A terahertz suppressed fringe interference imaging method and system based on complex domain circle construction method

Abstract

The present invention relates to a terahertz suppression fringe interference imaging method and system based on a complex domain circle construction method. The method comprises: a terahertz transceiver, a focusing optical path lens assembly, an imaging object, a three-dimensional displacement stage and an electronic computer. The terahertz wave is emitted by the transceiver, refracted by the focusing optical path lens assembly, irradiated to the imaging object, reflected by the imaging object, and returned to the original optical path and focused to the focus. The terahertz transceiver at the focus receives the signal and measures its intensity and phase. After the imaging object is imaged multiple times at different specific positions along the optical axis, multiple complex-valued images are obtained. The complex-valued image is estimated by the complex domain circle construction method and the background clutter signal is removed, thereby restoring the image. The present invention can well estimate the background clutter signal through a small number of multiple images, and obtain a clear and fringe-free terahertz image, which is superior to the traditional average method and background elimination method, and achieves better terahertz image fringe noise removal.

Description

Technical Field

[0001] The invention relates to the field of terahertz imaging, and in particular to a terahertz suppression fringe interference imaging method and system based on a complex domain circle construction method. Background Art

[0002] Terahertz waves refer to electromagnetic wave radiation with a frequency in the 0.1-10THz frequency band. Terahertz waves have a series of unique characteristics due to their special position in the electromagnetic spectrum, and are widely used in biomedical detection, material property detection and other fields. For example, terahertz photons have low energy and non-ionizing properties, which can achieve non-destructive testing; terahertz has the ability to penetrate many non-metallic materials and can be used to detect hidden objects in security inspections. However, there is often unnecessary fringe interference in the obtained terahertz intensity image. This fringe interference will reduce the clarity of the image and even drown out the target signal. There is an extremely urgent need to explore the causes of fringe interference and propose a fast and low-cost de-fringe imaging method in the field of terahertz imaging.

[0003] The fringes in the terahertz image are caused by the interference between the target signal and the coherent interference signal. The coherent interference signal, also known as the clutter signal, is often introduced by the additional reflection generated when the imaging system or the sample is placed. For the terahertz reflection imaging system composed of the same transceiver, the use of a focusing parabolic mirror can obtain a smaller imaging spot and a higher imaging resolution, but it also brings additional reflection interference signals, which can come from the reflection of imaging devices such as parabolic mirrors and optical brackets. If the reflection energy of the imaging device is greater than the target reflection wave energy, the resulting image will be severely interfered by the interference fringe noise and cannot be recognized. Removing the fringe interference essentially requires removing the background clutter signal generated by the additional reflection of the optical imaging system. Therefore, studying the background clutter signal suppression method is of great significance for terahertz reflection measurement.

[0004] Among the existing clutter signal suppression technologies, the background elimination method has the advantages of simple principle and low computational complexity. The algorithm first collects background information that does not contain the target, and then cancels the detection echo containing the target with the background signal. However, it is often difficult to obtain background information in advance in actual detection environments. The average method can effectively suppress background clutter by taking the average value through multiple measurements, but the number of measurements required is relatively large. Therefore, at this stage, there is an urgent need for a low-cost terahertz imaging method that can better estimate and remove background clutter. Summary of the invention

[0005] In view of the above technical deficiencies, the present invention provides a terahertz suppressed fringe interference imaging method based on the complex domain circle construction method. The method uses a terahertz continuous wave imaging system that can simultaneously measure signal amplitude and phase information, obtains a number of complex-valued images within a certain range before and after the focus on the optical axis, estimates the background clutter signal intensity through the complex domain circle construction method and removes it, thereby restoring a smooth and clear image.

[0006] The technical solution adopted by the present invention to solve its technical problem is:

[0007] A terahertz suppression fringe interference imaging method based on a complex domain circle construction method comprises the following steps:

[0008] S1. Setting a terahertz transceiver at the origin of a three-dimensional coordinate system, setting a focusing optical path lens assembly along the terahertz wave emission direction, and setting an imaging object at the focus of the lens assembly;

[0009] S2. Calculate the moving threshold range before and after the focus according to the spot size, and control the imaging object to move to a number of sampling points at a set step interval within the moving threshold range; control the terahertz transceiver to transmit terahertz waves to the imaging object at each point, control the imaging object to move according to a preset trajectory in the plane where it is located, and collect a series of complex value sequences reflected by the imaging object after being irradiated by the terahertz wave;

[0010] S3. Splice a series of complex-valued sequences into terahertz images corresponding to several sampling points, and extract the intensity signal of the complex-valued sequence values ​​collected at different points of the same pixel coordinates in turn using the circle construction method in the complex domain, and then reconstruct a clear amplitude image of the imaging object illuminated by the terahertz wave to filter out the fringe interference clutter.

[0011] The controlling the imaging object to move along a preset trajectory in the plane where it is located includes: adjusting the position of the imaging object in the xy two-dimensional plane by a three-dimensional displacement stage so that the terahertz wave illuminates each pixel position of the imaging object row by row and column by column.

[0012] The terahertz wave is made to illuminate each pixel position of the imaging object row by row and column by column to generate a complex pixel matrix.

[0013] The step spacing of the three-dimensional translation stage moving in the xy two-dimensional plane is the width value of a single row or column of the restored image, and the window size for a single sampling point to complete sliding irradiation and acquisition is equivalent to the complex-valued pixel matrix of the final restored image.

[0014] The method of calculating the moving threshold range before and after the focus according to the spot size includes: when the imaging object moves on the optical axis, according to the Rayleigh method, the change of the diffuse spot size before and after the preset focus position is required not to exceed one quarter of the wavelength, and the moving threshold range before and after the focus is calculated, and the number of sampling positions K is greater than 3.

[0015] The circle construction method is: the complex sequence values ​​of different sampling points of the same pixel in a series of images are expressed on the complex plane, and the circle is fitted using the least squares method. The center of the circle obtained by fitting is used as the background clutter signal, and the circle radius is the intensity of the terahertz wave reflected by the imaging object.

[0016] A terahertz suppressed fringe interference imaging system based on a complex domain circle construction method, comprising: a terahertz transceiver, a focusing optical path lens assembly, an imaging object, a three-dimensional displacement stage and an electronic computer;

[0017] The terahertz transceiver is arranged at the origin of the three-dimensional coordinate system, and the focusing optical path lens assembly and the image object are arranged in sequence in the terahertz wave emission direction; the terahertz transceiver transmits terahertz waves and collects complex value sequences reflected by the imaging object after being irradiated by the terahertz waves;

[0018] The three-dimensional displacement stage includes a servo motor, a driver, an x-axis transverse linear motion module, a y-axis longitudinal linear motion module, and a z-axis terahertz wave emission direction motion module. The imaging object is fixed on the transverse motion module, and the servo motor drives the transverse linear motion module and the longitudinal linear motion module to move, thereby adjusting the position of the imaging object in the xy two-dimensional plane, so that the terahertz wave illuminates each pixel position of the imaging object row by row and column by column, and the servo motor drives the z-axis terahertz wave emission direction motion module to change the sampling position of each step spacing residing on the z-axis; the driver is connected to an electronic computer through a communication interface;

[0019] The electronic computer internally comprises a storage unit and a processing unit. The storage unit stores a program, and the processing unit loads the program to execute the steps of the method described above.

[0020] The focusing optical path lens assembly is N concave reflectors arranged in sequence along the terahertz square wave emission direction, so that the terahertz wave is reflected from the previous concave reflector into the next concave reflector in sequence and finally focused on the imaging object.

[0021] The first N-1 are collimating parabolic mirrors, and the Nth is a converging parabolic mirror.

[0022] The present invention has the following beneficial effects and advantages:

[0023] The present invention adopts a multi-point position image measurement method on the optical axis, and can estimate the background clutter signal strength through a small number of complex-valued images at specific positions to obtain a stripe noise-free image. It is superior to the traditional average method and background elimination method, and can achieve better terahertz image stripe noise removal. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is the overall structural diagram of the present invention;

[0025] Figure 2 It is the algorithm flow chart of the complex field circle construction method;

[0026] Figure 3 It is a schematic diagram of the complex domain source construction method;

[0027] Figure 4 It is the original optical image of the example of the present invention.

[0028] Figure 5 It is a comparison diagram of the measurement diagram of the example of the present invention and the results of different processing methods. DETAILED DESCRIPTION

[0029] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is further described in detail below in conjunction with embodiments.

[0030] like Figure 1 The structure diagram of a terahertz suppression fringe interference imaging system based on the complex domain circle construction method of the present invention is shown. The imaging system can realize the electrically controlled movement of metal samples along the optical axis and imaging in a two-dimensional scanning manner at a series of specific optical axis positions, and control the terahertz transceiver antenna to obtain a series of imaging objects through the reflection of terahertz waves to obtain signal intensity values ​​and phase values, and the electronic computer 6 automatically completes the restoration of the terahertz image according to the algorithm program. It includes: a terahertz transceiver 1, a collimating parabolic mirror 2 that collimates the terahertz spherical wave into a plane wave, a converging parabolic mirror 3 that focuses the plane wave, an imaging object 4, a three-dimensional displacement stage 5, and an electronic computer 6.

[0031] The imaging object 4 is mounted on a three-dimensional displacement stage 5. The step length of the imaging object can be precisely controlled by the electronic control program, and the imaging object is programmed to translate in the direction of the optical axis. The intensity value and phase value of the terahertz wave reflection signal are synchronously read at the antenna end of the terahertz transceiver 1. The imaging object is initially placed at the focus of the converging parabolic mirror. After two-dimensional scanning imaging, the three-dimensional displacement stage 5 drives the imaging object to move along the optical axis away from the converging parabolic mirror 3. The change in the size of the diffuse spot within the moving range should not exceed one-quarter of the wavelength. The forward and backward movement range of the focus can be calculated by the Gaussian beam model. After each movement, the same two-dimensional scanning method is used for imaging. The number of movements K should be greater than 3. After completion, K two-dimensional complex value image measurement data of the same target at different sampling positions can be obtained. The two-dimensional scanning method is to adjust the position of the imaging object in the xy two-dimensional plane so that the terahertz wave illuminates each pixel position of the imaging object row by row and column by column.

[0032] The three-dimensional displacement stage 5 includes a servo motor, a driver, an x-axis transverse linear motion module, a y-axis longitudinal linear motion module, and a z-axis terahertz wave emission direction motion module. The imaging object is fixed on the transverse motion module, and the transverse linear motion module and the longitudinal linear motion module are driven by the servo motor to move, thereby adjusting the position of the imaging object in the xy two-dimensional plane, so that the terahertz wave illuminates each pixel position of the imaging object row by row and column by column, and the z-axis terahertz wave emission direction motion module is driven by the servo motor to change the sampling position of each step spacing residing on the z-axis; the driver is connected to the electronic computer 6 through a communication interface.

[0033] The specific imaging process is as follows: a terahertz wave of a certain frequency is generated by the terahertz transceiver antenna 1, the terahertz spherical wave is collimated into a uniform plane wave by the collimating parabolic mirror 2, and then the plane wave is focused on the imaging object by the converging parabolic mirror 3, and the reflected terahertz wave containing the object information is then converged to the terahertz transceiver antenna through the collimating parabolic mirror 3 and the converging parabolic mirror 2. The imaging object is two-dimensionally scanned at multiple preset positions within a certain range before and after the focus on the optical axis. The two-dimensional scanning terahertz reflection intensity and phase measurement values ​​at different optical axis positions are obtained by reading the amplitude and phase values ​​of the terahertz transceiver antenna. After the measurement value is processed by the complex domain circle construction method, the background clutter signal is estimated and removed, and finally the terahertz image is obtained.

[0034] In order to suppress background clutter signals and facilitate the removal of image stripes, the present invention uses the complex domain circle construction method to process the collected data. Figure 2 The flowchart of the algorithm is shown in Figure 1. The specific steps are as follows:

[0035] 1. Construct a complex valued signal based on the received amplitude and phase values {E s(k,i,j)} is a series of complex-valued signals measured at the kth position of the optical axis at the pixel at coordinate (i,j) in the image, k = 1, 2, ..., K. The image size is M×N, the image row number i = 1, 2, ..., M, and the column number j = 1, 2, ..., N.

[0036] 2. For each coordinate (i, j), E s (k,i,j) is represented by scattered points on the complex plane, and the horizontal coordinate of the kth scattered point is the complex number E s The real part of (k,i,j), the ordinate is the complex number E s The imaginary part of (k,i,j).

[0037] 3. For each coordinate (i, j), such as Figure 3 The least square method is used to find the scattered points {E s (k,i,j)} to fit a circle, and the center of the fitted circle is E c (i,j) is a complex-valued clutter signal. The radius of the circle is the target reflection signal strength at coordinate (i,j) |E F (i,j)|.

[0038] 4. According to |E F (i, j)| forms a terahertz image, reconstructs a clear amplitude image of the imaging object illuminated by the terahertz wave, and realizes the filtering of fringe interference clutter.

[0039] Example verification

[0040] In order to verify the effectiveness and superiority of the method for suppressing background clutter signals by the terahertz imaging system of the present invention, we used the terahertz imaging system of the present invention to perform terahertz imaging on a metal blade. Figure 4 is the original optical picture of the imaging object. Figure 5 (a) is the true value image obtained by using the threshold method based on the optical image. Figure 5 (b) is a single terahertz image obtained by measurement. Figure 5 (c)-(e) are terahertz images restored using the multi-image amplitude average method, with the number of images used being 3, 10, and 40, respectively. Figure 5 (f)-(h) are terahertz images restored using the complex domain circle construction method. Figures (f)-(h) show the results of restoration using 3, 10, and 40 images, respectively, under the condition that the moving interval on the optical axis is 0.4 mm. In order to objectively compare the imaging quality of the algorithm, we use the optical truth value Figure 5 (a) Divide the image into blade regions Ω t and background area Ω b , respectively count the mean square error and mean of these two regions Where X(i,j) is the gray value of the image, Num(Ω p ) is the number of pixels in the area. Then construct the normalized mean square error function: NMSE = (σ t +σ b ) / (μ t -μ b ), σ t ,σ b are the mean square error values ​​of the blade area and background area of ​​the imaging object, μ t ,μ b are the mean values ​​of the blade area and background area of ​​the imaging object, respectively. As can be seen from the formula, the smaller the NMSE value, the smaller the internal numerical variance of the region and the greater the contrast between the background and the imaging target, which means that the region is clearer and smoother. The NMSE value of a single original measurement image is 14.37. The comparison of the normalized mean square error function NMSE of the restored images by the average method and the complex domain circle construction method is shown in the following table, where K is the number of images participating in the calculation.

[0041]

[0042] The quality of the restored terahertz images and the values ​​in the table show that the contour of the blade of the imaging object can be restored by measuring fewer images using the complex domain circle construction method. When the number of images is small, there are more noise points in the background area. As the number of images increases, the background noise points gradually decrease, and the images become clearer. This verifies the advantage of the imaging method of the present invention in restoring high-quality images from a small number of images.

[0043] The above is an implementation mode of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should be regarded as within the scope of protection of the present invention.

Claims

1. A terahertz suppressed fringe interferometry imaging method based on complex domain circle construction method, characterized in that: The following steps are involved: S1. Setting a terahertz transceiver at the origin of a three-dimensional coordinate system, setting a focusing optical path lens assembly along the terahertz wave emission direction, and setting an imaging object at the focus of the lens assembly; S2. Calculate the moving threshold range before and after the focus according to the spot size, and control the imaging object to move to a number of sampling points at a set step interval within the moving threshold range; control the terahertz transceiver to transmit terahertz waves to the imaging object at each point, control the imaging object to move according to a preset trajectory in the plane where it is located, and collect a series of complex value sequences reflected by the imaging object after being irradiated by the terahertz wave; S3. Splice a series of complex-valued sequences into terahertz images corresponding to several sampling points, and extract the intensity signal of the complex-valued sequence values ​​collected at different points of the same pixel coordinates in turn using the circle construction method in the complex domain, and then reconstruct a clear amplitude image of the imaging object illuminated by the terahertz wave to filter out the fringe interference clutter.

2. The terahertz suppression fringe interferometry imaging method based on the complex domain circle construction method according to claim 1, characterized in that: The controlling the imaging object to move along a preset trajectory in the plane where it is located includes: adjusting the position of the imaging object in the xy two-dimensional plane by a three-dimensional displacement stage so that the terahertz wave illuminates each pixel position of the imaging object row by row and column by column.

3. The terahertz suppression fringe interferometry imaging method based on the complex domain circle construction method according to claim 2, characterized in that: The terahertz wave is made to illuminate each pixel position of the imaging object row by row and column by column to generate a complex pixel matrix.

4. The terahertz suppression fringe interference imaging method based on the complex domain circle construction method according to claim 2, characterized in that: The step spacing of the three-dimensional translation stage moving in the xy two-dimensional plane is the width value of a single row or column of the restored image, and the window size for a single sampling point to complete sliding irradiation and acquisition is equivalent to the complex-valued pixel matrix of the final restored image.

5. The terahertz suppression fringe interference imaging method based on the complex domain circle construction method according to claim 1, characterized in that: The method of calculating the moving threshold range before and after the focus according to the spot size includes: when the imaging object moves on the optical axis, according to the Rayleigh method, the change of the diffuse spot size before and after the preset focus position is required not to exceed one quarter of the wavelength, and the moving threshold range before and after the focus is calculated, and the number of sampling positions K is greater than 3.

6. The terahertz suppression fringe interference imaging method based on the complex domain circle construction method according to claim 1, characterized in that: The circle construction method is: the complex sequence values ​​of different sampling points of the same pixel in a series of images are expressed on the complex plane, and the circle is fitted using the least squares method. The center of the circle obtained by fitting is used as the background clutter signal, and the circle radius is the intensity of the terahertz wave reflected by the imaging object.

7. A terahertz suppression fringe interferometry imaging system based on complex domain circle construction method, characterized in that: include: A terahertz transceiver, a focusing optical path lens assembly, an imaging object, a three-dimensional translation stage, and an electronic computer; The terahertz transceiver is arranged at the origin of the three-dimensional coordinate system, and the focusing optical path lens assembly and the image object are arranged in sequence in the terahertz wave emission direction; the terahertz transceiver transmits terahertz waves and collects complex value sequences reflected by the imaging object after being irradiated by the terahertz waves; The three-dimensional displacement stage includes a servo motor, a driver, an x-axis transverse linear motion module, a y-axis longitudinal linear motion module, and a z-axis terahertz wave emission direction motion module. The imaging object is fixed on the transverse motion module, and the servo motor drives the transverse linear motion module and the longitudinal linear motion module to move, thereby adjusting the position of the imaging object in the xy two-dimensional plane, so that the terahertz wave illuminates each pixel position of the imaging object row by row and column by column, and the servo motor drives the z-axis terahertz wave emission direction motion module to change the sampling position of each step spacing residing on the z-axis; the driver is connected to an electronic computer through a communication interface; The electronic computer comprises a storage unit and a processing unit, the storage unit stores a program, and the processing unit loads the program to execute the steps of the method according to any one of claims 1 to 6.

8. The system according to claim 7, characterized in that The focusing optical path lens assembly is N concave reflectors arranged in sequence along the terahertz square wave emission direction, so that the terahertz wave is reflected from the previous concave reflector into the next concave reflector in sequence and finally focused on the imaging object.

9. The system according to claim 8, characterized in that The first N-1 are collimating parabolic mirrors, and the Nth is a converging parabolic mirror.

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