Metal object screening method and system based on terahertz security scanning

Abstract

The present invention relates to the field of security inspection technology, and discloses a method and system for screening metal objects based on terahertz security inspection scanning, including comparison of spectral data with preset thresholds, comparison of suspected thresholds, and definition of the best frequency band for penetration. The present invention uses a terahertz security inspection device to scan the carried items of the person to be inspected, forms spectral data through the reflection signal of the carried items, compares this spectral data with the spectral data of metal contraband in the terahertz system, and judges whether the carried items contain metal contraband based on the comparison results. In addition, a secondary judgment is made on the carried items close to the preset threshold, a suspected threshold less than the preset threshold is set, and the items exceeding the suspected threshold are marked as suspected metal contraband, and then it is judged whether there are shielding items on the suspected metal contraband, and the terahertz wave frequency band that can penetrate the shielding items is found, and the shielding items are scanned using this frequency band to obtain a clear image, thereby judging whether the suspected metal contraband is prohibited.

Description

Technical Field

[0001] The present invention relates to the field of security inspection technology, and in particular to a method and system for screening metal objects based on terahertz security inspection scanning. Background Art

[0002] Terahertz refers to the electromagnetic wave band with a frequency between 0.1THz and 10THz, which is between microwaves and infrared. It can penetrate many non-conductive materials such as paper, textiles, plastics and ceramics, which makes it very useful in non-destructive testing and security scanning. In the field of security scanning, terahertz technology has great potential, mainly because it can penetrate many common non-metallic materials while providing safe imaging.

[0003] The overall process of terahertz security scanner detection is: the inspected person stands at the designated position. The security scanner emits terahertz waves, and the beam scans the inspected object. The sensor receives the terahertz waves reflected from the inspected object. The analysis software automatically identifies and marks potential threat items. Among them, the detection of metal objects is particularly important, because metal objects can easily cause security objects, among which there are many prohibited items, such as knives, blades, sharp metals, etc. Summary of the invention

[0004] In view of the problems existing in the above-mentioned prior art, the purpose of the present invention is to provide a method and system for screening metal objects based on terahertz security scanning, so as to make an accurate judgment on whether there are any metal contraband in the carried objects.

[0005] In order to achieve the above-mentioned purpose, the present invention provides the following technical solutions: a method for screening metal objects based on terahertz security scanning, comprising the following steps: step 1, the terahertz security scanner emits terahertz waves to scan the objects carried by the inspected person, and the system forms spectral data according to the reflected signal; step 2, based on the obtained spectral data of the carried objects, the spectral data is compared with the spectral data of metal contraband in the terahertz system, and different response strategies are obtained according to the comparison results. If the spectral data exceeds the preset threshold of the spectral data of metal contraband, the system issues an early warning; if the spectral data does not exceed the preset threshold of the spectral data of metal contraband, the next step is executed; step 3, a suspected threshold value lower than the preset threshold value is set, and the spectral data is compared with the suspected threshold value. If the spectral data is lower than the suspected threshold value, it is released; if the spectral data exceeds the suspected threshold value, the carried object is marked as a suspected metal contraband, and the suspected metal contraband is judged whether there is an obstructing object. If there is an obstructing object, the terahertz wave frequency band that can penetrate it is found, and the obstructing object is scanned using this frequency band to obtain a clear image, thereby judging whether the suspected metal contraband is prohibited.

[0006] In some embodiments, whether the suspected metal contraband is obstructed by polarization detection is determined. Specifically, terahertz waves are emitted in a polarized state to illuminate the object. The polarization state is affected by the object, and the affected terahertz waves are measured by a polarization detector. The polarization characteristics of known materials in a database are used as a reference to compare the two, thereby identifying the obstructing object that causes the change in polarization state.

[0007] In some embodiments, a specific method for obtaining a terahertz frequency band that can penetrate obstructed objects is: setting the terahertz wave of the terahertz security scanner to a broadband terahertz wave, allowing the terahertz security scanner to use the broadband terahertz wave to scan the obstructed objects, and obtaining the terahertz frequency band with the highest transmittance to the obstructed objects.

[0008] In some embodiments, a specific method of using broadband terahertz for scanning is: the frequency range of the broadband terahertz wave is 0.1 MHz to 10 MHz, and the broadband range is evenly divided into 10 frequency bands, each of which is 0.1 THz to 1 THz, 1 THz to 2 THz, 2 THz to 3 THz, 3 THz to 4 THz, 4 THz to 5 THz, 5 THz to 6 THz, 6 THz to 7 THz, 7 THz to 8 THz, 8 THz to 9 THz, and 9 THz to 10 THz, and the terahertz security scanner uses the terahertz of each frequency band to scan the shielded objects respectively.

[0009] In some embodiments, an optimal threshold for penetration effect is set. If the penetration effects obtained by scanning the shielding object in 10 frequency bands within the wide frequency range do not exceed the optimal threshold, the frequency band with the best penetration effect is selected and the frequency band is divided into n detection frequency bands. The terahertz security scanner uses the n detection frequency bands to scan the shielding object separately.

[0010] In some embodiments, after the obstructing object is identified through the change in polarization state, it can also be labeled according to the type of the obstructing object. The identification includes reasonable existence identification and unreasonable existence identification. For obstructing objects with unreasonable existence identification, there is no need to perform terahertz sub-frequency band penetration detection, and manual inspection can be performed directly; for obstructing objects with reasonable existence identification, terahertz sub-frequency band penetration detection is performed.

[0011] In some embodiments, when the type of the masking object is but not limited to polymer plastics, ceramic materials, and composite materials, the masking object is marked as a reasonable existence mark; when the type of the masking object is but not limited to water-containing materials, biomolecular materials, mineral and powder materials, and biological materials, the masking object is marked as an unreasonable existence mark.

[0012] The present invention also provides the following technical solutions:

[0013] The present invention further provides a metal object screening system based on terahertz security scanning, comprising: a data acquisition module, which uses a terahertz security scanner to emit terahertz waves to scan items carried by inspected persons, so as to form spectral data according to the reflection signals of the carried items; a data comparison module, which compares the spectral data with a preset threshold in the terahertz system, and makes a response strategy of whether to issue an early warning based on the comparison result; a strategy response module, which compares the spectral data with a suspected threshold, and makes a response of releasing or judging whether there are obstructing items based on the comparison result.

[0014] The present invention further provides a terahertz security scanning device, comprising a memory and a processor coupled to the memory, wherein the processor is configured to execute the above-mentioned metal object screening method based on terahertz security scanning based on instructions stored in the memory.

[0015] The present invention further provides a computer-readable storage medium, wherein the computer-readable storage medium stores a computer program, and the computer program is executed by a processor to implement the above-mentioned method for screening metal objects based on terahertz security scanning.

[0016] Compared with the prior art, the technical solution provided by the present invention has the following beneficial effects:

[0017] First, the terahertz security scanner is used to scan the items carried by the person to be inspected, and the reflected signals of the items are used to form spectral data. This spectral data is compared with the spectral data of metal contraband in the terahertz system, and the comparison result is used to determine whether the items contain metal contraband.

[0018] Secondly, a secondary judgment is made on the carried items that are close to the preset threshold. A suspected threshold that is smaller than the preset threshold is set, and items that exceed the suspected threshold are marked as suspected metal contraband. Then, it is determined whether there are any shielding items on the suspected metal contraband, and the terahertz wave frequency band that can penetrate the shielding items is found. This frequency band is used to scan the shielding items to obtain a clear image, thereby determining whether the suspected metal contraband is prohibited.

[0019] Third, when the terahertz waves of all frequency bands do not penetrate suspected metal contraband very well, resulting in unclear displayed images, the people to be inspected will be stopped and the suspected metal contraband will be manually inspected to prevent any metal contraband from successfully passing security checks, avoid safety hazards, and prevent them before they occur. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a schematic diagram of the steps of the present invention;

[0021] Figure 2 The figure is a logical schematic diagram of the method of the present invention. DETAILED DESCRIPTION

[0022] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0023] It is to be understood that the term "one" should be understood as "at least one" or "one or more", that is, in one embodiment, the number of an element may be one, while in another embodiment, the number of the element may be multiple, and the term "one" should not be understood as a limitation on the quantity.

[0024] The present invention provides a method for detecting metal objects based on terahertz security scanning, such as Figure 1 and Figure 2 As shown, the following steps are included:

[0025] First, during the inspection, the inspected person stands at the designated position, the terahertz security scanner emits terahertz waves to scan the items carried by the inspected person, and the sensor receives the terahertz waves reflected by the items carried by the inspected person, and forms spectrum data in the system based on the reflected terahertz wave reflection signal. Among the above, it is worth mentioning that the terahertz security scanner emits terahertz waves from multiple angles to avoid blocking terahertz waves in a certain direction.

[0026] Secondly, the spectral data of the items carried by the inspected person is compared with the spectral data of metal contraband stored in the terahertz system, and specific responses are made according to the matching degree obtained by the comparison. If the spectral data of the items carried exceeds the preset threshold of the spectral data of metal contraband, it indicates that the items carried by the inspected person are very likely to be metal contraband, and the system issues an early warning, prompting the security inspector to manually check the items carried by the inspected person. If the spectral data of the items carried does not exceed the preset threshold of the spectral data of metal contraband, it cannot be completely determined that the items carried are definitely not metal contraband. Because if there are shielding items on the items carried, it is also possible that the spectral data of the items carried does not exceed the preset threshold of the spectral data of metal contraband. For example, the preset threshold is 90%. When the spectral data of the items carried by the inspected person exceeds 90%, it is very likely to be metal contraband. When the spectral data of the items carried by the inspected person is lower than 90%, but is 85%, 80%, etc. close to the preset threshold, it is necessary to consider whether there are shielding items on the items carried that affect the spectral data. In this case, the specific operation is to set a suspected threshold value lower than the preset threshold value, and release the items carried by the inspected personnel whose spectral data is lower than the suspected threshold value. The items carried by the inspected personnel whose spectral data is lower than the preset threshold value but higher than the suspected threshold value are marked as suspected metal contraband. Polarization detection is used to determine whether the items carried by the suspected metal contraband are shielded. When it is determined that there are shielding items, the next step is performed. The reason why shielding items can form shielding is that the terahertz waves emitted by the terahertz security inspection instrument cannot completely penetrate such shielding items. Because under normal conditions, the frequency of the terahertz waves emitted by the terahertz security inspection instrument is generally between 0.1THz and 1THz. The terahertz waves at this low frequency have better penetration ability and can be used for security inspection of most daily items. However, when facing shielding items, the terahertz waves at this frequency cannot penetrate well, because the absorption, reflection or scattering characteristics of the shielding material to the terahertz waves make the terahertz waves unable to penetrate completely, resulting in interference or attenuation of the signals detected by the instrument. Therefore, to solve this problem, it is necessary to set up a broadband terahertz wave and select the best frequency band for further detection. Through further detection, the suspected metal contraband can be scanned to obtain the situation inside the shielded object, so as to determine whether the object covered inside the shielded object is a metal contraband.

[0027] The specific method of using broadband terahertz waves for further detection is: adjust the terahertz frequency of the terahertz security scanner from 0.1THz to 1THz to 0.1THz to 10THz, and divide the 0.1THz to 10THz frequency band into 10 different frequency bands, namely 0.1THz to 1THz, 1THz to 2THz, 2THz to 3THz, 3THz to 4THz, 4THz to 5THz, 5THz to 6THz, 6THz to 7THz, 7THz to 8THz, 8THz to 9THz, 9THz to 10THz. Use the terahertz waves of each frequency band to scan the shielding objects respectively, obtain the penetration effect of different frequency bands on the shielding objects, compare the different penetration effects, and select the frequency band with the best penetration effect. The terahertz security scanner uses this frequency band with the best penetration effect to scan the shielding objects with higher resolution to obtain a clearer imaging effect, and judge whether it is a metal contraband through the final imaging effect. If yes, the person carrying the device will be stopped and manually checked to confirm. If no, the person to be inspected will be released. Assuming that the terahertz security scanner uses 10 different frequency bands of 0.1THz to 1THz, 1THz to 2THz, 2THz to 3THz, 3THz to 4THz, 4THz to 5THz, 5THz to 6THz, 6THz to 7THz, 7THz to 8THz, 8THz to 9THz, and 9THz to 10THz, the penetration effects are shown in Table 1 below (in the above 10 frequency bands, the same frequency band takes the higher frequency band, such as the terahertz wave frequency band corresponding to 2THz is 2MHz-3MHz):

[0028] Table 1

[0029] Frequency band (THz) 0.1-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 Penetration effect (%) 20% 30% 80% 60% 10% 5% 35% 25% 45% 65%

[0030] According to the table, the 80% with the best penetration effect corresponds to the terahertz wave frequency band of 2MHz-3MHz. The terahertz security scanner is set to this frequency band, and then emits terahertz waves to scan the obstructed objects to obtain a clearer image.

[0031] Theoretically, the better the terahertz wave's penetration effect on an object, the clearer the image obtained. The threshold of the penetration effect corresponding to the clearest image obtained by the terahertz system is set as the optimal threshold. Based on the above different terahertz frequency bands, if the penetration effect obtained by all terahertz frequency bands does not reach the optimal threshold, select the frequency band with the penetration effect closest to the optimal threshold as the reference frequency band, and divide the reference frequency band into n detection frequency bands, so that the terahertz security scanner can scan the obstructed objects separately under n different detection frequency bands to obtain the detection frequency band with the best penetration effect. Use this detection frequency band to scan the obstructed objects with higher resolution and obtain a higher-definition image. Assuming that the optimal threshold is 90%, according to the above example, the best penetration effect is 80%, which does not reach the optimal threshold. The terahertz frequency band corresponding to the 80% penetration effect is 2THz to 3THz. Taking 2THz to 3THz as the reference frequency band, it is divided into 10 frequency bands, namely 2THz to 2.1THz, 2.1THz to 2.2THz, 2.2THz to 2.3THz, 2.3THz to 2.4THz, 2.4THz to 2.5THz, 2.5THz to 2.6THz, 2.6THz to 2.7THz, 2.7THz to 2.8THz, 2.8THz to 2.9THz, 2.9THz to 3THz. The penetration effects of these 10 frequency bands are shown in Table 2 below (in the above 10 frequency bands, the same frequency band takes the higher frequency band, such as the terahertz wave frequency band corresponding to 2.5THz is 2.5MHz-2.6MHz):

[0032] Table 2

[0033] Frequency band (THz) 2-2.1 2.1-2.2 2.2-2.3 2.3-2.4 2.4-2.5 2.5-2.6 2.6-2.7 2.7-2.8 2.8-2.9 2.9-3 Penetration effect (%) 81% 83% 85% 87% 89% 91% 35% 25% 45% 65%

[0034] According to the table, the penetration effect exceeds the optimal threshold of 91%, and the corresponding frequency band is 2.5MHz-2.6MHz. The terahertz security scanner sets the frequency band to 2.5MHz-2.6MHz, and then scans the shielding objects to obtain the clearest image. If the terahertz waves of all frequency bands do not have a good penetration effect on suspected metal contraband, resulting in unclear displayed images, the person to be inspected will be stopped and the suspected metal contraband will be manually inspected to prevent any metal contraband from successfully passing the security check, avoid potential safety hazards, and prevent them before they happen.

[0035] It is worth mentioning that during manual inspection, since there is a high possibility that it is contraband, the inspection should be as detailed and meticulous as possible. First, security personnel will make a preliminary assessment of the item through visual observation to check whether its appearance is consistent with common metal contraband, such as shape, color and logo. Visual inspection helps to quickly identify obviously suspicious items and provide preliminary direction for subsequent inspections. Next, security personnel will use manual inspection methods, such as touching and moving items to perceive the weight, texture and structure of the items. These physical characteristics can provide clues about the material and possible use of the items. In many cases, especially for complex or large items, handheld metal detectors are an important auxiliary tool. Metal detectors can help security personnel identify whether there are metal components in the items and further verify the system detection results. Another common method is to use X-ray equipment to perform perspective imaging of the items. X-rays can provide detailed images of the internal structure of the items, allowing security personnel to determine whether the items contain unexpected components or hidden contraband. For more complex detection needs, it is necessary to combine Raman spectrometers, mass spectrometers and other equipment to obtain information on the chemical composition of the items, especially when it is suspected that the items may involve chemical contraband or explosives. In addition, in high-security situations, odor detectors or well-trained security dogs can also be used, especially when detecting explosives or drugs, which can provide additional security. Throughout the process, security personnel need to maintain a high degree of vigilance and professional judgment, and any abnormal situation should be recorded in a timely manner and necessary safety measures should be taken. The ultimate goal of manual detection is to minimize misjudgment and interference with legitimate items and personnel on the basis of ensuring safety.

[0036] When polarization detection is used to determine whether suspected metal contraband has an obstruction, the process mainly includes several key steps. First, the terahertz wave source emits a terahertz wave with a specific polarization state, such as linear polarization or circular polarization, and irradiates it onto the inspected object. At this time, the surface characteristics and internal structure of the object will affect the polarization state of the terahertz wave, causing changes in reflection, transmission or scattering. Next, a polarization detector is used to measure the reflected or transmitted terahertz wave and analyze the change in the polarization state of the wave. Using the polarization characteristics of known materials in the database as a reference, the polarization characteristics changes caused by the obstruction can be identified by comparing them with the detected polarization characteristics through algorithm analysis. This change is usually manifested as a significant difference in reflectivity, transmittance or phase. Finally, the data is processed and analyzed by the algorithm to determine whether there is a non-metallic obstruction on the surface of the object.

[0037] In addition, as an auxiliary judgment, after the shielding object is identified through the change of polarization state, it can also be marked according to the type of shielding object. The marking includes reasonable existence marking and unreasonable existence marking. For shielding objects with unreasonable existence marking, there is no need to perform terahertz sub-band penetration detection, and manual inspection is directly performed; for shielding objects with reasonable existence marking, terahertz sub-band penetration detection is performed. When the types of shielding objects belong to but are not limited to polymer plastics, ceramic materials and composite materials, the shielding objects are marked as reasonable existence markings; when the types of shielding objects belong to but are not limited to water-containing materials, biomolecular materials, minerals and powder materials, and biological materials, the shielding objects are marked as unreasonable existence markings. Because the common classifications of materials that absorb terahertz waves include the following: 1. Organic materials: First, water-containing materials: water itself and materials with high water content such as gel, wet paper, wet cloth, etc. usually strongly absorb terahertz waves. Second, polymers and plastics: Some plastics such as polyethylene and polypropylene exhibit strong absorption characteristics within a specific frequency range. Third, biological molecules: biological molecules such as glucose, amino acids and proteins have high absorption of terahertz waves. 2. Inorganic materials: First, ceramic materials: some ceramic materials, especially those oxide ceramics with polar bonds such as aluminum oxide and zirconium oxide, have absorption characteristics for terahertz waves under certain conditions. Second, minerals and powders: some minerals and powdered materials absorb energy at specific terahertz wave frequencies. 3. Composite materials: First, absorbent materials: materials specially designed to absorb electromagnetic waves, usually containing carbon, ferrite or conductive polymers, may absorb terahertz waves. Second, impurity-containing materials: For example, glass or ceramics doped with different elements may show enhanced absorption characteristics. 4. Biomaterials: First, human tissue: due to high water content, most human tissues show significant absorption of terahertz waves. Second, plant and cellulose materials: plant fibers and cellulose-based materials usually absorb terahertz waves, especially when wet. Based on the above categories, the reasons for marking water-containing materials, biomolecular materials, minerals and powder materials, and biological materials as unreasonable are: for water-containing materials, it is obviously unconventional to carry water-containing materials such as wet wipes and wet cloths without packaging when going through security checks, because these water-containing materials will wet clothes or other luggage. For biomolecular materials, such as glucose, protein, etc., which are either liquids or foods, even if these things are carried, they should not be carried without packaging, and they should not be used as wrappings. For minerals and powder materials, they should not be allowed to exist alone without packaging. For example, carrying a handful of powder in a pocket or suitcase is obviously unconventional.As for human tissue materials among biological materials, it is obviously not in line with the norm to carry with you or put a piece of human tissue that can be used for wrapping in a suitcase; as for plant and cellulose materials among biological materials, if they are to be used as covering items, these materials must be in a moist state. For example, it is reasonable to carry plant fibers such as rattan pillows normally, but it is also not in line with the norm to put a moist rattan pillow in a suitcase.

[0038] In addition to the above: On one hand, the present invention provides a terahertz security scanning device, including a memory and a processor coupled to the memory, the processor being configured to execute the above-mentioned method for screening metal objects based on terahertz security scanning based on instructions stored in the memory. On the other hand, a computer-readable storage medium is provided, the computer-readable storage medium storing a computer program, the computer program being executed by the processor to implement the above-mentioned method for screening metal objects based on terahertz security scanning.

[0039] In the embodiments disclosed in the present invention, the processes described above with reference to the flowchart can be implemented as a computer software program. The embodiments disclosed in the present invention include a computer program product, which includes a computer program carried on a computer-readable medium, and the computer program contains program code for executing the method shown in the flowchart. In such an embodiment, the computer program can be downloaded and installed from the network through the communication part, and / or installed from a removable medium. When the computer program is executed by the central processing unit, the above functions defined in the method of the present application are executed. It should be noted that the computer-readable medium mentioned above in the present application can be a computer-readable signal medium or a computer-readable storage medium or any combination of the above two. The computer-readable storage medium can be, for example, but not limited to, an electrical, magnetic, optical, electromagnetic, infrared or semiconductor system, device or device, or any combination of the above. More specific examples of computer-readable storage media can include, but are not limited to: an electrical connection with one or more wire segments, a portable computer disk, a hard disk, a random access memory, a read-only memory, an erasable programmable read-only memory, an optical fiber, a portable compact disk read-only memory, an optical storage device, a magnetic storage device, or any suitable combination of the above. In the present application, a computer-readable storage medium may be any tangible medium containing or storing a program that can be used by or in combination with an instruction execution system, apparatus, or device. In the present application, a computer-readable signal medium may include a data signal propagated in a baseband or as part of a carrier wave, which carries a computer-readable program code. Such propagated data signals may take a variety of forms, including but not limited to electromagnetic signals, optical signals, or any suitable combination of the above. A computer-readable signal medium may also be any computer-readable medium other than a computer-readable storage medium, which may send, propagate, or transmit a program for use by or in combination with an instruction execution system, apparatus, or device. The program code contained on the computer-readable medium may be transmitted using any suitable medium, including but not limited to: wireless segments, wire segments, optical cables, RF, etc., or any suitable combination of the above.

[0040] The flowcharts and block diagrams in the accompanying drawings illustrate the possible architecture, functions and operations of the systems, methods and computer program products according to various embodiments of the present invention. In this regard, each box in the flowchart or block diagram can represent a module, a program segment or a part of a code, and the module, a program segment or a part of a code contains one or more executable instructions for implementing the specified logical functions. It should also be noted that in some alternative implementations, the functions marked in the boxes can also occur in a different order than the order marked in the accompanying drawings. For example, two boxes represented in succession can actually be executed substantially in parallel, and they can sometimes be executed in the opposite order, depending on the functions involved. It should also be noted that each box in the block diagram and / or flowchart, and the combination of boxes in the block diagram and / or flowchart, can be implemented with a dedicated hardware-based system that performs the specified function or operation, or can be implemented with a combination of dedicated hardware and computer instructions.

[0041] Those skilled in the art should understand that the above description is only a specific implementation mode of the present application, but the protection scope of the present application is not limited thereto. Any technician familiar with the technical field can easily think of changes or substitutions within the technical scope disclosed in the present application, which should be covered by the protection scope of the present application.

Claims

1. A method for detecting metal objects based on terahertz security scanning, characterized in that: The following steps are involved: Step 1: The terahertz security scanner emits terahertz waves to scan items carried by the inspected person, and the system forms spectral data based on the reflected signal; Step 2: Based on the obtained spectral data of the carried items, the spectral data is compared with the spectral data of the metal contraband in the terahertz system, and different response strategies are obtained according to the comparison results. If the spectral data exceeds the preset threshold of the spectral data of the metal contraband, the system will issue an alarm; If the spectrum data does not exceed the preset threshold of the spectrum data of metal contraband, proceed to the next step; Step 3: Set a suspected threshold value lower than the preset threshold value, compare the spectral data with the suspected threshold value, and release it if the spectral data is lower than the suspected threshold value; If the spectral data exceeds the suspected threshold, the carried items will be marked as suspected metal contraband, and the suspected metal contraband will be judged whether there are any obstructing objects. If there are obstructing objects, the terahertz wave frequency band that can penetrate them will be found, and this frequency band will be used to scan the obstructing objects to obtain a clear image, thereby determining whether the suspected metal contraband is prohibited.

2. The method for detecting metal objects based on terahertz security scanning according to claim 1, characterized in that: Polarization detection is used to determine whether there are any obstructing objects on suspected metal contraband. Specifically, terahertz waves are used to emit polarized terahertz waves to illuminate the objects. The polarization state is affected by the objects, and the affected terahertz waves are measured by the polarization detector. The polarization characteristics of known materials in the database are used as a reference to compare the two, thereby identifying the obstructing objects that cause the polarization state change.

3. The method for detecting metal objects based on terahertz security scanning according to claim 2 is characterized in that: The specific method for obtaining the terahertz frequency band that can penetrate the obstructed object is: setting the terahertz wave of the terahertz security scanner to a broadband terahertz wave, so that the terahertz security scanner uses the broadband terahertz wave to scan the obstructed object, and obtaining the terahertz frequency band with the highest transmittance to the obstructed object.

4. The method for detecting metal objects based on terahertz security scanning according to claim 3 is characterized in that: The specific method of using broadband terahertz waves for scanning is: the frequency range of broadband terahertz waves is 0.1MHz to 10MHz, and the broadband range is divided into 10 frequency bands, each of which is 0.1THz to 1THz, 1THz to 2THz, 2THz to 3THz, 3THz to 4THz, 4THz to 5THz, 5THz to 6THz, 6THz to 7THz, 7THz to 8THz, 8THz to 9THz, and 9THz to 10THz. The terahertz security scanner uses the terahertz in each frequency band to scan the shielding objects respectively.

5. The method for detecting metal objects based on terahertz security scanning according to claim 4, characterized in that: Set the optimal threshold for penetration effect. If the penetration effects of scanning the shielding object by the 10 frequency bands within the wide frequency range do not exceed the optimal threshold, select the frequency band with the best penetration effect and divide it into n detection frequency bands. The terahertz security scanner uses the n detection frequency bands to scan the shielding object separately.

6. The method for detecting metal objects based on terahertz security scanning according to claim 2, characterized in that: After the obstructing objects are identified through the change in polarization state, they can also be labeled according to the type of the obstructing objects. The identification includes reasonable existence identification and unreasonable existence identification. For obstructing objects with unreasonable existence identification, there is no need to perform terahertz sub-frequency band penetration detection, and manual inspection can be carried out directly; for obstructing objects with reasonable existence identification, terahertz sub-frequency band penetration detection is performed.

7. The method for detecting metal objects based on terahertz security scanning according to claim 6, characterized in that: When the types of masking objects include but are not limited to polymer plastics, ceramic materials and composite materials, the masking objects will be marked as reasonable existence marks; when the types of masking objects include but are not limited to water-containing materials, biomolecular materials, mineral and powder materials, and biological materials, the masking objects will be marked as unreasonable existence marks.

8. A metal object screening system based on terahertz security scanning, characterized in that: A method for screening metal objects based on terahertz security scanning according to any one of claims 1 to 7, comprising: A data acquisition module, which uses a terahertz security scanner to emit terahertz waves to scan items carried by the inspected person, so as to form spectral data according to the reflection signals of the items carried; The data comparison module compares the spectral data with the preset threshold in the terahertz system and makes a response strategy based on the comparison results to determine whether to issue an early warning. The strategy response module compares the spectral data with the suspected threshold and makes a decision to release or determine whether there are any obstructions based on the comparison results.

9. A terahertz security scanning device, characterized in that: It comprises a memory and a processor coupled to the memory, wherein the processor is configured to execute a metal object screening method based on terahertz security scanning as described in any one of claims 1 to 7 based on instructions stored in the memory.

10. A computer-readable storage medium, characterized in that: The computer-readable storage medium stores a computer program, and the computer program is executed by a processor to implement a method for screening metal objects based on terahertz security scanning as described in any one of claims 1 to 7.

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