A three-dimensional defocus error correction method and device based on stable frequency band constraint

Abstract

The present application relates to the technical field of error correction, in particular to a three-dimensional defocus error correction method and device based on stable frequency band constraint, which first carries out data cleaning on the broadband measurement signal of the to-be-measured structure at different spatial positions; secondly, frequency component analysis is carried out based on the cleaned broadband measurement signal to obtain frequency resolution spatial response; thirdly, spatial spectrum analysis is carried out on the frequency resolution spatial response to generate periodic shift at different detection frequencies; then, the stable frequency band is determined according to the periodic shift at different detection frequencies; finally, the stable frequency band is corrected by constraint. The present application takes the real scale of the periodic structure as the geometric scale reference, converts the imaging degradation induced by defocus into apparent periodic shift, and carries out frequency band screening, weight constraint and error suppression on the three-dimensional reconstruction process of different layer positions or local areas based on the stable frequency band boundary, so as to reduce scale shift, layer position deviation and reconstruction artifacts, and realize three-dimensional defocus error correction with geometric scale constraint.

Description

Technical Field

[0001] This invention relates to the field of error correction technology, and in particular to a three-dimensional defocus error correction method and apparatus based on stable frequency band constraints. Background Technology

[0002] Three-dimensional imaging non-destructive testing technology is widely applicable to many fields such as polymers, ceramics, foams, composite materials, tablet coating, semiconductor packaging, and cultural relic defect detection. Terahertz imaging, infrared imaging, and other frequency-resolved imaging methods can extract multi-band amplitude, phase, time delay, and spatial response characteristics to achieve non-contact, non-destructive three-dimensional reconstruction of the sample surface and internal structure, which is the mainstream technology for precision non-destructive testing.

[0003] In 3D imaging nondestructive testing, the focal plane position of a sample is usually determined by indirect indicators such as signal intensity, image sharpness, temporal peak value, or spectral response, which cannot achieve precise and quantitative focal plane calibration. When the sample surface or internal layers shift axially relative to the system focal plane, the propagation conditions, phase relationships, spatial frequency transfer weights, and diffraction order contributions of different frequency components change. This leads to lateral scale shifts, boundary blurring, depth layer deviations, and misjudgments of feature dimensions in the 3D reconstruction results, severely affecting the accuracy of structural measurements and the reliability of defect identification.

[0004] For a sample with a periodic structure or regular characteristic scale, its spatial spectrum includes the fundamental frequency, higher harmonics, and sidelobe components. Defocus distortion will destroy the phase matching relationship and system transfer weight of each frequency component, so that the apparent characteristic scale of the reconstructed image no longer corresponds to the true geometric size of the sample, but is affected by the coupling of sample structural characteristics, light field propagation conditions, system transfer function and detection response, further aggravating the measurement error.

[0005] Currently, defocus correction techniques for 3D imaging mainly include mainstream solutions such as image sharpness optimization, autofocus algorithms, point spread function deconvolution, and empirical frequency band selection. These solutions can improve image quality degradation caused by defocus to a certain extent, but they still have multiple inherent technical defects in high-precision quantitative nondestructive testing scenarios. They are still difficult to effectively reduce scale shift, layer deviation, and reconstruction artifacts. The specific problems can be summarized into five aspects.

[0006] First, focal plane determination relies on indirect characterization indicators, making it impossible to quantify scale measurement errors. Traditional focal plane positioning methods for frequency-resolved 3D imaging such as terahertz and infrared rely heavily on indirect indicators such as time-domain peak intensity, image sharpness, spectral amplitude, or manual visual judgment. These parameters can only intuitively reflect signal strength and image visual quality, but cannot directly characterize the structural feature scale deviation caused by defocusing, nor can they determine the imaging reliability of spatial structures corresponding to different frequency components, thus lacking quantitative error assessment criteria.

[0007] Secondly, the uniform processing of broadband signals can easily introduce unstable frequency bands, causing imaging distortion. Multi-frequency, broadband 3D imaging signals contain a large number of frequency components, and the imaging response of different frequencies under defocus conditions exhibits significant differences: low-frequency components are easily affected by imaging boundaries, signal envelopes, and finite sampling window effects, while high-frequency components are prone to problems such as high-order harmonic competition and phase matching failure. Most existing methods reconstruct the entire frequency band signal as a whole, without distinguishing the differences in defocus stability of different frequency components. This easily introduces low-reliability unstable frequency bands into the 3D reconstruction process, further exacerbating defects such as structural scale shift and image artifacts.

[0008] Third, traditional correction methods only optimize visual effects and lack constraints on geometric scale errors. Autofocus algorithms based on maximizing image sharpness and deconvolution restoration methods based on point spread functions primarily aim to improve image visual clarity and reduce blur, but they do not consider core geometric parameters such as sample periodic structure, aperture size, and interlayer spacing as optimization constraints. These methods can only improve the visual appeal of the image but cannot guarantee the measurement accuracy of structural features, making them unsuitable for the quantitative measurement needs of non-destructive testing.

[0009] Fourth, there is a lack of stable frequency band evaluation indicators and no clear boundaries for effective imaging frequency bands. Current technologies have not established a quantitative mapping relationship between defocus, imaging frequency, and structural apparent scale offset, nor have they defined continuous and stable frequency band evaluation indicators that can be used for high-precision measurements. In detection scenarios where the focal plane position is uncertain, it is impossible to accurately select the optimal effective imaging frequency band, making it difficult to achieve accurate reconstruction and error correction based on frequency band characteristics.

[0010] Fifth, it cannot adapt to local defocus differences in three-dimensional layers, lacking layered correction capabilities. Actual tested samples generally possess complex morphologies such as multi-layered depth structures, tilted surfaces, and non-flat curved surfaces. The axial offset relative to the system focal plane varies significantly between different depth layers and local regions. Existing defocus correction methods mostly employ a globally uniform processing strategy, failing to perform differentiated stable frequency band identification and defocus error correction for slices at different depths and in different local regions, making it difficult to solve the layered imaging distortion problem of complex structural samples.

[0011] Given the aforementioned technical shortcomings, designing a three-dimensional defocus error correction method that can reduce scale shift, layer deviation, and reconstruction artifacts has become an urgent technical problem to be solved in this field. Summary of the Invention

[0012] The purpose of this invention is to provide a three-dimensional defocus error correction method and device based on stable frequency band constraints, so as to reduce scale shift, layer deviation and reconstruction artifacts, and realize three-dimensional defocus error correction with geometric scale constraints.

[0013] To achieve the above objectives, the present invention provides a three-dimensional defocus error correction method based on stable frequency band constraints, the method comprising:

[0014] Acquire broadband measurement signals of the structure under test at different spatial locations;

[0015] The broadband measurement signal is cleaned to obtain a cleaned broadband measurement signal;

[0016] Frequency component analysis is performed on the cleaned broadband measurement signal to obtain the frequency-resolved spatial response.

[0017] Spatial spectrum analysis is performed on the frequency-resolved spatial response to generate period offsets at different detection frequencies;

[0018] The stable frequency band is determined based on the period offset at different detection frequencies;

[0019] The stable frequency band is constrained and corrected to obtain the correction result.

[0020] Optionally, the step of performing frequency component analysis based on the cleaned broadband measurement signal to obtain the frequency-resolved spatial response specifically includes:

[0021] Determine whether the cleaned broadband measurement signal is a time-domain signal; if the cleaned broadband measurement signal is a time-domain signal, obtain the frequency domain signal corresponding to each spatial position at the k-th detection frequency through Fourier transform; if the cleaned broadband measurement signal is not a time-domain signal, read the frequency domain signal corresponding to each spatial position at the k-th detection frequency; k is a positive integer greater than or equal to 1.

[0022] Frequency channels are extracted based on the frequency domain signals corresponding to each spatial location at the kth detection frequency.

[0023] A frequency-resolved spatial response is constructed based on the frequency domain signal and frequency channel corresponding to each spatial location at the k-th detection frequency.

[0024] Optionally, the step of performing spatial spectrum analysis on the frequency-resolved spatial response to generate period offsets at different detection frequencies specifically includes:

[0025] Spatial spectrum analysis is performed on the frequency-resolved spatial response to obtain the spatial spectrum peak values ​​corresponding to each detection frequency; the spatial spectrum peak values ​​include multiple peak values.

[0026] Select the main peak from the spatial spectrum peak corresponding to the i-th detection frequency;

[0027] Determine whether the difference between the main peak and the adjacent secondary peak is less than a set threshold; if it is greater than or equal to the set threshold, extract the spatial frequency corresponding to the main peak as the equivalent spatial frequency; if it is less than the set threshold, combine the peak amplitude to perform a weighted calculation on the spatial frequency corresponding to the main peak and the spatial frequency corresponding to the adjacent secondary peak to obtain the equivalent spatial frequency.

[0028] Based on the reciprocal relationship between equivalent spatial frequency and period, the apparent period at different detection frequencies is calculated.

[0029] The period offset at different detection frequencies is calculated based on the apparent period at different detection frequencies.

[0030] Optionally, the calculation of the period offset at different detection frequencies based on the apparent period at different detection frequencies is specifically formulated as follows:

[0031]

[0032] in, To detect the apparent period at frequency f, To detect the periodic offset at frequency f and in defocus state d, The true geometric period in the reference scale.

[0033] Optionally, determining the stable frequency band based on the period offset at different detection frequencies includes:

[0034] The condition corresponding to the stable region is determined based on the period offset at different detection frequencies. If the condition corresponding to the stable region is met, at least one stable region is output, and the stable frequency band is determined based on the longest stable region in the continuous frequency band.

[0035] Optionally, the structure under test is at least one of a metal periodic grating, a periodic aperture array, and a regular line grating.

[0036] The present invention also provides a three-dimensional defocus error correction device based on stable frequency band constraints, the device further comprising:

[0037] The frequency-resolved detection module is used to acquire broadband measurement signals of the structure under test at different spatial locations;

[0038] The data processing computer is connected to the frequency resolution detection module and is used to perform three-dimensional defocus error correction using the aforementioned three-dimensional defocus error correction method based on stable frequency band constraints.

[0039] Optionally, the frequency resolution detection module includes:

[0040] The system comprises a broadband light source, a transmitting lens group, a structure under test, a receiving lens group, and a broadband detector; these components are arranged in a corresponding manner.

[0041] The present invention also provides a computer-readable storage medium, characterized in that it stores a computer program, which, when executed by a processor, implements the steps of the above-described three-dimensional defocus error correction method based on stable frequency band constraints.

[0042] The present invention also provides a computer device, including a memory and a processor, wherein the memory stores a computer program, characterized in that: when the processor executes the computer program, it implements the steps of the above-described three-dimensional defocus error correction method based on stable frequency band constraints.

[0043] According to specific embodiments provided by the present invention, the present invention discloses the following technical effects:

[0044] This invention discloses a method and apparatus for three-dimensional defocus error correction based on stable frequency band constraints. First, broadband measurement signals of the structure under test at different spatial locations are cleaned. Second, frequency component analysis is performed on the cleaned broadband measurement signals to obtain the frequency-resolved spatial response. Third, spatial spectrum analysis is performed on the frequency-resolved spatial response to generate periodic offsets at different detection frequencies. Then, a stable frequency band is determined based on the periodic offsets at different detection frequencies. Finally, constraint correction is applied to the stable frequency band to obtain the correction result. This invention uses the true scale of the periodic structure as the geometric scale benchmark, transforming defocus-induced imaging degradation into an apparent periodic offset. Based on the stable frequency band boundary, frequency band selection, weight constraints, and error suppression are applied to the three-dimensional reconstruction process of different layers or local regions, thereby reducing scale shift, layer deviation, and reconstruction artifacts, achieving three-dimensional defocus error correction with geometric scale constraints. Attached Figure Description

[0045] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0046] Figure 1 This is a flowchart of a three-dimensional defocus error correction method based on stable frequency band constraints according to an embodiment of the present invention;

[0047] Figure 2 This is a diagram of a three-dimensional defocus error correction structure based on stable frequency band constraints according to an embodiment of the present invention.

[0048] Figure 3This is a graph showing the change of period offset with detection frequency in an embodiment of the present invention.

[0049] The components include: 1. Broadband light source; 2. Emitting lens group; 3. Structure under test; 4. Reference focal plane; 5. Receiving lens group; 6. Broadband detector; and 7. Data processing computer. Detailed Implementation

[0050] 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.

[0051] The purpose of this invention is to provide a three-dimensional defocus error correction method and device based on stable frequency band constraints, so as to reduce scale offset, layer deviation and reconstruction artifacts, and realize three-dimensional defocus error correction with geometric scale constraints.

[0052] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0053] like Figure 1 As shown, the present invention also provides a three-dimensional defocus error correction method based on stable frequency band constraints, the method comprising:

[0054] Step S1: Acquire broadband measurement signals of the structure under test at different spatial locations; the structure under test can be a metal periodic grating, a periodic aperture array, a regular wire grating, or other structures with definite periodic parameters; the above-mentioned structure under test is a periodic structure with a known period, and its true geometric period is denoted as . .

[0055] In actual testing, the structure under test can be either the object itself or an external periodic reference film that is in the same or approximately the same focal plane as the object. When the object itself does not have a periodic structure, a stable frequency band determination result can be obtained through an external periodic reference film that is in the same or approximately the same focal plane as the structure under test. This stable frequency band is then used as the basis for frequency band selection, weight allocation, and error suppression for the object under test. The acquired measurement signals are processed to form frequency-resolved spatial response data, which is used for subsequent apparent period extraction, period offset calculation, stable frequency band identification, frequency band selection, and error suppression.

[0056] Step S2: Perform data cleaning on the broadband measurement signal to obtain the cleaned broadband measurement signal. The cleaned data is used for subsequent single-frequency spatial response construction to reduce the impact of abnormal acquisition points, defective data, missing data, and system background on the subsequent periodic extraction results. Data cleaning includes outlier removal, defective data deletion, missing value correction, background subtraction, and normalization.

[0057] Step S3: Perform frequency component analysis on the cleaned broadband measurement signal to obtain the frequency-resolved spatial response, specifically including:

[0058] Step S31: Determine whether the cleaned broadband measurement signal is a time-domain signal; if the cleaned broadband measurement signal is a time-domain signal, obtain the frequency domain signal corresponding to each spatial location at the k-th detection frequency through Fourier transform; if the cleaned broadband measurement signal is not a time-domain signal, read the frequency domain signal corresponding to each spatial location at the k-th detection frequency.

[0059] Step S32: Extract the frequency channel based on the frequency domain signal corresponding to each spatial location at the k-th detection frequency.

[0060] Step S33: Construct a frequency-resolved spatial response based on the frequency domain signal and frequency channel corresponding to each spatial location at the k-th detection frequency. The frequency-resolved spatial response includes at least one of amplitude response, phase response, intensity response, and complex frequency domain response.

[0061] If the structure under test in this invention is a one-dimensional periodic structure, the frequency-resolved spatial response constructed along the periodic direction by the above method is a one-dimensional spatial response curve; for two-dimensional periodic arrays or regular aperture arrays, it can also be extended to a two-dimensional spatial response image.

[0062] Step S4: Perform spatial spectrum analysis on the frequency-resolved spatial response to generate period offsets at different detection frequencies, specifically including:

[0063] Step S41: Perform spatial spectrum analysis on the frequency-resolved spatial response to obtain the spatial spectrum peak value corresponding to each detection frequency; the spatial spectrum peak value includes multiple peak values.

[0064] Step S42: Select the main peak from the spatial spectrum peaks corresponding to the i-th detection frequency.

[0065] Step S43: Determine whether the difference between the main peak and the adjacent secondary peak is less than a set threshold; if it is greater than or equal to the set threshold, extract the spatial frequency corresponding to the main peak as the equivalent spatial frequency; if it is less than the set threshold, combine the peak amplitude to perform a weighted calculation on the spatial frequency corresponding to the main peak and the spatial frequency corresponding to the adjacent secondary peak to obtain the equivalent spatial frequency.

[0066] Step S44: Calculate the apparent period at different detection frequencies based on the reciprocal relationship between the equivalent spatial frequency and the period.

[0067] Step S45: Calculate the period offset at different detection frequencies based on the apparent period at different detection frequencies. The specific formula is as follows:

[0068]

[0069] in, To detect the apparent period at frequency f, To detect the periodic offset at frequency f and in defocus state d, The true geometric period in the reference scale.

[0070] To improve peak location accuracy, this invention can also refine the spectral lines near the main peak through interpolation.

[0071] Step S5: Determine the stable frequency band based on the period offset at different detection frequencies, specifically including:

[0072] Step S51: Determine whether the conditions corresponding to the stable region are met based on the period offset at different detection frequencies; if the conditions corresponding to the stable region are met, output at least one stable region and determine the stable frequency band based on the longest stable region in the continuous frequency band; if the conditions corresponding to the stable region are not met, proceed to step S52.

[0073] Step S52: Determine whether the conditions corresponding to the fluctuation zone are met based on the period offset at different detection frequencies; if the conditions corresponding to the fluctuation zone are met, then it is determined to be a fluctuation zone; if the conditions corresponding to the fluctuation zone are not met, then proceed to step S53.

[0074] Step S53: Determine whether the conditions corresponding to the competition zone are met based on the period offset at different detection frequencies; if the conditions corresponding to the competition zone are met, then it is determined to be a competition zone.

[0075] The stable region of this invention is defined as a frequency range in which the period offset is below a preset error threshold and is continuously distributed. The competitive region is defined as a frequency range in which the period offset is below the preset error threshold and is continuously distributed. The competitive region is defined as a frequency range in which the period offset increases again due to harmonic competition, sidelobe enhancement, or phase mismatch on the high-frequency side. The fluctuating region is defined as a frequency range in which the period offset fluctuates significantly.

[0076] like Figure 3As shown, based on the curve of period offset versus detection frequency, the frequency response is divided into a fluctuation region, a stable region, and a competition region. To further quantitatively characterize the impact of defocusing on the stability of periodic extraction, [the following is taken]... As a preset error threshold for the stable zone, under a given out-of-focus state If a certain detection frequency Corresponding period offset Not greater than the preset error threshold If the apparent periodicity extraction result at this frequency satisfies the conditions corresponding to the stable region, then:

[0077] ;

[0078] in, Represents frequency Out-of-focus state The period offset below.

[0079] Among all detection frequencies, frequency points satisfying the above conditions may be distributed within one or more continuous stable regions. To obtain the most stable and suitable frequency range as an effective measurement band, the longest continuous frequency band is selected as the stable frequency band, denoted as:

[0080] ;

[0081] The upper and lower boundaries of the stable frequency band are denoted as follows: and The corresponding stable bandwidth is :

[0082] ;

[0083] Stable bandwidth This is used to characterize the effective frequency range within which the system can still reliably perform periodic measurements under the current defocused state. The larger the value, the wider the usable frequency range under the condition, and the higher the stability of the period measurement results to defocus disturbances; The smaller the value, the more the stable measurement window is compressed, increasing the risk of scale errors caused by defocusing.

[0084] The stable frequency band obtained by the above method serves as the effective frequency band for three-dimensional imaging, period measurement, and scale error correction.

[0085] Step S6: Perform constraint correction on the stable frequency band to obtain the correction results and scale error evaluation results.

[0086] Specifically, the 3D data to be measured is divided into multiple data units to be calibrated according to depth layers, local spatial regions, or structural units. Based on the periodic offset distribution and stable frequency band boundaries of each data unit under the current defocus state, the corresponding effective frequency range is determined. During 3D reconstruction, period measurement, or scale analysis, frequency components within the stable frequency band are preferentially used in the calculation, and corresponding weights are set according to the periodic offset. The smaller the periodic offset, the weaker the influence of the current defocus state on the frequency component, and the higher its corresponding weight. The closer the periodic offset is to or exceeds the preset error threshold, the stronger the influence of the current defocus state on the frequency component, and the lower its corresponding weight. Through the above frequency band selection and weight constraints, the calibration result is mainly contributed by frequency components with smaller scale offsets and more stable spatial spectral responses, thereby reducing lateral scale offsets, layer identification deviations, and reconstruction artifacts caused by defocus.

[0087] For cases where different defocus states exist at different depth levels or in different local areas, the stable frequency bands corresponding to each level or local area are determined, and frequency band filtering and weight constraints are performed respectively to form a correction result for local defocus differences.

[0088] The method disclosed in this invention has the following advantages compared with existing methods:

[0089] 1. Improve the accuracy of scale measurement under defocus conditions.

[0090] This invention calculates the period offset using a frequency-resolved method, directly evaluating the scale measurement error at different detection frequencies, thus avoiding the reliance on image clarity or signal strength alone to determine measurement reliability.

[0091] 2. Determine the effective measurement frequency band under the condition of focal plane uncertainty.

[0092] This invention uses stable frequency band determination to select frequency ranges with smaller period offsets and more stable scale measurement results under defocus conditions, providing effective frequency band basis for subsequent imaging, period measurement and error correction.

[0093] 3. Suppress the influence of unstable frequency bands on measurement results.

[0094] By stabilizing the frequency band, the interference of low-frequency fluctuations and high-frequency competition regions on period extraction, scale measurement, and 3D imaging results can be reduced, thereby reducing apparent period shift, lateral scale error, and imaging artifacts.

[0095] 4. Improve the reliability of three-dimensional non-destructive testing.

[0096] This method can be used for 3D inspection of objects such as periodic structures, microstructure arrays, layered materials, composite materials, and packaged devices, improving the reliability of feature size, periodic parameters, and structural position measurements.

[0097] 5. Expand the applicability of imaging systems in different bands.

[0098] This method is not limited to a single band and can be used in terahertz, infrared and other broadband or multi-frequency imaging systems with frequency-resolved imaging capabilities.

[0099] This invention proposes a three-dimensional defocus error correction method based on stable frequency band constraints. This method extracts the periodic offset using a frequency-resolved approach and divides the frequency response into fluctuation, stable, and competing regions based on the periodic offset. The stable region corresponds to effective frequency bands with smaller apparent scale offsets, more stable spatial frequency component propagation, and suitability for three-dimensional reconstruction and error correction. The boundaries and bandwidth of the stable frequency band can be used to characterize the impact of defocus on scale measurement stability and provide a basis for subsequent frequency band selection and error suppression, thereby reducing three-dimensional scale errors, toposcopic errors, and reconstruction instability caused by focal plane uncertainty. The imaging bands applicable to this invention include, but are not limited to, terahertz, infrared, and other electromagnetic bands with frequency-resolved imaging capabilities.

[0100] like Figure 2 As shown, this invention discloses a three-dimensional defocus error correction device based on stable frequency band constraints. The device includes: a frequency resolution detection module and a data processing computer 7; the frequency resolution detection module is used to acquire broadband measurement signals of the structure under test 3 at different spatial positions; the data processing computer 7 is connected to the frequency resolution detection module and is used to perform three-dimensional defocus error correction using the above-mentioned three-dimensional defocus error correction method.

[0101] The frequency-resolved detection module includes a broadband light source 1, a transmitting lens group 2, a structure under test 3, a receiving lens group 5, and a broadband detector 6. The broadband light source 1, transmitting lens group 2, structure under test 3, reference focal plane 4, receiving lens group 5, and broadband detector 6 are arranged correspondingly to each other. The broadband detector 6 is connected to a data processing computer 7. The broadband light source 1 outputs a broadband signal for detection, including but not limited to terahertz waves, infrared radiation, or other electromagnetic waves that can be used for frequency-resolved imaging. The transmitting lens group 2 collimates, focuses, or beam-shapes the broadband signal, so that the processed beam illuminates the structure under test 3. The structure under test 3 is mounted on a three-dimensional displacement platform, resulting in axial defocusing relative to the reference focal plane 4, and the defocusing amount is obtained. The reference focal plane 4 represents the measurement focal plane position corresponding to the ideal imaging of the system, and the defocusing amount d characterizes the degree of deviation of the structure under test 3 from the reference focal plane 4.

[0102] The broadband signal transmitted, reflected, or scattered by the structure under test 3 is collected by the receiving lens group 5 and transmitted to the broadband detector 6. The broadband detector 6 is used to output the broadband measurement signal of the structure under test at different spatial locations. The broadband measurement signal can be time-domain waveform, frequency-domain response, peak amplitude, or intensity response data. The broadband measurement signal is further input into the data processing computer 7, which performs data cleaning, frequency component analysis, spatial response construction, apparent period calculation, stable frequency band identification, and stable frequency band constraint correction.

[0103] In one alternative embodiment, the broadband light source 1 may be a terahertz transmitter and the broadband detector 6 may be a terahertz receiver; in another embodiment, the broadband light source 1 may be an infrared light source and the broadband detector 6 may be an infrared detector or other imaging device with frequency resolution measurement capability.

[0104] Specific examples:

[0105] This invention employs a terahertz time-domain spectral imaging system, a three-dimensional scanning platform, a calibration plate with a known period, and a data processing module. First, response data of the calibration plate or the sample under test are acquired at different spatial locations and frequencies. Apparent periods or characteristic scales at different frequencies are extracted, and stable frequency bands are determined based on scale offsets. Subsequently, frequency band filtering or weight allocation is performed on the three-dimensional imaging data based on the stable frequency bands, and error suppression is applied to the period measurement results, lateral scale measurement results, and imaging artifacts.

[0106] In one implementation, known periodic structures such as 300 μm, 600 μm, or 900 μm can be used as reference samples to compare the variation patterns of stable frequency bands under different known defocus conditions. This is used to evaluate the impact of different defocus conditions on the stability of scale measurements and to guide subsequent frequency band selection. In another implementation, regular structures in the sample under test or external periodic reference plates can be directly used as reference objects to achieve three-dimensional error correction under focal plane uncertainty conditions during nondestructive testing.

[0107] Through the above embodiments, the present invention can reduce lateral scale shift, depth layer error and reconstruction artifacts caused by defocusing without changing the main structure of the terahertz imaging system, thereby improving the reliability of terahertz three-dimensional measurement results.

[0108] The above description is only a part of the embodiments of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention can have various modifications and variations. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

[0109] The present invention also provides a computer-readable storage medium storing a computer program, which, when executed by a processor, implements the steps of the above-described three-dimensional defocus error correction method based on stable frequency band constraints.

[0110] The present invention also provides a computer device, including a memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to implement the steps of the above-described three-dimensional defocus error correction method based on stable frequency band constraints.

[0111] Embodiments of the present invention may be provided as methods, systems, or computer program products. The invention may take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the invention may take the form of a computer program product implemented on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0112] This invention is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart illustrations and / or block diagrams. Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0113] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0114] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0115] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the systems disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the descriptions are relatively simple; relevant parts can be referred to the method section.

[0116] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. Furthermore, those skilled in the art will recognize that, based on the ideas of the present invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A three-dimensional defocus error correction method based on stable frequency band constraints, characterized in that, The method includes: Acquire broadband measurement signals of the structure under test at different spatial locations; The broadband measurement signal is cleaned to obtain a cleaned broadband measurement signal; Frequency component analysis is performed on the cleaned broadband measurement signal to obtain the frequency-resolved spatial response. Spatial spectrum analysis is performed on the frequency-resolved spatial response to generate period offsets at different detection frequencies; The stable frequency band is determined based on the period offset at different detection frequencies; The stable frequency band is constrained and corrected to obtain the correction result.

2. The three-dimensional defocus error correction method based on stable frequency band constraints according to claim 1, characterized in that, The step of performing frequency component analysis based on the cleaned broadband measurement signal to obtain the frequency-resolved spatial response specifically includes: Determine whether the cleaned broadband measurement signal is a time-domain signal; if the cleaned broadband measurement signal is a time-domain signal, obtain the frequency domain signal corresponding to each spatial position at the k-th detection frequency through Fourier transform; if the cleaned broadband measurement signal is not a time-domain signal, read the frequency domain signal corresponding to each spatial position at the k-th detection frequency; k is a positive integer greater than or equal to 1. Frequency channels are extracted based on the frequency domain signals corresponding to each spatial location at the kth detection frequency. A frequency-resolved spatial response is constructed based on the frequency domain signal and frequency channel corresponding to each spatial location at the k-th detection frequency.

3. The three-dimensional defocus error correction method based on stable frequency band constraints according to claim 1, characterized in that, The step of performing spatial spectrum analysis on the frequency-resolved spatial response to generate period offsets at different detection frequencies specifically includes: Spatial spectrum analysis is performed on the frequency-resolved spatial response to obtain the spatial spectrum peak values ​​corresponding to each detection frequency; the spatial spectrum peak values ​​include multiple peak values. Select the main peak from the spatial spectrum peaks corresponding to the i-th detection frequency; i is a positive integer greater than or equal to 1; Determine whether the difference between the main peak and the adjacent secondary peak is less than a set threshold; if it is greater than or equal to the set threshold, extract the spatial frequency corresponding to the main peak as the equivalent spatial frequency; if it is less than the set threshold, combine the peak amplitude to perform a weighted calculation on the spatial frequency corresponding to the main peak and the spatial frequency corresponding to the adjacent secondary peak to obtain the equivalent spatial frequency. Based on the reciprocal relationship between equivalent spatial frequency and period, the apparent period at different detection frequencies is calculated. The period offset at different detection frequencies is calculated based on the apparent period at different detection frequencies.

4. The three-dimensional defocus error correction method based on stable frequency band constraints according to claim 3, characterized in that, The formula for calculating the period offset at different detection frequencies based on the apparent period at different detection frequencies is as follows: in, To detect the apparent period at frequency f, To detect the periodic offset at frequency f and in defocus state d, The true geometric period in the reference scale.

5. The three-dimensional defocus error correction method based on stable frequency band constraints according to claim 1, characterized in that, The step of determining the stable frequency band based on the period offset at different detection frequencies includes: The condition corresponding to the stable region is determined based on the period offset at different detection frequencies. If the condition corresponding to the stable region is met, at least one stable region is output, and the stable frequency band is determined based on the longest stable region in the continuous frequency band.

6. The three-dimensional defocus error correction method based on stable frequency band constraints according to claim 1, characterized in that, The structure under test is at least one of a metal periodic grating, a periodic aperture array, and a regular line grating.

7. A three-dimensional defocus error correction device based on stable frequency band constraints, characterized in that, The device further includes: The frequency-resolved detection module is used to acquire broadband measurement signals of the structure under test at different spatial locations; The data processing computer is connected to the frequency resolution detection module and is used to perform three-dimensional defocus error correction using a three-dimensional defocus error correction method based on stable frequency band constraints as described in any one of claims 1-6.

8. The three-dimensional defocus error correction device based on stable frequency band constraints according to claim 7, characterized in that, The frequency resolution detection module includes: The system comprises a broadband light source, a transmitting lens group, a structure under test, a receiving lens group, and a broadband detector; these components are arranged in a corresponding manner.

9. A computer-readable storage medium, characterized in that, The device contains a computer program that, when executed by a processor, implements the steps of a three-dimensional defocus error correction method based on stable frequency band constraints as described in any one of claims 1-6.

10. A computer device comprising a memory and a processor, wherein the memory stores a computer program, characterized in that: When the processor executes the computer program, it implements the steps of the three-dimensional defocus error correction method based on stable frequency band constraints as described in any one of claims 1-6.

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