Time domain wide spectrum double-layer medium spectral parameter field measurement method with multi-distance detection

Abstract

A method for measuring spectral parameter fields of double-layer media with time-domain wide spectrum and multi-distance detection. The present application uses the selection of wide-spectrum multi-band light sources to provide sufficient incident spectral information, and uses a time-domain model to provide rich time-varying detection signals. By detecting the incident and boundary exit radiation signals of light sources at different positions, a reconstruction optimization algorithm is combined to analyze the spectral information in the boundary measurement signal. Based on the acquisition and analysis of rich wide-spectrum multi-distance time-varying detection signals, the pathological nature of the inverse problem reconstruction of the spectral parameters of the double-layer medium model is overcome, and the problem of measuring the spectral parameters of the double-layer medium can be solved more accurately, avoiding the problem that the uniform medium model with single-point detection in the prior art can obtain limited information and cannot well reflect the real parameter distribution in the medium, thereby improving the accuracy of spectral parameter reconstruction.

Description

Technical Field

[0001] The invention relates to the technical field of spectral imaging, in particular to a method for measuring spectral parameter fields of double-layer media with a time domain wide spectrum and multi-distance detection. Background Art

[0002] The wide spectrum measurement technology uses multiple visible or near-infrared lights of different wavelengths to illuminate the medium, and obtains the wide spectrum parameters and structural information inside the medium by measuring and analyzing the medium boundary detection signal. In most cases, the spectral parameter distribution inside the medium cannot be directly obtained through measurement. When the laser is incident on the medium, the boundary detection signal on the surface of the medium can be directly measured. The spectral information in the boundary measurement signal can be analyzed by combining the reconstruction optimization algorithm to indirectly reconstruct the spectral parameters inside the medium. The wide spectrum measurement technology can provide rich spectral information and play an important role in non-destructive detection, combustion temperature measurement, infrared remote sensing, medical imaging, etc.

[0003] According to the different laser light sources, the wide spectrum measurement technology can be divided into steady-state model (using continuous laser incidence), frequency domain model (using frequency modulated laser incidence), and time domain model (using pulsed laser incidence). Among the three radiation transmission models, the solution process of steady-state and frequency domain modes is relatively simple and the calculation efficiency is high, but the measurement data provided is less and the reconstruction accuracy of spectral parameters is low. The solution of time domain mode is more complicated and the reconstruction efficiency is low. However, it can provide rich time-varying detection signals, thereby obtaining higher quality reconstruction results.

[0004] In recent years, the rich time-varying optical signals provided by ultrashort pulse lasers have promoted the development of wide spectrum measurement technology. However, considering the complexity of time-domain wide spectrum measurement and the pathological nature of the spectral parameter reconstruction problem of the double-layer medium model, current time-domain wide spectrum measurement research is mostly based on a uniform medium model with single-point detection. However, the information that can be obtained from the uniform medium model with single-point detection is limited and cannot well reflect the real parameter distribution in the medium, resulting in low accuracy of spectral parameter reconstruction. Summary of the invention

[0005] The purpose of the present invention is to propose a time-domain wide-spectrum double-layer medium spectral parameter field measurement method with multi-distance detection in order to address the problem that the information that can be obtained by the uniform medium model of single-point detection in the prior art is limited and cannot well reflect the actual parameter distribution in the medium, which in turn leads to low accuracy of spectral parameter reconstruction.

[0006] The technical solution adopted by the present invention to solve the above technical problems is:

[0007] A method for measuring spectral parameter fields of double-layer media with wide spectrum in time domain for multi-distance detection is implemented based on a measuring device:

[0008] The measuring device comprises: a broadband picosecond pulse continuous light source 1, a prism-based band selector 2, a variable neutral density attenuator 3, an optical switch 4, a photodetector 6 and a data acquisition and processing system 7;

[0009] The method comprises the following steps:

[0010] Step 1: Turn on the broadband picosecond pulse continuous light source 1. The light source output by the broadband picosecond pulse continuous light source 1 passes through the prism-based band selector 2 to obtain pulsed lasers of w different bands. The pulsed laser enters the variable neutral density attenuator 3. After that, the light source output by the variable neutral density attenuator 3 enters the optical switch 4 through the optical fiber, and is irradiated onto the double-layer medium 5. By switching the optical switch 4, d incident light sources at different positions on the double-layer medium 5 are obtained. After each switching of the optical switch 4, the photodetector 6 collects the outgoing radiation signal of the double-layer medium 5 once, and then the photodetector 6 sends all the collected outgoing radiation signals to the data acquisition and processing system 7. The data acquisition and processing system 7 processes the outgoing radiation signal to obtain the spectral outgoing radiation signal emitted by the double-layer medium 5. Where, i=1,2,...w, j=1,2,...d, w≥2, d≥2;

[0011] Step 2: Select the pulsed laser of the i-th band and set the initial distribution value of the spectral parameter field in the double-layer medium 5 to be Afterwards Substitute the time-domain diffusion approximation equation of the double-layer medium to obtain the initial value of the outgoing radiation signal at the medium boundary under the pulse laser incident in the i-th band That is, the outgoing radiation simulation signal, repeat the above steps to obtain the initial values ​​of the outgoing radiation signals of all medium boundaries under the pulse laser incident in the i-th band, and use the initial values ​​of the outgoing radiation signals of all medium boundaries and the spectral outgoing radiation signal in step 1 Get the initial objective function

[0012] Step 3: Update the distribution value of the spectral parameter field in the double-layer medium 5 according to the conjugate gradient method to obtain the distribution value of the spectral parameter field obtained in the k-th iteration. It is expressed as:

[0013]

[0014] Where k represents the number of iterations, Δμ i represents the change in the spectral parameter field, represents the distribution value of the spectral parameter field obtained at the kth iteration, represents the distribution value of the spectral parameter field obtained after k-1 iterations;

[0015] Step 4: Substitute the time-domain diffusion approximation equation of the double-layer medium to obtain the updated outgoing radiation signal of the medium boundary under the pulse laser incident in the i-th band Repeat the above steps to obtain all updated outgoing radiation signals of the medium boundary, and use all updated outgoing radiation signals of the medium boundary and the spectral outgoing radiation signal in step 1 to obtain the outgoing radiation signal of the medium boundary. Get the updated objective function F(μ i ), and finally determine the objective function F(μ i ) is less than the set minimum value ε, if the objective function F(μ i ) is less than the set minimum value ε, then execute step 6, otherwise, execute step 5;

[0016] Step 5: Determine whether the number of iterations k reaches the maximum number of iterations K. If the number of iterations k reaches the maximum number of iterations K, use the current spectral parameter field distribution value as the initial value and re-execute step 2, that is, If the number of iterations k does not reach the maximum number of iterations K, execute step 3;

[0017] Step 6: According to the current iteration result, the distribution value of the spectral parameter field in the i-th band is obtained;

[0018] Step 7: Repeat steps 2 to 6 to obtain the distribution values ​​of all spectral parameter fields in w different bands, and use the conjugate gradient method to update the distribution values ​​of the spectral parameter fields until convergence to obtain the distribution values ​​of the final spectral parameter fields in w different bands.

[0019] Furthermore, the time domain diffusion approximation equation of the double-layer medium is expressed as:

[0020]

[0021] Among them, D i (r) represents the diffusion coefficient, is the absorption coefficient, represents the effective scattering coefficient, represents the sign of partial differential, r represents position, c represents the speed of light in the tissue medium, t represents time, S i (r, t) represents the radiation source term introduced by the incident laser, g i represents the scattering asymmetry factor, represents the scattering coefficient, G i (r, t) represents the projected radiation force and n represents the refractive index.

[0022] Furthermore, the initial objective function It is expressed as:

[0023]

[0024] in, is the spectral parameter field of the target side, is the absorption coefficient, represents the scattering coefficient.

[0025] Furthermore, the objective function F(μ i ) is expressed as:

[0026]

[0027] Further, the conjugate gradient method is expressed as:

[0028]

[0029] in, represents the step size of the kth iteration of the ith light wave, Indicates the descending direction of the kth iteration of the i-th light wave.

[0030] Furthermore, the spectral parameter field includes the absorption coefficient μ a Spectral parameter field and scattering coefficient μ s Spectral parameter field.

[0031] Furthermore, the broadband picosecond pulse continuous light source 1 is a broadband picosecond pulse NKT continuous light source.

[0032] Furthermore, the broadband picosecond pulse continuous light source 1 outputs laser light with a spectral range of 400-2400nm.

[0033] Furthermore, the photodetector 6 is a silicon photomultiplier tube.

[0034] The beneficial effects of the present invention are:

[0035] The present application uses the selection of wide-spectrum multi-band light sources to provide sufficient incident spectral information, uses the time domain model to provide rich time-varying detection signals, and detects the incident and boundary exit radiation signals of light sources at different positions, combined with the reconstruction optimization algorithm to analyze the spectral information in the boundary measurement signal. Based on the acquisition and analysis of rich wide-spectrum multi-distance time-varying detection signals, the pathological nature of the inverse problem reconstruction of the spectral parameters of the double-layer medium model is overcome, and the problem of measuring the spectral parameters of the double-layer medium can be solved more accurately, avoiding the problem that the uniform medium model of single-point detection in the prior art can obtain limited information and cannot well reflect the real parameter distribution in the medium, thereby improving the accuracy of spectral parameter reconstruction.

[0036] The present application can be applied to wide-spectrum nondestructive detection inside a medium, combustion process diagnosis, infrared remote sensing detection, biomedical imaging, etc., and is of great significance to the field of spectral imaging technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is a schematic diagram of the structure of the time-domain wide spectrum double-layer medium spectral parameter measurement system based on multi-distance detection;

[0038] Figure 2 Flow chart of spectral parameter measurement of double-layer media in time domain wide spectrum based on multi-distance detection. DETAILED DESCRIPTION

[0039] It should be particularly noted that, in the absence of conflict, the various embodiments disclosed in this application can be combined with each other.

[0040] Specific implementation method 1: refer to Figure 1 Specifically describing this embodiment, the multi-distance detection time domain wide spectrum double-layer medium spectral parameter field measurement method described in this embodiment is based on the following device, which includes:

[0041] Broadband picosecond pulse NKT continuous light source 1, prism-based band selector 2, variable neutral density attenuator 3, optical switch 4, double-layer medium 5, photodetector 6, data acquisition and processing system 7;

[0042] The broadband picosecond pulse continuous light source 1 is a broadband picosecond pulse NKT continuous light source, and the photodetector 6 is a silicon photomultiplier tube.

[0043] The method comprises the following steps:

[0044] Step 1: A broadband picosecond pulse continuous light source 1 outputs a laser with a spectral range of 400-2400nm. The laser with a variable wavelength is selected by an automatic rotating prism 2 to obtain pulse lasers with w different wavelengths, where w ≥ 2. The light wave is transmitted to a variable neutral density attenuator 3 through an optical fiber to keep the number of detection photons at 5×10 5 Photons / second. Then, the light source enters the optical switch 4 through the optical fiber, and d light sources (where d≥2) are obtained by switching the optical switch. The output optical fibers of the corresponding light sources are fixed at different positions of the detection board on the surface of the double-layer medium 5, and the detection optical fiber connected to the photodetector 6 is fixed on one side of the detection board. The double-layer medium 5 is placed above the detection board so that the optical fiber is in close contact with the double-layer medium 5. The data acquisition and processing system 7 is connected to the prism-based band selector 2 and the silicon photomultiplier tube 6 to automatically control the detection laser and collect the detection signal.

[0045] Each time the optical switch 4 emits a pulse laser, the silicon photomultiplier tube 6 collects the outgoing radiation signal from the double-layer medium surface, and then sends all the obtained outgoing radiation signals to the data acquisition and processing system 7;

[0046] The data acquisition and processing system 7 processes the obtained outgoing radiation signals respectively to obtain the spectral outgoing radiation signals emitted from each boundary of the double-layer medium 5. As the measurement signal when the pulse laser is incident, w represents the selected pulse laser band number, d represents the light source position number, i = 1, 2, ... w, j = 1, 2, ... d;

[0047] Step 2: Assume that the initial value of the spectral parameter field of the double-layer medium is Will Substitute the time-domain diffusion approximate equation of the double-layer medium into the calculated outgoing radiation signal of the medium boundary under different wavelength laser incidence The measured signal in step 1 Constructing the objective function

[0048] Step 3: Update the distribution value of the diffuse medium spectral parameter field according to the conjugate gradient method: k represents the number of iterations, Δμ i represents the change in the spectral parameter field, represents the distribution value of the spectral parameter field obtained at the kth iteration, represents the distribution value of the spectral parameter field obtained after k-1 iterations;

[0049] Step 4: Spectral parameter distribution obtained according to the k-th iteration The outgoing radiation signal at the medium boundary is obtained by calculating the time-domain diffusion approximation equation of the double-layer medium. Calculate the objective function F(μ i ), if the objective function value is less than the minimum value ε, execute step 6; otherwise, execute step 5;

[0050] Step 5: If the number of iterations k reaches the maximum number of iterations K, use the current spectral parameter field distribution as the initial value and restart the iteration, that is, Coming Soon The value of Execute step 2; otherwise, execute step 3;

[0051] Step 6: Use the spectral parameter field obtained in the current iteration as the reconstruction result and end the inversion process.

[0052] When the light source is projected into the medium to be tested, only one light source should be projected into the medium to be tested. By turning the light switch on and off, the position of the light source to be turned on is changed, thereby adjusting the distance between the light source and the detector. Each time the distance between the light source and the silicon photomultiplier tube is adjusted, a signal is collected.

[0053] Specific implementation method 2: This implementation method is a further explanation of the specific implementation method 1. The difference between this implementation method and the specific implementation method 1 is that the time domain diffusion approximation equation of the double-layer medium is expressed as:

[0054]

[0055] Among them, D i (r) represents the diffusion coefficient, is the absorption coefficient, represents the effective scattering coefficient, represents the differential sign, r represents the position, c represents the speed of light in the tissue medium, t represents the time, S i (r, t) represents the radiation source term introduced by the incident laser, g i represents the scattering asymmetry factor, represents the scattering coefficient, G i (r, t) represents the projected radiation force and n represents the refractive index.

[0056] Specific implementation method three: This implementation method is a further explanation of specific implementation method two. The difference between this implementation method and specific implementation method two is that the initial objective function It is expressed as:

[0057]

[0058] in, is the spectral parameter field of the target side, and the boundary signal is calculated by the double-layer medium time domain diffusion approximation model. is the absorption coefficient, represents the scattering coefficient.

[0059] The objective function F(μ i ) is expressed as:

[0060]

[0061] Specific implementation method 4: This implementation method is a further explanation of specific implementation method 3. The difference between this implementation method and specific implementation method 3 is that the conjugate gradient method is expressed as:

[0062]

[0063] in, represents the step size of the kth iteration of the ith light wave, which can be obtained by one-dimensional search, It represents the descent direction of the kth iteration of the i-th light wave, which is determined by the current objective function gradient and the descent direction of the previous iteration.

[0064] Specific implementation mode 5: This implementation mode is a further explanation of specific implementation mode 4. The difference between this implementation mode and specific implementation mode 4 is that the spectral parameter field includes the absorption coefficient μ a Spectral parameter field and scattering coefficient μ s Spectral parameter field.

[0065] According to the time domain wide spectrum double-layer medium spectral parameter measurement technology, it is characterized in that the multi-parameter field μ of the medium to be measured to be reconstructed includes the absorption coefficient μ a and the scattering coefficient μ s Spectral parameter field, and the two parts of the parameter field are reconstructed simultaneously under the same spectrum.

[0066] Specific implementation example 6: This implementation example is a further description of specific implementation example 5. The difference between this implementation example and specific implementation example 5 is that the broadband picosecond pulse continuous light source 1 is a broadband picosecond pulse NKT continuous light source.

[0067] Specific implementation method 7: This implementation method is a further explanation of specific implementation method 6. The difference between this implementation method and specific implementation method 6 is that the broadband picosecond pulse continuous light source 1 outputs a laser with a spectral range of 400-2400nm.

[0068] Specific implementation example eight: This implementation example is a further description of specific implementation example seven. The difference between this implementation example and specific implementation example seven is that the photodetector 6 is a silicon photomultiplier tube.

[0069] It should be noted that the specific implementation is only an explanation and description of the technical solution of the present invention, and cannot be used to limit the scope of protection of the rights. Any partial changes made according to the claims and description of the present invention should still fall within the scope of protection of the present invention.

Claims

1. A method for measuring spectral parameter fields of double-layer media with a wide spectrum in a time domain with multi-distance detection, characterized in that The method is implemented based on a measuring device: The measuring device comprises: a broadband picosecond pulse continuous light source (1), a prism-based band selector (2), a variable neutral density attenuator (3), an optical switch (4), a photodetector (6), and a data acquisition and processing system (7); The method comprises the following steps: Step 1: Turn on the broadband picosecond pulse continuous light source (1). The light source output by the broadband picosecond pulse continuous light source (1) passes through a prism-based band selector (2) to obtain pulsed lasers of w different bands. The pulsed lasers enter a variable neutral density attenuator (3). After that, the light source output by the variable neutral density attenuator (3) enters an optical switch (4) through an optical fiber and irradiates the double-layer medium (5). By switching the optical switch (4), incident light sources at d different positions on the double-layer medium (5) are obtained. After each switching of the optical switch (4), the photodetector (6) collects an outgoing radiation signal of the double-layer medium (5). Then, the photodetector (6) sends all collected outgoing radiation signals to a data acquisition and processing system (7). The data acquisition and processing system (7) processes the outgoing radiation signal to obtain a spectral outgoing radiation signal emitted by the double-layer medium (5). Where, i=1,2,...w, j=1,2,...d, w≥2, d≥2; Step 2: Select the pulsed laser of the i-th band and set the initial distribution value of the spectral parameter field in the double-layer medium (5) to be Afterwards Substitute the time-domain diffusion approximation equation of the double-layer medium to obtain the initial value of the outgoing radiation signal at the medium boundary under the pulse laser incident in the i-th band That is, the outgoing radiation simulation signal, repeat the above steps to obtain the initial values ​​of the outgoing radiation signals of all medium boundaries under the pulse laser incident in the i-th band, and use the initial values ​​of the outgoing radiation signals of all medium boundaries and the spectral outgoing radiation signal in step 1 Get the initial objective function Step 3: Update the distribution value of the spectral parameter field in the double-layer medium (5) according to the conjugate gradient method to obtain the distribution value of the spectral parameter field obtained in the k-th iteration. It is expressed as: Where k represents the number of iterations, Δμ i represents the change in the spectral parameter field, represents the distribution value of the spectral parameter field obtained at the kth iteration, represents the distribution value of the spectral parameter field obtained after k-1 iterations; Step 4: Substitute the time-domain diffusion approximation equation of the double-layer medium to obtain the updated outgoing radiation signal of the medium boundary under the pulse laser incident in the i-th band Repeat the above steps to obtain all updated outgoing radiation signals of the medium boundary, and use all updated outgoing radiation signals of the medium boundary and the spectral outgoing radiation signal in step 1 to obtain the outgoing radiation signal of the medium boundary. Get the updated objective function F(μ i ), and finally determine the objective function F(μ i ) is less than the set minimum value ε, if the objective function F(μ i ) is less than the set minimum value ε, then execute step 6, otherwise, execute step 5; Step 5: Determine whether the number of iterations k reaches the maximum number of iterations K. If the number of iterations k reaches the maximum number of iterations K, use the current spectral parameter field distribution value as the initial value and re-execute step 2, that is, If the number of iterations k does not reach the maximum number of iterations K, execute step 3; Step 6: According to the current iteration result, the distribution value of the spectral parameter field in the i-th band is obtained; Step 7: Repeat steps 2 to 6 to obtain the distribution values ​​of all spectral parameter fields under w different bands, and use the conjugate gradient method to update the distribution values ​​of the spectral parameter fields until convergence, and obtain the distribution values ​​of the final spectral parameter fields under w different bands; The time domain diffusion approximation equation of the double-layer medium is expressed as: Among them, D i (r) represents the diffusion coefficient, is the absorption coefficient, represents the effective scattering coefficient, represents the sign of partial differential, r represents position, c represents the speed of light in the tissue medium, t represents time, S i (r, t) represents the radiation source term introduced by the incident laser, g i represents the scattering asymmetry factor, represents the scattering coefficient, G i (r, t) represents the projected radiation force and n represents the refractive index.

2. The method for measuring spectral parameter field of double-layer medium with time domain wide spectrum and multi-distance detection according to claim 1 is characterized in that The initial objective function It is expressed as: in, is the spectral parameter field of the target side, is the absorption coefficient, represents the scattering coefficient.

3. The method for measuring spectral parameter field of double-layer medium with time domain wide spectrum and multi-distance detection according to claim 2 is characterized in that The objective function F(μ i ) is expressed as:

4. The method for measuring spectral parameter field of double-layer medium with time domain wide spectrum and multi-distance detection according to claim 3 is characterized in that The conjugate gradient method is expressed as: in, represents the step size of the kth iteration of the ith light wave, Indicates the descending direction of the kth iteration of the i-th light wave.

5. The method for measuring spectral parameter field of double-layer medium with time domain wide spectrum and multi-distance detection according to claim 1 is characterized in that The spectral parameter field includes the absorption coefficient μ a Spectral parameter field and scattering coefficient μ s Spectral parameter field.

6. The method for measuring spectral parameter field of double-layer medium with time domain wide spectrum and multi-distance detection according to claim 1 is characterized in that The broadband picosecond pulse continuous light source (1) is a broadband picosecond pulse NKT continuous light source.

7. The method for measuring spectral parameter field of double-layer medium with time domain wide spectrum and multi-distance detection according to claim 1 is characterized in that The broadband picosecond pulse continuous light source (1) outputs laser light with a spectral range of 400-2400nm.

8. The method for measuring spectral parameter field of double-layer medium with time domain wide spectrum and multi-distance detection according to claim 1 is characterized in that The photodetector (6) is a silicon photomultiplier tube.

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