Visible light wide-spectrum polarization imaging system and method based on liquid crystal variable phase delayer

Abstract

A visible light wide-spectrum polarization imaging system based on a liquid crystal variable phase retarder comprises a light source unit, and a polarization modulation unit, an imaging unit and a polarization information processing and correcting unit are sequentially arranged in the light emitting direction of the light source unit. The polarization modulation unit comprises a first achromatic 1 / 4 wave plate, a liquid crystal variable phase delayer and a second achromatic 1 / 4 wave plate which are sequentially arranged in the light emitting direction of a simulated sunlight source; the imaging unit comprises a first linear polarizer and a color sensor which are sequentially arranged in the light emitting direction of the second achromatic 1 / 4 wave plate, and is used for acquiring a polarization image of a to-be-detected target; the polarization information processing and correcting unit comprises an upper computer and is used for correcting the wide spectrum response of the liquid crystal variable phase delayer; compared with a traditional mechanical modulation system, the device and the imaging method have the advantages of no mechanical motion, high modulation speed and wide spectrum adaptability, and can be widely applied to the fields of biological tissue imaging, material characterization, industrial detection and the like.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical imaging, and in particular relates to a visible light wide-band polarization imaging system and an imaging method based on a liquid crystal variable phase retarder. Background Art

[0002] Polarization imaging is a novel computational optical imaging method that analyzes the polarization distribution of reflected or transmitted light from a target to determine its physical properties, including its material, morphology, and surface roughness. Based on the mathematical framework of Stokes vectors and Mueller matrices, this technology can quantitatively extract parameters such as the degree of polarization (DoP) and angle of polarization (AoP), breaking through the information dimensionality limitations of traditional intensity imaging. Its core advantages lie in its label-free detection capabilities and microstructure sensitivity. Typical applications include biomedicine and tissue engineering, industrial testing and materials characterization, remote sensing monitoring, and extreme environment imaging.

[0003] For example, abnormal cell density in tumor tissue can lead to enhanced depolarization. Polarization differential imaging (PDI) can be used to achieve noninvasive screening for early-stage cancer [SL Jacques, "Polarized light imaging of biological tissues," Proc. SPIE, 2013.]; in 2020, a full-field polarization OCT system developed by a team at the Massachusetts Institute of Technology successfully achieved three-dimensional reconstruction of corneal collagen fibers, providing a new tool for diagnosing ophthalmic diseases [MIT News, "New polarization-sensitive OCT system captures 3D images of collagen," 2020.]; in optical thin film manufacturing, polarization imaging can non-contactly measure the thickness and refractive index distribution of multilayer films by analyzing the ellipticity angle changes of reflected light, with sub-nanometer accuracy [ZEISS Technical White Paper, "Application of Lumera Polarization Interferometer in Optical Coating Inspection," 2018.]; Shao Xiaopeng's team at Xidian University further established a physical model of underwater polarization transmission, achieving target reconstruction under conditions without active illumination [Shao Xiaopeng et al., "Underwater Polarization Imaging Method Based on Physical Model," Acta Optica Sinica, 2019.].

[0004] However, traditional time-sharing polarization imaging systems rely on mechanically rotating wave plates or switching polarizer angles, which inherently hinders them, including slow modulation speeds (typically >100ms / frame), image misalignment, and sensitivity to environmental vibrations. For example, the MSPI system deployed by NASA in 2005, while capable of multi-angle polarization detection, its mechanical modulation structure resulted in a frame rate of less than 5Hz, making it difficult to meet the demands of dynamic scenes [DJ Diner et al., "MSPI: A multiangle imaging spectropolarimeter for aerosol characterization," IEEE TGRS, 2007].

[0005] To address these bottlenecks, electronic modulation technology based on liquid crystal variable retarders (LCVRs) has become a research hotspot. LCVRs achieve rapid switching of phase retardation through electrically controlled birefringence, eliminating the need for mechanical moving parts. Key technological breakthroughs include a wide-band correction method. Li Kewu's team at North University of China proposed a collaborative modulation scheme combining LCVRs with an acousto-optic tunable filter (AOTF), achieving full Stokes parameter resolution with a spectral resolution of 10 nm in the 450-650 nm range [Li Kewu et al., "Hyperspectral Polarization Imaging System Based on LCVR and AOTF," China Laser, 2017]. Another innovation in system integration is the dual-LCVR orthogonal beam splitting system designed by Yang Wei et al. in 2021. This system increases the imaging frame rate to 60 Hz through dual-CMOS simultaneous acquisition. However, due to the wavelength dependence of the liquid crystal material, nonlinear errors still exceed 8% across the wide spectral range [Yang Wei et al., "Design of a Dual-LCVR Orthogonal Beam Splitting Polarization Imaging System," Optical Precision Engineering, 2021]. It is clear that the core challenge of current LCVR systems lies in controlling the consistency of phase retardation for wide-band imaging.

[0006] Based on this, and addressing the bottleneck problem of LCVR wide-band polarization imaging, the present invention proposes a visible light (450nm-650nm) wide-band polarization imaging system device and method based on a liquid crystal variable phase retarder (LCVR). By accurately measuring and correcting the LCVR's phase-locking characteristics and combining it with a multi-band polarization correction algorithm, polarization imaging in a wide spectral range of 450nm-650nm is achieved. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to overcome the defects of the above-mentioned prior art and provide a visible light wide-band polarization imaging system based on a liquid crystal variable phase retarder, which is used to realize polarization imaging measurement of visible light wide-band without moving parts, so as to avoid the problems of rotation error and poor reliability caused by moving parts in the polarization imaging system, improve the stability and detection accuracy of the system, and provide a corresponding imaging method.

[0008] The technical solution adopted to solve the above technical problems is: a visible light broadband polarization imaging system based on a liquid crystal variable phase retarder, comprising a light source unit, wherein the light source unit is provided with a polarization modulation unit, an imaging unit, and a polarization information processing and correction unit in the emission direction thereof;

[0009] The light source unit includes a simulated sunlight source for providing illumination in a broad spectrum of visible light with a wavelength λ of 450nm to 650nm;

[0010] The polarization modulation unit includes a first achromatic quarter-wave plate, a liquid crystal variable phase retarder, and a second achromatic quarter-wave plate, which are arranged in sequence to simulate the emission direction of sunlight.

[0011] The imaging unit includes a first linear polarizer and a color sensor sequentially arranged in the light emitting direction of the second achromatic quarter-wave plate, and is used to collect a polarization image of the target to be measured;

[0012] The polarization information processing and correction unit includes a host computer for correcting the wide-band response of the liquid crystal variable phase retarder to ensure that each wavelength in the wide-band visible light achieves the same phase delay under the same conditions.

[0013] The fast axis directions of the first achromatic quarter-wave plate and the second achromatic quarter-wave plate of the present invention are consistent, both are horizontal, and maintain an angle of 45° with the fast axis of the liquid crystal variable phase retarder.

[0014] The first linear polarizing plate of the present invention is placed with its light transmission axis in a horizontal direction.

[0015] The wavelength range of the simulated sunlight source of the present invention is 280nm-2500nm, and the simulation uncertainty is 12%.

[0016] The imaging method of the visible light broadband polarization imaging system based on the liquid crystal variable phase retarder of the present invention comprises the following steps:

[0017] Step 1: Start the simulated sunlight source, adjust the simulated sunlight source to provide a continuous spectrum similar to natural sunlight, and ensure that the light source is stable;

[0018] Step 2: Place the target to be measured directly in front of the optical path of the imaging unit, ensure that the target to be measured is in the optimal imaging position of the optical system, and adjust the optical path of the device so that the incident light is perpendicular to the target surface;

[0019] Step 3: Start the polarization modulation unit and change the phase delay of the scene reflected light passing through the optical path by the driving voltage of the liquid crystal variable phase retarder, so that the phase delay is φ=0, The reflected light of the target scene to be measured is modulated into three different polarization states in sequence; after passing through the polarization modulation unit composed of the first achromatic quarter-wave plate, the liquid crystal variable phase retarder, and the second achromatic quarter-wave plate, it is irradiated to the focal plane of the color sensor;

[0020] Step 4: Use the color sensor to collect images of different polarization states of the target I a , each set of two-dimensional image data contains three image information I(φ), where φ is the phase delay, which is 0,

[0021] Step 5: Collect the background response image data I of the system b , the collection method is the same as step 4;

[0022] Step 6: The collected two-dimensional image data I a and background response image data I b The image is transmitted to the polarization information processing and correction unit for processing and correction, and the corrected image is used for polarization information processing to achieve high-precision polarization imaging in a wide visible light band.

[0023] Step 6 of the present invention comprises the following steps:

[0024] Step 6.1: Collect single-wavelength two-dimensional image data containing target polarization information through steps 4 and 5. and background response image data I b , using the polarization information processing unit to and I b Perform differential processing to obtain the denoised polarization intensity data I at each wavelength. λ :

[0025] I λ =I λ a -I b

[0026] Where λ is the wavelength, which can be 450, 455...645, 650 nm.

[0027] Step 6.2: The polarization information processing unit is used to integrate the polarization azimuth angles of each band affected by the polarization modulation unit that have been measured and calibrated in advance, and the three phase delays are obtained as follows: φ = 0, The curve of the full-band polarization azimuth angle changing with wavelength;

[0028] Step 6.3: Denoised polarization state parameter I obtained in step 6.1 λ , determine the Stokes vectors S0, S1, S2 obtained by the Mueller matrix correction method:

[0029]

[0030] Where, is the phase delay The corresponding light intensity value, i takes 0, 1, 2, which respectively means the phase delay is φ = 0, Three situations, Directly collected by the color sensor; C mn is the algebraic cofactor of the matrix M, where m is 1, 2, or 3, n is 1, 2, or 3, det(M) is the determinant of the matrix M, and the matrix M is The coefficient matrix of S0, S1, S2;

[0031] Step 6.4: The image data whose polarization state is changed by the polarization modulation unit Denoted as I SET , set up a control group to collect image data that is not affected by the polarization modulation unit under the same conditions Denoted as I FIR , and perform denoising in step 6.1 to obtain I′ SET and I′ FIR Transmitted to the host computer, the RGB three-color channel correction is performed using Malus's law, and the phase delay correction coefficient β of the RGB three-color channel is ij Determined by the following formula:

[0032]

[0033] Where i is 0, 1, and 2, which correspond to phase delays of φ=0, j represents any one of the three color channels R, G, and B;

[0034] Step 6.5: Get the correction coefficient β corresponding to the RGB three color channels from step 6.4 ij , use the color channel correction coefficient to correct the data I obtained in step 6.1 λ Perform channel correction and determine the Stokes vectors S′0, S′1, S′2 obtained by the color channel correction method:

[0035]

[0036] Where, β 0j , β 1j , β 2j Indicates that the phase delay is φ = 0, The correction coefficient of the phase delay of any one of the three RGB color channels corresponding to the time is determined by step 6.4; λ (β 0j ), I λ (β 1j ), Iλ (β 2j ) is β 0j , β 1j , β 2j The corresponding light intensity component is measured experimentally; C' mn The algebraic cofactor of the matrix M′, det(M′) is the determinant of the matrix M′, and the matrix M′ is I λ (β 0j ), I λ (β 1j ), I λ (β 2j ) The coefficient matrix of S′0, S′1, S′2;

[0037] Step 6.6: Based on the Stokes vectors S0, S1, and S2 obtained in step 6.3, determine the degree of polarization DoP and the angle of polarization AoP obtained by the Mueller matrix correction method:

[0038]

[0039] Where S 0cal is the total light intensity after correction by Mueller matrix correction method, i.e. the sum of polarized light and unpolarized light, S 1cal It represents the difference between the horizontal and vertical polarization components of the light after correction by the Mueller matrix correction method, S 2cal It represents the difference in polarization components of light at 45° and -45° after correction by the Mueller matrix correction method, S 3cal Represents the difference in circular polarization components of light after correction by the Mueller matrix correction method;

[0040] Step 6.7: Based on the Stokes vectors S′0, S′1, and S′2 obtained in step 6.5, determine the degree of polarization DoP′ and the angle of polarization AoP′ obtained by the color channel correction method:

[0041]

[0042] Among them, S′ 0cal , S′ 1cal , S′ 2cal is the Stokes vector corrected by the color channel correction method;

[0043] Step 6.8. Compare the results obtained in step 6.6 and step 6.7, output the more optimal polarization degree and polarization angle in the dual-path analysis results, construct the wide-band polarization imaging data of the target to be measured based on the optimized polarization parameter distribution, and generate polarization degree and polarization angle diagrams of the target to be measured at different wavelengths; realize the display of the wide-band polarization characteristics of the target through visualization processing, including polarization degree distribution and polarization angle distribution.

[0044] The matrix M in step 6.3 of the present invention is:

[0045]

[0046] Where, The phase delay is φ=0, The experimentally obtained phase delay with error corresponding to is determined by the following formula:

[0047]

[0048] Where, is the actual Mueller matrix of the liquid crystal variable phase retarder, 2θ is the angle between the optical axis of the liquid crystal variable phase retarder and the horizontal reference axis, which is 45°; is the Mueller matrix of the first achromatic quarter-wave plate, 2θ1 is the angle between the optical axis of the first achromatic quarter-wave plate and the horizontal reference axis, which is 0°; is the Mueller matrix of the second achromatic quarter-wave plate, 2θ2 is the angle between the optical axis of the second achromatic quarter-wave plate and the horizontal reference axis, which is 0°; M total is the actual Mueller matrix of the polarization modulation unit, which is as follows:

[0049]

[0050] Comparing the experimentally measured Mueller matrix with the above formula, the specific

[0051] The matrix M′ in step 6.5 of the present invention is:

[0052]

[0053] In step 6.6 of the present invention, S 3cal =0;S 0cal , S 1cal , S 2cal Determined by the following formula:

[0054]

[0055] Where, is the phase delay The deviation angle of polarization azimuth at each wavelength is measured by calibration experiment. is the phase delay The light intensity weight proportional coefficient at each wavelength is determined by the following formula:

[0056]

[0057] In step 6.7 of the present invention, S' 3cal =0; S′0cal , S′ 1cal ,S′ 2cal Determined by the following formula:

[0058]

[0059]

[0060] Where S′ 0cal is the total light intensity after correction by color channel correction method, i.e. the sum of polarized light and unpolarized light, S′ 1cal is the difference between the horizontal and vertical polarization components of the light after correction by the color channel correction method, S′ 2cal The polarization component difference of the light at 45° and -45° directions after correction by the color channel correction method.

[0061] Compared with the prior art, the present invention has the following advantages:

[0062] 1. The present invention provides a wide-band visible light polarization imaging system based on a liquid crystal variable phase retarder (LCVR). This system utilizes the LCVR to dynamically modulate the phase delay of light, thus avoiding the vibration errors introduced by traditional mechanical rotating components. The system has a simple overall structure and has the advantages of high stability and fast modulation speed, while also improving the system's environmental adaptability.

[0063] 2. The system of the present invention provides a wide-band visible light output in the range of 450nm to 650nm by simulating a solar light source. Combined with a polarization modulation unit, an imaging unit, and a polarization information processing unit, it can efficiently collect and correct the polarization information of the LCVR within the wide-band visible light spectrum, providing a reliable guarantee for high-precision polarization measurement.

[0064] 3. The visible light wide-band polarization imaging method based on liquid crystal variable phase retarder (LCVR) provided by the present invention, combined with the polarization difference algorithm and the phase-change characteristic correction model, can eliminate the nonlinear response error of the system. Through the correction of the RGB three-color space and the integration of multi-wavelength data, it can achieve accurate measurement of the degree of polarization (DOP) and angle of polarization (AOP) of the target in the visible light wide-band range.

[0065] In summary, compared with traditional mechanical modulation systems, this device has the advantages of no mechanical motion, high modulation speed, and wide spectral adaptability, and can be widely used in biological tissue imaging, material characterization, and industrial detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0066] Figure 1 It is a structural diagram of an embodiment of the present invention.

[0067] Figure 2This is an experimental optical path diagram used by the present invention to verify the applicability of Malus's law to correct the intensity component of a wide spectrum of visible light in a liquid crystal variable phase retarder.

[0068] Figure 3 The diagram is a schematic diagram of the experimental optical path and working principle for measuring the Mueller matrix of the polarization modulation unit according to the present invention.

[0069] Figure 4 The diagram is a schematic diagram of the experimental optical path and working principle for measuring the 3V phase characteristic of the liquid crystal variable phase retarder according to the present invention.

[0070] Figure 5 The graph is a graph showing the change in polarization rotation angle of the reflected light from the scene passing through the liquid crystal variable phase retarder 3. (a) shows the change in polarization rotation angle when the phase delay of the liquid crystal variable phase retarder 3 is φ=0, and (b) shows the change in polarization rotation angle when the phase delay of the liquid crystal variable phase retarder 3 is φ=0. The polarization rotation angle change curve of the liquid crystal variable phase retarder 3 is The curve of the polarization rotation angle change when .

[0071] Figure 6 This is a simulated polarization degree reference diagram of the final polarization imaging result in an embodiment of the visible light wide-band polarization imaging device based on a liquid crystal variable phase retarder (LCVR) of the present invention, where P is the polarization degree, R, G, and B are the red, green, and blue three-channel color spaces, respectively. The background is unpolarized light, and the polarization degree is shown as P=0. "SNNU" represents the measured target, and its polarization degree is P=1.

[0072] Figure 7 This is a simulated polarization degree calculation correction diagram for the final polarization imaging result of the present invention, where P is the polarization degree, R, G, and B are the red, green, and blue three-channel color spaces respectively, and the background is unpolarized light. As shown in the figure, the polarization degree is P=0, and "SNNU" represents the measured target, whose polarization degree is P=1.

[0073] Figure 8 This is a simulated polarization azimuth reference diagram of the final polarization imaging result of the present invention, where A is the polarization azimuth, R, G, and B are the red, green, and blue three-channel color spaces, respectively. The background is unpolarized light with no polarization azimuth. "SNNU" represents the measured target, whose polarization azimuth A=1.

[0074] Figure 9 This is a simulated polarization azimuth calculation correction diagram for the final polarization imaging result of the present invention, where A is the polarization azimuth, R, G, and B are the red, green, and blue three-channel color spaces, respectively. The background is unpolarized light with no polarization azimuth. "SNNU" represents the measured target, whose polarization azimuth A=1.

[0075] Figure 10 These are uncorrected polarization degree and polarization angle diagrams for performing wide-band polarization imaging using the optical path device described in this embodiment, where the left diagram is the polarization degree diagram and the right diagram is the polarization angle diagram.

[0076] Figure 11 These are the polarization degree and polarization angle diagrams obtained by using the optimized color channel correction method after performing wide-band polarization imaging using the optical path device described in this embodiment. The left figure is the polarization degree diagram, and the right figure is the polarization angle diagram.

[0077] In the figure: 1. Simulated sunlight source; 2. First achromatic 1 / 4 wave plate; 3. Liquid crystal variable phase retarder; 4. Second achromatic 1 / 4 wave plate; 5. First linear polarizer; 6. Color sensor; 7. Host computer; 8. Polarizer; 9. 50:50 spectroscope; 10. Polarizer; 11. Power meter; 12. Second linear polarizer; 13. Third achromatic 1 / 4 wave plate; 14. Fourth achromatic 1 / 4 wave plate; 15. Third linear polarizer; 16. Analyzer. DETAILED DESCRIPTION

[0078] The present invention will be further described in detail below with reference to the accompanying drawings and examples, but the present invention is not limited to these examples.

[0079] Example 1

[0080] exist Figure 1 The present invention relates to a visible light wide-band polarization imaging system based on a liquid crystal variable phase retarder, comprising a light source unit, wherein a polarization modulation unit, an imaging unit, and a polarization information processing and correction unit are sequentially arranged in the emission direction of the light source unit;

[0081] Specifically, the light source unit includes a simulated solar light source 1 for providing a wide spectrum of visible light with a wavelength λ of 450nm to 650nm. It can provide a continuous spectrum similar to natural sunlight to be suitable for polarization characteristic measurement of different targets. The wavelength range of the simulated solar light source 1 is 280nm to 2500nm, and the simulation uncertainty is 12%, which meets the spectral mismatch Class B standard in JJF 1615-2017.

[0082] The polarization modulation unit includes a first achromatic quarter-wave plate 2, a liquid crystal variable phase retarder 3, and a second achromatic quarter-wave plate 4, which are arranged in sequence to simulate the light emission direction of the sunlight source 1; the first achromatic quarter-wave plate 2 and the second achromatic quarter-wave plate 4 are used to change the phase delay of the incident light and convert linearly polarized light into different polarization states, including linearly polarized light, circularly polarized light, and elliptically polarized light; the liquid crystal variable phase retarder 3 is a nematic liquid crystal (NLC), and the liquid crystal variable phase retarder 3 is loaded with three different voltages, namely a volt (the voltage when the phase delay of the liquid crystal variable phase retarder 3 is φ=0), b volt (the voltage when the phase delay of the liquid crystal variable phase retarder 3 is φ=0), and b volt (the voltage when the phase delay of the liquid crystal variable phase retarder 3 is φ=0). voltage at the time of ) and c volts (making the phase delay of the liquid crystal variable phase retarder 3 The voltage at the time of the change is used to adjust the orientation of the liquid crystal molecules and achieve precise phase delay control. Its response time is less than 10ms, and the delay range is from 0 to π, which is suitable for fast modulation of multiple polarization states in a wide spectrum. In order to reduce the impact of ambient temperature changes on the performance of the liquid crystal retarder 3, a uniform ambient temperature of 23°C is adopted in the laboratory. The first achromatic 1 / 4 wave plate 2, the second achromatic 1 / 4 wave plate 4 and the liquid crystal variable phase retarder 3 are combined to achieve precise rotation of the polarization state of the scene reflected light and generate output light beams with different polarization states to meet the measurement requirements of multiple scenes. During the experiment, the target is placed in front of the optical path, at the optimal imaging position of the optical path. The fast axes of the first achromatic 1 / 4 wave plate 2 and the second achromatic 1 / 4 wave plate 4 are consistent in direction, both horizontal, and maintain a 45° angle with the fast axis of the liquid crystal variable phase retarder 3, which is used to assist the liquid crystal variable phase retarder 3 in achieving high-precision rotation and modulation of the polarization state.

[0083] The imaging unit includes a first linear polarizer 5 and a color sensor 6 arranged in sequence in the light emitting direction of the second achromatic 1 / 4 wave plate 4, which are used to collect polarization images of the target to be measured; specifically, the first linear polarizer 5 is located at the light emitting end of the polarization modulation unit, and is placed to ensure that the transmission axis direction is horizontal, and is used to selectively transmit the modulated light beam, filter out light in non-target polarization states, and suppress the interference of stray light on imaging; the color sensor 6 is close to the light emitting end of the first linear polarizer 5, and the focal plane of the color sensor 6 coincides with the transmitted light path of the first linear polarizer 5, and collects light intensity images of the target to be measured in different polarization states. The photosensitive chip of the color sensor 6 receives the modulated light beam and completes image recording, and the imaging resolution can meet the requirements of high-precision polarization measurement in a wide spectrum band. The color sensor 6 transmits the collected light intensity image to the polarization information processing and correction unit for further processing and analysis.

[0084] The polarization information processing and correction unit includes a host computer 7, the polarization information processing unit uses a multi-wavelength Stokes vector solution algorithm to convert the three collected polarization state intensity images into a target non-biased wide-band polarization state intensity image; the correction unit combines Figure 4 The measured voltage-phase characteristic data and Figure 3 The Mueller matrix data of the polarization modulation unit is measured to correct the nonlinear response error of the liquid crystal variable phase retarder at different wavelengths to ensure the accuracy of polarization parameters in a wide spectral range. Figure 3 In the embodiment, the polarizer is composed of a second linear polarizer 12 and a third achromatic 1 / 4 wave plate. First, the unpolarized light emitted from the light source first passes through the combination of the second linear polarizer 12 and the third achromatic 1 / 4 wave plate 13 to generate linearly polarized light. The polarizer is adjusted to four polarization states of 0°, 45°, 90° linear polarization and right-handed circular polarization (that is, when the second linear polarizer 12 is 0°, the third achromatic 1 / 4 wave plate 13 is placed horizontally, when the second linear polarizer 12 is 45°, the third achromatic 1 / 4 wave plate 13 is placed at 45°, when the second linear polarizer 12 is 90°, the third achromatic 1 / 4 wave plate 13 is placed at 90°, and when the second linear polarizer 12 is 0°, the third achromatic 1 / 4 wave plate 13 is placed at 45°). The four different polarization states of light are sequentially passed through the polarization modulation unit. In the polarization modulation unit, the polarized light sequentially passes through the first achromatic 1 / 4 wave plate 2, the liquid crystal variable phase retarder 3, and the second achromatic 1 / 4 wave plate 4. The phase delay amount of the liquid crystal variable phase retarder 3 is changed by applying an external voltage to the liquid crystal variable phase retarder 3. To achieve precise control of the polarization state of light; the modulated light beam is then passed through the analyzer for polarization state analysis. The analyzer is composed of a fourth achromatic quarter wave plate 14 and a third linear polarizer 15. The analyzer is also adjusted to four polarization states: 0°, 45°, 90° linear polarization and right circular polarization (when the third linear polarizer 15 is 0°, the fourth achromatic quarter wave plate 14 is placed horizontally; when the third linear polarizer 15 is 45°, the fourth achromatic quarter wave plate 14 is placed at 45°, and the third linear polarizer 15 is 90°). When the plate 15 is at 90°, the fourth achromatic 1 / 4 wave plate 14 is placed at 90°, and when the third linear polarizer 15 is at 0°, the fourth achromatic 1 / 4 wave plate 14 is placed at 45° to form a complete polarization combination; each polarized light state is combined with each polarized light state to obtain light intensity measurement values ​​under 16 polarization configurations. In this process, the polarimeter 10 is used to record each set of light intensity data, and the Mueller matrix of the polarization modulation unit is inverted according to the Stokes vectors of the incident light and the outgoing light using the Mueller matrix theory.

[0085] The imaging method of the above-mentioned visible light broadband polarization imaging system based on liquid crystal variable phase retarder comprises the following steps:

[0086] Step 1: Start the simulated sunlight source 1, adjust the simulated sunlight source 1 to provide a continuous spectrum similar to natural sunlight, and ensure that the light source is stable;

[0087] Step 2: Place the target to be measured directly in front of the optical path of the imaging unit, ensure that the target to be measured is in the optimal imaging position of the optical system, and adjust the optical path of the device so that the incident light is perpendicular to the target surface;

[0088] Step 3: Start the polarization modulation unit and change the phase delay of the scene reflected light passing through the optical path by the driving voltage of the liquid crystal variable phase retarder 3, and adjust its voltage to a volt (using Figure 4 The device measures the phase characteristics of the liquid crystal variable phase retarder at 3 volts and obtains the voltage when the phase retardation is φ = 0), and obtains the light intensity component diagram filtered by the analyzer when the liquid crystal phase retardation is 0; then adjusts its voltage to b volts (using Figure 4 The device measures the phase characteristics of the liquid crystal variable phase retarder at 3 volts and the phase delay is: The voltage at the time of the liquid crystal phase delay is 45, and the light intensity component diagram filtered by the analyzer is obtained; finally, the voltage is adjusted to c volts (using Figure 4 The device measures the phase characteristics of the liquid crystal variable phase retarder at 3 volts and the phase delay is: The voltage at the time of the liquid crystal phase delay is 90, and the light intensity component diagram filtered by the analyzer is obtained. The ideal light intensity component expression is as follows:

[0089]

[0090] The reflected light from the target scene to be measured is modulated into three specified polarization states in sequence after passing through the polarization modulation unit composed of the first achromatic quarter-wave plate 2, the liquid crystal variable phase retarder 3, and the second achromatic quarter-wave plate 4, and is irradiated onto the focal plane of the color sensor 6;

[0091] Step 4: Use the color sensor 6 to collect images of different polarization states of the target. a , each set of two-dimensional image data contains three image information I(φ), where φ is the phase delay, which is 0,

[0092] Step 5: Collect the background response image data I of the system b , the acquisition method is the same as step 4; in order to effectively eliminate system noise and reduce environmental interference, thereby ensuring the accuracy and reliability of the measurement data.

[0093] Step 6: The collected two-dimensional image data I a and background response image data I bThe image is transmitted to the polarization information processing and correction unit for processing and correction, and the corrected image is used for polarization information processing to achieve high-precision polarization imaging in a wide visible light band.

[0094] The specific steps are as follows:

[0095] Step 6.1: Collect single-wavelength two-dimensional image data containing target polarization information through steps 4 and 5. and background response image data I b , using the polarization information processing unit to and I b Perform differential processing to obtain the denoised polarization intensity data Ix for each set of wavelengths:

[0096]

[0097] Where λ is the wavelength, which can be 450, 455...645, 650 nm.

[0098] Step 6.2: The polarization information processing unit is used to integrate the polarization azimuth angles of each band affected by the polarization modulation unit that have been measured and calibrated in advance, and the three phase delays are obtained as follows: φ = 0, The curve of the full-band polarization azimuth angle changing with wavelength is shown in the figure below. Figure 5 As shown;

[0099] Step 6.3: Denoised polarization state parameter I obtained in step 6.1 λ , determine the Stokes vectors S0, S1, S2 obtained by the Mueller matrix correction method:

[0100]

[0101] Where, is the phase delay The corresponding light intensity value, i takes 0, 1, and 2, which respectively represent the phase delay of φ = 0, Three situations, Directly collected by the color sensor 6; C mn is the algebraic cofactor of the matrix M, where m is 1, 2, or 3, n is 1, 2, or 3, det(M) is the determinant of the matrix M, and the matrix M is The coefficient matrix of S0, S1, S2; the matrix M is:

[0102]

[0103] Where, The phase delay is φ=0, The experimentally obtained phase delay with error corresponding to is determined by the following formula:

[0104]

[0105] Where, is the actual Mueller matrix of the liquid crystal variable phase retarder 3, 2θ is the angle between the optical axis of the liquid crystal variable phase retarder 3 and the horizontal reference axis, which is 45°; is the Mueller matrix of the first achromatic quarter-wave plate 2, 2θ1 is the angle between the optical axis of the first achromatic quarter-wave plate 2 and the horizontal reference axis, which is 0°; is the Mueller matrix of the second achromatic quarter-wave plate 4, 2θ2 is the angle between the optical axis of the second achromatic quarter-wave plate 4 and the horizontal reference axis, which is 0°; M total is the actual Mueller matrix of the polarization modulation unit, which is as follows:

[0106]

[0107] Comparing the experimentally measured Mueller matrix with the above formula, the specific

[0108] Step 6.4: The image data whose polarization state is changed by the polarization modulation unit Denoted as I SET , set up a control group to collect image data that is not affected by the polarization modulation unit under the same conditions Denoted as I FIR , and perform denoising in step 6.1 to obtain I′ SET and I′ FIR Transmit to host computer 7, use Figure 2 Malus's law is used to calibrate the RGB three-color channels, correcting the polarization response errors of different wavelengths to the ideal state, effectively improving the accuracy of polarization measurement in a wide spectrum. The phase delay correction coefficient β of the RGB three-color channels is ij Determined by the following formula:

[0109]

[0110] Where i is 0, 1, and 2, which correspond to phase delays of φ=0, j represents any one of the three color channels R, G, and B;

[0111] Specifically, using Figure 2 The device, Figure 5The rotation angles of the polarized light passing through the liquid crystal variable phase retarder 3 at three different phase retardation values ​​were measured and verified. The unpolarized light emitted from the light source passes through the polarizer 8, generating linearly polarized light in a specific direction. The polarizer 8 is adjusted to 0° and remains unchanged. The direction of the polarizer 8 is then adjusted, with a range of 0° to 180°, and the polarizer 8 is rotated every 10°, starting from 0°, so that the polarization direction of the incident light is fixed to the reference direction. The polarized light passes through the polarization modulation unit module, and the retardation value of the liquid crystal variable phase retarder 3 is adjusted to a volt (using an external voltage). Figure 4 The device measures the phase characteristics of the liquid crystal variable phase retarder at 3 volts and obtains the voltage when the phase delay is φ = 0), and then adjusts the voltage to b volts (using Figure 4 The device measures the phase characteristics of the liquid crystal variable phase retarder at 3 volts and the phase delay is: The voltage at the time of the test is adjusted to c volts (using Figure 4 The device measures the phase characteristics of the liquid crystal variable phase retarder at 3 volts and the phase delay is: The voltage at the time of the polarization is set (the voltage at the time of the polarization is set), causing the polarized light to produce three different phase delays after passing through this portion, thereby obtaining three sets of light intensity data with different delay amounts. Furthermore, the polarized light passes through a 50:50 beamsplitter 9, with part of the beam entering a power meter 11 for intensity measurement, while the remaining portion enters an analyzer 16 as a control group. The resulting light intensity data can be used to verify whether Malus's law meets the experimental requirements.

[0112] Step 6.5: Get the correction coefficient β corresponding to the RGB three color channels from step 6.4 ij , use the color channel correction coefficient to correct the data I obtained in step 6.1 λ Perform channel correction and determine the Stokes vectors S′0, S′1, S′2 obtained by the color channel correction method:

[0113]

[0114] Where, β 0j , β 1j , β 2j Indicates that the phase delay is φ = 0, The correction coefficient of the phase delay of any one of the three RGB color channels corresponding to the time is determined by step 6.4; λ (β 0j ), I λ (β 1j ), I λ (β 2j ) is β 0j , β 1j , β 2j The corresponding light intensity component is measured experimentally; C' mnThe algebraic cofactor of the matrix M′, det(M′) is the determinant of the matrix M′, and the matrix M′ is I λ (β 0j ), I λ (β 1j ), I λ (β 2j ) Regarding the coefficient matrix of S′0, S′1, S′2, the matrix M′ is:

[0115]

[0116] Step 6.6: Based on the Stokes vectors S0, S1, and S2 obtained in step 6.3, determine the degree of polarization DoP and the angle of polarization AoP obtained by the Mueller matrix correction method:

[0117]

[0118] Where S 0cal is the total light intensity after correction by Mueller matrix correction method, i.e. the sum of polarized light and unpolarized light, S 1cal It represents the difference between the horizontal and vertical polarization components of the light after correction by the Mueller matrix correction method, S 2cal It represents the difference in polarization components of light at 45° and -45° after correction by the Mueller matrix correction method, S 3cal Represents the difference in circular polarization components of light after correction by the Mueller matrix correction method;

[0119] In this experiment, S 3cal =0;S 0cal , S 1cal , S 2cal Determined by the following formula:

[0120]

[0121] Where, is the phase delay The deviation angle of polarization azimuth at each wavelength is measured by calibration experiment, and the results are as follows: Figure 5 As shown, is the phase delay The weight ratio coefficient of the light intensity at each wavelength to the total light intensity is determined by the following formula:

[0122]

[0123] Step 6.7: Based on the Stokes vectors S′0, S′1, and S′2 obtained in step 6.5, determine the degree of polarization DoP′ and the angle of polarization AoP′ obtained by the color channel correction method:

[0124]

[0125] Where S′ 0cal is the total light intensity after correction by color channel correction method, i.e. the sum of polarized light and unpolarized light, S′ 1cal is the difference between the horizontal and vertical polarization components of the light after correction by the color channel correction method, S′ 2cal The polarization component difference of the light at 45° and -45° after correction by the color channel correction method. 3cal =0; S′ 0cal , S′ 1cal , S′ 2cal Determined by the following formula:

[0126]

[0127] Where S′(j) is determined by step 6.5.

[0128] Step 6.8: Compare the results from Step 6.6 and Step 6.7, and output the dual-path analysis result with the better degree of polarization and angle of polarization. If a true reference value is available, the closer the result is to the true value, the better. If no true reference value is available, the following criteria can be used for judgment: Determine whether the degree of polarization or angle of polarization of the result set exceeds the physically reasonable range; if it does, the result is poor; whether the result graph shows sudden changes, discontinuities, or irregular jumps; a smoother distribution is preferred; whether the result shows a reasonable correlation between the degree of polarization and the angle of polarization, such as a concentrated distribution of polarization angles in high-degree-of-polarization regions; if one group has a concentrated distribution and the other has a random and disordered distribution, the former group is preferred; and whether the result graph is noisy, with a large amount of random variation; after comparison, the result with a lower noise level is preferred. Based on the optimized polarization parameter distribution, the wide-band polarization imaging data of the target to be measured is constructed, and polarization degree and polarization angle diagrams of the target to be measured at different wavelengths are generated. The filtering algorithm is used to reduce the noise signal in the data and improve the imaging quality. The optical compensation algorithm is applied to refine the corrected polarization imaging data to ensure the accuracy of the results at high resolution. Through visualization processing, the wide-band polarization characteristics of the target are displayed, including polarization degree distribution and polarization angle distribution.

[0129] The above step 6.8 can obtain the optimized wide-band polarization imaging results. The specific visualization results are as follows: Figures 6 to 9 As shown, Figure 6 and Figure 8 The reference diagram of the three-channel polarization degree and polarization angle simulation results is obtained by using the light intensity data obtained in step 6.1 to construct a full-band simulation diagram and then using the optimized color channel correction algorithm. Figure 7 and Figure 9 In order to construct a full-band simulation map using the light intensity data obtained in step 6.1 and then use the optimized color channel correction algorithm to obtain the three-channel polarization degree and polarization angle simulation result correction map, Figure 10and Figure 11 Comparative results show that the LCVR-based wide-band polarization imaging using this method achieves superior imaging quality and higher precision. Compared to traditional polarization imaging using mechanical rotating parts, this method offers faster imaging speeds, avoids manual rotation of mechanical parts, and reduces errors. This method effectively overcomes the bottleneck of LCVR being limited to single-wavelength polarization imaging, achieving high-precision polarization imaging over a wide band of visible light. Furthermore, compared to traditional mechanically modulated polarization imaging systems, this device offers the advantages of no mechanical motion, high modulation speed, and wide spectral adaptability, providing a practical and feasible technical path for wide-band polarization imaging without moving parts, improving its reliability and further broadening the application areas of polarization imaging technology.

Claims

1. A visible light broadband polarization imaging system based on a liquid crystal variable phase retarder, characterized by: It includes a light source unit, wherein a polarization modulation unit, an imaging unit, and a polarization information processing and correction unit are sequentially arranged in the emission direction of the light source unit; The light source unit comprises a simulated sunlight source (1) for providing illumination in a broad spectrum of visible light with a wavelength λ of 450nm to 650nm; The polarization modulation unit comprises a first achromatic quarter-wave plate (2), a liquid crystal variable phase retarder (3), and a second achromatic quarter-wave plate (4) which are arranged in sequence in the light emission direction of a simulated sunlight light source (1); The imaging unit comprises a first linear polarizing plate (5) and a color sensor (6) arranged in sequence in the light emitting direction of a second achromatic quarter-wave plate (4), and is used to collect a polarization image of a target to be measured; The polarization information processing and correction unit includes a host computer (7) for correcting the wide-band response of the liquid crystal variable phase retarder (3) to ensure that each wavelength in the wide-band visible light achieves the same phase delay under the same conditions.

2. The visible light broadband polarization imaging system based on a liquid crystal variable phase retarder according to claim 1, characterized in that: The fast axis directions of the first achromatic quarter-wave plate (2) and the second achromatic quarter-wave plate (4) are consistent, both are horizontal, and maintain an angle of 45° with the fast axis of the liquid crystal variable phase retarder (3).

3. The visible light broadband polarization imaging system based on a liquid crystal variable phase retarder according to claim 1, characterized in that: The first linear polarizing plate (5) is placed so as to ensure that the light transmission axis is horizontal.

4. The visible light broadband polarization imaging system based on a liquid crystal variable phase retarder according to claim 1, characterized in that: The wavelength range of the simulated sunlight source (1) is 280nm to 2500nm, and the simulation uncertainty is 12%.

5. The imaging method of the visible light broadband polarization imaging system based on liquid crystal variable phase retarder according to claim 1, characterized in that The following steps are involved: Step 1, starting the simulated sunlight source (1), adjusting the simulated sunlight source (1) to provide a continuous spectrum similar to natural sunlight, and ensuring that the light source is stable; Step 2: Place the target to be measured directly in front of the optical path of the imaging unit, ensure that the target to be measured is in the optimal imaging position of the optical system, and adjust the optical path of the device so that the incident light is perpendicular to the target surface; Step 3: Start the polarization modulation unit and change the phase delay of the scene reflected light passing through the optical path by the driving voltage of the liquid crystal variable phase retarder (3), so that the phase delay is φ=0, The light is modulated into three different polarization states in sequence; the reflected light of the target scene to be measured passes through a polarization modulation unit composed of a first achromatic quarter-wave plate (2), a liquid crystal variable phase retarder (3), and a second achromatic quarter-wave plate (4), and then irradiates the focal plane of the color sensor (6); Step 4: Use the color sensor (6) to collect images of different polarization states of the target to be measured. a , each set of two-dimensional image data contains three image information I(φ), where φ is the phase delay, which is 0, Step 5: Collect the background response image data I of the system b , the collection method is the same as step 4; Step 6: The collected two-dimensional image data I a and background response image data I b The image is transmitted to the polarization information processing and correction unit for processing and correction, and the corrected image is used for polarization information processing to achieve high-precision polarization imaging in a wide visible light band.

6. The imaging method of the visible light broadband polarization imaging system based on liquid crystal variable phase retarder according to claim 5, characterized in that The step 6 comprises the following steps: Step 6.1: Collect single-wavelength two-dimensional image data containing target polarization information through steps 4 and 5. and background response image data I b , using the polarization information processing unit to and I b Perform differential processing to obtain the denoised polarization intensity data I at each wavelength. λ : I λ =I λ a -I b Where λ is the wavelength, which can be 450, 455...645, 650 nm. Step 6.2: The polarization information processing unit is used to integrate the polarization azimuth angles of each band affected by the polarization modulation unit that have been measured and calibrated in advance, and the three phase delays are obtained as follows: φ = 0, The curve of the full-band polarization azimuth angle changing with wavelength; Step 6.3: Denoised polarization state parameter I obtained in step 6.1 λ , determine the Stokes vectors S0, S1, S2 obtained by the Mueller matrix correction method: Where, Phase delay The corresponding light intensity value, i takes 0, 1, 2, which respectively means the phase delay is Φ = 0, Three situations, Directly collected by the color sensor (6); C mn is the algebraic cofactor of the matrix M, where m is 1, 2, or 3, n is 1, 2, or 3, det(M) is the determinant of the matrix M, and the matrix M is The coefficient matrix of S0, S1, S2; Step 6.4: The image data whose polarization state is changed by the polarization modulation unit Denoted as I SET , set up a control group to collect image data that is not affected by the polarization modulation unit under the same conditions Denoted as I FIR , and perform denoising in step 6.1 to obtain I′ SET and I′ FIR Transmitted to the host computer (7), the RGB three-color channel correction is performed using Malus's law, and the phase delay correction coefficient β of the RGB three-color channel is ij Determined by the following formula: Where i is 0, 1, and 2, which correspond to phase delays of φ=0, j represents any one of the three color channels R, G, and B; Step 6.5: Get the correction coefficient β corresponding to the RGB three color channels from step 6.4 ij , use the color channel correction coefficient to correct the data I obtained in step 6.1 λ Perform channel correction and determine the Stokes vectors S0′, S1′, S2′ obtained by the color channel correction method: Where, β 0j , β 1j , β 2j Indicates that the phase delay is φ = 0, The correction coefficient of the phase delay of any one of the three RGB color channels corresponding to the time is determined by step 6.4; λ (β 0j ), I λ (β 1j ), I λ (β 2j ) is β 0j , β 1j , β 2j The corresponding light intensity component is measured experimentally; C′ mn The algebraic cofactor of the matrix M′, det(M′) is the determinant of the matrix M′, and the matrix M′ is I λ (β 0j ), I λ (β 1j ), I λ (β 2j ) The coefficient matrix of S0′, S1′, S2′; Step 6.6: Based on the Stokes vectors S0, S1, and S2 obtained in step 6.3, determine the degree of polarization DoP and the angle of polarization AoP obtained by the Mueller matrix correction method: Where S 0cal is the total light intensity after correction by Mueller matrix correction method, i.e. the sum of polarized light and unpolarized light, S 1cal It represents the difference between the horizontal and vertical polarization components of the light after correction by the Mueller matrix correction method, S 2cal It represents the difference in polarization components of light at 45° and -45° after correction by the Mueller matrix correction method, S 3cal Represents the difference in circular polarization components of light after correction by the Mueller matrix correction method; Step 6.7: Based on the Stokes vectors S0′, S1′, and S2′ obtained in step 6.5, determine the degree of polarization DoP′ and the angle of polarization AoP′ obtained by the color channel correction method: Among them, S′ 0cal , S′ 1cal , S′ 2cal is the Stokes vector corrected by the color channel correction method; Step 6.

8. Compare the results obtained in step 6.6 and step 6.7, output the more optimal polarization degree and polarization angle in the dual-path analysis results, construct the wide-band polarization imaging data of the target to be measured based on the optimized polarization parameter distribution, and generate polarization degree and polarization angle diagrams of the target to be measured at different wavelengths; realize the display of the wide-band polarization characteristics of the target through visualization processing, including polarization degree distribution and polarization angle distribution.

7. The imaging method of the visible light broadband polarization imaging system based on liquid crystal variable phase retarder according to claim 6, characterized in that The matrix M in step 6.3 is: Where, The phase delay is φ=0, The experimentally obtained phase delay with error corresponding to is determined by the following formula: Where, is the actual Mueller matrix of the liquid crystal variable phase retarder (3), 2θ is the angle between the optical axis of the liquid crystal variable phase retarder (3) and the horizontal reference axis, which is 45°; is the Mueller matrix of the first achromatic 1 / 4 wave plate (2), 2θ1 is the angle between the optical axis of the first achromatic 1 / 4 wave plate (2) and the horizontal reference axis, which is 0°; is the Mueller matrix of the second achromatic quarter wave plate (4), 2θ2 is the angle between the optical axis of the second achromatic quarter wave plate (4) and the horizontal reference axis, which is 0°; M total is the actual Mueller matrix of the polarization modulation unit, which is as follows: Comparing the experimentally measured Mueller matrix with the above formula, the specific 8. The imaging method of the visible light broadband polarization imaging system based on liquid crystal variable phase retarder according to claim 6, characterized in that The matrix M′ in step 6.5 is:

9. The imaging method of the visible light broadband polarization imaging system based on liquid crystal variable phase retarder according to claim 7, characterized in that As described in step 6.6 3cal =0;S 0cal , S 1cal , S 2cal Determined by the following formula: Where, Phase delay The deviation angle of polarization azimuth at each wavelength is measured by calibration experiment. Phase delay The light intensity weight proportional coefficient at each wavelength is determined by the following formula:

10. The imaging method of the visible light broadband polarization imaging system based on liquid crystal variable phase retarder according to claim 6, characterized in that S' in step 6.7 3cal =0; S′ 0cal , S′ 1cal ,S′ 2cal Determined by the following formula: Where S′ 0cal is the total light intensity after correction by color channel correction method, i.e. the sum of polarized light and unpolarized light, S′ 1cal is the difference between the horizontal and vertical polarization components of the light after correction by the color channel correction method, S′ 2cal The polarization component difference of the light at 45° and -45° directions after correction by the color channel correction method.

Images