A double-label-based hidden point centroid positioning error evaluation method

Abstract

The present application relates to the field of terahertz imaging, and discloses a hidden point centroid positioning error evaluation method based on double labels, comprising: building a terahertz imaging system, including a femtosecond laser system, a beam splitter, an optical delay unit, a high reflector, a terahertz probe, an insulating tile sample, a metal label, a skin, a stepping motor, an adapter and a computer; sticking the double metal labels on the surface of the skin, and then bonding the skin with the insulating tile sample; making the metal labels in different positions of the terahertz light spot; emitting terahertz waves to the metal labels and the skin surface, receiving the reflected time-domain signals, displaying and recording the waveforms by the computer, performing imaging processing, and calculating the imaging centroid spacing; and performing centroid positioning error evaluation; the present application solves the problem that in the prior art, in the actual application scene of small-caliber target detection, it is difficult to meet the demand of obtaining the absolute coordinates of the metal target, and the hidden point centroid positioning error of the terahertz imaging system cannot be accurately evaluated.

Description

Technical Field

[0001] This invention relates to the field of terahertz imaging, and more particularly to a method for evaluating the centroid localization error of hidden points based on dual labels. Background Technology

[0002] Terahertz waves are high-frequency electromagnetic waves characterized by low photon energy, strong penetrating power, and non-destructive behavior towards the object being tested. Therefore, they have significant application value in fields such as non-destructive testing, terahertz imaging, and target recognition. Currently, terahertz technology has made some progress in industrial inspection, playing an irreplaceable role in areas such as pharmaceutical testing, airport security checks, and drug identification.

[0003] Thanks to their high penetration and low photon energy, terahertz waves exhibit unique advantages in the non-destructive testing of aerospace thermal insulation materials. Compared to X-rays, they emit no ionizing radiation; compared to ultrasound, they offer higher resolution. Because terahertz waves have a high frequency, and conventional imaging resolution is typically positively correlated with frequency, terahertz imaging generally offers superior resolution compared to millimeter-wave imaging. Nevertheless, the application of terahertz technology in the aerospace field is still in its early stages, primarily limited by its current limited imaging resolution, high transmission path loss, and slow imaging speed.

[0004] In existing terahertz imaging research, scholars have focused primarily on improving image quality and enhancing resolution through algorithms. Reflective terahertz imaging systems can image, extract edges, and locate the centroid of metallic targets. However, the centroid acquired by these systems is typically a relative coordinate. In small-aperture target detection scenarios, obtaining the absolute coordinates of the metal tags is of significant practical importance. Therefore, this invention proposes a method for evaluating the positioning error of terahertz imaging based on dual tags. This method uses calipers to precisely calibrate the distance between the two tags, then acquires images of the two tags using a terahertz imaging system, and calculates the imaging coordinates between the two tags using edge detection and centroid localization algorithms. By comparing the relative distance between the centroids of the two tags obtained from imaging with the actual measured distance, the positioning error of the terahertz imaging system can be evaluated.

[0005] The centroid positioning error evaluation method based on dual labels proposed in this invention has good application potential in the fields of terahertz imaging, target recognition, and precise positioning of concealed points. Summary of the Invention

[0006] The present invention aims to provide a method for evaluating the centroid positioning error of concealed points based on dual labels, in order to solve the problem that in practical application scenarios such as small-aperture target detection, it is difficult to meet the requirement of obtaining the absolute coordinates of metal targets, and it is also impossible to accurately evaluate the centroid positioning error of concealed points in terahertz imaging systems.

[0007] To achieve the above objectives, the present invention provides the following method:

[0008] This invention provides a method for evaluating the centroid localization error of hidden points based on dual labels, comprising:

[0009] S1: Construct a terahertz imaging system, which consists of a femtosecond laser system, a beam splitter, an optical delay unit, a high-reflectivity mirror, a terahertz probe, a heat-insulating tile sample, a metal label, a skin, a stepper motor, an adapter, and a computer. The terahertz probe is equipped with four off-axis parabolic mirrors and two photoconductive antennas.

[0010] S2: Use vernier calipers to accurately calibrate the actual centroid distance between the two metal labels;

[0011] S3: Adhere the two metal tags to the surface of the skin, then attach the skin with the metal tags to the heat insulation tile sample, and fix the skin to the stepper motor through the 3D printed adapter.

[0012] S4: Drive the stepper motor through the 3D motion control program written in LabVIEW on the computer, so that the skin and the metal tag move in the XY plane, and the metal tag is placed at different positions of the terahertz light spot;

[0013] S5: Control the terahertz probe to emit terahertz waves toward the metal tag and the skin surface, and receive the time-domain signals reflected from each surface, which are then displayed and recorded by the computer;

[0014] S6: The time-domain signal recorded by the computer is processed for imaging, and the centroid distance between the two metal tags is calculated by edge detection and centroid positioning algorithm.

[0015] S7: Compare the imaging centroid spacing with the actual centroid spacing to evaluate the centroid positioning error of the terahertz imaging system for hidden points.

[0016] Preferably, the metal tag has a size of 2mm×2mm, the two metal tags are placed with a spacing of 10.5mm, and the actual centroid spacing after calibration is 8.5mm; the metal tag has a three-layer structure from top to bottom, namely a 100nm thick gold functional layer, a 20nm thick chromium intermediate layer, and a 500μm thick silicon dioxide substrate.

[0017] Preferably, the terahertz optical path of the terahertz imaging system is configured as follows: the terahertz pulse generated by the femtosecond laser system is split into a probe beam and a pump beam by the beam splitter. The probe beam is introduced with a delay and then received by photoconductive antenna one and transmitted to the computer. The pump beam is guided by the high-reflectivity mirror to photoconductive antenna two, then passes through the off-axis parabolic mirrors second OAP and fourth OAP and is focused on the surface of the skin after passing through the heat insulation tile sample. After being reflected by the metal label or skin, the terahertz beam is reflected back to the off-axis parabolic mirrors third OAP and first OAP at a small angle. The reflected beam passes sequentially through the heat insulation tile sample and the parabolic mirrors and is received by photoconductive antenna one and transmitted to the computer. The aperture of the off-axis parabolic mirrors first OAP and second OAP is 2 inches and the focal length is 5 cm. The aperture of the off-axis parabolic mirrors third OAP and fourth OAP is 2 inches and the focal length is 200 mm.

[0018] Preferably, the thickness of the heat insulation tile sample is 4cm, the distance from the terahertz probe to the skin surface is 16.6cm, the distance from the center of the third and fourth off-axis parabolic mirrors to the front surface of the terahertz probe is 3.4cm, and the diameter of the terahertz spot is 18mm.

[0019] Preferably, the scanning parameters of the terahertz probe in step S5 are as follows: the scanning area is a 20mm×20mm square, with the center of the first 2mm×2mm square grid in the lower left corner as the scanning starting point, the scanning step is 2mm, and a line-by-line reciprocating scanning path is adopted. The stepper motor controls the skin to step 2mm in the opposite direction of the coordinate axis, which is equivalent to the terahertz probe stepping 2mm in the positive direction of the axis. The signal at 135ps of the terahertz reflection pulse is collected for imaging.

[0020] Preferably, the specific process of the imaging processing in step S6 is as follows: interpolation processing is performed on the original imaging image obtained by the acquired signal, and then the interpolated data matrix is ​​linearly mapped to the 0-1 interval to complete the grayscale processing to obtain a grayscale image; the Canny operator is used to perform edge detection on the grayscale image, and a high threshold Th is set to confirm strong edges and a low threshold Tl to retain weak edges during edge detection. Only when a weak edge is connected to a strong edge is it retained, and the edge detection effect of different threshold ranges is tested cyclically.

[0021] Preferably, after edge detection in step S6, the image further includes an optimization step: performing dilation, erosion, and closure processing on the image with discontinuous edges in sequence. The dilation processing is used to connect broken edges, the erosion processing is used to restore the original size of the image, and the closure processing is used to smooth the image edges.

[0022] Preferably, the optimized edge detection image is filled with regions to form a solid binary region, and then the centroid localization algorithm is used to locate the centroid of the binary region to obtain the centroid coordinates of the metal tag. The centroid coordinates are the weighted average of the coordinates of all pixels in the binary region.

[0023] Preferably, steps S6 and S7 are repeated three times. The distance between the imaging centroids of the two metal tags is calculated in each of the three repeated experiments. The distance between the imaging centroids is compared with the actual centroid distance to obtain the single positioning error. The average positioning error and error fluctuation value of the three experiments are then calculated to complete the comprehensive evaluation of the centroid positioning error of the terahertz imaging system for the hidden point.

[0024] The beneficial effects of this invention are as follows: This invention images two hidden metal tags under aviation thermal insulation tiles using terahertz imaging. Contour extraction and edge detection are used to measure the centroid coordinates of the two tags. The absolute distance between the two tags is calibrated using calipers. By comparing the centroid distance of the two metal tags measured by terahertz imaging with the actual centroid distance, the error of terahertz imaging relative to the measurement of the centroid of the hidden point can be calibrated. This invention patent proposes a method for calibrating the centroid positioning error based on dual tags, which has potential application value in the identification and positioning of hidden points under aviation thermal insulation tiles. Attached Figure Description

[0025] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn to scale.

[0026] Figure 1 A schematic diagram of the terahertz imaging system structure provided in an embodiment of the present invention;

[0027] Figure 2 This is a schematic diagram showing the positional relationship between the heat-insulating tile and the label provided in an embodiment of the present invention;

[0028] Figure 3 This is a schematic diagram of the label structure provided in an embodiment of the present invention;

[0029] Figure 4 These are actual images of two 2mm metal tags provided in an embodiment of the present invention;

[0030] Figure 5 This is a schematic diagram of the stepper motor motion path in terahertz imaging provided in an embodiment of the present invention;

[0031] Figure 6The centroid localization results of the dual-tag provided in the embodiments of the present invention are as follows: (a) is the first scan image, (b) is the centroid localization image of the tag in (a); (c) is the second scan image, (d) is the centroid localization image of the tag in (c); (e) is the third scan image, and (f) is the centroid localization image of the tag in (e).

[0032] Figure 7 This is a partial coordinate diagram of the labels from three experiments provided in an embodiment of the present invention.

[0033] Figure reference numerals: 1-Stepper motor, 2-Adapter, 3-Skin, 4-Metal tag, 401-Functional layer, 402-Intermediate layer, 403-Substrate, 5-Insulation tile sample, 6-Computer, 7-Fourth OAP, 8-Third OAP, 9-Second OAP, 10-First OAP, 11-Photoconductive antenna two, 12-Photoconductive antenna one, 13-High reflectivity mirror, 14-Femtosecond laser system, 15-Beam splitter, 16-Terahertz spot. Detailed Implementation

[0034] To enable those skilled in the art to better understand the present invention, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0035] The terms "first," "second," etc., used in the specification, claims, and accompanying drawings of this invention are used to distinguish different objects, not to describe a specific order. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, apparatus, product, or end that includes a series of steps or units is not limited to the listed steps or units, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to these processes, methods, products, or ends.

[0036] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of the invention. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0037] In existing terahertz imaging research, scholars have focused primarily on improving image quality and enhancing resolution through algorithms. Reflective terahertz imaging systems can image, extract edges, and locate the centroid of metallic targets. However, the centroid acquired by these systems is typically a relative coordinate. In small-aperture target detection scenarios, obtaining the absolute coordinates of the metal tags is of significant practical importance. Therefore, this invention proposes a method for evaluating the positioning error of terahertz imaging based on dual tags. This method uses calipers to precisely calibrate the distance between the two tags, then acquires images of the two tags using a terahertz imaging system, and calculates the imaging coordinates between the two tags using edge detection and centroid localization algorithms. By comparing the relative distance between the centroids of the two tags obtained from imaging with the actual measured distance, the positioning error of the terahertz imaging system can be evaluated.

[0038] The centroid positioning error evaluation method based on dual labels proposed in this invention has good application potential in the fields of terahertz imaging, target recognition, and precise positioning of concealed points.

[0039] The present invention aims to provide a method for evaluating the centroid positioning error of concealed points based on dual labels, in order to solve the problem that in practical application scenarios such as small-aperture target detection, it is difficult to meet the requirement of obtaining the absolute coordinates of metal targets, and it is also impossible to accurately evaluate the centroid positioning error of concealed points in terahertz imaging systems.

[0040] like Figure 1 As shown, a specific embodiment of the present invention provides a method for evaluating the centroid positioning error of hidden points based on dual labels, including:

[0041] S1: Construct a terahertz imaging system. The terahertz imaging system consists of a femtosecond laser system 14, a beam splitter 15, an optical delay unit, a high-reflectivity mirror 13, a terahertz probe, a heat insulation tile sample 5, a metal label 4, a skin 3, a stepper motor 1, an adapter 2, and a computer 6. The terahertz probe is equipped with four off-axis parabolic mirrors and two photoconductive antennas.

[0042] S2: Use vernier calipers to accurately calibrate the actual centroid distance of the bimetallic label 4.

[0043] S3: Adhere the bimetallic label 4 to the surface of the skin 3, then attach the skin 3 with the metal label 4 to the heat insulation tile sample 5, and fix the skin 3 and the stepper motor 1 to the stepper motor 1 through the 3D printed adapter 2.

[0044] In this embodiment of the invention, the metal tag 4 is 2mm×2mm in size, the bimetallic tags 4 are placed at intervals of 10.5mm, and the actual centroid spacing after calibration is 8.5mm; the metal tag 4 has a three-layer structure from top to bottom, namely a 100nm thick gold functional layer 401, a 20nm thick chromium intermediate layer 402, and a 500μm thick silicon dioxide substrate 403.

[0045] S4: The stepper motor 1 is driven by the 3D motion control program written in LabVIEW on computer 6, which moves the skin 3 and the metal tag 4 in the XY plane, so that the metal tag 4 is in different positions of the terahertz light spot 16.

[0046] S5: Controls the terahertz probe to emit terahertz waves to the metal tag 4 and the skin 3, and receives the time-domain signals reflected from each surface, which are then displayed and recorded by the computer 6.

[0047] In this embodiment of the invention, the terahertz optical path of the terahertz imaging system is configured as follows: the terahertz pulse generated by the femtosecond laser system 14 is split into a probe beam and a pump beam by the beam splitter 15. The probe beam is introduced with a delay and then received by the photoconductive antenna 12 and transmitted to the computer 6. The pump beam is guided by the high-reflectivity mirror 13 to the photoconductive antenna 11, and then passes through the second OAP9 and the fourth OAP7 of the off-axis parabolic mirrors, passing through the heat insulation tile sample 5 and focusing onto the surface of the skin 3. After being reflected by the metal tag 4 or the skin 3, the terahertz beam is reflected back to the third OAP8 and the first OAP10 of the off-axis parabolic mirrors at a small angle. The reflected beam passes through the heat insulation tile sample 5 and the parabolic mirrors in sequence and is received by the photoconductive antenna 12 and transmitted to the computer 6. The first OAP10 and the second OAP9 of the off-axis parabolic mirrors have an aperture of 2 inches and a focal length of 5 cm. The third OAP8 and fourth OAP7 of the parabolic mirrors have an aperture of 2 inches and a focal length of 200 mm; the thickness of the heat insulation tile sample 5 is 4 cm; the distance from the terahertz probe to the skin 3 is 16.6 cm; the distance from the center of the third OAP8 and fourth OAP7 of the off-axis parabolic mirrors to the front surface of the terahertz probe is 3.4 cm; the diameter of the terahertz spot 16 is 18 mm; the scanning parameters of the terahertz probe are as follows: the scanning area is a 20 mm × 20 mm square, with the center of the first 2 mm × 2 mm square grid in the lower left corner as the scanning starting point; the scanning step is 2 mm; a line-by-line reciprocating scanning path is adopted; the stepper motor 1 controls the skin 3 to step 2 mm in the opposite direction of the coordinate axis, which is equivalent to the terahertz probe stepping 2 mm in the positive direction of the axis; the signal at 135 ps of the terahertz reflection pulse is collected for imaging.

[0048] S6: The time-domain signal recorded by computer 6 is processed for imaging, and the centroid spacing of the bimetallic tag 4 is calculated by edge detection and centroid localization algorithm.

[0049] In this embodiment of the invention, the specific process of imaging processing is as follows: interpolation processing is performed on the original imaging image obtained from the acquired signal, and then the interpolated data matrix is ​​linearly mapped to the 0-1 interval to complete grayscale processing, resulting in a grayscale image; the Canny operator is used to perform edge detection on the grayscale image, and a high threshold Th is set to confirm strong edges and a low threshold Tl to retain weak edges during edge detection. Only when a weak edge is connected to a strong edge is it retained, and the edge detection effect of different threshold ranges is tested cyclically; after edge detection, the process also includes an optimization step for the edge detection image: dilation processing, erosion processing, and closure processing are performed sequentially on the image with discontinuous edges. Dilation processing is used to connect broken edges, erosion processing is used to restore the original size of the image, and closure processing is used to smooth the image edges; the optimized edge detection image is filled with regions to form a solid binary region, and then the centroid localization algorithm is used to locate the centroid of the binary region to obtain the centroid coordinates of the metal tag 4. The centroid coordinates are the weighted average of the coordinates of all pixels in the binary region.

[0050] S7: Compare the distance between the centroids of the imaging system with the actual distance between the centroids to evaluate the centroid positioning error of the terahertz imaging system for hidden points.

[0051] In this embodiment of the invention, steps S6 and S7 are performed three times in a repeated experiment. The centroid spacing of the bimetallic tag 4 in the three repeated experiments is calculated respectively. The centroid spacing of each imaging experiment is compared with the actual centroid spacing to obtain the single positioning error. Then, the average positioning error and error fluctuation value of the three experiments are calculated to complete the comprehensive evaluation of the centroid positioning error of the terahertz imaging system for the hidden point.

[0052] Example 1

[0053] like Figure 1 As shown, Figure 1This is a schematic diagram of the terahertz imaging system constructed according to an embodiment of the present invention. The system consists of a femtosecond laser system 14, a beam splitter 15, an optical delay unit, a high-reflectivity mirror 13, a terahertz probe, a heat-insulating tile sample 5, a metal tag 4, a skin 3, a stepper motor 1, an adapter 2, and a computer 6. The probe mainly contains four high-axis parabolic mirrors and two photoconductive antennas. The terahertz optical path of this system is as follows: the terahertz pulse generated by the femtosecond laser system 14 is split into a probe beam and a pump beam by the beam splitter 15. The probe beam is received by the photoconductive antenna 12 after a certain delay, and then transmitted to the personal computer 6 for signal processing. The pump beam is guided by the high-reflectivity mirror 13 to the photoconductive antenna 11, then passes through the off-axis parabolic mirrors (second OAP9 and fourth OAP7) and is focused on the surface of the skin 3. After being reflected by the label or the skin 3, the terahertz beam is reflected back to the off-axis parabolic mirrors (third OAP8 and first OAP10) at a small angle. The reflected beam passes through the heat insulation tile and the parabolic mirror in sequence, and is finally received by the photoconductive antenna 12 and transmitted to the personal computer 6 for signal processing. Figure 1 The first OAP10 and second OAP9 at the bottom center have a diameter of 2 inches and a focal length of 5 cm, while the third OAP8 and fourth OAP7 at the top have a diameter of 2 inches and a focal length of 200 mm. A 3D-printed adapter 2 connects the stepper motor 1 to the skin 3 surface, allowing the skin 3 surface to be fixed to the motor's screw holes. A 3D motion control program written in LabVIEW drives the label and skin 3 to move to different positions of the light spot. The probe emits terahertz waves towards the label and skin 3 surface and receives the time-domain signals reflected from each surface; the waveforms are displayed and recorded by computer 6.

[0054] Taking two metal tags 4 as an example, the system is used for imaging and positioning of tags on a 4cm thick sample (the sample is a heat insulation tile). Its simplified structural diagram is shown below. Figure 2 As shown, Figure 2 This is a schematic diagram illustrating the positional relationship between the heat-insulating tile and the label provided in this embodiment of the invention. The metal label 4 has a size of 2mm × 2mm. The red circle represents the terahertz light spot 16, with a spot diameter of 18mm. The metal label 4 is adhered to the skin 3, with two metal labels 4 placed 10.5mm apart. The center-to-center distance between the two metal labels 4 is 8.5mm. The distance from the probe to the surface of the skin 3 is 16.6cm, and the distance from the center of the two upper third OAP8 and fourth OAP7 to the front surface of the terahertz probe is 3.4cm. The skin 3 is connected to the stepper motor 1 via a 3D printer component. The stepper motor 1 is controlled to move in the XY plane using LabVIEW software on the computer 6, thereby moving the label and skin 3 to different positions of the light spot. This is in principle the same as controlling the terahertz probe to scan a certain area on the skin 3.

[0055] like Figure 3 As shown, Figure 3 This is a schematic diagram of the label structure provided in an embodiment of the present invention. The label is 2mm long and wide, and consists of three layers from top to bottom: a functional layer 401 (Au), an intermediate layer 402 (Cr), and a substrate 403 (SiO2). The functional layer 401, located on the top layer of the label, is a 100nm thick gold (Au) film and is the key structure for reflecting terahertz waves. Gold has excellent reflection characteristics in the terahertz frequency band, directly determining the reflected signal of the label under terahertz wave irradiation, thereby achieving accurate label positioning. The intermediate layer 402 is an ultra-thin chromium (Cr) film, only 20nm thick. Its function is to fill the bonding gap between the silicon dioxide substrate 403 and the top metal film, enhancing the adhesion between the two, effectively preventing the top metal film from peeling off and extending the label's service life. The 403 substrate (SiO2 layer), which serves as the support carrier for the entire label, is made of silicon dioxide (SiO2) and has a thickness of 500μm. It is the thickest part of the three-layer structure and can provide physical support for the label, preventing deformation and damage during use and ensuring its long-term stability. Figure 4 The image shows two 2mm metal tags 4 provided in an embodiment of the present invention. The center-to-center distance between the two tags is measured to be 8.5mm.

[0056] like Figure 5 As shown, Figure 5 This diagram illustrates the motion path of the stepper motor 1 in terahertz imaging according to an embodiment of the present invention. It shows the scanning path of the terahertz time-domain waveform, with the center of the first 2mm×2mm square grid at the lower left corner as the scanning starting point. The displacement between adjacent coordinate points (the centers of two adjacent square grids) is achieved through the movement of the stepper motor 1. At each coordinate point (x, y), the system records the corresponding terahertz pulse time-domain waveform. The scanning area is a 20mm×20mm square, with the smallest grid unit being 2mm×2mm. Each time, the stepper motor 1 controls the skin 3 to step 2mm in the opposite direction along the coordinate axis (X or Y), which is equivalent to the probe stepping 2mm in the positive direction of that axis. The entire scanning plane has a range of 20mm on both the X-axis (horizontal) and Y-axis (vertical). The dashed line in the diagram represents the probe's motion path, and the arrows represent the probe's scanning direction. A "line-by-line scanning" method is used; after completing one line scan along the X-axis, the probe moves one step along the Y-axis and then reverses direction to perform the next line scan. This back-and-forth scanning path maximizes the coverage of the entire 20mm×20mm area. The orange squares in the image represent two labels within the scanned area, each measuring 2mm x 2mm.

[0057] Two 2mm × 2mm tags were imaged using terahertz pulses, with an imaging range of 20mm × 20mm and a scanning step of 2mm. The experiment was repeated three times, and the imaging results and centroid coordinates of the tags were obtained for each experiment. The results are as follows: Figure 6As shown. After the probe scans, imaging is performed on signals near the reflection signal of skin 3. Therefore, the signal at 135 ps of the terahertz reflection pulse is acquired for imaging. The image after interpolation and grayscale processing of the original image is shown below. Figure 6 As shown in (a), (c), and (e), the X and Y axes of the images both cover a region of 0-20 mm, perfectly corresponding to the previously scanned 20 mm × 20 mm detection range. The color bars on the right side of the image represent signal intensity, with white corresponding to a high reflectance signal (close to 0.9) and black corresponding to a low reflectance signal (close to 0). There are two distinct black areas in the image, corresponding to the positions of the two labels. This indicates that at 135 ps, due to label occlusion, the label area generated a weaker skin 3 reflectance signal. In the image, the upper part (color close to white corresponding to a strong signal) has a stronger reflectance signal than the lower part (color close to gray, corresponding to a weaker signal), which is due to the unbalanced placement of skin 3.

[0058] After the probe scans, the signal at 135 ps is taken to obtain the original image of the terahertz imaging. The original image is interpolated and the image becomes smooth after interpolation. Then, the image grayscale is processed to obtain the interpolated grayscale image. The interpolated grayscale image can be used to perform edge detection using the Canny operator. After edge detection, the centroid of the image can be located to obtain the centroid coordinates of the label. The edge detection algorithm is as follows: (1) Grayscale processing - the interpolated data matrix is ​​linearly mapped to the 0-1 interval. The data matrix I is converted into a grayscale image, which can provide a standard 0-1 threshold input for the Canny operator. (2) Setting threshold - a high threshold Th confirms strong edges, and a low threshold Tl retains weak edges. Only when a weak edge is connected to a strong edge is it retained. (3) Loop processing of multiple thresholds - the edge detection effect of different threshold ranges can be tested.

[0059] After edge detection, the centroid coordinates of the label are obtained by the contour centroid localization algorithm. The algorithm process is as follows: 1. Edge optimization processing (when the edges are discontinuous) - forming continuous edges. (1) Dilation - connecting broken edges. (2) Erosion - restoring the original size. (3) Closure - smoothing the edges. 2. Region filling - forming a solid binary region. 3. Centroid calculation, the centroid of the region is defined as the weighted average of the coordinates of all pixels in the region. For the binary region R, its centroid coordinates (Cx, Cy) can be expressed as:

[0060] ;

[0061] Depend on Figure 6The results show that the distance between the positioning coordinates of the two labels in the three experiments were 8.74 mm, 8.75 mm, and 8.52 mm, respectively. Compared with the actual distance of 8.5 mm between the two labels, the errors in the three experiments were 0.24 mm, 0.25 mm, and 0.02 mm, respectively. The lateral coordinate of the centroid of the two labels obtained from the three measurements was 8.5 ± 0.17 mm, and the error in the lateral coordinate of the centroid of the two labels obtained from the three measurements was 0.17 mm. Figure 6 The centroid coordinates of the terahertz tag measured in the middle are plotted. Figure 7 The centroid coordinates and average coordinates of the label were measured three times. Figure 7 The coordinate distribution of the three experiments can be seen more intuitively, which can better reflect the error of the measured coordinates.

[0062] The beneficial effects of this invention are as follows: This invention images two hidden metal tags under aviation thermal insulation tiles using terahertz imaging. Contour extraction and edge detection are used to measure the centroid coordinates of the two tags. The absolute distance between the two tags is calibrated using calipers. By comparing the centroid distance of the two metal tags measured by terahertz imaging with the actual centroid distance, the error of terahertz imaging relative to the measurement of the centroid of the hidden point can be calibrated. This invention patent proposes a method for calibrating the centroid positioning error based on dual tags, which has potential application value in the identification and positioning of hidden points under aviation thermal insulation tiles.

[0063] The above descriptions are merely embodiments of the present invention. Commonly known technical solutions or characteristics are not described in detail here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A method for evaluating the centroid localization error of hidden points based on dual labels, characterized in that, The system includes: S1: Construct a terahertz imaging system, which consists of a femtosecond laser system (14), a beam splitter (15), an optical delay unit, a high-reflectivity mirror (13), a terahertz probe, a heat insulation tile sample (5), a metal tag (4), a skin (3), a stepper motor (1), an adapter (2), and a computer (6). The terahertz probe is equipped with four off-axis parabolic mirrors and two photoconductive antennas. S2: The actual centroid distance between the two metal labels (4) is precisely calibrated using a vernier caliper; S3: Adhere the two metal labels (4) to the surface of the skin (3), and then attach the skin (3) with the metal labels (4) to the heat insulation tile sample (5). The skin (3) and the stepper motor (1) are fixedly connected by the 3D printed adapter (2). S4: Drive the stepper motor (1) through the three-dimensional motion control program written in LabVIEW in the computer (6), so that the skin (3) and the metal tag (4) move in the XY plane, and the metal tag (4) is placed at different positions of the terahertz light spot (16); S5: Control the terahertz probe to emit terahertz waves to the metal tag (4) and skin (3) surface, and receive the time domain signals reflected by each surface, and the computer (6) displays and records the waveforms; S6: The time-domain signal recorded by the computer (6) is processed for imaging, and the imaging centroid distance between the two metal tags (4) is calculated by edge detection and centroid positioning algorithm. S7: Compare the imaging centroid spacing with the actual centroid spacing to evaluate the centroid positioning error of the terahertz imaging system for hidden points.

2. The method for evaluating the centroid positioning error of hidden points based on dual labels according to claim 1, characterized in that: The metal tag (4) has a size of 2mm×2mm, and the two metal tags (4) are placed with a spacing of 10.5mm. The actual centroid spacing after calibration is 8.5mm. The metal tag (4) has a three-layer structure from top to bottom, namely a 100nm thick gold functional layer (401), a 20nm thick chromium intermediate layer (402), and a 500μm thick silicon dioxide substrate (403).

3. The method for evaluating the centroid positioning error of hidden points based on dual labels according to claim 1, characterized in that: The terahertz optical path of the terahertz imaging system is configured as follows: the terahertz pulse generated by the femtosecond laser system (14) is split into a probe beam and a pump beam by the beam splitter (15). The probe beam is introduced with a delay and then received by the first photoconductive antenna (12) and transmitted to the computer (6). The pump beam is guided by the high-reflectivity mirror (13) to the second photoconductive antenna (11), and then passes through the second OAP (9) and the fourth OAP (7) of the off-axis parabolic mirror and is focused onto the surface of the skin (3) of the heat insulation tile sample (5). The terahertz beam passes through the... After being reflected by the metal tag (4) or skin (3), the light beam is reflected back to the third OAP (8) and the first OAP (10) of the off-axis parabolic mirror at a small angle. The reflected light beam passes through the heat insulation tile sample (5) and the parabolic mirror in sequence and is received by the photoconductive antenna (12) and transmitted to the computer (6). The first OAP (10) and the second OAP (9) of the off-axis parabolic mirror have an aperture of 2 inches and a focal length of 5 cm. The third OAP (8) and the fourth OAP (7) of the off-axis parabolic mirror have an aperture of 2 inches and a focal length of 200 mm.

4. The method for evaluating the centroid positioning error of hidden points based on dual labels according to claim 3, characterized in that: The thickness of the heat insulation tile sample (5) is 4cm, the distance from the terahertz probe to the skin (3) surface is 16.6cm, the distance from the center of the third OAP (8) and the fourth OAP (7) of the off-axis parabolic mirror to the front surface of the terahertz probe is 3.4cm, and the diameter of the terahertz spot (16) is 18mm.

5. The method for evaluating the centroid positioning error of hidden points based on dual labels according to claim 1, characterized in that: The scanning parameters of the terahertz probe in step S5 are as follows: the scanning area is a 20mm×20mm square, with the center of the first 2mm×2mm square grid in the lower left corner as the scanning starting point, the scanning step is 2mm, and a line-by-line reciprocating scanning path is adopted. The stepper motor (1) controls the skin (3) to step 2mm in the opposite direction of the coordinate axis, which is equivalent to the terahertz probe stepping 2mm in the positive direction of the axis. The signal at 135ps of the terahertz reflection pulse is collected for imaging.

6. The method for evaluating the centroid positioning error of hidden points based on dual labels according to claim 5, characterized in that: The specific process of imaging processing in step S6 is as follows: interpolation processing is performed on the original imaging image obtained by the acquired signal, and then the interpolated data matrix is ​​linearly mapped to the 0-1 interval to complete the grayscale processing and obtain the grayscale image. The Canny operator is used to perform edge detection on the grayscale image. During edge detection, a high threshold Th is set to confirm strong edges and a low threshold Tl to retain weak edges. Only weak edges that are connected to strong edges are retained. The edge detection effect of different threshold ranges is tested cyclically.

7. The method for evaluating the centroid positioning error of hidden points based on dual labels according to claim 6, characterized in that: After edge detection in step S6, the process further includes an optimization step for the edge-detected image: performing dilation, erosion, and closure processing on the image with discontinuous edges in sequence. The dilation processing is used to connect broken edges, the erosion processing is used to restore the original size of the image, and the closure processing is used to smooth the image edges.

8. The method for evaluating the centroid positioning error of hidden points based on dual labels according to claim 7, characterized in that: The optimized edge detection image is filled with regions to form a solid binary region. Then, the centroid localization algorithm is used to locate the centroid of the binary region to obtain the centroid coordinates of the metal tag (4). The centroid coordinates are the weighted average of the coordinates of all pixels in the binary region.

9. The method for evaluating the centroid positioning error of hidden points based on dual labels according to claim 8, characterized in that: Steps S6 and S7 are repeated three times. The distance between the imaging centroids of the two metal tags (4) in the three repeated experiments is calculated respectively. The distance between the imaging centroids is compared with the actual centroid distance each time to obtain the single positioning error. Then the average positioning error and error fluctuation value of the three experiments are calculated to complete the comprehensive evaluation of the centroid positioning error of the terahertz imaging system for the hidden point.

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