Abstract:
A cargo inspection apparatus and process includes scanning containers with x-rays along two different planes. Outputs from x-ray sensors along the two different planes are collected for use in establishing the presence of contraband within the container. Using the sensor output data, images of the container are provided and suspicious areas and background areas are identified on the images. By using representative suspicious area and background area geometry and compensating in the suspicious area for the effective estimated background thereat, the average atomic number and density of the suspicious contraband is established. The average atomic number and density is then compared with known atomic numbers and densities of contraband materials and an output indicative of whether the suspicious area falls within the parameters of actual contraband material is provided.

Description:
This application claims priority under 35 USC § 119 (e)(1) of Provisional Application No. 60/159,614 filed Oct. 14, 1999. 
    
    
     TECHNICAL FIELD 
     The present invention relates to the technical field of detecting contraband such as weapons, explosives, drugs, etc., in containers such as large cargo containers and small baggage. More particularly, the present invention is directed to an apparatus and process of non-intrusive x-ray inspection of containers through which contraband can be detected. 
     BACKGROUND 
     The transport of contraband has become and continues to be a worldwide problem. For safety, compliance of laws and other numerous reasons, it is desirable to inspect containers for establishing whether or not contraband is contained therein. Because it is impractical to open and physically inspect every container traveling through, for example, airports and harbors, it is desirable that the containers be inspected in a non-intrusive manner, that is, without opening and physically inspecting. Numerous attempts and devices have heretofor been suggested for accomplishing such non-intrusive inspections. Some such devices and/or components of such devices known to applicant are disclosed in the following references: 
     1. U.S. Pat. No. 5,442,672, August 1995; 
     2. Richard F. Eilbert and Kristoph D. Kug, SPIE Vol. 1824 (1992)/127-143; 
     3. U.S. Pat No. 5,319,547, June 1994; 
     4. U.S. Pat No. 5,490,218, February 1996; 
     5. U.S. Pat No. 5,600,700, February 1997; 
     6. U.S. Pat No. 5,642,393, June 1997; 
     7. Pratt R. H., Tseng H. K., Lee C. M. Atom. Data. Nucl. Data. Tables. 1977. O1. 20, No. 2.P. 175-209; and, 
     8. Russian Federation Patent No. 2115914, 23.04.1997. 
     Prior non-intrusive inspection apparatus and systems, however, are inaccurate and are thus impractical because they either are incapable of detecting contraband or can not distinguish between contraband and other materials, thereby passing over and not detecting contraband or mistaking other materials for contraband and causing false alarms. 
     Accordingly, a need exists for a more accurate cargo inspection apparatus and process through which contraband can more accurately be detected with minimal false alarms. 
     SUMMARY OF THE INVENTION 
     It is the principal object of the present invention to overcome the above-discussed disadvantages associated with prior cargo inspection apparatus and processes. 
     The present invention, in summary, is an apparatus and process through which cargo and baggage containers are scanned with x-rays along two different planes. While scanning along the first plane, a first sequence of one dimensional arrays from outputs of a plurality of x-ray sensors representative of a first x-ray intensity spectrum is obtained and a second sequence of one dimensional arrays from outputs of a plurality of x-ray sensors representative of a second x-ray intensity spectrum is also obtained. Similarly, while scanning along the second plane, a second sequence of one dimensional arrays representative of a first x-ray intensity spectrum and a second sequence of one dimensional arrays representative of a second x-ray intensity spectrum are also obtained. By using the first intensity spectrum and second intensity spectrum sequence of one dimensional arrays, atomic number sequences of one dimensional arrays are calculated for both the first and second planes. By combining at least one of the first plane first or second one dimensional arrays, a first plane image is provided. Similarly, by combing at least one of the second plane first or second sequence of one dimensional arrays, a second plane image is provided. A suspicious area and a background area are then identified in at least one of the first plane image or the second plane image. By using a representative suspicious area and representative background area geometry along with the atomic number and mass thickness values of the suspicious areas and background areas and by assuming that the effect of the background area is the same in the projection through the suspicious area, the background values are subtracted or otherwise compensated for in the suspicious area values for thereby effectively removing the background and calculating the more precise average atomic number and density of the suspicious contraband. Thereafter, the average atomic number and density of the suspicious contraband is compared with known atomic numbers and densities of actual contraband materials and a visual or audible output is provided indicative of whether the suspicious contraband falls within the parameters of actual contraband material. 
     Preferably, in addition to the first and second plane images, first and second plane atomic number display images are also provided for aiding in the identification of suspicious contraband. More preferably, the first and second plane atomic numbers display images provide a color display of the scanned container with the various materials corresponding to different groups of atomic numbers being displayed in different colors. 
     In one form thereof, the present invention is directed to a process of detecting contraband within a container. The process includes the steps of scanning the container with x-rays along a first plane and obtaining a first sequence of one dimensional arrays from outputs of a plurality of x-rays sensors representative of a first x-ray intensity spectrum. A second sequence of one dimensional arrays from outputs of a plurality of x-ray sensors representative of a second x-ray intensity spectrum is also obtained along the first plane. The container is further scanned with x-rays along a second plane and a second sequence of one dimensional arrays from outputs of a plurality of x-ray sensors representative of a first x-ray intensity spectrum is obtained. A second sequence of one dimensional arrays from outputs of a plurality of x-rays sensors representative of second x-ray intensity spectrum along the second plane is also obtained. By using the first plane first intensity spectrum and the second intensity spectrum sequence of one dimensional arrays, a first plane atomic number sequence of one dimensional arrays is calculated. By using the second plane first intensity spectrum and the second intensity spectrum sequence of one dimensional arrays, a second plane atomic number sequence of one dimensional arrays is calculated. A first plane image is then provided by combining at least one of the first plane first or second one dimensional arrays. A second plane image is also provided by combining at least one of the second plane first or second one dimensional arrays. A suspicious area and a background area are then identified in at least one of the first plane image or the second plane image. Using a representative suspicious area and background area geometry, along with the atomic number and mass thickness values of the suspicious areas and background areas, the average atomic number and density of the suspicious contraband is calculated. The average atomic number and density of the suspicious contraband are then compared with known atomic numbers and densities of actual contraband materials and an output is provided indicative or whether the suspicious contraband falls within the parameters of actual contraband material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features and objects of this invention and the manner of obtaining them will become more apparent and the invention itself will be better understood by reference to the following description of the embodiments of the invention taken in conjunction with accompanying drawings wherein: 
     FIG. 1 is a diagrammatic plan view of a cargo inspection apparatus constructed in accordance with the principles of the present invention; 
     FIG. 2 is a diagrammatic sectional view of a cargo inspection apparatus constructed in accordance with the principles of the present invention, and further diagrammatically showing a container being scanned along two different planes and sensors providing outputs along the two different planes for creating sequences of one dimensional arrays which are then stored and used for computing and establishing the presence of contraband; 
     FIG. 3 is a depiction of matrices of one dimensional array outputs of the sensors along the two different planes; 
     FIG. 4 is an example of gray scale x-ray images of a container and wherein each image is along a different plane; 
     FIG. 5 is a diagrammatic view of sensors arranged along two orthogonal planes and depicting filtering of every other sensor for thereby obtaining outputs representative of two different x-ray intensity spectrums; 
     FIG. 6 depicts matrices of outputs from every other sensor (odd sensors); 
     FIG. 7 depicts matrices of outputs from every other sensor (even sensors); 
     FIG. 8 depicts matrices of calculated atomic numbers along both of the planes; 
     FIG. 9 is a radiographic inspection procedure flow chart; 
     FIGS. 10 a  and  10   b  are diagrammatic views of side and plan image views of a container and depicting the identification of a suspicious area and background area and further depicting identifying by rectangle or square the representative suspicious areas and background areas for further analysis and interrogation; 
     FIG. 11 is a diagrammatic side view of a cargo inspection apparatus and showing a container being scanned, and further depicting the x-ray lines of travel therethrough as ultimately would create an output by the sensors; 
     FIGS. 12-16 are flow charts depicting the inspection process in accordance with the present invention; and, 
     FIG. 17 is a diagrammatic view depicting x-ray radiation passing through a volume containing suspicious materials. 
     Corresponding reference characters indicate corresponding parts throughout the several views of the drawings. 
     The exemplifications set out herein illustrate preferred embodiments of the invention in one form thereof and such exemplifications are not to be construed as limiting the scope of the disclosure or the scope of the invention in any manner. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A cargo inspection apparatus constructed in accordance with the principles of the present invention is diagrammatically shown and generally designated by the numeral  1  in FIGS. 1 and 2. 
     Apparatus  1  includes a conveyor  2  for transporting cargo such as an object or package  3  in known and customary manner. Conveyor  2  extends through an inspection chamber housing  4  made of radiation shielding material such as lead. As more fully discussed hereinbelow, apparatus  1  and the process of operation thereof are provided so that the various packages and/or objects  3  may be passed through the inspection chamber housing  4  on conveyor  2  whereat they are inspected, typically for determining whether they contain contraband such as weapons, explosives, drugs, etc. 
     Referring more particularly to the diagrammatic cross-section of the inspection chamber  4  of FIG. 2, apparatus  1  is further provided with an x-ray source  5  with focus S 1  for selectively providing x-ray beam γ V  through a window  6 . As shown, the beam of x-rays γ V  from source  5  projects or travels generally horizontally across the inspection chamber housing  4 , whereat it travels through window  7  and falls on or is received by the vertical multi-channel detector  8 . Multi-channel detector  8  is essentially made up of a plurality of x-ray elementary detectors (sensors) N V  spaced vertically as shown and having an output for each sensor which is generally proportional to the intensity of the x-ray beam being detected. Thus, for each x-ray beam portion, each of the sensors N V  provides an output which is dependent on the density and x-ray absorbing ability of the object  3  and its contents. Naturally, if no object is in the path of the x-ray beam γ V , all of the sensors N V  of detector  8  will have an output which is substantially the same. Additionally, if a very dense or highly x-ray absorbing item is within the object  3 , the x-ray beam γ V  is substantially attenuated by that item and the sensors therebehind have a proportionally lower output. 
     A second x-ray source  9  with focus S 2  is also provided and selectively provides beams of x-rays γ H  through a window  10  generally vertically downwardly as shown. The beam γ H  of x-rays from source  9  travels through the object  3 , conveyor  2  and window  11  whereat it falls or is received and is detected by the horizontally disposed multi-channel detector  12 . Multi-channel detector  12  is similarly made up of a plurality of individual x-ray elementary detectors (sensors) N H  which are horizontally disposed as shown and have an output proportional to the intensity of the x-ray beam received thereat. 
     Each of the x-ray sources  5  and  9  are housed by, for example, shielding elements  13  to prevent radiation leakage. 
     The outputs of each of the sensors N V  and N H  of the multi-channel detectors  8  and  12  are received by analog to digital circuitry diagrammatically depicted by the numeral  14  and which converts, at any given point in time, the analog outputs from each of sensors N V  and N H  of detectors  8  and  12  into digital values. More specifically, at any point in time, analog to digital converter  14  converts and provides a one dimensional digital array of values for the vertically situated sensors of detector  8 , and another one dimensional digital array of values for the horizontally situated sensors of detector  12 . 
     These one dimensional digital arrays of values are processed and saved by a computer  15  for displaying images of the object on computer screens  16  and  17 . A computer input device  18  such as a keyboard or a mouse is also provided and is connected to the computer  15  for use and control by the operator  19 . (See FIG. 1) 
     In operation, as an object  3  is traveling through the inspection chamber  4  on conveyor  2 , it is bombarded by x-ray beams γ V  and γ H  from both x-ray sources  5  and  9 . The output signals of detectors  8  and  12  are provided at a frequency which is preselected depending on the speed at which object  3  is traveling and the desired resolution or accuracy of the apparatus. Nevertheless, for each x-ray beam or during each period, a one-dimensional digital array is created and stored by computer  15  for each detector  8  and  12 . As object  3  is carried past detectors  8  and  12 , a sequence of one dimensional arrays are created and stored, thus forming a signal matrix for each of the detectors  8  and  12 . Each of the values in the signal matrices are converted by the computer to a matrix of signals U(M V , N V ) or U(M H , N H ) which can be displayed on a computer screen. The structure of these signal matrices and some other ones (for example, B(M V , N V ), B(M H , N H ), X(M V , N V ) X(M H , N H ), etc.) to be mentioned below is shown in FIG. 3. M V  and M H  are the numerals of interrogations (read-outs) of the detectors  8  and  12  respectively. The M V  and M H  interrogations (read-outs) apply to a particular cross sections of the object  3  by the x-ray beams γ V  and γ H  (see FIG. 2) while the object  3  is crossing the x-ray beam plane(s). N V  and N H  are the numerals of elementary detectors (sensors) in the detectors  8  and  12 . Here and below the values noted by the index V apply to screening the object  3  by the x-ray beam γ V , and the index H applies to screening the object  3  by the x-ray beam γ H . Thus, the signal matrix U(M V , N V ) created from the vertical detector  8  is used for creating an x-ray image or view as would be seen from x-ray source  5  which is essentially a side view of object  3 . This side view x-ray image is viewed by the operator on the computer screen  16  in a gray scale, and showing the more dense items within object  3  darker or more black than those items which are less dense. Similarly, the signal matrix U(M H , N H ) created from the signals from the horizontal detector  12  is used for creating an x-ray image of the contents of object  3  as seen from x-ray source  9  which is essentially a plan view thereof. Preferably, both the side and plan view images are displayed on a single computer screen, namely, screen  16 . An example of these side and plan x-ray images as are seen by the operator is shown in FIG.  4 . 
     It is known that the sensor output signals referred to as DR which are read out from the detectors  8  and  12  and consequently the signal values stored in the form of the U(M V , N V ) or U(M H , N H ) matrices are a function of the atomic number Z and mass thickness X of object  3  material along the x-ray path from the x-ray source focus S 1  or S 2  to the particular elementary detector N V  or N H  of multi-channel detector  8  or  12 : 
     
       
           DR=f ( Z, X ). 
       
     
     In addition, the DR value at a given Z and X depends also on an x-ray intensity spectrum J(E) generated by the sources  5  and  9 . The J(E) spectrum describes the energy distribution of x-ray quanta in the beam γ V  or γ H  and can be characterized by its shape and effective energy boundaries E min  and E max .The lower boundary E min  is governed by material and thickness of a x-ray filter placed between the source  5  or  9  anode and the detectors  8  or  12 . The higher boundary E max  is related to the source  5  or  9  anode high voltage. 
     To analyze the object  3  material, the so called dual energy inspection mode is employed when two differing J 1 )E) and J 2 (E) spectra are used. As a result at same Z and X values (or at the same object cross section passing through the x-ray beam plane) two sensor response values DR 1  and DR 2  are obtained: 
     
       
           DR   1   =f   1 ( Z, X ); 
       
     
     
       
           DR   2   =f   2 ( Z, X ).  ( 1 ) 
       
     
     Correspondingly two sets of image signal matrices are obtained: 
     U 1 (M V , N V ) and U 1 (M H , N H ) corresponding to J 1 (E) spectrum; and 
     U 2 (M V , N V ) and U 2 (M H , N H ) corresponding to J 2 (E) spectrum, 
     where the normalized signals defined as U 1 =DR 1 /DR 10  and U 2 =DR 2 /DR 20  are stored. DR 10  and DR 20  are the elementary detector N V  or N H  responses to J 1 )E) and J 2 (E) spectra respectively at X=0 (object  3  is absent). 
     There are two practical ways to implement a dual energy mode of object  3  inspection. The first is to vary the higher x-ray spectrum boundary E max , that is to have two different anode voltages. The second is to modify the lower spectrum boundary E min  which means to have two different x-ray filtration levels. 
     In the proposed radiographic apparatus  1  dual spectra mode is implemented by incorporating the grid filters F into the multi-channel detectors  8  and  12  (see FIG. 5) so that filter material overlaps every second or every other elementary detector (for example each even sensor N VE  or N HE ). Varying filter material and thickness provides for optimal x-ray spectrum modification. Preferably, copper grid filters of filtering thickness of 1 mm are used. The elementary detectors with odd numerals N VO  or N HO  remain open (are not screened by the filter material). 
     For this specific approach the structure of the U 1 (M V , N VO ); U 1 (M H , N HO ) and U 2 (M V , N VE ) and U 2 (M H , N HE ) matrices is illustrated in FIGS. 6 and 7. 
     To create the black and white image of side view, the two matrices U 1 (M V , N VO ) and U 2 (M V , N VE ) are used instead of the one matrix U(M V , N V ) described above for single energy inspection mode. To transform these two matrices into one B(M V , N V ), which is displayed as gray brightness levels onto the monitor screen  16 , the specially developed correlative function h(U 1 , U 2 ) is used. By analogy two matrices U 1 (M H , N HO ) and U 2 (M H , N HE ) are used to form the B(M H , N H ) matrix and the corresponding black and white image of the plan view. The resulted black and white image is the same as shown in FIG.  4 . 
     The calculation of the Z and X values proceeds in computer  15  in parallel with preparing the black and white image. 
     For the system  1  the problem of determining Z and X values, which consists in solving equation system (1), is considered in general form (irrespectively of technical dual energy mode realization), with elementary detector responses DR 1 , and DR 2  being described analytically. The obtained solution in the form of the analytic algorithm with a set of free parameters is based on mathematical simulation of the elementary detector response, and therefore is valid for both mentioned above methods of implementing the dual energy inspection mode. The details of the simulation procedure and the approach to using the simulation results for approximating Z and X as a function of elementary detector responses U 1 , and U 2  in dual energy inspection mode are described in Appendix 1. 
     The particular algorithms FZ(U 1 ,U 2 ) and FX(U 1 ,U 2 ) with the parameters adjusted for the preferred technical implementation of the apparatus  1  is obtained and included in the system  1  software for calculating Z and X values (see Appendix 2). 
     Each Z or X value resulted from combined processing of signals U 1  and U 2  detected by elementary detectors N V  in multi-channel detector  8  is stored in the Z(M V , N V ) or X(M V , N V ) matrix having the structure same as shown in FIG.  3 . By analogy the Z(M H , N H ) and X(M H , N H ) matrices are created on the basis of processing the signals U 1  and U 2  detected by the sensors N H  of multi-channel detector  12 . 
     In the proposed technical realization of dual energy inspection mode each Z or X value resulted from combined processing of signals detected by a pair of adjacent elementary detectors N VO  and N VE  (one of odd numeral and one of even numeral) in multi-channel detector  8  is stored in the Z(M V , K V ) or X(M V , K V ) matrix. K V  is a sensor pair (N VO  and N VE ) numeral. The structure of these matrices is shown in FIG.  8 . By analogy the Z(M H , K H ) and X(M H , K H ) matrices are created on the basis of processing the signals detected by the sensors N HO  and N HE  of multi-channel detector  12 . 
     The Z(M V , N V ) and Z(M H , N H ) (or Z(M V , K V ) and Z(M H , K H ) in proposed system) matrices are converted by the computer  15  into the C(M V , N V ) and C(M H , N H ) (or C(M V , K V ) and C(M H , K H ) in the proposed system) so that to display them on the monitor screen  17  as a Z-image of side and plan views . Preferably, the effective atomic numbers recorded in the Z(M V , K V ) and Z(M H , K H ) matrices are converted in color codes and displayed on the screen  17  in accordance with the following scheme C(Z): 
     1. When Z is less that 8, representative of most organic substances, the screen color shown is gray; 
     2. When Z is 8-12, representative of most explosives and drugs, the screen color shown is red; 
     3. When Z is 12-16, representative of light metals such as aluminum, the screen color shown is green; 
     4. When Z is greater than 16, representative of most heavier metals such as iron and copper, the screen color shown is blue. 
     The intervals of Z may be changed and adjusted by the operator. 
     The flowchart of the described dual energy and dual view inspection mode is presented in FIG.  9 . 
     The Z-image displayed on the screen  17  reflects the effective atomic numbers averaged over the object  3  material along the x-ray path across the object  3  when passing from the focus S 1  or S 2  of x-ray sources  5  or  9  to the elementary detectors N V  or N H . Although the plan and side image views of Z-image shown on the computer screen  17  are helpful to the operator in establishing if, for example, the inspected object contains weapons, explosives or drugs, as can be appreciated, because only the effective atomic numbers of object  3  as a whole are displayed but not effective atomic numbers of items inside the object  3 , it is not conclusive and can be deceiving. These images are, however, helpful to the operator in determining a potential threat. 
     By viewing both computer screens  16  and  17  of the images created as described hereinabove, the operator can establish or determine suspicious or potential threatening and/or unlawful items or substances within an object  3 . When a potential threat is recognized, the operator proceeds to further analyze such potential threat as described hereinbelow. In this regard, FIGS. 10 a  and  10   b  diagrammatically show x-ray black and white side and plan view images of two items inside an object  3  as are, for example, seen by the operator on computer screen  16 . FIG. 11 schematically illustrates location of these items inside the object  3  applying to image shown in FIGS. 10 a  and  10   b.    
     By viewing these images as well as the atomic number images thereof on screen  17  (not shown), the operator identifies the potential threat such as, for example, the darker area  19  in the side view and darker area  20  in the plan view both corresponding to item  25  (see FIG.  11 ). At that point, the operator, using a mouse or other computer input device  18 , makes a target designation, that is points to the darker area  19  in the side view and darker area  20  in the plan view. Two implementations of target designation are realized in apparatus  1 : 
     1. Manual Target Designation Mode 
     Operator draws a rectangle or square frame  21  on the suspicious area  19  in the side view of FIG. 10 a  and also draws a corresponding rectangle or square  22  in the suspicious or threatening area  20  of the plan view as shown in FIG. 10 b.    
     2. Automated Target Designation Mode 
     Operator points or clicks with a computer mouse on the suspicious area  19  in the side view image and the rectangle frame  21  appears around the clicking point automatically. Then operator clicks on the suspicious area  20  and the rectangle frame  22  appears around the clicking point automatically. Simultaneously the background target designation proceeds in both image projections (see below). 
     Framing  21  initiates calculation of the average effective atomic number Z Vs , average mass thickness X Vs , and average geometric size (along the γ H  beam path) R H  of the framed part of object  3  as it is viewed from focus S 1  of the x-ray source  5 . Concurrently errors ΔZ Vs  and ΔX Vs  are determined. Framed part of object  3  contains material of potential threat  25  and other materials against or in front of and behind potential threat  25  along the path of the x-ray beam γ V  through the object  3 . The matrices Z(M V , N V ) and X(M V , N V ) (or Z(M V , K V ) and X(M V , K V ) for proposed system) are used for calculation. Averaging is performed over the matrix elements which fall inside boundaries specified by the frame  21 , that is, over the matrix elements with interrogation numerals M V  from M 1V  to M 2V  and elementary detector numerals N V  from N 1V  to N 2V  (see FIGS. 10 a  and  11 ). 
     Framing  22  initiates calculation of the average effective atomic number Z Hs , average mass thickness X Hs , and average geometric size (along the γ V  beam path) R V  of the framed part of object  3  as it is viewed from focus S 2  of the x-ray source  9 . Concurrently errors ΔZ Hs  and ΔX Hs  are determined. Framed part of object  3  contains material of potential threat  25  and other materials against or in front of and behind potential threat  25  along the path of the x-ray beam γ H  through the object  3 . The matrices Z(M H , N H ) and X(M H , N H ) (or Z(M H , K H ) and X(M H , K H ) for proposed system) are used for calculation. Averaging is performed over the matrix elements which fall inside boundaries specified by the frame  22 , that is over the matrix elements with interrogation numerals M H  from M 1H  to M 2H  and elementary detector numerals N H  from N 1H  to N 2H  (see FIGS. 10 b  and  11 ). 
     The flowchart of calculating the average effective atomic numbers Z Vs  and Z Hs , average mass thicknesses X Vs  and X Hs  is shown in FIG.  12 . 
     Values of N 1V , N 2V  and N 1H , N 2H  are used for determining geometrical coordinates Q V , Q H  of the threat object  25  center. The middle detector numerals N mV  and N mH  are specified (see FIG.  11 ). Then the specially designed software block GEOMETRY is employed, which calculates the distances from the object  25  center to the detectors  8  and  12  on the basis for the specific dimensions of the inspection chamber  4  and elementary detectors N V  and N H , for example in cm. After that, geometric sizes GL V  and GL H  of object  25  along the vertical and horizontal directions are determined in same units. Then calculating the angle of the ray from focus S 1  or S 2  to elementary detector N mV  and N mH  with respect to multi-channel detector  8  or  12  the geometrical size R H  or R V  of framed part  22  or  21  of the potential threat  25  is evaluated. The flowchart of geometrical thickness R H  and R V  evaluation is shown in FIG.  13 . 
     Next is the procedure of background target designation. Also two implementations of this procedure are realized. 
     1. Manual Target Designation Mode 
     The operator, using a mouse or other computer input device  18 , draws a rectangle or square  23  on the area of object  3  close to suspect area  19  in the side view of FIG. 10 a . The material of object  3  corresponding to image area framed by rectangle or square  23  is considered to be equivalent to screening material against and behind potential threat  25  along the path of the x-ray beam γ V  through the object  3 . Then the operator draws a rectangle or square  24  on the area of object  3  close to suspect area  20  in the plan view of FIG. 10 b . The material of object  3  corresponding to image area framed by rectangle or square  24  is considered to be equivalent to screening material against or in front of and behind potential threat  25  along the path of the x-ray beam γ H  through the object  3 . Materials of object  3  corresponding to the image areas framed by rectangles or squares  23  and  24  are called background. 
     2. Automated Target Designation Mode 
     In this case the background concept is the same as described for Manual Target Designation procedure. 
     Clicking on suspicious area  19  initiates the automatic analysis of the Z(M V , N V ) and X(M V , N V ) (or Z(M V , K V ) and X(M V , K V ) for proposed system) matrices according to the preset algorithm of determining background area in the side view. That is, the matrix elements, which are considered to correspond to the background material are selected. Chosen matrix elements form a set  23 ′. (not shown) 
     Clicking on suspicious area  20  initiates the automatic analysis of the Z(M H , N H ) and X(M H , N H ) (or Z(M V , K H ) and X(M V , K H ) for proposed system) matrices according to the preset algorithm of determining background area in the plan view. That is, the matrix elements, which are considered to correspond to the background material are selected. Chosen matrix elements form a set  24 ′. (not shown) 
     Drawing frame  23  or specifying set  23 ′ initiates calculation of the average effective atomic number Z Vb  and average mass thickness X Vb  of the background material. Concurrently errors ΔZ Vb  and ΔX Vb  are determined. The matrices Z(M V , N V ) and X(M V , N V ) (or Z(M V , K V ) and X(M V , K V ) for proposed system) are used for calculation. Averaging is performed over the matrix elements which fall inside boundaries specified by the frame  23  or belong to set  23 ′. FIG. 10 a  illustrates the Manual Target Designation Mode when averaging is carried out over the matrix elements with interrogation numerals M V  from M 1Vb  to M 2Vb  and elementary detector numerals N V  from N 1Vb  to N 2Vb . 
     Drawing frame  24  or specifying set  24 ′ initiates calculation of the average effective atomic number Z hb  and average mass thickness X Hb  of the background material. Concurrently errors ΔZ Hb  and ΔX Hb  are determined. The matrices Z(M H , N H ) and X(M H , N H ) (or Z(M H , K H ) and X(M H , K H ) for proposed system) are used for calculation. Averaging is performed over the matrix elements which fall inside boundaries specified by the frame  24  or belong to set  24 ′. FIG. 10 b  illustrates the Manual Target Designation Mode when averaging is carried out over the matrix elements with interrogation numerals M H  from M 1Hb  to M 2Hb  and elementary detector numerals N H  from N 1Hb  to N 2Hb . 
     The flowchart of calculating the average effective atomic numbers Z Vb  and Z Hb , average mass thicknesses X Vb  and X Hb  of threat object  25  background is shown in FIG. 14 for Manual Target Designation Mode. 
     The average mass thickness X Vi  of the potential threat  25  in side view is calculated: 
     
       
         
           X 
           Vi 
           =X 
           Vs 
           −X 
           Vb. 
         
       
     
     The average mass thickness X Hi  of the potential threat  25  in plan view is calculated: 
     
       
         
           X 
           Hi 
           =X 
           Hs 
           −X 
           hb. 
         
       
     
     Concurrently the mass thickness errors ΔX Vi  and ΔX Hi  are determined. 
     The average effective atomic number of the potential threat  25  is calculated on the basis of side view data processing: 
     
       
           Z   Vi ={[( Z   vs ) a   X   Vs −( Z   Vb ) c   X   Vb ]/( X   Vs   −X   Vb )} 1/a . 
       
     
     The average effective atomic number of the potential threat  25  is calculated on the basis of plan view data processing: 
     
       
           Z   Hi ={[( Z   Hs ) a   X   Hs −( Z   Hb ) c   X   Hb ]( X   Hs   −X   Hb )} 1/a . 
       
     
     Free parameters a and c are adjusted for the proposed technical realization of system  1 . 
     Concurrently the average atomic number errors ΔZ Vi  and ΔZ Hi  are determined. 
     The concept of background subtracting procedure is described in Appendix 3. 
     The average atomic number Z i  of potential threat  25  is calculated as a weighted average of Z iV  and Z Hi . Weighting factors are assigned according to ΔZ Vi  and ΔZ Hi  values. 
     The densities D Vi  and D Hi  of potential threat  25  material are calculated as follows: 
     
       
         
           D 
           Vi 
           =X 
           Vi 
           /R 
           Vi. 
         
       
     
     
       
         
           D 
           Hi 
           =X 
           Hi 
           /R 
           Hi. 
         
       
     
     Simultaneously the density errors ΔD Vi  and ΔD Hi  are determined. 
     The density D i  of potential threat  25  is calculated as a weighted average of D iV  and D iH . Weighting factors are assigned according to ΔD Vi  and ΔD Hi  values. 
     The flowchart of calculation of the average atomic number Z i ±ΔZ i  and average density D i ±ΔD i  of the threat object  25  is shown in FIG.  15 . 
     The last step of potential threat analysis is to compare the obtained physical parameters (effective atomic number and density) of the potential threat object  25  with the physical parameters known for real threats (explosives, drugs, etc.). As a result of this comparison the decision is made whether the object  25  belongs to the particular preset group of threat materials or not. 
     Preferably, the several ranges of physical parameters (Z and D) characteristic for threat materials are preset and stored in the computer  15  memory in the form of data file BANK. Each range corresponds to particular interval (Z 1 , Z 2 )of Z and/or particular interval (D 1 , D 2 )of D values. The threat parameter ranges and boundaries Z 1  and Z 2 ; D 1  and D 2  of the corresponding Z and D intervals were pre-determined according to the results of testing a great number of real threat materials and objects. The tests were carried out with the use of proposed radiographic system and can be considered as the system calibration. 
     Numerical values of Z i ±ΔZ i  and D i ±ΔD i  are displayed on the left monitor screen  16  under the inspected object image. 
     If the calculated Z and D values fall within one of the threat group ranges (a specific couple of Z and D intervals); that is if Z belongs to the preset (Z 1 , Z 2 )interval and simultaneously D belongs to the corresponding (D 1 , D 2 )interval, then the prompt appears on the monitor screen  16  and the alarm signal is generated by the computer  15 . The displayed prompt includes the estimated value of alarm signal reliability in percents. The reliability percentage is evaluated on the basis of comparing the ±ΔZ i  and ±ΔD i  intervals with the corresponding (Z 1 , Z 2 )and (D 1 , D 2 )intervals. The flowchart of the alarm signal generation and evaluation of alarm signal reliability is shown in FIG.  16 . 
     The proposed inspection procedure can be implemented in any dual-energy dual-projection x-ray system independent of: 
     1. X-ray source operation time mode (pulse or continuous). 
     2. Maximal energy(ies) of the used x-ray radiation spectrum (spectra). 
     3. Method of technical implementation of dual energy inspection mode. 
     In the particular case of dual projection single-energy apparatus, the function of determining the average density of the suspicious volume  25  inside the inspected baggage  3  can be implemented without Z-analysis. 
     In particular case of single projection dual-energy apparatus the functions of determining the average effective number Z i  and average mass thickness X i  of suspicious volume  25  inside the baggage  3  can be used without density analysis. This inspection mode is also available in the proposed apparatus  1  and is recommended when multiple objects overlap one another in the inspected baggage  3  or the mass thickness values of the suspected volume  25  are extreme in one of projections (for example, in plan view). In such case the average effective atomic number Z Vi  and the average mass thickness X Vi  of the said volume are determined in the other projection (for example, in side view). 
     APPENDIX 1 
     In the described system  1  analytical approximation process to solve the problem of restoration of effective atomic number of the x-ray absorber during inspection using different energy radiation spectra is used. 
     The direct problem of mathematical simulation of elementary detector response is formulated as follows:                DR   1     =     k                     ∫     E   min1       E   max1                J   1          (   E   )                     η                   (   E   )                   exp                   (       -       ∫   0   X          μ                   (     E   ,       Z        (   x   )               x         )             E           ,                     (A1.1)                                
     here DR 1  is the response of elementary detector (sensor) N V  or N H  of detectors  8  or  12 ; 
     k—proportionality factor; 
     J 1 (E) is the energy spectrum of x-ray intensity; 
     E is the x-ray quantum energy; 
     x is the integration variable in mass thickness of the inspected object along the beam passing through focus S 1  of the x-ray source and the elementary detector; 
     Z(x) is the inspected object  3  material atomic number at the point “x”; 
     μ(E, Z(x)) is the absorption mass factor of x-ray quanta having energy E by the material having atomic number Z(x); 
     X is the cumulative mass thickness of the inspected object  3  along the penetrating beam; 
     η(E) is the efficiency of x-ray detection by sensors N V  or N H  of detectors  8  or  12 ; 
     In the expression (A1.1) it is assumed that J 1 (E)=0 if E&lt;E min1  or E&gt;E max1 . 
     If the entire layer having thickness X is assigned an effective atomic number Z such that                    ∫   0   X          μ                   (     E   ,     Z        (   x   )         )             x         =     μ                   (     E   ,   Z     )        X       ,           (A1.2)                                
     then expression (A2.1) can be transformed in the following manner                  DR   1          (     Z   ,   X     )       =     k                     ∫     E   min1       E   max1                J   1          (   E   )                     η                   (   E   )                   exp                   (       -   μ                     (     E   ,   Z     )        X     )               E     .                   (A1.3)                                
     If inspection is performed in the spectral mode different to the initial spectral mode, i.e. if instead of spectrum J 1 (E) another spectrum J 2 (E) is used having different effective boundaries E min2  and E max2 , then the corresponding detector response by analogy to expression (A1.3) will be presented by the following expression:                  DR   2          (     Z   ,   X     )       =     k                     ∫     E   min2       E   max2                J   2          (   E   )                     η                   (   E   )                   exp                   (       -   μ                     (     E   ,   Z     )        X     )               E     .                   (A1.4)                                
     When practically building and processing x-ray images it is feasible to use normalized values of detector responses U 1 (Z, X) and U 2 (Z, X) such that: 
     
       
           U   1 ( Z, X )= DR   1 ( Z, X )/ DR   10 ,  (A1.5) 
       
     
     
       
           U   2 ( Z, X )= DR   2 ( Z, X )/ DR   20 ,  (A1.6) 
       
     
     where                  DR   10     =     k                     ∫     E   min1       E   max1                J   1          (   E   )                     η                   (   E   )                        E             ,           (A1.7)                                              DR   20     =     k                     ∫     E   min2       E   max2                J   2          (   E   )                     η                   (   E   )                          E     .                   (A1.8)                                 
     From expressions (A1.5) and (A1.6) follows that U 1 (Z, X)≦1 and U 2 (Z, X)≦1. At X=0 U 1  and U 2  are equal to 1. 
     There are two practical technical solutions for dual energy x-ray inspection mode: variation of the upper spectrum boundary, i.e. performance at two different x-ray tube anode voltages, and variation of the lower spectrum boundary, i.e. performance at two different x-ray radiation filtration levels. Using expressions (A1.5) and (A1.6) and data on initial x-ray spectra [ 7 ] a direct mathematical simulation in a wide range of Z, X, and E values, which is typical for x-ray inspection was performed. It is convenient to present simulation results in terms of: 
     
       
           gg   1 ( Z, X )= ln( 1/ U   1 ( Z, X )),  (A1.9) 
       
     
     
       
           gg   2 ( Z, X )= ln( 1/ U   2 ( Z, X )),  (A1.10) 
       
     
     
       
           gg   1 ( Z, X )&gt; gg   2 ( Z, X ), if  X &gt;0  (A1.1) 
       
     
     
       
           cfg ( Z, X )= gg   1 ( Z, X )/ gg   2 ( Z, X )−1, or  (A1.12) 
       
     
     
       
           cfg ( Z, X )=( U   2 ( Z, X )/ U   1 ( Z, X )−1)/ gg   2 ( Z, X ).  (A1.12′) 
       
     
     If only such materials, for which K-boundary of x-ray absorption is less than minimal meaningful energy in the spectrum, i.e. E K &lt;E min1  and E K &lt;E min2  are considered, then from the obtained results it follows that for any set of gg 1  or gg 2  values there exists a simple monotonous function cfg(Z). This proves the existence of inverse function Z(gg 1 , gg 2 ) where gg 1  and gg 2  are related to the responses U 1  and U 2  obtained in detector interrogations. 
     The mentioned inverse dependency can be expressed in the form of analytical approximation with acceptable accuracy. 
     APPENDIX 2 
     Specifically, for the proposed device  1  with additional x-ray radiation filtration implemented as grid-like filters F (see FIG. 5) following inverse dependency, i.e. algorithm establishing relationship between the value of effective atomic number Z and signal values U 1  and U 2  has been selected: 
     
       
           cfex=U   2   /U 1−1. 
       
     
     
       
           gg 2 =alog (1./ U 2) 
       
     
     
       
           cfg=cfex/gg 2 
       
     
     
       
           act 1=(0.83 /gg 2)**0.363/(1.+( gg 2/6.)**1.5) 
       
     
     
       
           act 2=0.5/(1.+( gg 2/2.4)**0.75) 
       
     
     
       
           atu 2=0.4+0.17 *gg 2**0.84 
       
     
     
       
           afi   1=(   cfg/act 1)**4 
       
     
     
       
           afi   2=(   cfg/act 2)** atu 2 
       
     
     
       
           tat 1=11. 
       
     
     
       
           tat 2=9.7 
       
     
     
       
           hkf =0.74 
       
     
     
       
           fuf=cfex **2/(1.+( gg 2/6.)**4) 
       
     
     
       
           hah 11 =tat 1 *afi 1 *hkf *(1.+ fuf*hkg ) 
       
     
     
       
           hah 2 =tat 2 *afi 2 
       
     
       hhaa=hah 2 +hah 11 
     
       
           trt =( hhaa /14.)**4 
       
     
     
       
           ffrr =2.4 *trt /(1 .+trt **2) 
       
     
     
       
           hah 1 =hah 11/(1 .+ffrr ) 
       
     
     
       
           Z=hah 1 +hah 2  (A2.1) 
       
     
     Algorithm (A2.1) is notified FZ(U 1 ,U 2 ) on FIG.  9 . 
     It is convenient to present the analytical dependency of mass thickness X on signal U 1  and U 2  values in the form of function of two arguments: gg 2  and Z, the value of which has been determined in the previous step: 
     
       
           rr 1=0.15+(1.+5.3*( Z /13.)**3.6)/100. 
       
     
     
       
           rd 1=1 ./rr 1 
       
     
     
       
           rqq 3=(13.1 /Z )**8 
       
     
     
       
           rd 2=0.07+0.43/(1 .+rqq 3) 
       
     
     
       
           X=gg 2*( rd 1 +rd 2 *sqrt ( gg 2))  (A2.2) 
       
     
     Algorithm (A2.2) is notified FX(U 1 ,U 2 ) on FIG.  9 . 
     When selecting parameters for X approximation (algorithm (A2.2)) it was assumed that ratio Z/A, where A is the atomic weight, for pure substances (elements) is a monotonous function of atomic number. Analysis shows that the error, which is introduced by deviation of ratio Z/A from monotonous dependency for all real materials (with the exception of hydrogen) does not exceed several per cent. 
     The free parameters of presented algorithms (A2.1) and (A2.2) were adjusted for the following conditions: 
     1. The x-ray J 1 (E) spectrum is generated in the tungsten anode of the x-ray tube and filtered by layers of tungsten having 0.01 mm thickness and aluminum having thickness of 2 mm and thus generating spectrum (see expression 3). 
     2. Spectrum J 2 (E) is formed when x-rays with J 1 (E) spectrum pass through a 1 mm thick copper filter. 
     3. X-ray radiation detection effectiveness is assumed to be η(E)=1. 
     4. Maximal energy of both x-ray spectra J 2 (E) and J 1 (E) is the same and E max1 =E max2 =130 keV. 
     In the presented above algorithms (A2.1) and (A2.2) notation of variables typical for programming languages and mathematical operations is used. 
     Some results of mathematical simulation of the gg 1 , gg 2 , and cfg functions related to detector responses U 1  and U 2  are shown in Tables 1-3. The corresponding Z and X values restored according to algorithms (A2.1) and (A2.2) are also presented. Results for carbon absorber are in Table 1, for aluminum absorber, in Table 2, and for iron absorber, in Table 3. In these tables the following notations are used: 
     Z (1)  is the real absorber material atomic number; 
     X (1)  is the real absorber material mass thickness; 
     Z is the restored absorber material atomic number determined by signal U 1  and U 2  values; 
     X is the absorber mass thickness determined by values of gg 2  and Z. 
     
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Results of mathematical simulation for carbon absorber (Z (1)  = 6) 
               
             
          
           
               
                   
                 X (1) , 
                   
                   
                   
                   
                   
               
               
                   
                 g/cm 2   
                 gg 1   
                 gg 2   
                 Cfg 
                 Z 
                 X/X (1)   
               
               
                   
                   
               
             
          
           
               
                   
                 0.42 
                 0.078 
                 0.068 
                 0.1527 
                 6.076 
                 0.992 
               
               
                   
                 0.84 
                 0.156 
                 0.136 
                 0.1516 
                 6.075 
                 0.993 
               
               
                   
                 1.68 
                 0.311 
                 0.271 
                 0.1495 
                 6.063 
                 0.994 
               
               
                   
                 3.36 
                 0.618 
                 0.542 
                 0.1457 
                 6.027 
                 0.996 
               
               
                   
                 6.30 
                 1.148 
                 1.015 
                 0.1399 
                 5.963 
                 0.999 
               
               
                   
                 8.40 
                 1.521 
                 1.352 
                 0.1362 
                 5.921 
                 1.000 
               
               
                   
                 12.60 
                 2.258 
                 2.025 
                 0.1297 
                 5.852 
                 1.002 
               
               
                   
                 21.00 
                 3.699 
                 3.363 
                 0.1189 
                 5.764 
                 1.004 
               
               
                   
                 29.40 
                 5.109 
                 4.692 
                 0.1102 
                 5.740 
                 1.005 
               
               
                   
                 37.80 
                 6.496 
                 6.014 
                 0.1028 
                 5.792 
                 1.004 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Results of mathematical simulation for aluminum absorber (Z (1)  = 13) 
               
             
          
           
               
                   
                 X (1) , 
                   
                   
                   
                   
                   
               
               
                   
                 g/cm 2   
                 gg 1   
                 gg 2   
                 Cfg 
                 Z 
                 X/X (1)   
               
               
                   
                   
               
             
          
           
               
                   
                 0.37 
                 0.147 
                 0.078 
                 0.9120 
                 12.97 
                 1.013 
               
               
                   
                 0.74 
                 0.279 
                 0.156 
                 0.8376 
                 12.97 
                 1.016 
               
               
                   
                 1.48 
                 0.515 
                 0.310 
                 0.7333 
                 12.97 
                 1.019 
               
               
                   
                 2.96 
                 0.930 
                 0.614 
                 0.6060 
                 12.95 
                 1.021 
               
               
                   
                 5.55 
                 1.566 
                 1.131 
                 0.4824 
                 12.94 
                 1.020 
               
               
                   
                 7.40 
                 1.984 
                 1.492 
                 0.4265 
                 12.94 
                 1.018 
               
               
                   
                 11.10 
                 2.768 
                 2.196 
                 0.3514 
                 12.96 
                 1.013 
               
               
                   
                 18.50 
                 4.223 
                 3.558 
                 0.2656 
                 13.04 
                 1.002 
               
               
                   
                 25.90 
                 5.594 
                 4.875 
                 0.2157 
                 13.09 
                 0.996 
               
               
                   
                 33.30 
                 6.917 
                 6.161 
                 0.1832 
                 13.21 
                 0.990 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Results of mathematical simulation for iron absorber (Z (1)  = 26) 
               
             
          
           
               
                   
                 X (1) , 
                   
                   
                   
                   
                   
               
               
                   
                 g/cm 2   
                 gg 1   
                 gg 2   
                 Cfg 
                 Z 
                 X/X (1)   
               
               
                   
                   
               
             
          
           
               
                   
                 0.18 
                 0.341 
                 0.129 
                 1.818 
                 22.74 
                 1.397 
               
               
                   
                 0.37 
                 0.575 
                 0.251 
                 1.520 
                 24.98 
                 1.130 
               
               
                   
                 0.73 
                 0.932 
                 0.478 
                 1.200 
                 26.57 
                 0.990 
               
               
                   
                 1.46 
                 1.471 
                 0.885 
                 0.898 
                 27.02 
                 0.957 
               
               
                   
                 2.75 
                 2.199 
                 1.509 
                 0.658 
                 26.34 
                 0.996 
               
               
                   
                 3.66 
                 2.643 
                 1.912 
                 0.563 
                 25.97 
                 1.013 
               
               
                   
                 5.49 
                 3.436 
                 2.655 
                 0.446 
                 25.62 
                 1.021 
               
               
                   
                 9.15 
                 4.827 
                 3.996 
                 0.324 
                 25.29 
                 1.024 
               
               
                   
                 12.81 
                 6.088 
                 5.232 
                 0.259 
                 25.01 
                 1.033 
               
               
                   
                 16.47 
                 7.277 
                 6.404 
                 0.218 
                 24.68 
                 1.051 
               
               
                   
                   
               
             
          
         
       
     
     When practically implementing these principles in a specific x-ray inspection system it is necessary to introduce some corrections of algorithm free parameters. These corrections account for difference between real conditions and assumptions that had been made in the mathematical simulation of radiography inspection process. Basically, introduction of such corrections is a procedure of physical model calibration and its mathematical formalization. 
     APPENDIX 3 
     Physical considerations for background subtraction procedure are illustrated in FIG.  17 . When quanta of x-ray beams γ V  and γ H  pass through the inspected object  3  they are partially absorbed by it. Detectors  8  and  12  register quanta that are not absorbed or scattered. Index γ s  in FIG. 17 conditionally denotes intensity of x-ray radiation that has passed through the material in front of volume  25 , material of volume  25 , and material behind volume  25 ., Index γ b  denotes intensity of x-ray radiation that has passed through contents of the inspected object  3  bypassing the area with the suspicious volume  25 . 
     It can be assumed that in reality suspicious volume  25  will be freely placed between walls of the inspected object  3  or inserted into the background contents, e.g. into clothing, causing tighter packing of the background items. In both cases background mass thickness in the direction indicated by the arrow with index γ s  will be equal to the background mass thickness along the direction indicated by the arrow with index γ b . Mathematically this assumption can be expressed as: 
     
       
           X   b   =X   b   (1)   +X   b   (2) ,  (A3.1) 
       
     
     here X b  is the cumulative background mass thickness outside the suspicious volume  25  location; 
     X b   (1)  is background mass thickness before the suspicious volume  25 ; 
     X b   (2)  is background mass thickness behind the suspicious volume  25 . 
     Based on the above considerations and formula (A4.1) mass thickness of the suspicious volume  25  is determined as follows: 
     
       
           X   Vi   =X   Vs   −X   Vb —in vertical projection;  (A3.2) 
       
     
     
       
           X   Hi   =X   Hs   −X   Hb —in horizontal projection.  (A3.3) 
       
     
     EXPLANATION OF ABBREVIATIONS 
     M V  ordinal number of interrogation of multi-channel radiation detector in the first projection 
     M H  ordinal number of interrogation of multi-channel radiation detector in the second projection 
     N V  ordinal number of the elementary detector in the multi-channel radiation detector in the first projection 
     N H  ordinal number of the elementary detector in the multi-channel radiation detector in the second projection 
     U 1  normalized response of the elementary detector in the first energy model 
     U 2  normalized response of the elementary detector in the second energy model 
     U 1 (M V , N V ) two-dimensional array of normalized responses of elementary radiation detectors of the first projection in the first spectral mode 
     U 2 (M V , N V ) two-dimensional array of normalized responses of elementary radiation detectors of the first projection in the second spectral mode 
     U 1 (M H , N H ) two-dimensional array of normalized responses of elementary radiation detectors of the second projection in the first spectral mode 
     U 2 (M H , N H ) two-dimensional array of normalized responses of elementary radiation detectors of the second projection in the second spectral mode 
     Z effective atomic number of the substance 
     X,g/sq.cm mass thickness of the substance in the direction of x-ray beam 
     FZ(U 1 , U 2 )analytical approximation of the dependence of Z from U 1  and U 2  in a specific dual spectra mode in a specific radiographic device 
     FX(U 1 , U 2 )analytical approximation of the dependence of x from U 1  and U 2  in a specific dual spectra mode in a specific radiographic device 
     Z(M V , N V ) two-dimensional array of effective atomic number values in the first projection 
     X(M V , N V ) two-dimensional array of mass thickness values in the first projection 
     Z(M H , N H ) two-dimensional array of effective atomic number values in the second projection 
     X(M H , N H ) two-dimensional array of mass thickness values in the second projection 
     B(U 1 , U 2 )dependence of video monitor screen gray glow brightness on the value of normalized signal in each spectral mode 
     C(Z) dependence of coloring on the video monitor screen on the effective atomic number 
     B(M V , N V ) shadow image in the first projection 
     C(M V , N V ) Z-image in the first projection 
     B(M H , N H ) shadow image in the second projection 
     C(M H , N H ) Z-image in the second projection 
     N 1V , N 2V  image boundaries of the rectangular fragment of the potential threat in the first projection 
     N mV  coordinate of the center of the potential threat in the first projection 
     N 1H , N 2H  image boundaries of the rectangular fragment of the potential threat in the second projection 
     N mH  coordinate of the center of the potential threat in the second projection 
     N 1V , N 2V , M 1V , M 2V  set of four numbers that characterize the coordinates of the rectangular fragment in the image of the potential threat in the first projection 
     N 1H , N 2H , M 1H , M 2H  set of four numbers that characterize the coordinates of the rectangular fragment in the image of the potential threat in the second projection 
     N 1Vb , N 2Vb , M 1Vb , M 2Vb  set of four numbers that characterize the coordinates of the rectangular fragment of the background in the first projection 
     N 1Hb , N 2Hb , M 1Hb , M 2Hb  set of four numbers that characterize the coordinates of the rectangular fragment of the background in the second projection 
     AVERAGE calculation of the average value within the rectangular fragment of video image 
     ERROR calculation of the mean-square error of the value within the rectangular fragment of video image 
     GEOMETRY set of quantitative relationships that bind geometrical coordinates of any point inside the inspection chamber with the coordinate N V  in the first projection and its coordinate N H  in the second projection 
     Q V , Q H  geometrical coordinates of the center of the potential threat inside the inspection chamber 
     GL V  geometrical thickness of the potential threat, which is determined by the location of its fragment boundaries N 1V  and N 2V  in the first projection 
     GL H  geometrical thickness of the potential threat, which is determined by the location of its fragment boundaries N 1H  and N 2H  in the first projection 
     R V  geometrical thickness of the potential threat along the x-ray beam path in the first projection 
     R H  geometrical thickness of the potential threat along the x-ray beam path in the second projection 
     Z Vs  mean effective atomic number of the potential threat fragment in the first projection 
     ΔZ Vs  Z Vs  error 
     X Vs  mean mass thickness of the potential threat fragment in the first projection 
     ΔX Vs  X Vs  error 
     Z Hs  mean effective atomic number of the potential threat fragment in the second projection 
     ΔZ Hs  Z Hs  error 
     X Hs  mean mass thickness of the potential threat fragment in the second projection 
     ΔX Hs  X Hs  error 
     Z i =φ(Z s , X s , Z b , X b ) analytical expression of the function that approximates dependence of the mean effective atomic number of the potential threat on physical parameters of the fragment, where the potential threat and the background overlap, and physical parameters of the background (“background elimination”) 
     D Vi =X Vi /R V  mean density of the potential threat determined taking into account its mass thickness in the first projection 
     ΔD Vi  D Vi  error 
     D Hi =X Hi /R H  mean density of the potential threat determined taking into account its mass thickness in the second projection 
     ΔD Hi  D Hi  error 
     AVWEIGHT calculation of weighted average value 
     ERWEIGHT calculation of weighted average value error 
     Z 1  weighted average effective atomic number of the potential threat 
     ΔZ 1  Z i  error 
     D 1  weighted average density of the potential threat 
     ΔD 1  D i  error 
     BANK((Z 1 &lt;Z i &lt;Z 2 ; D 1 &lt;D i &lt;D 2 ); array of two-dimensional (effective atomic number—(Z 3 &lt;Z i &lt;Z 4 ; D 3 &lt;D i &lt;D 4 ); . . . ) substance density) ranges of physical parameters that are characteristic for contraband substances or substance groups 
     (Z i , D i ) couple of parameters of the potential threat 
     (ΔZ i , ΔD 1 ) potential threat physical parameters&#39; couple errors 
     COMPARISON checking whether the couple (Z i , D i ) falls into any of the ranges of two-dimensional range array BANK 
     RELIABILITY, % relative reliability of decision