Millimeter-wave terahertz imaging equipment and object recognition and classification methods

Abstract

A millimeter-wave terahertz imaging device for security inspection of an object includes a focusing lens, a micropolarizer array, a detector array, and an image processing unit. The focusing lens is disposed between the object and the micropolarizer array and configured to focus millimeter-wave terahertz waves spontaneously radiated or reflected from the object onto the detector array. The micropolarizer array is disposed on the side of the detector array facing the focusing lens and positioned close to the detector array. The detector array is disposed on the focal plane of the focusing lens and configured to convert the millimeter-wave terahertz waves transmitted through the micropolarizer array into a polarized image of the object. The image processing unit is disposed on the side of the detector array away from the micropolarizer array and configured to process the polarized image to identify and classify the object.

Description

[0001] This application is a divisional application of application number 201811654173.X, filed on December 29, 2018, entitled "Millimeter-wave terahertz imaging device and object recognition and classification method". Technical Field

[0002] This disclosure relates to the field of security inspection technology, and in particular to a millimeter-wave terahertz imaging device, and a method for detecting objects using the aforementioned millimeter-wave terahertz imaging device for object identification and classification. Background Technology

[0003] Existing passive millimeter-wave terahertz imaging is similar to optical photography, utilizing a two-dimensional array (each array element's detector (or radiometer, or detector, which can be direct or indirect detection) corresponds to one pixel, and the array elements in the form of an array form an array), to stare at the target's field of view, without the need for scanning, and can achieve real-time imaging.

[0004] Considering the cost of millimeter-wave terahertz detectors, adopting a purely two-dimensional focal plane direct imaging method would result in an extremely expensive system. Therefore, in practical applications, to balance system cost and imaging rate requirements, current mainstream systems for two-dimensional imaging employ a certain number of radiometers combined with mechanical scanning to achieve full field-of-view coverage. This reduces the need for a larger number of detectors by sacrificing imaging time, thereby lowering the overall system cost.

[0005] Existing passive millimeter-wave terahertz imaging security inspection devices based on focal plane imaging, whether employing direct detection by radiometers or indirect detection by heterodyne methods, can only display the image shape of suspicious objects (such as mobile phones, banknotes, knives, and pistols) based on the temperature difference between the object and the human body, thus determining whether a person is carrying a suspicious object. They cannot perform object identification. Typically, the surface temperature of the human body is higher than that of a suspicious object, appearing white in the grayscale image while the suspicious object appears black. Generally, neither machine nor manual identification can distinguish between similarly shaped and sized items such as belt buckles, mobile phones, metal blocks, media blocks, and banknotes.

[0006] In addition, the resolution (object orientation) of current passive human body security screening transposition is generally only 2-3cm, which is not perfect for classifying and recognizing objects by size and shape. Summary of the Invention

[0007] The purpose of this disclosure is to solve at least one aspect of the aforementioned technical problems by providing a millimeter-wave terahertz imaging device and a method for object identification and classification using the same device. This millimeter-wave terahertz imaging device can identify and classify objects without producing harmful radiation to the human body, and the identified objects can be as small as millimeters.

[0008] According to one aspect of this disclosure, a millimeter-wave terahertz imaging device for security inspection of an object is provided, comprising a focusing lens, a micropolarizer array, a detector array, and an image processing device, wherein...

[0009] The focusing lens is disposed between the object under test and the micro-polarized plate array, and is configured to focus the millimeter-wave terahertz waves spontaneously radiated or reflected back by the object under test onto the detector array.

[0010] The micro-polarized plate array is disposed on the side of the detector array facing the focusing lens and is positioned close to the detector array;

[0011] The detector array is positioned on the focal plane of the focusing lens and is configured to convert millimeter-wave terahertz waves transmitted through the micropolarized array into a polarized image of the object under test; and

[0012] The image processing device is disposed on the side of the detector array away from the micro-polarization array and is configured to process the polarization image to identify and classify the object under inspection.

[0013] According to one embodiment of this disclosure, the micro-polarized plate array includes a plurality of micro-polarized plates, each of the plurality of micro-polarized plates being fully polarized, partially polarized, or unpolarized.

[0014] According to another embodiment of this disclosure, the detector array includes a plurality of wave-sensing units, the number of which is the same as the number of micro-polarizers, and the position of each wave-sensing unit on the detector array corresponds to the position of each micro-polarizer on the micro-polarizer array.

[0015] According to another embodiment of this disclosure, the micro-polarization array is a one-dimensional array, the detector array is a one-dimensional array, the one-dimensional micro-polarization array includes a plurality of linearly arranged macro-pixel units, wherein each macro-pixel unit is an N*1 micro-polarization array, where N is a positive integer and N≥3, and each macro-pixel unit includes at least N-1 different polarization angles.

[0016] According to another embodiment of this disclosure, the micro-polarization array is a two-dimensional array, the detector array is a two-dimensional array, the two-dimensional micro-polarization array includes a plurality of macro-pixel units arranged on a two-dimensional plane, wherein each macro-pixel unit is an M1*M2 micro-polarization array, wherein M1 and M2 are positive integers and M1 and M2 ≥ 2, and each macro-pixel unit includes at least N-1 different polarization angles, wherein N = M1*M2.

[0017] According to another embodiment of this disclosure, the N micro-polarizers of each macropixel unit include at least one of the following: N linearly polarized micro-polarizers; N-1 linearly polarized micro-polarizers and one circularly polarized micro-polarizer; N-1 linearly polarized micro-polarizers and one non-polarized micro-polarizer; N partially polarized micro-polarizers.

[0018] According to another embodiment of this disclosure, the polarization angles of the N linearly polarized micro-polarizers are Deg1, Deg2, Deg3, ..., DegN, where...

[0019]

[0020] Where i is a positive integer less than or equal to N.

[0021] According to another embodiment of this disclosure, the polarization angles of the N-1 linearly polarized micropolarizers are Deg1, Deg2, Deg3, ..., DegN-1, respectively.

[0022] or

[0023] Where i is a positive integer less than or equal to N-1;

[0024] Among them, circular polarization includes at least one of left-handed circular polarization and right-handed circular polarization.

[0025] According to another embodiment of this disclosure, a partially polarized micro-polarized sheet includes a portion of a micro-polarized sheet that is linearly polarized and the other portion is unpolarized, or a portion of a micro-polarized sheet that is circularly polarized and the other portion is unpolarized.

[0026] According to another embodiment of this disclosure, the polarization angle is fixed or adjustable.

[0027] According to another embodiment of this disclosure, the millimeter-wave terahertz imaging apparatus further includes a millimeter-wave terahertz radiation source for radiating millimeter-wave terahertz waves to the object under inspection.

[0028] According to another embodiment of this disclosure, the micro-polarized plate array is a one-dimensional array, the detector array is a one-dimensional array, and the millimeter-wave terahertz imaging device further includes a rotatable scanning mirror disposed in the optical path between the object under inspection and the focusing lens.

[0029] According to another embodiment of this disclosure, the rotatable scanning mirror can be rotated to image a specific portion of the object under inspection onto a specific wave-sensing unit of a one-dimensional detector array at a specific rotation angle.

[0030] According to another aspect of this disclosure, a method for object recognition and classification using a millimeter-wave terahertz imaging device according to a first aspect of this disclosure is provided, comprising:

[0031] The focusing lens allows the millimeter-wave terahertz waves spontaneously radiated or reflected from the object under test to pass through the micro-polarized plate array and be focused onto the detector array.

[0032] The detector array converts the millimeter-wave terahertz waves passing through the micro-polarized plate array into a polarization image of the object under test.

[0033] The polarization image is processed using the image processing device to obtain a high-resolution polarization image;

[0034] Based on the obtained high-resolution polarization images, an automatic recognition algorithm is used for object identification and classification.

[0035] According to one embodiment of this disclosure, both the micropolarized array and the detector array are two-dimensional arrays.

[0036] According to another embodiment of this disclosure, the micro-polarizer array includes a plurality of macro-pixel units, each macro-pixel unit including N micro-polarizers, the N micro-polarizers having at least N-1 polarization angles, and the detector array including wave-sensing units (N is a positive integer greater than or equal to 4) with the same number and corresponding positions as the micro-polarizers. The step of processing the polarized image using the image processing device to obtain a high-resolution polarized image includes:

[0037] S1: In the polarization image obtained by the detector array, extract N low-resolution polarization images from the pixels corresponding to multiple sensing units. Each low-resolution polarization image has a polarization angle and includes all pixels with the same polarization angle.

[0038] S2: Estimate the non-polarization intensity data of pixels at polarization angle positions in the polarization array to obtain a high-resolution non-polarization image. The resolution of the high-resolution non-polarization image is equal to the size of the micro-polarization array.

[0039] In each polarization unit of the high-resolution non-polarized image, the average value of the estimated non-polarized intensity data is calculated. This average value is used as the non-polarized intensity value of each polarization unit with the corresponding polarization angle. The same process is performed on the entire array range to obtain N low-resolution non-polarized images.

[0040] S3: Guided by the N low-resolution images obtained in step S1 and the low-resolution non-polarized images obtained in step S2, interpolation is used to obtain N intermediate images with different polarization angles. Then, the low-resolution non-polarized images are subtracted from the N intermediate images to obtain N low-resolution polarization difference images.

[0041] S4: The N low-resolution polarimetric difference images obtained in step S3 are processed using bilinear interpolation and upsampling to obtain N corresponding high-resolution polarimetric difference images; and

[0042] S5: Summate the N high-resolution polarization difference images obtained in step S4 with the high-resolution non-polarized images obtained in step S2 to finally obtain N high-resolution polarized images.

[0043] According to another embodiment of this disclosure, the step of processing the polarized image using the image processing apparatus to obtain a high-resolution polarized image further includes: S6: performing a super-resolution image processing algorithm to improve the resolution of the high-resolution polarized image with polarization information obtained in step S5.

[0044] In the millimeter-wave terahertz imaging device and object recognition and classification method using the device according to this disclosure, a polarization image of the inspected object can be obtained by setting a one-dimensional or two-dimensional micro-polarization array. After processing by an image processing device, this polarization image yields a high-resolution image with polarization information. Polarization imaging technology can detect not only the structural information of an object's surface, such as roughness and texture, but also information such as the object's electrical conductivity and refractive index. This approach provides more information than existing passive terahertz imagers (which can only detect surface intensity information), which is very useful for object classification and recognition. By acquiring polarization information, such as different surface textures, roughness, refractive index, and electrical conductivity of materials, suspicious objects of similar shapes and sizes can be distinguished, i.e., identified and classified. Furthermore, the millimeter-wave terahertz imaging device according to this disclosure can identify objects down to the millimeter level. Attached Figure Description

[0045] Figure 1 A passive millimeter-wave terahertz imaging apparatus according to one embodiment of the present disclosure is shown.

[0046] Figure 2An active millimeter-wave terahertz imaging device according to one embodiment of the present disclosure is shown.

[0047] Figure 3 A schematic diagram of the imaging principle of a millimeter-wave terahertz imaging device including a two-dimensional micro-polarization array according to an embodiment of the present disclosure is shown.

[0048] Figure 4 A schematic diagram of the imaging principle of a millimeter-wave terahertz imaging device including a one-dimensional micro-polarization array is shown according to an embodiment of the present disclosure.

[0049] Figure 5A and 5B A simplified schematic diagram of a macropixel unit of a two-dimensional micropolarized sheet array according to an embodiment of the present disclosure is shown.

[0050] Figure 6A and 6B A simplified schematic diagram of a macropixel unit of a two-dimensional micropolarized sheet array according to an embodiment of the present disclosure is shown.

[0051] Figure 7A and 7B A simplified schematic diagram of a macropixel unit of a two-dimensional micropolarized sheet array according to an embodiment of the present disclosure is shown.

[0052] Figure 8A , 8B Figure 8C shows a simplified schematic diagram of a macropixel unit of a two-dimensional micropolarized sheet array according to an embodiment of the present disclosure.

[0053] Figure 9 A simplified schematic diagram of a macropixel unit of a one-dimensional micropolarized sheet array according to an embodiment of the present disclosure is shown.

[0054] Figure 10 A simplified schematic diagram of a macropixel unit of a one-dimensional micropolarized sheet array according to an embodiment of the present disclosure is shown.

[0055] Figure 11A and 11B A simplified schematic diagram of a macropixel unit of a one-dimensional micropolarized sheet array according to an embodiment of the present disclosure is shown.

[0056] Figure 12 A simplified schematic diagram of a macropixel unit of a one-dimensional micropolarized sheet array according to an embodiment of the present disclosure is shown.

[0057] Figure 13 An image obtained by a detector array according to an embodiment of the present disclosure is shown.

[0058] Figure 14Four low-resolution polarization images extracted from images obtained from a detector array according to one embodiment of the present disclosure are shown. Detailed Implementation

[0059] While this disclosure will be fully described with reference to the accompanying drawings containing preferred embodiments, it should be understood before this description that those skilled in the art can modify the disclosure described herein to obtain the technical effects of this disclosure. Therefore, it should be understood that the above description is a broad disclosure to those skilled in the art and is not intended to limit the exemplary embodiments described herein.

[0060] Furthermore, in the following detailed description, numerous specific details are set forth for ease of explanation to provide a thorough understanding of the embodiments disclosed herein. However, it will be apparent that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and apparatuses are illustrated to simplify the figures.

[0061] Figure 1 A passive millimeter-wave terahertz imaging device according to this disclosure is shown. For example... Figure 1 As shown, a millimeter-wave terahertz imaging device is used for security inspection of an object 1. It includes a focusing lens 3, a micro-polarizer array 4, a detector array 5, and an image processing unit 6. The focusing lens 3 is positioned between the object 1 and the micro-polarizer array 4, and is configured to focus the spontaneously radiated or reflected millimeter-wave terahertz waves 2 from the object 1 onto the detector array 5. The micro-polarizer array 4 is positioned on the side of the detector array 5 facing the focusing lens 3 and is close to the detector array 5. Preferably, the micro-polarizer array 4 is positioned close to the detector array 5, minimizing the spacing between them. The size of the micro-polarizer is 0.01-1 mm, and the size of the polarizer needs to match the size of the detector; preferably, the size of the micro-polarizer is 0.03-0.1 mm. The detector array 5 is positioned on the focal plane of the focusing lens 3 and is configured to convert the millimeter-wave terahertz waves transmitted through the micro-polarizer array 4 into a polarized image of the object. The image processing device 6 is disposed on the side of the detector array 5 away from the micro-polarization array 4, and is configured to process the polarization image to obtain a high-resolution polarization image, thereby identifying and classifying the object under inspection based on the high-resolution polarization image.

[0062] In one embodiment according to this disclosure, the image processing apparatus 6 includes an analog signal processor 61, a digital-to-analog converter (D / A converter) 62, a digital signal processor 63, and an image display 64. The detector array 5 converts the incident millimeter-wave terahertz wave into an electrical signal at each pixel and sends it to the analog signal processor 61. The analog signal processor 61 receives the analog signal from the detector and sends it to the D / A converter 62. The D / A converter 62 receives the signal transmitted from the analog signal processor, performs digital-to-analog conversion on it, and then sends it to the digital signal processor 63. The digital signal processor 63 receives the converted information, performs de-mosaic processing on it, and then displays the de-mosaiced image on the image display 64. The de-mosaic processing method will be described in detail below.

[0063] In this disclosure, terahertz waves are electromagnetic waves with frequencies ranging from 100 GHz to 10 THz (10000 GHz). Terahertz waves lie between microwaves and visible light, overlapping with millimeter waves in the long-wavelength band and with infrared light in the short-wavelength band. Millimeter waves have a frequency band of 26.5 to 300 GHz, and the millimeter-wave terahertz waves described in this disclosure refer to electromagnetic waves with frequencies between 30 GHz and 1000 GHz. In the technical field of millimeter-wave terahertz imaging equipment, because the energy of millimeter-wave terahertz waves radiated or reflected by the human body is very low, millimeter-wave terahertz waves are suitable for use in security checks.

[0064] Figure 2 An active millimeter-wave terahertz imaging device according to this disclosure is shown. For example... Figure 2 As shown, the active millimeter-wave terahertz imaging device also includes a millimeter-wave terahertz radiation source 7, which is used to radiate millimeter-wave terahertz waves to the object under test 1, so that the object under test 1 reflects the millimeter-wave terahertz waves to the focusing lens 3.

[0065] In one embodiment according to the present disclosure, the micro-polarized plate array 4 includes a plurality of micro-polarized plates, each of the plurality of micro-polarized plates being fully polarized, partially polarized, or unpolarized, and each polarized micro-polarized plate having a specific polarization direction or polarization angle.

[0066] In one embodiment according to this disclosure, the detector array 5 includes a plurality of wave-sensing units, the number of which is the same as the number of micro-polarizers, and the position of each wave-sensing unit on the detector array corresponds to the position of each micro-polarizer on the micro-polarizer array. In the millimeter-wave terahertz imaging device according to this disclosure, the pixel spacing of the micro-polarizer array is matched with the pixel spacing of the detector array, and the polarizer attenuation ratio is designed to be greater than 100 to ensure the acquisition of effective polarization information; the crosstalk (mixed polarization information between adjacent pixels) between adjacent wave-sensing units of the detector array is minimized.

[0067] In one embodiment of the present disclosure, the micropolarized plate array can be a two-dimensional array or a one-dimensional array.

[0068] In one embodiment of the present disclosure, the two-dimensional micro-polarization array 4 includes a plurality of macro-pixel units arranged on a two-dimensional plane, wherein each macro-pixel unit is an M1*M2 micro-polarization array, where M1 and M2 are positive integers and M1, M2 ≥ 2, and each macro-pixel unit includes at least N-1 different polarization angles, where N = M1*M2. In a specific embodiment, M1 and M2 are both equal to 2, and each macro-pixel unit is a 2*2 micro-polarization array.

[0069] In one embodiment of the present disclosure, the one-dimensional micro-polarization array 4 includes a plurality of linearly arranged macro-pixel units, wherein each macro-pixel unit is an N*1 micro-polarization array, where N is a positive integer and N≥3, and each macro-pixel unit includes at least N-1 different polarization angles. In a specific embodiment, N equals 3, and a macro-pixel unit is a 3*1 micro-polarization array.

[0070] In one specific embodiment according to this disclosure, a micropolarizer array using terahertz waves with a center frequency of 650 GHz is employed. The array size is 120 × 160, each micropolarizer measures 1.15 mm × 1.15 mm, and the array size is 13.8 cm × 18.4 cm. This micropolarizer array is placed in front of a terahertz imaging detector. The terahertz detector has a pixel value of 120 × 160, and each pixel (sensing unit) measures 1.15 mm × 1.15 mm. The micropolarizer array and the detector are perfectly matched in size and dimensions, and the micropolarizer array and the detector are placed as close as possible.

[0071] Figure 3 A schematic diagram of the imaging principle of a millimeter-wave terahertz imaging device including a two-dimensional micro-polarization array according to an embodiment of the present disclosure is shown. Figure 4 A schematic diagram of the imaging principle of a millimeter-wave terahertz imaging device including a one-dimensional micro-polarization array is shown according to an embodiment of the present disclosure.

[0072] In one embodiment according to this disclosure, such as Figure 3 As shown, the micro-polarized array is a two-dimensional micro-polarized array, and the detector array is a two-dimensional detector array. The two-dimensional micro-polarized array is placed in front of and close to the plane of the two-dimensional detector array. For example, when the object being inspected is a person, millimeter-wave terahertz waves from the person's head are imaged onto one or more first sensing units of the two-dimensional detector array after passing through a lens and the two-dimensional micro-polarized array. Simultaneously, millimeter-wave terahertz waves from the person's chest are imaged onto one or more second sensing units of the two-dimensional detector array after passing through a lens and the two-dimensional micro-polarized array, and these second sensing units are located at different positions than the first sensing units. Therefore, the entire two-dimensional detector array can detect and image millimeter-wave terahertz waves from multiple different locations simultaneously. When detecting a suspicious object, it is possible to image millimeter-wave terahertz waves radiated or reflected from multiple locations on the suspicious object.

[0073] In one embodiment according to this disclosure, such as Figure 4 As shown, the micropolarized plate array is a one-dimensional micropolarized plate array, and the detector array is a one-dimensional detector array. The one-dimensional micropolarized plate array is placed in front of and close to the plane of the one-dimensional detector array. In this case, the millimeter-wave terahertz imaging device also includes a rotatable scanning mirror 8 disposed in the optical path between the subject 1 and the focusing lens 3. The rotatable scanning mirror 8 can rotate to image a specific part of the subject onto a specific sensing unit of the one-dimensional detector array at a specific rotation angle. For example, when the rotatable scanning mirror 8 is at a first rotation angle, the millimeter-wave terahertz imaging device images the head of the subject onto the first sensing unit of the one-dimensional detector array. When the rotatable scanning mirror 8 is at a second rotation angle different from the first rotation angle, the millimeter-wave terahertz imaging device images other parts of the subject, such as the chest, onto a second sensing unit of the one-dimensional detector array, which is different from the first sensing unit. The rotatable scanning mirror 8 is rotated repeatedly until a complete scan of the subject is achieved and each part is imaged onto the one-dimensional detector array. By incorporating this rotatable scanning mirror 8, the number of expensive detector units can be reduced, thereby saving costs.

[0074] The advantages of using the millimeter-wave terahertz imaging device according to this disclosure are mainly reflected in the following two aspects.

[0075] Firstly, the polarization information from the detected polarization images can be used for object classification and recognition. This is because polarization imaging technology can detect not only the structural information of an object's surface, such as roughness and texture, but also information such as the surface conductivity and refractive index. This approach provides more information than existing passive terahertz imagers (which can only detect surface intensity information), and this information is very useful for object classification and recognition. For example, when using a common passive millimeter-wave terahertz security scanner to detect suspicious items carried by a person, such as mobile phones, banknotes, knives, and handguns, the human body appears white in the grayscale image because its surface temperature is higher than that of the suspicious items, while the suspicious items appear as black blocks. Usually, neither machine recognition nor manual recognition can distinguish between belt buckles, mobile phones, metal blocks, dielectric blocks, and banknotes of similar shapes and sizes. We cannot identify suspicious items based on the shape of the black blocks. Using polarization imaging technology, the acquired polarization information (different surface textures, roughness, refractive index, conductivity, etc.) can be used to distinguish suspicious items of similar shapes and sizes.

[0076] On the other hand, super-resolution imaging can be achieved through super-resolution polarization imaging reconstruction algorithms, which improves the resolution by at least 4 times compared to existing imaging image modes (which cannot obtain polarization information). The resolution can reach the millimeter level, which is very effective for identifying suspicious objects with millimeter-level structures.

[0077] The following section will detail the polarization methods of one-dimensional and two-dimensional micropolarizer arrays. The polarization direction or polarization angle of the polarizer is its transmission direction. An incident wave can be decomposed into waves whose vibration direction is parallel to the transmission direction and waves whose vibration direction is perpendicular to the transmission direction. Waves whose vibration direction is perpendicular to the transmission direction cannot pass through, while waves whose vibration direction is parallel to the transmission direction can pass through. Therefore, we can obtain linearly polarized waves polarized along the transmission direction.

[0078] Figure 5A and 5B A simplified schematic diagram of a macropixel unit of a two-dimensional micropolarizer array according to an embodiment of the present disclosure is shown. In this embodiment, the macropixel unit includes N linearized micropolarizers with polarization angles of Deg1, Deg2, Deg3, ..., DegN, wherein... i is a positive integer less than or equal to N. For example... Figure 5A As shown, when the number of micro-polarizers in each macropixel unit is N=4, the macropixel arrangement of a macropixel unit is linear polarization of 0°, 45°, 90°, and -45°. For example... Figure 5B As shown, a macropixel unit has a macropixel arrangement with linear polarization of 30°, 75°, 120° and -15°.

[0079] Figure 6A and6B A simplified schematic diagram of a macropixel unit of a two-dimensional micropolarization array according to an embodiment of the present disclosure is shown. In this embodiment, the macropixel unit includes N-1 linearly polarized micropolarization plates and one circularly polarized micropolarization plate. The circular polarization can be left-handed or right-handed circular polarization. The N-1 linear polarization angles are Deg1, Deg2, Deg3, ... Deg(N-1), where... or i is a positive integer less than or equal to N-1. For example... Figure 6A As shown, when the number of micro-polarizers in each macropixel unit is N=4, the polarization angles of the four polarizers are 0° linear polarization, 60° linear polarization, 120° linear polarization, and circular polarization. Figure 6B As shown, when the number of micro-polarizers in each macropixel unit is N=4, the polarization angles of the four polarizers are 0° linear polarization, 45° linear polarization, 90° linear polarization, and circular polarization.

[0080] Figure 7A and 7B A simplified schematic diagram of a macropixel unit of a two-dimensional micropolarizer array according to an embodiment of the present disclosure is shown. In this embodiment, the macropixel unit includes N-1 linearly polarized micropolarizers and one non-polarized micropolarizer, wherein the N-1 linear polarization angles are Deg1, Deg2, Deg3, ..., Deg(N-1), and so on. or i is a positive integer less than or equal to N-1. For example... Figure 7A As shown, when the number of micro-polarizers in each macropixel unit is N=4, the polarization angles of the four polarizers are 0° linear polarization, 60° linear polarization, 120° linear polarization, and no polarization. Figure 7B As shown, when the number of micro-polarizers in each macropixel unit is N=4, the polarization angles of the four polarizers are 0° linear polarization, 45° linear polarization, 90° linear polarization, and no polarization. Introducing a non-polarized waveplate into the polarization unit ensures high transmittance for waves in different polarization states. Furthermore, while ensuring real-time acquisition of polarized images, it improves the wave sensitivity of the imaging device under low illumination or low exposure time, and effectively solves the saturation problem of different types of lenses. The size of the non-polarized waveplate region is adjusted according to the required transmittance.

[0081] Figure 8A , 8B Figure 8C shows a simplified schematic diagram of a macropixel unit of a two-dimensional micropolarizer array according to an embodiment of the present disclosure. In this embodiment, to increase the transmittance of the polarizer, the polarizer can be fabricated so that some regions are polarized and some regions are non-polarized. The size of the polarized region and the size of the non-polarized region can be adjusted by the transmittance as needed.

[0082] In one embodiment, such as Figure 8A As shown, when the number of micro-polarizers in each macropixel unit is N=4, the polarization angles of the four polarizers are partially 0° linear polarization, partially 45° linear polarization, partially 90° linear polarization, and partially -45° linear polarization.

[0083] In one embodiment, such as Figure 8B As shown, when the number of micro-polarizers in each macropixel unit is N=4, the polarization angles of the four polarizers are partially 0° linear polarization, partially 45° linear polarization, partially 90° linear polarization, and partially circular polarization.

[0084] In one embodiment, such as Figure 8C As shown, when the number of micro-polarizers in each macropixel unit is N=4, the polarization angles of the four polarizers are partially 0° linear polarization, partially 60° linear polarization, partially 120° linear polarization, and partially circular polarization.

[0085] Figure 9 A simplified schematic diagram of a macropixel unit of a one-dimensional micropolarizer array according to an embodiment of the present disclosure is shown. In this embodiment, the macropixel unit includes N linearly polarized micropolarizers with polarization angles of Deg1, Deg2, Deg3, ..., DegN, where... In one embodiment, such as Figure 9 As shown, when the number of micro-polarizers N=3 in a macropixel unit, the macropixel arrangement of a macropixel unit is linear polarization of 0°, 60° and 120°.

[0086] Figure 10 A simplified schematic diagram of a macropixel unit of a one-dimensional micropolarization array according to an embodiment of the present disclosure is shown. In this embodiment, the macropixel unit includes N-1 linearly polarized micropolarization plates and one circularly polarized micropolarization plate. The circular polarization can be left-handed or right-handed circular polarization. The N-1 linear polarization angles are Deg1, Deg2, Deg3, ... Deg(N-1), where... In one embodiment, such as Figure 10 As shown, when the number of micro-polarizers N=3 in a macropixel unit, the macropixel arrangement of a macropixel unit is 0° linear polarization, 90° linear polarization, and circular polarization.

[0087] Figure 11A and 11B A simplified schematic diagram of a macropixel unit of a one-dimensional micropolarizer array according to an embodiment of the present disclosure is shown. In this embodiment, the macropixel unit includes N-1 linearly polarized micropolarizers and one non-polarized micropolarizer, wherein the N-1 linear polarization angles are Deg1, Deg2, Deg3, ..., Deg(N-1), and so on. or In one embodiment, such as Figure 11A As shown, when the number of micro-polarizers in each macropixel unit is N=4, the polarization angles of the four polarizers are 0° linear polarization, 60° linear polarization, 120° linear polarization, and no polarization. In one embodiment, as... Figure 11B As shown, when the number of micro-polarizers in each macropixel unit is N=4, the polarization angles of the four polarizers are 0° linear polarization, 45° linear polarization, 90° linear polarization, and no polarization.

[0088] Figure 12 A simplified schematic diagram of a macropixel unit of a one-dimensional micropolarizer array according to an embodiment of the present disclosure is shown. In this embodiment, to increase the transmittance of the polarizer, the polarizer can be fabricated so that some regions are polarized and some regions are non-polarized. The size of the polarized region and the size of the non-polarized region can be adjusted by the transmittance as needed.

[0089] In one embodiment, such as Figure 12 As shown, when the number of micro-polarizers in each macropixel unit is N=3, the polarization angles of the three polarizers are partially 0° linear polarization, partially 60° linear polarization, and partially 120° linear polarization.

[0090] In one embodiment according to this disclosure, the polarization angle of each micropolarizer in the micropolarizer array is fixed after manufacturing. Three methods for fixing the polarization angle of the micropolarizers will be described below.

[0091] In the first approach, the polarizer used for low-frequency millimeter-wave polarization selection is a subwavelength periodically distributed metal wire grid. The orientation of the wire grid is used to select the polarization direction through which the wave can pass. A row of parallel metal wires is wound around a metal frame. The diameter of the metal wires is 1–10 μm, and the spacing between the wires is 10 μm–1 mm. The materials of the metal wires are typically aluminum and copper. This type of polarizer has advantages such as simple fabrication process, no need for a substrate, and high transmittance.

[0092] In the second approach, the polarizer suitable for wavelengths of 0.3mm-3mm is a metal grid line polarization substrate, comprising a metal grid and a substrate. The metal grid consists of multiple parallel metal lines, through which the wave it passes is called a linearly polarized wave. One side surface of the substrate has multiple grooves. The multiple metal lines are disposed within these grooves. The surfaces of the multiple metal lines are flush with the grooved side surface of the substrate. The metal grid is made of aluminum, titanium, or silver.

[0093] In the third method, the polyethylene polarizer suitable for high-frequency millimeter-wave terahertz polarization is designed based on the transmission wave principle of diffraction gratings. Its manufacturing process involves first etching periodic lines with a triangular shape onto a substrate, and then depositing a metal film (Al film) on the surface. The advantages of polyethylene grid polarizers over grating polarizers are their lower cost and wider band application (suitable for wavelengths of 0.3-3 mm).

[0094] In one embodiment according to this disclosure, the polarization angle of each micropolarizer in the micropolarizer array can be changed as needed after manufacturing. Two methods for adjusting the polarization angle will be described in detail below.

[0095] One way to achieve polarization angle control is to use programmable metamaterials to fabricate micropolarizers and adjust their polarization angle through computer programming.

[0096] Another possible implementation is a combination of a tunable waveplate and a conventional polarization plate. The tunable waveplate can be made of a functional material. Functional materials are materials that possess specific functions after being acted upon by waves, electricity, magnetism, heat, chemicals, biochemistry, etc. Functional materials can be liquid crystals, vanadium oxide, graphene, etc. For example, a structure with polarization asymmetry can be designed so that millimeter-wave and terahertz waves can produce birefringence when passing through it, and the polarization angle can be controlled by adjusting the refractive index of the waveplate using an external field.

[0097] According to another aspect of this disclosure, a method for object recognition and classification using the aforementioned millimeter-wave terahertz imaging device is also provided. The method includes the following steps: using the focusing lens, allowing millimeter-wave terahertz waves spontaneously radiated or reflected from the object to pass through the micro-polarization array and be focused onto the detector array; using the detector array, converting the millimeter-wave terahertz waves passing through the micro-polarization array into a polarization image of the object (e.g., as shown in the image). Figure 13 The polarization image shown is processed using the image processing device to obtain a high-resolution polarization image; based on the obtained high-resolution polarization image, an automatic recognition algorithm is used to identify and classify objects.

[0098] In one embodiment according to this disclosure, both the micropolarizer array and the detector array are two-dimensional arrays. It is understood that the micropolarizer array and the detector array may also be one-dimensional arrays. When both the micropolarizer array and the detector array are one-dimensional arrays, the millimeter-wave terahertz imaging device further includes a rotatable scanning mirror disposed between the focusing lens and the object under inspection. The function and operation of this rotatable scanning mirror have been described in detail above and will not be repeated here.

[0099] In one embodiment of this disclosure, where both the micropolarizer array and the detector array are two-dimensional arrays, a method for processing the polarized image to obtain a high-resolution polarized image using an image processing apparatus will be described in detail. In this embodiment, the micropolarizer array includes a plurality of macropixel units, each macropixel unit including N micropolarizers, the N micropolarizers having at least N-1 polarization angles, and the detector array including wave-sensing units (N is a positive integer greater than or equal to 4) that are equal in number and position to the micropolarizers.

[0100] In this embodiment, the polarization image is processed using the image processing device to obtain a high-resolution polarization image, thereby completing the demosaicing process of the original image. This process includes the following five steps. In step S1, the polarization image obtained by the detector array (e.g., as shown in the image) is processed... Figure 13 In the example shown), N low-resolution polarization images (e.g., as shown) are extracted from the pixels corresponding to multiple sensing units. Figure 14 The four images shown (a), (b), (c), and (d) each have a low-resolution polarization angle and include all pixels with the same polarization angle. For example, as... Figure 13 As shown, the detector array used includes 16 sensing units, thus the obtained polarization image resolution is 4*4. The corresponding micropolarimeter array includes 16 micropolarimeters, therefore the micropolarimeter array size is 4*4, and the polarization angles of the macropixel units in the micropolarimeter array are -45° linear polarization, 0° linear polarization, 45° linear polarization, and 90° linear polarization, and the resolution of the macropixel unit is 2*2. Therefore, as... Figure 14 As shown, the resolution of the four low-resolution polarized images obtained through step S1 is 2*2, and the polarization angle of image (a) is 0°, the polarization angle of image (b) is 45°, the polarization angle of image (c) is 90° and the polarization angle of image (d) is -45°.

[0101] In step S2, the non-polarization intensity data of pixels at polarization angle positions in the polarization array is estimated to obtain a high-resolution non-polarization image. The resolution of the high-resolution non-polarization image is equal to the size of the micro-polarization array. For example, as... Figure 14 As shown, in the illustrated embodiment, the resolution of the high-resolution polarization-free image is 4*4. Furthermore, in step S2, the average value of the estimated polarization intensity data is calculated for each polarization unit of the high-resolution polarization-free image. This average value is used as the polarization intensity value for each polarization unit with the corresponding polarization angle. This same process is performed across the entire array range to obtain N low-resolution polarization-free images. For example, as... Figure 14As shown, in the illustrated embodiment, the number of low-resolution unpolarized images is 4, and the resolution is 2*2.

[0102] In step S3, guided by the N low-resolution images obtained in step S1 and the low-resolution non-polarized image obtained in step S2, N intermediate images with different polarization angles are obtained through interpolation. Then, the low-resolution non-polarized image is subtracted from each of the N intermediate images to obtain N low-resolution polarization difference images. Figure 14 As shown, in the illustrated embodiment, four low-resolution polarization difference images with a resolution of 2*2 are obtained.

[0103] In step S4, bilinear interpolation and upsampling are used to process the N low-resolution polarimetric images obtained in step S3, resulting in N corresponding high-resolution polarimetric images. For example... Figure 14 As shown, in the illustrated embodiment, four high-resolution polarization difference images with a resolution of 4*4 are obtained.

[0104] In step S5, the N high-resolution polarization difference images obtained in step S4 are summed with the high-resolution non-polarization image obtained in step S2 to finally obtain N high-resolution polarization images. Figure 14 As shown, in the illustrated embodiment, four high-resolution polarization images were obtained.

[0105] In one embodiment according to this disclosure, to further improve the resolution of high-resolution polarimetric images, a super-resolution image processing algorithm can be applied to the high-resolution polarimetric image containing polarimetric information to improve the resolution. Super-resolution imaging can be achieved through a super-resolution polarimetric imaging reconstruction algorithm, increasing the resolution by at least four times compared to existing imaging image modes (which cannot obtain polarimetric information), reaching millimeter-level resolution. This is highly effective for identifying suspicious objects with millimeter-level structures.

[0106] Those skilled in the art will understand that the embodiments described above are exemplary and can be improved upon. The structures described in the various embodiments can be freely combined without causing any conflict in structure or principle.

[0107] After a detailed description of the preferred embodiments of this disclosure, those skilled in the art will clearly understand that various changes and modifications can be made without departing from the scope and spirit of the appended claims, and that this disclosure is not limited to the implementation of the exemplary embodiments described in the specification.

Claims

1. A millimeter-wave terahertz imaging device for security inspection of an object, comprising a focusing lens, a micropolarizer array, a detector array, and an image processing unit, wherein... The focusing lens is disposed between the object under test and the micro-polarized plate array, and is configured to focus the millimeter-wave terahertz waves spontaneously radiated or reflected back by the object under test onto the detector array. The micro-polarized plate array is disposed on the side of the detector array facing the focusing lens and is positioned close to the detector array; The detector array is disposed on the focal plane of the focusing lens and is configured to convert millimeter-wave terahertz waves passing through the micropolarizer array into a polarized image of the object under test. The image processing device is disposed on the side of the detector array away from the micropolarization array and is configured to process the polarization image to identify and classify the object under inspection; and The micro-polarization array is a two-dimensional array, the detector array is a two-dimensional array, and the two-dimensional micro-polarization array includes multiple macro-pixel units arranged on a two-dimensional plane, wherein each macro-pixel unit is an M1*M2 micro-polarization array, where M1 and M2 are positive integers and M1 and M2≥2, and each macro-pixel unit includes at least N-1 different polarization angles, where N=M1*M2; The image processing device is configured to estimate the non-polarization intensity data of pixels at polarization angle positions in the polarization array to obtain a high-resolution non-polarization image, the resolution of which is equal to the size of the micro-polarization array; and to average the estimated non-polarization intensity data in each polarization unit of the high-resolution non-polarization image, using this average value as the non-polarization intensity value for each polarization unit with the corresponding polarization angle, and to perform the same processing across the entire array range to obtain N low-resolution non-polarization images; and The image processing device is configured to obtain a high-resolution polarized image of the object under inspection based on the N low-resolution non-polarized images and polarized images from the detector array.

2. The millimeter-wave terahertz imaging device according to claim 1, wherein, The micro-polarized plate array includes multiple micro-polarized plates, each of which is fully polarized, partially polarized, or unpolarized.

3. The millimeter-wave terahertz imaging device according to claim 2, wherein, The detector array includes multiple sensing units, the number of which is the same as the number of micro-polarizers. The position of each sensing unit on the detector array corresponds to the position of each micro-polarizer on the micro-polarizer array.

4. The millimeter-wave terahertz imaging device according to claim 1, wherein, Each macropixel unit has N micro-polarizers, including at least one of the following: N linearly polarized micro-polarizers; N-1 linearly polarized micro-polarizers and one circularly polarized micro-polarizer; N-1 linearly polarized micro-polarizers and one non-polarized micro-polarizer; or N partially polarized micro-polarizers.

5. The millimeter-wave terahertz imaging device according to claim 4, wherein, The polarization angles of the N linearly polarized micro-polarizers are Deg1, Deg2, Deg3, ..., DegN, where... Where i is a positive integer less than or equal to N.

6. The millimeter-wave terahertz imaging device according to claim 4, wherein, The polarization angles of the N-1 linearly polarized micro-polarizers are Deg1, Deg2, Deg3, ..., DegN-1, respectively. or Where i is a positive integer less than or equal to N-1; Among them, circular polarization includes at least one of left-handed circular polarization and right-handed circular polarization.

7. The millimeter-wave terahertz imaging device according to claim 4, wherein the partially polarized micropolarizer comprises a portion of a micropolarizer that is linearly polarized and the other portion is unpolarized, or a portion of a micropolarizer that is circularly polarized and the other portion is unpolarized.

8. The millimeter-wave terahertz imaging device according to claim 1, wherein the polarization angle is fixed or adjustable.

9. The millimeter-wave terahertz imaging device according to claim 1 further includes a millimeter-wave terahertz radiation source for radiating millimeter-wave terahertz waves to the object under inspection.

10. A method for object recognition and classification using the millimeter-wave terahertz imaging device according to claim 1, comprising: The focusing lens allows the millimeter-wave terahertz waves spontaneously radiated or reflected from the object under test to pass through the micro-polarized plate array and be focused onto the detector array. The detector array converts the millimeter-wave terahertz waves passing through the micro-polarized plate array into a polarization image of the object under test. The polarization image is processed using an image processing device to obtain a high-resolution polarization image; Based on the obtained high-resolution polarization images, an automatic recognition algorithm is used for object identification and classification.

11. The object recognition and classification method according to claim 10, wherein, The micropolarizer array includes multiple macropixel units, each macropixel unit includes N micropolarizers, and the N micropolarizers have at least N-1 polarization angles. The detector array includes wave-sensing units equal in number and position to the micropolarizers, where N is a positive integer greater than or equal to 4. The step of processing the polarized image using the image processing device to obtain a high-resolution polarized image includes: S1: In the polarization image obtained by the detector array, extract N low-resolution polarization images from the pixels corresponding to multiple sensing units. Each low-resolution polarization image has a polarization angle and includes all pixels with the same polarization angle. S2: Estimate the non-polarization intensity data of pixels at polarization angle positions in the polarization array to obtain a high-resolution non-polarization image. The resolution of the high-resolution non-polarization image is equal to the size of the micro-polarization array. In each polarization unit of the high-resolution non-polarized image, the average value of the estimated non-polarized intensity data is calculated. This average value is used as the non-polarized intensity value of each polarization unit with the corresponding polarization angle. The same process is performed on the entire array range to obtain N low-resolution non-polarized images. S3: Guided by the N low-resolution polarized images obtained in step S1 and the low-resolution non-polarized image obtained in step S2, interpolation is used to obtain N intermediate images with different polarization angles. Then, the low-resolution non-polarized image is subtracted from each of the N intermediate images to obtain N low-resolution polarization difference images. S4: The N low-resolution polarimetric difference images obtained in step S3 are processed using bilinear interpolation and upsampling to obtain N corresponding high-resolution polarimetric difference images; and S5: Summing the N high-resolution polarization difference images obtained in step S4 with the high-resolution non-polarized images obtained in step S2, finally obtaining N high-resolution polarized images.

12. The object recognition and classification method according to claim 11, wherein, The step of processing the polarized image using the image processing apparatus to obtain a high-resolution polarized image further includes: S6: Use a super-resolution image processing algorithm to improve the resolution of the high-resolution polarized image with polarization information obtained in step S5.

Images