the example described earlier, a comparison was performed between the X-ray signature of the liquid product and the X-ray signature of the area  212 . This could work in instances where the tray  200  has a single reference area, however in situations where the tray  200  has more than one reference area it may be difficult to determine which reference area of the set of reference areas on the tray should be compared to the X-ray signature of the liquid product. In this case, it is advantageous to determine the location of each reference area in the tray and read the X-ray signature of each reference area and then compare it with the X-ray signature of the liquid product. 
     The final step of the processing operation is the determination of the threat status of the liquid product on the basis of X-ray signature comparison. Several possibilities exist.
         1. The threat status can be implicitly determined when the X-ray signature comparison allows identifying the liquid product. For example, the operator  130  has operational knowledge that the area  212  has a reference X-ray signature of water, therefore if the there is a match between the X-ray signatures of the area  212  and of the liquid product, the latter can be assumed to be water as well, hence safe to carry on a plane, train or any other public transportation. Conversely, the area  212  can be provided as a reference for a dangerous substance, say hydrogen peroxide. If a match is found, then the liquid  104  in the container  102  is identified as being hydrogen peroxide and the operator concludes that this is a threatening substance.   2. The threat status can be derived without an explicit determination of the identity of the material in the container  102 . For instance the area  212  is designed with an X-ray signature to screen for a specific one or a class of substances that are deemed threatening. Therefore, the operator  130  does not need to know what those substances are. If there is a match between the X-ray signature of the area  212  and the X-ray signature of the liquid product, then the operator  130  concludes that the liquid product presents a threat. In a similar fashion, the area  212  with an X-ray signature that screens for “safe” products rather than “unsafe” products will show a match when “safe” products are put in the tray  200 . In this case, the match will be used as an indication that the liquid product does not present a safety risk.       

     In a possible variant, the tray  200  is provided with machine readable and/or human readable indicia to facilitate the liquid product screening operation. Several possibilities exist.
         1. The tray  200  can be provided with a human readable identification of the materials that are being screened. For instance, each area  208 ,  210 ,  212  and  214  bears a label showing the operator  130  which material is associated with that area. The label may say “water”, “orange juice” or any other. The label is visible to the operator  130  such as when the tray  200  and the liquid product it supports is placed in the X-ray apparatus  100  the operator  130  can visually see which one of the reference areas  208 ,  210 ,  212  and  214  will need to be compared with the liquid product. Another possibility is to make those labels visible only in the X-ray image. The labels can be created by placing inserts in the tray  200 , near the respective areas  208 ,  210 ,  212  and  214  which create a contrasting label in the X-ray image. In this fashion, the operator  130  would see the product that is associated with each area  208 ,  210 ,  212  and  214  and can manually designate the relevant area  208 ,  210 ,  212  and  214  that is to be compared with the liquid product.   2. The tray  200  is provided with machine readable indicia. The machine readable indicia can be in the form of a bar code or any other suitable machine readable code that is provided at an appropriate location in the tray. Preferably, the machine readable indicia are not visible to the human eye but shows on the X-ray image. This can be done by printing the tray surface with inks that create a high degree of contrast in an X-ray image or by placing an insert in the tray that carries the indicia made from a high density material that will easily show in the X-ray image. Examples of the type of information that the indicia conveys, include:
           a. The identity of the materials associated with the respective areas  208 ,  210 ,  212  and  214 . In addition to the identity information the indicia may specify the location in the tray of each area  208 ,  210 ,  212  and  214  with respect to a certain reference, which can be the indicia itself. The indicia is read by the image processing software and the information on the identity of the materials, in the case there is a match between the X-ray signatures of one of the areas  208 ,  210 ,  212  and  214  and the liquid product on the tray  200  can be displayed to the operator  130  on the same monitor showing the X-ray image or on a different monitor.   b. Information on the threat status. Instead of showing material identity information, the indicia convey information on the threat status.   c. The indicia can be used as an index to search a database that provides additional info to the operator  130  about the liquid product associated with the reference area  208 ,  210 ,  212  and  214 .   
               

     The graphical user interface on the console  128  displays the results of the comparison operation. The information that can be shown includes:
         1. The identity of the material;   2. The threat status;   3. The degree of confidence in the assessment based on the degree of match between the X-ray signatures.       

     The example of implementation shown in  FIG. 2  depicts the areas  208 ,  210 ,  212  and  214  placed in the respective corners of the tray  200 . This is done in order to reduce the likelihood of obscuring anyone of those areas  208 ,  210 ,  212  and  214  by an article that is placed in the tray. For instance, if an article is put in the tray immediately above anyone of those areas  208 ,  210 ,  212  and  214 , the X-ray signature of that area may not be correctly read since the X-ray image will be the result of a composite response (the area  208 ,  210 ,  212  and  214  and the article on top of it). In order to further reduce the possibility of obscuring the areas  208 ,  210 ,  212  and  214  it is possible to place the areas  208 ,  210 ,  212  and  214  at a location that is outside the zone in the tray where the articles to be screened are located. An example of such embodiment is shown in  FIG. 7 . The tray  700  defines a central article receiving area  702  in which are placed the articles to be screened. The article receiving area  702  is surrounded by a rim portion  704  that extends peripherally and fully encircles the article receiving area  702 . The rim portion  704  has a top area  706  that is flat and that is sufficiently wide such as to accept the reference areas  208 ,  210 ,  212  and  214 . In this fashion, articles to be screened are unlikely obscure anyone of the areas  208 ,  210 ,  212  and  214  that remain outside the central article receiving area. 
     In a possible variant, the tray  200  is used to provide a material reference during the X-ray scanning process to limit or avoid altogether machine induced variations in the results. Since in practice different X-ray apparatuses are never identical and manifest some variations that can be either at the level of the X-ray detectors elsewhere in the machine, those variations can impact the detection results. 
     Under this variant, the tray  200  is used as a known reference for the X-ray scanning apparatus. Accordingly, when the X-ray scanning process is performed the X-ray apparatus  100  can use the X-ray signature of the tray  200  to self-calibrate or compensate the image data for variations. 
     Since in the course of an X-ray scanning operation the tray  200  will be used repeatedly, the self-calibration operation occurs with regularity, thus enhancing the performance of the X-ray apparatus in terms accuracy in identifying security threats. 
     Under this variant, the reference areas  208 ,  210 ,  212  and  214  are not compared to anyone of the articles that are put in the tray  200  during the scanning operation. Rather, the processing module  500  senses the X-ray signatures of the reference areas  208 ,  210 ,  212  and  214  and determines if there is any variation from what those signatures are expected to be. Recall that since the reference areas  208 ,  210 ,  212  and  214  are made from known materials, hence their x-ray signatures are known, the module  500  can determine if there is any variation between the nominal X-ray signatures (the expected signatures) and those read by the X-ray apparatus  100 . 
     If deviations are observed, the module  500  can perform a corrective action. Such corrective action may include compensating the signatures to X-rays observed in connection with items in the tray that are being scanned. 
     After the X-ray scanning process has been completed, the locations of reference areas  208 ,  210 ,  212  and  214  in the X-ray image are identified by anyone of the techniques described earlier. Subsequently, the X-ray signatures of the reference areas  208 ,  210 ,  212  and  214  are determined. 
     In one specific example, the X-ray signatures that are being read can be expressed in term of gray scale values. If the reference areas  208 ,  210 ,  212  and  214  are all uniform, the X-ray signature of each one of them can be expressed as a single gray scale value. The nominal X-ray signatures of the reference areas  208 ,  210 ,  212  and  214  can be stored in the memory of the module  500  or they can carried or expressed on the tray  200  for automatic reading by the image processing software of the module  500 . The nominal gray scale values can be expressed on the tray as bar codes or any other encoding that can be read by the image processing software. 
     If the nominal X-ray signatures are stored in the memory of the module  500 , they can be arranged in a database, such as database  134 , particularly if a number of different X-ray signatures are to be maintained. To locate the proper entry in the database the image processing software in the module  500  can use several different techniques. In one case, the image processing software determines an identifier of a reference area  208 ,  210 ,  212  and  214  and uses that identifier to find the proper entry in the database. The identifier can be printed or otherwise marked on the tray  200 , in the manner discussed earlier such that it appears in the X-ray image. 
     The other option, where the X-ray signatures of the reference areas  208 ,  210 ,  212  and  214  are marked on the tray  200  itself, those X-ray signatures are represented by any suitable method such as a bar code or any other machine readable format that can be read by the image processing software in the module  500 . 
     Irrespective of the option chosen, the module  500  determines the X-ray signature of the reference areas  208 ,  210 ,  212  and  214  by processing the image data generated by the X-ray apparatus  100  and then determines the nominal X-ray signatures of the  208 ,  210 ,  212  and  214 . Both sets of X-ray signatures are then compared. If there is a match within a certain tolerance, the X-ray apparatus is deemed to be calibrated. On the other hand, if there is a variation outside the tolerance range, then a corrective action is taken. 
     The corrective action may vary depending upon the particular implementation chosen. When the tray  200  has several reference areas having different X-ray signatures, it is possible to determine the X-ray machine induced variations for a range of different materials (reference areas). In that case, the corrective action may be a simple global compensation of the image data, which produces compensated image date where each portion of the image is compensated in the same fashion. The degree of compensation is determined on the basis of a computed average of the variations between the nominal X-ray signatures and the X-ray signatures that are being measured from the image data. 
     A more sophisticated approach is to compensate parts of the image differently based on how close they are to the reference areas  208 ,  210 ,  212  and  214 . For instance, the module  500  determines a specific compensation to carry out in connection with each reference area  208 ,  210 ,  212  and  214  and then applies those compensations selectively to different image portions depending on how close the image portions are to the respective reference areas  208 ,  210 ,  212  and  214 . In this fashion, the overall compensation more accurately tracks the X-ray induced machine variations. 
     The identification of the different areas in the image and their association to the respective reference areas  208 ,  210 ,  212  and  214  which determines which compensation to apply, can be done by segmenting the image. Such segmentation can be done in any arbitrary fashion, such as by dividing the image in to regular blocks and then associating each block to a different reference area  208 ,  210 ,  212  and  214 . The association can be done by processing the image information in each block and determining how close the gray level values are to anyone of the gray level values of the reference areas  208 ,  210 ,  212  and  214 . Once the comparison is made, each block is assigned to anyone of the reference areas  208 ,  210 ,  212  and  214  and the compensation for anyone of those areas is then applied to the block. 
     Once the compensated image is produced it can be processed by using any suitable techniques to determine the security status of the articles placed in the tray  200 . Those techniques are automated techniques and they may include determining the density, effective atomic number, diffraction/scattering signature or a combination thereof on the basis of software processing of the compensated image data. 
     Note that in simple situations, the tray  200  may not need to be provided with multiple reference areas  208 ,  210 ,  212  and  214 . The tray  200  itself may be made from a material that constitutes a reference area. In this case the tray  200  has a single reference area. 
     Although various embodiments have been illustrated, this was for the purpose of describing, but not limiting, the invention. Various modifications will become apparent to those skilled in the art and are within the scope of this invention. Specifically, note that while the examples of implementation provided earlier are directed to the assessment of the security status of liquid products, the invention is not limited to the screening of liquid products and can be used for screening other products as well. In particular, the invention can be used to identify and/or assess the threat status of materials in bulk form. This could be done by placing a sample of the material in a tray and performing the assessment as described earlier to identify the material/and or assess its security status. Also, the screening process described above is not limited to the use of X-rays. Other penetrating radiation can be used.