CN115078299B - Terahertz computed tomography three-dimensional imaging method based on galvanometer scanning system - Google Patents
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Abstract
The invention relates to a terahertz computed tomography three-dimensional imaging method based on a galvanometer scanning system, which is characterized in that terahertz light beams are rapidly turned through the galvanometer scanning system, so that high-quality and uniform array terahertz light beams are provided, the terahertz light beams deflected by the galvanometer can vertically penetrate through a sample through a galvanometer distortion correction method, further, intensity information is obtained in an area array detector, and finally, a computed tomography reconstruction algorithm is used for reconstructing a three-dimensional internal structure absorption diagram of the sample without scanning distortion. Compared with the traditional point scanning terahertz computed tomography, the invention ensures the reconstruction quality and improves the imaging speed, and is a novel high-fidelity and rapid continuous terahertz wave computed tomography three-dimensional imaging solution.
Description
Technical Field
The invention relates to a continuous terahertz wave computed tomography three-dimensional imaging technology based on a galvanometer scanning system, which is mainly used for objects which are easy to penetrate by terahertz waves, and enables terahertz light beams to be rapidly turned through the galvanometer scanning system, so that high-quality and uniform area array terahertz light beams are provided, the area array terahertz light beams can vertically penetrate through a sample through a galvanometer distortion correction method, and a computed tomography reconstruction algorithm is utilized to reconstruct a three-dimensional internal structure absorption diagram of the sample with high fidelity.
Background
The photon energy of the terahertz wave is only millielectron volts and is between photons and electrons, and is far smaller than the energy of X-rays, so that the imaging sample is not damaged. And because some dielectric materials, such as polymers or ceramics, have relatively low absorption coefficients in the terahertz band. Terahertz imaging represents a contactless and harmless solution compared to other techniques, such as ultrasound or X-ray imaging, suitable for many different potential non-destructive testing applications, such as maintenance or manufacturing industries of composite materials, food quality inspection, artistic protection and authentication or biomedical science, etc.
Terahertz computed tomography is currently available for three-dimensional imaging of objects, but unlike conventional optical microscopy or infrared imaging, is limited by the current technical maturity of area array sensors, and most terahertz computed tomography is performed in a slow and laborious raster scanning manner in a controlled environment in order to ensure the signal-to-noise ratio of the acquired images. However, this technique is still very time consuming to obtain a clearer three-dimensional reconstruction, requiring several hours of measurement for a two-dimensional image, while several days are required to complete the image stacking required for three-dimensional tomographic reconstruction. This cumbersome process is far from quality control requirements in an industrial setting and frame rates required for non-destructive inspection applications.
The galvanometer scanning is used as a vector scanning mode, and has the advantages of small volume, high scanning speed, high positioning precision, small moment of inertia and the like. The high-precision galvanometer scanning device is used for replacing the traditional mechanical point-by-point scanning system, so that the instantaneity and the integration level of the imaging system can be remarkably improved. The galvanometer scanning system can realize high-speed and high-resolution scanning by precisely controlling the rotation of the reflecting mirror to change the direction of the light beam, and can provide very high positioning precision and repetition precision at the same time, thereby realizing high-speed and high-precision measurement of an object.
Disclosure of Invention
The invention solves the problem of too slow single-point scanning acquisition rate in the traditional continuous terahertz wave computed tomography three-dimensional imaging technology, and compared with the traditional mechanical movement mode of continuous terahertz wave computed tomography three-dimensional imaging, the vibrating mirror system uses light beam steering to replace mechanical movement, thereby breaking through the system rate limit of mechanical movement.
In view of the above, the invention provides a terahertz computed tomography three-dimensional imaging method based on a galvanometer scanning system, which can quickly reconstruct a high-fidelity three-dimensional internal structure absorption diagram of a sample.
The invention relates to a continuous terahertz computed tomography three-dimensional imaging system, which comprises a continuous terahertz wave laser, a 45-degree off-axis parabolic mirror L1, a double-shaft scanning vibrating mirror, a 45-degree off-axis parabolic mirror L2, an electric control rotating table, an experimental sample, a 45-degree off-axis parabolic mirror L3 and a pyroelectric image detector, wherein the whole system is divided into two modules which are respectively an optical path device module formed by the 45-degree off-axis parabolic mirror L1, the 45-degree off-axis parabolic mirror L2 and the 45-degree off-axis parabolic mirror L3 and a hardware module formed by the double-shaft scanning vibrating mirror, the electric control rotating table and the pyroelectric image detector,
The optical path device module is used for realizing the conversion of light beams, the hardware module is mainly used for mechanical control and data acquisition of hardware, the two modules jointly form the system, spherical waves emitted by the continuous terahertz wave laser are converted into parallel light through the off-axis parabolic mirror and are incident into the double-axis scanning galvanometer, the double-axis scanning galvanometer comprises two dimensional galvanometers, the positions and angles of the light beams entering the 45-degree off-axis parabolic mirror L2 are changed through the two dimensional galvanometer, the parallel light is changed into converging spherical waves through the 45-degree off-axis parabolic mirror L2, the experimental sample is located at a converging focus, wherein the experimental sample is placed on the electric control rotating table, the light beams are irradiated on the 45-degree off-axis parabolic mirror L3 after penetrating through the sample, the diverging spherical waves are converted into parallel light through the 45-degree off-axis parabolic mirror L3, and the parallel light is irradiated on the pyroelectric image detector to receive the intensity value.
When a galvanometer is used for scanning, scanned image distortion often exists, the method uses a continuous terahertz computed tomography three-dimensional imaging distortion correction method to correct the position of a distorted area scanned by the galvanometer, and the method comprises the following steps:
Firstly, establishing a two-dimensional coordinate system on a scanning plane based on the fact that the position of a light beam on the scanning plane is an O point when a vibrating mirror returns to zero (namely, the initial point when X, Y vibrating mirrors are 45 degrees), so that the positional relationship between the deflection angle of the X, Y vibrating mirror and the position of the light beam on the scanning plane is established:
Wherein alpha and beta are deflection angles of the light beam on the X, Y vibrating mirror respectively, X and Y are two-dimensional coordinates of a scanning plane respectively, d is a vertical distance from the center of the Y vibrating mirror to the point O, and e is a distance from the center point of the X vibrating mirror to the center point of the Y vibrating mirror.
The formula (1) controls the illumination position of the light beam on the scanning plane by controlling the rotation angle of the vibrating mirror X, Y, and the specific deduction process is as follows:
Defining the deflection angles of the X and Y vibrating mirrors to be theta X and theta Y respectively, analyzing the vibrating mirror X, wherein when the vibrating mirror X rotates, the deflection angle theta X of the vibrating mirror lens is unequal to the deflection angle alpha of the reflected light beam. Let the initial incident angle of the incident ray P be λ, and the reflection angle after reflection by the lens be After the reflector deflects θx, the incident angle becomes λ+θx, and P' and p″ are reflected light rays before and after the deflection, and the incident angle is equal to the reflected angle, which means that the deflection is before/>Post-deflection/>The relationship between the deflection angle of the reflected light beam and the deflection angle of the reflecting mirror can be obtained:
a＝2θx (2)
for galvanometer Y, the same can be said:
β＝2θy (3)
i.e. the deflection angle of the reflected beam is twice the deflection angle of the mirror.
When the beam is directed to point O, the entire optical path l=d+e, and when the beam is directed to any point P (x, y), the optical path changes: in ΔAOB when only the Y galvanometer rotates
When the X galvanometer and the Y galvanometer are rotated simultaneously, in Δcap:
The optical path L is:
Thus, the formula (1) can be obtained.
And secondly, establishing a plane perpendicular to the light beam according to the focal point of the sample, wherein the plane is defined as a scanning plane, and the scanning plane is used for placing a calibration plate or the sample, and the calibration plate is provided with N multiplied by N equidistant calibration points.
And thirdly, driving the galvanometer to scan the calibration plate of the scanning plane point by point, recording the scanning position of the calibration plate, and calibrating the position as a characteristic point.
And fourthly, calculating coordinate errors of all the characteristic points by making differences between equidistant calibration points on the calibration plate and the scanned characteristic points, and respectively establishing an error table in the X direction and an error table in the Y direction so as to form error characteristic description.
The error feature description can be described by a distortion error model, an error caused by adopting an approximate control strategy is defined as a method error, other errors are defined as non-method errors, and the method errors in the X direction and the Y direction are respectively defined as Deltax and Deltay, and then:
Wherein x and y represent theoretical position coordinates, Representing actual position coordinates;
Let the non-method errors in X and Y directions be δx and δy, respectively, and δx is far smaller than Δx and δy is far smaller than Δy, then the total errors Dx and Dy are respectively:
Dx＝△x+δx (10)
Dy＝△y+δy (11)
And fifthly, executing a compensation algorithm of a linear interpolation method to carry out compensation correction, wherein the method is shown as [1], the error table in the X direction and the Y direction can be converted into a voltage value required to be compensated for a vibration mirror X, Y shaft by the method, and the voltage difference value is sent to the vibration mirror to be driven, so that scanning position correction can be realized.
[1] Zhu Tieshuang, zhang Chengrui vision-aided laser galvanometer processing distortion correction and precision division [ J/OL ]. Computer integrated manufacturing system.
The specific three-dimensional imaging steps are as follows:
The invention uses a specific continuous terahertz computed tomography three-dimensional imaging method, uses the system after the correction method to carry out three-dimensional imaging, can be used under the condition of not using a distortion correction method, but the reconstructed three-dimensional image has distortion, and comprises the following steps:
firstly, removing a calibration plate in the method, and placing an experimental sample at a scanning plane where the calibration plate is positioned;
And secondly, driving the vibrating mirror corrected by the method to uniformly and stably scan the sample, wherein the whole scanning is Z-shaped, namely, a two-dimensional area to be scanned is divided into N multiplied by N lattices, the scanning is performed from the upper left corner to the lower right corner, in order to ensure that the light beams at each position in the terahertz light beam array are uniform, the light beams stay for 40ms in each area, the light beam intensity information at the moment is recorded at the detector, the moving time of each area is set to be 0.5ms, the scanning period from the upper left scanning to the lower right scanning is a scanning plane, the scanning is stopped after one period, and N multiplied by N intensity values are recorded at the detector.
And thirdly, driving the rotary table to rotate the sample after the second step is completed, wherein the rotation angle is generally 5 degrees each time, repeating the operation of the second step again after rotating the sample, recording N multiplied by N intensity values under the rotation angle, stopping the rotation angle of the sample when the rotation angle reaches 180 degrees, and storing the intensity values recorded under all the rotation angles.
And fourthly, obtaining a high-fidelity three-dimensional internal structure absorption diagram of the sample by using an FBP three-dimensional reconstruction algorithm for the N multiplied by N intensity values under each recorded angle.
Advantageous effects
Compared with the prior art, the invention has the following remarkable advantages: besides the nondestructive testing advantage of the continuous terahertz wave computed tomography three-dimensional imaging technology, the terahertz wave beam can be rapidly turned by using the galvanometer scanning system, so that the high-quality and uniform area array terahertz wave beam is provided, compared with the previous mechanical scanning mode, the acquisition and reconstruction rate can be remarkably improved, and the real-time performance of the system is improved; through the distortion correction of the vibrating mirror, the area array terahertz wave beam can vertically penetrate through the sample, the absorption diagram of the three-dimensional internal structure of the sample with higher fidelity is reconstructed, and the three-dimensional imaging reconstruction quality is improved.
Drawings
FIG. 1 is a design of a continuous terahertz wave computed tomography three-dimensional imaging system based on a galvanometer scanning system.
FIG. 2 is a schematic diagram of the transformation of the scanning coordinate system of the galvanometer and the angular relationship of beam deflection.
Fig. 3 is a flow chart of a method for correcting distortion of continuous terahertz computed tomography three-dimensional imaging.
Fig. 4 is a flow chart of a continuous terahertz computed tomography method of the present invention.
Detailed Description
Exemplary embodiments of the present invention and features thereof are described in detail below with reference to the accompanying drawings.
The invention relates to a continuous terahertz computed tomography three-dimensional imaging method, which utilizes a scanning galvanometer to quickly turn terahertz light beams, thereby providing high-quality and uniform area array terahertz wave light beams, enabling the area array terahertz wave light beams to vertically penetrate through a sample by using a galvanometer distortion correction method, and finally reconstructing a high-fidelity three-dimensional internal structure absorption diagram of the sample by using a computed tomography reconstruction algorithm.
The system design comprises 1, an avalanche diode terahertz source (IMMPATT, TERASENCE); 2. 45-degree off-axis parabolic mirror L1, gold-plated film, and focal length of 50.8mm (for converging divergent spherical waves emitted by terahertz source into parallel light); 3. biaxial scanning galvanometer (Thorlabs, GVS 112), mirror surface gold plating film; 4. 45-degree off-axis parabolic mirror L2, gold-plated film, focal length of 50.8mm (for converting parallel light into converging spherical waves); 5. electronically controlled rotary tables (Thorlabs, PRMTZ8 _m); 6. experimental samples (terahertz wave easily penetrating samples); 7. 45-degree off-axis parabolic mirror L3, gold-plated film, focal length of 50.8mm (for converting converging spherical waves into parallel light); 8. pyroelectric image detector. The avalanche diode terahertz source frequency is 278.6GHz (corresponding to a center wavelength of 1.08 mm), which can generate continuous terahertz waves with the highest power of 26mW, and the output light spot size of the laser is 1.8mm. The deflection angle of the two-dimensional scanning galvanometer is +/-20. The angles of the used off-axis parabolic mirrors L1, L2 and L3 are 45 degrees, the surface of the off-axis parabolic mirrors is coated with gold, the radius of the off-axis parabolic mirrors is 50.8mm, the focal length of the off-axis parabolic mirrors is 50.8mm, and the off-axis parabolic mirrors are suitable for terahertz wave bands. The pyroelectric image detector had 320×320 pixels in number, 80 μm×80 μm in pixel size, and a sampling frequency of 50Hz. The whole system needs to use a PC end to control a scanning galvanometer, a rotary table and an image detector.
In order to apply continuous voltage signals to control the galvanometer to scan by using the PC end, labview software and an NI-DAQ module thereof are needed, and the specific control method is as follows:
The PC is used for driving Labview software, a needed periodic voltage waveform is generated according to a formula (1), then an NI-DAQ module is called in Labview, DAQmx tasks are created, a voltage output option is selected, the waveform is selected as a square wave, the maximum voltage value is input to be +/-5V, the voltage waveform is written into DAQmx-write sub VI, and then a voltage signal can be output to a DAQ acquisition card, so that a galvanometer motion control module is driven, and Z-type scanning control on a two-dimensional scanning galvanometer can be realized.
In order to realize the position correction of the distortion area for scanning the galvanometer, the specific implementation method is characterized by comprising the following steps:
Firstly, zeroing a vibration mirror, namely, setting the vibration mirror X, Y at 45 degrees, at the moment, placing the center of a sample to be measured at the focus of a light spot, wherein a plane perpendicular to a light beam is a scanning plane, and setting up a two-dimensional coordinate system on the scanning plane by taking the focus as an O point;
Secondly, establishing a control model of X, Y galvanometer deflection angles on the position relation of the light beam on the scanning plane;
thirdly, placing a calibration plate on a scanning plane, wherein the calibration plate is provided with 10 multiplied by 10 equidistant calibration points;
driving a vibrating mirror to uniformly and stably scan a scanning plane, wherein the whole scanning is Z-shaped, namely dividing a two-dimensional area to be scanned into 10X 10 lattices, wherein the direct distance of each dot is 1mm, the scanning direction is a scanning period from the upper left corner to the lower right corner to the upper right corner, namely a scanning plane, the scanning is stopped after one period, the scanning position of a calibration plate is recorded, the position of the calibration plate is calibrated as characteristic points, and the total number of the characteristic points is 100;
Fifthly, making differences between equidistant calibration points on the calibration plate and scanned feature points, calculating coordinate errors of all the feature points, and respectively establishing an error table in the X direction and an error table in the Y direction, wherein the error table comprises 100 error values in the X direction and the Y direction;
A sixth step of executing a linear interpolation compensation algorithm to carry out compensation correction, and converting an error table in the X direction and the Y direction into a voltage value required to be compensated of a vibration mirror X, Y shaft by the control model obtained in the second step, and sending the voltage difference value to the vibration mirror to drive, so that scanning position correction can be realized;
seventh, the fifth and sixth steps are performed again, and it is verified whether the error is within the allowable range, that is, the error in both the X direction and the Y direction is not greater than 0.01mm, if so, the correction step is ended, and if not, the seventh step is performed again.
The system corrected by the correction method can be used for three-dimensional imaging, and can also be used without the correction method, but the reconstructed three-dimensional image has distortion, and the specific three-dimensional imaging method after distortion correction is as follows:
firstly, removing a calibration plate in the method, and placing an experimental sample at a scanning plane where the calibration plate is positioned;
The second step, the vibrating mirror corrected by the method is driven to uniformly and stably scan the sample, the whole scanning is Z-shaped, namely, a two-dimensional area to be scanned is divided into 10X 10 lattices, the direct distance of each lattice is 1mm, the scanning direction is scanned from the upper left corner to the lower right corner, the light beam intensity information of each lattice is recorded at a detector, the scanning period from the upper left scanning to the lower right scanning is a scanning plane, the scanning is stopped after one period, and 10X 10 intensity values are recorded at the detector;
third, driving the rotary table to rotate the sample after finishing the second step, wherein each rotation angle is 5 degrees, repeating the operation of the second step again after rotating the sample, recording 10 multiplied by 10 intensity values under the rotation angle, stopping when the rotation angle of the sample reaches 180 degrees, and storing the intensity values recorded under all rotation angles;
And fourthly, using an FBP three-dimensional reconstruction algorithm for 10 multiplied by 10 intensity values under each recorded angle to obtain the high-fidelity three-dimensional internal structure absorption diagram of the sample. The imaging result has higher acquisition and reconstruction efficiency compared with the computed tomography of point scanning under the condition of unchanged precision.
The invention aims to provide a novel rapid terahertz computed tomography technology, which enables terahertz light beams to be rapidly turned through a galvanometer scanning system, so that high-quality and uniform array terahertz light beams are provided, the terahertz light beams deflected by a galvanometer can vertically penetrate through a sample through a galvanometer distortion correction method, further, intensity information is obtained in an area array detector, and finally, a three-dimensional internal structure absorption diagram of the sample without scanning distortion is reconstructed through a computed tomography reconstruction algorithm. Compared with the traditional point scanning terahertz computed tomography, the method ensures the reconstruction quality and improves the imaging speed, and is a novel high-fidelity and rapid continuous terahertz wave computed tomography three-dimensional imaging method.
Although the present invention has been described in detail with reference to particular embodiments, the embodiments of the invention described herein are not intended to be exhaustive or to be limited to the precise forms disclosed. Rather, the embodiments chosen to illustrate the problem are chosen to enable one skilled in the art to practice the invention. Variations and modifications exist without departing from the true scope of the invention as described and defined in the following claims.
Claims (2)
1. A terahertz computed tomography three-dimensional imaging method based on a galvanometer scanning system comprises a continuous terahertz wave laser, a 45-degree off-axis parabolic mirror L1, a double-axis scanning galvanometer, a 45-degree off-axis parabolic mirror L2, an electric control rotating table, an experimental sample, a 45-degree off-axis parabolic mirror L3 and a pyroelectric image detector, wherein the whole system is divided into two modules, namely an optical path device module formed by the 45-degree off-axis parabolic mirror L1, the 45-degree off-axis parabolic mirror L2 and the 45-degree off-axis parabolic mirror L3, and a hardware module formed by the double-axis scanning galvanometer, the electric control rotating table and the pyroelectric image detector, wherein the optical path device module is used for realizing the transformation of light beams, and the hardware module is used for mechanical control and data acquisition of hardware; the double-shaft scanning galvanometer comprises an X galvanometer and a Y galvanometer; firstly, converting spherical waves emitted by a continuous terahertz wave laser into parallel light through a 45-degree off-axis parabolic mirror, and then, making the parallel light enter the 45-degree off-axis parabolic mirror L2 through the two-dimensional oscillating mirrors, and then, converting the parallel light into converging spherical waves through the 45-degree off-axis parabolic mirror L2, wherein, the experimental sample is positioned at a focus point where the spherical waves converge, wherein, the experimental sample is arranged on an electric control rotating table, the light beam is irradiated on the 45-degree off-axis parabolic mirror L3 after penetrating the sample, and the diverging spherical waves are converted into parallel light through the 45-degree off-axis parabolic mirror L3 to be received as intensity values on a pyroelectric image detector, and the method is characterized in that:
The three-dimensional imaging system corrected by the correction method is used for three-dimensional imaging, and the specific correction method comprises the following steps:
Firstly, zeroing a vibration mirror, namely, setting the vibration mirror X, Y at 45 degrees, at the moment, placing the center of a sample to be measured at the focus of a light spot, wherein a plane perpendicular to a light beam is a scanning plane, and setting up a two-dimensional coordinate system on the scanning plane by taking the focus as an O point;
Secondly, establishing a control model of X, Y galvanometer deflection angles on the positional relationship of the light beam on the scanning plane:
Wherein alpha and beta are respectively the deflection angles of X, Y vibrating mirrors, X and Y are respectively the two-dimensional coordinates of a scanning plane, d is the vertical distance from the center of the Y vibrating mirror to the point O, and e is the distance from the center point of the X vibrating mirror to the center point of the Y vibrating mirror;
thirdly, placing a calibration plate on a scanning plane, wherein the calibration plate is provided with N multiplied by N equidistant calibration points;
Driving the galvanometer to scan the calibration plate on the scanning plane point by point, recording the scanning position of the calibration plate, and calibrating the position as a characteristic point;
fifthly, making differences between equidistant calibration points on the calibration plate and scanned feature points, calculating coordinate errors of the feature points, and respectively establishing an error table in the X direction and an error table in the Y direction;
A sixth step of executing a linear interpolation compensation algorithm to carry out compensation correction, converting an error table in the X direction and the Y direction into a voltage difference value required to be compensated of a vibrating mirror X, Y shaft through the linear interpolation compensation algorithm and the control model obtained in the second step, and sending the voltage difference value to the vibrating mirror to drive, so that scanning position correction can be realized;
Seventh, executing the fifth step and the sixth step again, verifying whether the error is within the allowable range, if so, ending the correction step, and if not, executing the seventh step again;
the terahertz computed tomography three-dimensional imaging method based on the galvanometer scanning system comprises the following steps of:
Firstly, removing a calibration plate in the correction method, and placing an experimental sample at a scanning plane where the calibration plate is positioned;
The second step, the vibrating mirror corrected by the correction method is driven to uniformly and stably scan the sample, the whole is Z-shaped scanning, namely, a two-dimensional area to be scanned is divided into N multiplied by N lattices, the upper left corner is scanned to the lower right corner, the light beam intensity information of each lattice is recorded at a detector, the scanning period from the upper left scanning to the lower right is a scanning plane, the scanning is stopped after one period, and N multiplied by N intensity values are recorded at the detector;
Thirdly, driving the rotary table to rotate the sample after the second step is completed, wherein each rotation angle is 5 degrees, repeating the operation of the second step again after rotating the sample, recording N multiplied by N intensity values under the rotation angle, stopping the rotation angle of the sample when the rotation angle reaches 180 degrees, and storing the intensity values recorded under all the rotation angles;
And fourthly, obtaining a high-fidelity three-dimensional internal structure absorption diagram of the sample by using an FBP three-dimensional reconstruction algorithm for the N multiplied by N intensity values under each recorded angle.
2. The terahertz computed tomography method based on the galvanometer scanning system according to claim 1, wherein: when Z-type scanning is carried out, stay for 40ms in each area and are used for ensuring that the light beams at each position in the terahertz light beam array are uniform; the movement time of each zone is set to 0.5ms.
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