A spatially partitioned polarization modulation light source and a Mueller matrix imaging system based on the light source

Abstract

The present invention provides a spatially partitioned polarization modulation light source, which includes an illumination unit and a polarization modulation unit. The illumination unit cooperates with the polarization modulation unit to switch the illuminated range by switching the illumination area of the illumination unit, so as to achieve a rapid switch between different polarization states of the incident polarized light. By spatially partitioning and cooperating with time-sequence modulation, a variety of incident polarized lights with specific polarization states are generated to illuminate the sample. Through the light source of the present invention, the problem of the complex and costly modulation structure of different polarization states in the existing Mueller matrix measurement device is solved. The present invention also provides a Mueller matrix imaging system based on the spatially partitioned polarization modulation light source using the above light source, and its calibration method, Mueller matrix calculation method and background noise removal method.

Description

Technical Field

[0001] The present invention belongs to the technical field of polarization optical imaging, and relates to a Mueller matrix imaging system and method using a spatially partitioned polarization modulation light source as a polarizer. Background Art

[0002] When using polarization measurement methods to detect samples, a large amount of polarization optical information that cannot be obtained by conventional optical measurement means will be obtained. There is a certain correspondence between this polarization optical information and the physical structure of the sample. Therefore, physical structure characteristics that cannot be recognized by conventional optical measurement means can be obtained through polarization measurement. Polarization measurement methods are widely used in the fields of military, underwater detection, medical diagnosis, meteorological detection, remote sensing, material identification, etc.

[0003] Among them, Mueller matrix measurement is an important method in the field of polarization measurement. In recent years, more and more researchers have obtained sample structure information by measuring the Mueller matrix of samples, and it has been widely developed and applied in various fields, especially in the field of biomedical engineering. The Mueller matrix measurement method requires active imaging. After the polarization modulation of the light source by the polarizer, at least four independent incident polarization states must be generated, and the analyzer can detect the polarization state of each outgoing light.

[0004] Existing Mueller matrix measurement devices often use a single light source, combined with a polarizer and an analyzer to form a system. Therefore, the modulation of different polarization states is often carried out by rotating polarization optical elements or using voltage-controlled optical elements. Such as using polarizing films, wave plates, etc. This type of combination requires the introduction of moving parts such as motors, which increases the complexity and modulation time of the system; such as using polarizing films and liquid crystal modulators, photoelastic modulators, etc., the phase delay needs to be adjusted through voltage regulating elements. The former moving parts increase the measurement time and structural complexity, and the latter elements are more sensitive and will be interfered by many factors such as their own characteristics and environmental conditions. The complex requirements for the system structure limit the application range of the imaging system.

[0005] In summary, the existing technology has the following problems:

[0006] (1) The moving parts such as motors used in the existing polarizer structure increase the complexity and modulation time of the system, while the voltage modulation elements such as liquid crystal modulators and photoelastic modulators are easily interfered by environmental factors and their own characteristics.

[0007] (2) Whether the polarizer uses moving parts or sensitive voltage-controlled elements, it will increase the complexity of the system and reduce the robustness, restricting the application range of the system.

[0008] (3) The motors or voltage-controlled optical elements used in the existing polarizer structure are expensive.

[0009] It should be noted that the information disclosed in the above background art section is only for understanding the background of the present application, and thus may include information that does not constitute the prior art known to those of ordinary skill in the art. Summary of the Invention

[0010] To solve the above problems, the present invention proposes a spatially partitioned polarization modulation light source and a Mueller matrix imaging system based on this light source. This light source is equivalent to different partitions of the same light source composed of multiple polarization light sources. And for the first time, a light source with this structure is used to build a Mueller matrix imaging system.

[0011] To achieve the above object, the present invention adopts the following technical solutions:

[0012] A spatially partitioned polarization modulation light source includes an illumination unit and a polarization modulation unit. The illumination unit cooperates with the polarization modulation unit to switch the illuminated range by switching the illumination area of the illumination unit, so as to realize the rapid switching of incident polarized light between different polarization states, and generate multiple incident polarized lights with specific polarization states for illuminating the sample through spatial partitioning and timing modulation.

[0013] Further, the illumination unit is directly connected to the polarization modulation unit. The unit partitions at different spatial positions of the polarization modulation unit can modulate different incident polarization states. The illumination area division of the illumination unit corresponds one-to-one with the unit partitions, so that when different illumination areas are illuminated, incident polarized light with different polarization states can be generated.

[0014] Further, the illumination unit includes a light source, a control circuit, a power supply, and a host computer. Among them, the light source is spatially divided as a whole to obtain multiple illumination areas. The voltage required by the light source is provided by connecting the power supply through the control circuit, and the host computer controls the illumination or extinction of different illumination areas through the control circuit.

[0015] Further, the polarization modulation unit includes a linear polarizer, a wave plate, and a fixing element. Among them, the polarizer and the wave plate are divided correspondingly in the same area according to the multiple illumination areas obtained by spatially dividing the light source, resulting in multiple unit partitions. Each unit partition corresponds to an area of the polarizer and an area of the wave plate. The polarizer is located between the light source and the wave plate. The axes of the light source, the polarizer, and the wave plate coincide and are parallel to each other, and are cooperated with the fixing element to ensure tight combination without misalignment.

[0016] Further, the angles of each area of the polarizer and the wave plate can be selected and optimized according to indexes such as the condition number of the instrument matrix or the equal-weight variance of the polarization modulation unit.

[0017] A Mueller matrix imaging system based on a spatially partitioned polarization modulation light source, comprising the above-mentioned spatially partitioned polarization modulation light source, an analyzer modulation unit, an acquisition control unit, and a data processing unit; wherein, the analyzer modulation unit receives the reflected or scattered light from the sample under illumination by different incident polarized lights and detects its polarization state; the acquisition control unit is used to control the illumination unit to switch its lighting range, and at the same time, the acquisition control system is also used to control the analyzer modulation unit to synchronously acquire the sample image with the polarization modulation unit; the data processing unit is used to process the sample image to obtain the Mueller matrix.

[0018] Further, the acquisition control unit is respectively connected to the illumination unit and the analyzer modulation unit through a control circuit and a data line, and is used to control the lighting range of the illumination unit, control the analyzer modulation unit to acquire images, and control the two to work alternately and cooperate with each other;

[0019] The acquisition control unit can be divided into the following two parts: the control module of the illumination unit and the analyzer control module.

[0020] Among them, the control module of the illumination unit covers the upper computer and the control circuit in the illumination unit, and executes the timing switching of the lighting area of the illumination unit; the analyzer control module is used to execute the image acquisition of the analyzer modulation unit. The two control modules share the same upper computer, so that the switching of the lighting area and the acquisition of images are carried out alternately until enough images are acquired for calculating the Mueller matrix.

[0021] Further, the data processing unit is connected to the analyzer modulation unit through a data line and calculates the Mueller matrix according to the acquired images.

[0022] The samples applicable to this system can be processed experimental samples, or artificial or natural samples without special treatment (such as walls, desktops, plants, various sundries, etc.), and there is no fixed requirement for the spatial position of the samples. The incident angle of the polarization modulation unit and the receiving angle of the analyzer modulation unit can be selected according to the needs;

[0023] A calibration method for calibrating the above-mentioned Mueller matrix imaging system based on a spatially partitioned polarization modulation light source, comprising the following steps:

[0024] S1: Calibrate the mean values of the intensity and polarization state of the incident light from each region received by the sample plane to calibrate the instrument matrix of the system;

[0025] S2: Calibrate the intensity distribution error introduced by the spatially partitioned polarization modulation light source. Further, step S1 specifically includes the following steps:

[0026] S1-1: Place the analyzer modulation unit at the position of the sample point to be calibrated,

[0027] and image at the analyzer modulation unit end after each switching of the illumination area;

[0028] S1-2: Integrate the image points of the images at the analyzer modulation unit end after each illumination area switching to obtain a Stokes vector in digital quantity form that characterizes the actual intensity and polarization state of the incident light at the position of this sample point for each area;

[0029] S1-3: Stack the Stokes vectors of the incident light corresponding to all areas traversed in sequence column by column to form an instrument matrix of the incident light of the spatially partitioned polarization modulation light source at the position of this sample point;

[0030] When the intensity distribution error of the spatial polarization modulation light source is relatively low, or the distance is far enough so that the error caused by the structure of this light source is relatively low, the instrument matrix obtained by the mean calibration method can be substituted as the instrument matrix of the entire plane for Mueller matrix imaging.

[0031] Further, step S2 specifically includes the following steps:

[0032] S2-1: Perform a mean calibration in step S1 once and normalize the mean calibration result;

[0033] S2-2: Remove the polarization modulation unit of the spatial partitioned polarization modulation light source and perform a mean calibration without the polarization modulation unit once with the calibration method in step S1;

[0034] S2-3: Calculate the light intensity transfer coefficients of different illumination areas through the mean calibration results of step S2-1 and step S2-2.

[0035] Further, step S2 also includes the following steps performed after changing the sampling distance,

[0036] S2-4: Before measuring the Mueller matrix of the sample, remove the polarization modulation unit and obtain the light intensity distribution of the sample surface corresponding to different illumination areas at the current sampling distance through the analyzer modulation unit to obtain the intensity distribution matrix of the incident light of the sample at the current distance;

[0037] S2-5: Calculate the incident light instrument matrix distribution within the entire imaging plane according to the normalization result of the mean calibration in step S2-1, the light intensity transfer coefficient, and the light intensity distribution matrix.

[0038] A method for calculating the Mueller matrix of a sample, used for the above-mentioned Mueller matrix imaging system based on a spatially partitioned polarization modulation light source, includes the following steps:

[0039] A1: According to the structure of the analyzer selected during the actual setup of the system, use a mature method to calibrate the analyzer separately to obtain its instrument matrix;

[0040] A2: Obtain the instrument matrix S of the sample incident light by using the calibration method according to any one of claims 9 - 12; in ;

[0041] A3: The lighting unit sequentially switches and lights up each illumination area to ensure that all incident polarization states are traversed. A measurement is taken at the analyzer modulation unit end before each illumination area switch. After the traversal, calculate the sample output light matrix S based on the image obtained by the analyzer modulation unit; out ;

[0042] A4: Calculate the Mueller matrix M of the sample sample = S out ·pinv(S in ), where pinv(S in ) represents the pseudo - inverse matrix of the instrument matrix S of the sample incident light in .

[0043] Furthermore, step A2 includes the following steps:

[0044] A2 - 1: Determine whether to use the instrument matrix of the polarizer modulation unit obtained by the mean calibration method or the instrument matrix obtained by the distribution calibration method according to the sample structure or accuracy requirements;

[0045] A2 - 2: Obtain the instrument matrix of the sample incident light according to the calibration method determined in step A2 - 1; if the instrument matrix obtained by the distribution calibration method is used, when the sample or sampling distance changes, it is necessary to re - calibrate using the distribution calibration method.

[0046] A method for removing background noise, which is used for the above - mentioned Mueller matrix imaging system based on spatially - partitioned polarization - modulated light source, specifically:

[0047] Before the lighting unit is lit or after it is completely turned off, with the system spatial position unchanged, the analyzer modulation unit takes a set of sample images under only background light illumination. This image serves as the background noise image. Subtract each pixel of the image obtained by the analyzer after each lighting unit switch from this background noise image (i.e., subtract each pixel of the image under polarizer + background light illumination from the image under only background light illumination) and use it as the theoretical true value in this measurement and substitute it into the Mueller matrix calculation.

[0048] The present invention has the following beneficial effects:

[0049] (1) When the spatial partition polarization modulation light source of the present invention is used for Mueller matrix imaging, the polarization modulation unit performs polarization by switching the light-emitting area of the light source, with fast measurement speed, no sensitive voltage-controlled optical elements, and is not easily affected by the environment; the polarizer adopting the spatial partition polarization modulation light source has a simple structure, does not require expensive motors or voltage-controlled optical elements, and can reduce costs.

[0050] (2) When the spatial partition polarization modulation light source of the present invention is combined with any analyzer to form a Mueller matrix imaging system, the polarization modulation unit has a simple structure, strong robustness, and a wide range of application scenarios. Description of the Drawings

[0051] The drawings described herein are used to provide a further understanding of the present invention, form a part of this application, and the schematic embodiments of the present invention and their descriptions are used to explain the present invention, and do not constitute an improper limitation to the present invention. In the drawings:

[0052] Figure 1 It is a front view schematic diagram of the partitioned light source of the lighting unit in the embodiment of the present invention;

[0053] Figure 2 It is a variant front view schematic diagram of the partitioned light source of the lighting unit in the embodiment of the present invention;

[0054] Figure 3 It is a schematic diagram of the cooperation between the lighting unit and the polarization modulation unit of the present invention;

[0055] Figure 4 It is a connection diagram of the overall structure of the system of the present invention;

[0056] Figure 5 It is an internal structure connection diagram of the cooperation structure between the lighting unit and the polarization modulation unit of the present invention;

[0057] Figure 6 It is a flowchart of the mean calibration method for the sample plane of the present invention;

[0058] Figure 7 It is a flowchart of the distribution calibration method for the sample imaging plane of the present invention;

[0059] Figure 8 It is a flowchart for calculating the Mueller matrix of the sample;

[0060] Figure 9 It is the Mueller matrix of two kinds of samples (A) metal (B) wood board;

[0061] Figure 10 It is the average RMSE (variance from the true value) of 16 elements of the Mueller matrix measured for a uniform paper with the system of the present invention;

[0062] Figure 11 It is the result demonstration of the method for removing background noise in the present invention;

[0063] In the figure, 100: spatially partitioned polarization modulation light source; 200: analyzer modulation unit; 300: acquisition control unit; 400: data processing unit; 500: sample; 1001: illumination unit; 1002: polarization modulation unit; 101: light source; 102: linear polarizer; 103: quarter-wave plate; 104: acquisition control circuit board; 105: power supply; 106: fixing element. Specific embodiments

[0064] The following provides a detailed description of the embodiments of the present invention. It should be emphasized that the following description is merely exemplary and not intended to limit the scope of the present invention and its applications.

[0065] As Figure 4 , an embodiment of the present invention provides a Mueller matrix imaging system based on a spatially partitioned polarization modulation light source, including a spatially partitioned polarization modulation light source 100, an analyzer modulation unit 200, an acquisition control unit 300, a data processing unit 400, and a sample 500;

[0066] As Figure 5 , in some embodiments, the spatially partitioned polarization modulation light source 100 includes an illumination unit and a polarization modulation unit. The illumination unit includes: a light source 101, an acquisition control circuit board 104, a power supply 105, and a host computer. The polarization modulation unit includes: a linear polarizer 102 and a quarter-wave plate 103.

[0067] The acquisition control circuit board 104 is separately connected to the light source 101, the power supply 105, and the host computer. Among them, the host computer is connected to the acquisition control circuit board by a data line, and the acquisition control circuit board is connected to the power supply and the light source by metal wires for energy transmission.

[0068] As Figure 1 shown, the light source is spatially divided as a whole (taking a surface light source composed of an LED array as an example, which means that the surface light source is spatially divided into multiple regions, and each region contains a certain number of LED arrays). The voltage required by the light source is provided by connecting to an independent power supply via a control circuit. The host computer transmits data to the control circuit through a data line, and then controls whether different light source regions are powered on. Thus, the host computer can control the lighting or extinguishing of each region of the light source.

[0069] Polarization modulation unit (polarizer):

[0070] The polarization modulation unit can be divided into the following three parts: a linear polarizer, a wave plate, and a fixing element 106.

[0071] The polarizer and wave plate are segmented correspondingly according to the spatial regions of the light source. Each region corresponds to a polarizer and a wave plate, and the polarizer is sandwiched between the light source and the wave plate. The angles of the polarizer and the wave plate can be selected as needed to ensure obtaining an optimized incident light instrument matrix with a minimized condition number. The axes of the three are coincident and parallel to each other. They are tightly combined without misalignment through fixed components, such as Figure 3 as shown.

[0072] Analyzer modulation unit (analyzer):

[0073] The analyzer modulation unit can be any type of polarization state analyzer.

[0074] Acquisition control unit:

[0075] The acquisition control unit can be divided into the following two parts: the control module of the illumination unit and the analyzer control module.

[0076] Among them, the control module of the illumination unit covers the host computer and the control circuit in the illumination unit, and executes the timing switching of the illuminated areas; the analyzer control module is used to execute the image acquisition of the analyzer modulation unit. The two control modules share the same host computer, enabling the switching of the illuminated areas and the image acquisition to alternate until a sufficient number of images are collected for calculating the Mueller matrix.

[0077] Data processing unit:

[0078] Analyze and process the data collected by the two analyzers, and then calculate the 16 Mueller matrix elements images of the sample.

[0079] Such as Figure 1 shown is a specific embodiment of the present invention, specifically a front view schematic diagram of the partitioned light source of the adoptable illumination unit.

[0080] The light source is divided into four partitions, and each partition is defined as a region (quadrant) here. Each region (quadrant) is composed of 36 LEDs (not all shown in the figure, only for illustration, and the number of LEDs is not limited). The diameter of the adopted illumination unit is 10 cm, and the shortest interval distance between the LEDs in each region (quadrant) is 1.5 cm. The light sources in the four regions can be individually lit or extinguished without interference. During the actual calibration and measurement process, the four regions of the light source will be lit in sequence according to the timing (it is required that only one region is lit at the same time, and the previous region is extinguished when the next region is lit)

[0081] All the dimensions and numbers in the figure are only for demonstration, and any changes can be made during the actual construction.

[0082] Such as Figure 2The figure shows a specific implementation of the present invention, specifically a front view schematic diagram of a zoned light source of another adoptable lighting unit.

[0083] The figure shows a variant of a four-zoned light source. In the figure, the lighting unit is divided into 36 independent cells, each containing four LEDs. According to the orientation, it is divided into four regions (quadrants) (distinguished by gray scale in the figure). The same quadrant (represented by the same gray scale) in each cell is regarded as the same lighting area and is simultaneously lit or extinguished during lighting (that is, the same quadrant in 36 cells will be simultaneously lit or extinguished). If polarizers with the same angle are installed on the same quadrant of different grids subsequently, the lighting unit at this time is still equivalent to Figure 1 the four-zoned light source in, except that the same quadrant of different grids is used as the same zone, and the actual spatial distance between zones is equivalent to the spatial distance of the four LEDs in a single cell.

[0084] All the dimensions and numbers in the figure are only for demonstration, and any changes can be made during actual construction. As Figure 3 The figure shows a specific implementation of the present invention. The four zones of the lighting unit 1001 correspond spatially to the four zones of the linear polarizer 102 and the quarter-wave plate 103 in the polarization modulation unit 1002. The light transmission angles of the linear polarizers in the four zones are all 0° relative to the x-axis, and the major axis angles of the quarter-wave plates in the four zones are -19.6°, 19.6°, -45°, and 45° respectively relative to the x-axis.

[0085] Figure 3 The lighting unit used in the demonstration in is Figure 1 the four-zoned light source in. If the lighting unit adopts the Figure 2 form of the grid-based four-zoned light source in, then Figure 2 the cooperation between the lighting unit and the polarization modulation unit in each grid can also be demonstrated by Figure 3 .

[0086] For the system built based on the above specific implementation, an example analysis of the working principle of measuring the Mueller matrix of a sample is as follows:

[0087] (1) The method for mean calibration of the sample plane, such as Figure 6 :

[0088] According to the principle of the mean calibration method of the sample plane in the foregoing invention content, the following specific implementation can be carried out:

[0089] Place the lighting unit and the polarization modulation unit that is connected to and fixed with the lighting unit at the position where the measurement is to be made, and place the analyzer modulation unit at the position of the sample to be calibrated. It is required that the coordinate system of the polarization modulation unit coincides with the coordinate system of the analyzer modulation unit, and it is required that one of the coordinate axes is horizontal with the ground (if a non-anisotropic plane mirror is introduced for calibration, place the plane mirror at the position of the sample to be calibrated). Adjust the orientation of the analyzer modulation unit so that the polarization modulation unit is at the exact center of the analyzer imaging.

[0090] (Step X1):

[0091] According to the structure of the analyzer used, adopt the corresponding calibration method and obtain the instrument matrix A of the analyzer modulation unit PSA .

[0092] (Steps X2 - X6):

[0093] Light up each area of the polarization modulation unit successively. When the i-th area is lit, record the set of images I collected by the analyzer modulation unit CCDi . After the collection is completed, switch to the next area. Do this until all areas have been traversed and the analyzer modulation unit has recorded the images corresponding to all areas.

[0094] (Step X7):

[0095] For each set of images obtained by traversing the analyzer modulation unit, calculate the Stokes vector of the light detected at each pixel point of the analyzer modulation unit:

[0096] S' out = pinv(A PSA )·[I CCD

[0097] (where pinv(A PSA ) is the pseudo-inverse matrix of the analyzer instrument matrix A PSA , and [I CCD is the light intensity value of each pixel point detected by the analyzer)

[0098] For each set of images, integrate and sum up the Stokes vectors of the light detected at all pixel points within the image, and we have:

[0099]

[0100] (where S ini represents the integration of the Stokes vector of this set of images, I, Q, U, V respectively represent the four elements of the Stokes vector corresponding to this set of images after Stokes integration, and S0, S1, S2, S3 represent the four elements of the Stokes vector of each pixel point before integration.) ​

[0101] (Step X8):

[0102] Integrating the Stokes vectors of the group of images corresponding to each region and stacking them together in the form of a column vector in chronological order can constitute the mean instrument matrix of the incident light corresponding to the sample spatial position under this imaging optical path (taking the illumination unit divided into four regions as an example):

[0103]

[0104] (where I i , Q i , U i , V i respectively represent the integration of the Stokes vectors of the group of images detected at the analyzer end corresponding to the i-th region)

[0105] For the case where the change in the incident light instrument matrix received by the sample plane can be ignored (such as when the light intensity distribution difference of the spatially polarized modulation light source is small or the imaging distance is far), the mean value A' PSG can be considered as the incident light instrument matrix of the entire sample plane.

[0106] (2) Distribution calibration method for the sample imaging plane, such as Figure 7 :

[0107] (taking the illumination unit divided into four regions as an example):

[0108] (Step Y1):

[0109] Such as Figure 7 , extract the intensity proportionality coefficients at the output ends of the polarizers in the four regions from the first row of the instrument matrix A' PSG obtained from the mean calibration:

[0110]

[0111] The normalized Stokes matrix of the incident light is:

[0112] Stokes PSG = A' PSG ·(P1) -1 .

[0113] (Step Y2):

[0114] Remove the analyzer part of the spatially polarized modulation light source and perform a mean calibration without an analyzer to obtain the incident light instrument matrix A' LED of the pure light source, and extract the first row of A' LED as the intensity proportionality coefficients of the light source in the four regions:

[0115]

[0116] Calculate the light intensity transfer coefficient of the polarizer:

[0117] K P = P1·(P2) -1 ,

[0118] The above calibration steps only need to be carried out once.

[0119] (Steps Y3, Y4):

[0120] At this time, it is judged whether the sample or the sampling distance has changed. If there is no change, the incident light instrument matrix distribution A of the spatially partitioned polarization modulation light source measured by the previous distribution calibration method is used PSG That's it. If there is a change, the following steps need to be carried out again:

[0121] (Step Y5):

[0122] When the lighting unit switches following the time sequence in the lit area, the analyzer modulation unit synchronously obtains the intensity distribution matrices [k1] width*height , [k2] width*height , [k3] width*height , [k4] width*height corresponding to each pixel point in the lighting area, and integrates the four intensity matrices into a coefficient matrix:

[0123]

[0124] (Step Y6):

[0125] Then the instrument matrix distribution of the spatially polarized modulation light source on the entire imaging plane can be expressed as:

[0126] A PSG = M flipped ·Stokes PSG ·k width×height ·K P ,

[0127] where the expansion form of M flipped is:

[0128]

[0129] The result demonstration of the calibration method of the spatially partitioned polarization modulation illumination Mueller matrix imaging system is as Figure 9 - 10 shown,

[0130] Figure 9In (a), it is the theoretical true value measured by other mature systems. (b), (c), and (d) are the results measured by the spatial partition polarization modulation illumination system using different calibration methods. (b) performs distribution calibration; (c) performs mean calibration; (d) does not perform calibration. RSME_A and RSME_B represent the variances between the measured values of metal and wood board and the true values.

[0131] It can be seen from both the images and the RMSE that the error of distribution calibration is the smallest and the error of not performing calibration is the largest.

[0132] Figure 10 In it, the horizontal axis is the sampling distance and the vertical axis is the magnitude of RMSE. The blue line represents the result of not performing calibration; the green line represents the result of using mean calibration; the red line represents the result of using distribution calibration. It can be seen that the result of distribution calibration is the best and the result of not performing calibration is the worst.

[0133] (3) Calculation of the Mueller matrix of the sample, such as Figure 8 :

[0134] (Taking the illumination unit divided into four regions as an example):

[0135] (Step C1):

[0136] According to the structure of the analyzer used, adopt the corresponding calibration method and obtain the instrument matrix A of the analyzer modulation unit PSA .

[0137] (Step C2):

[0138] According to the structure of the measured sample or the measurement accuracy requirements, decide to adopt the mean calibration method to obtain the instrument matrix A' of the incident light PSG or the instrument matrix A obtained by the distribution calibration method PSG . No matter which method is adopted, here the instrument matrix of the incident light is uniformly denoted as A PSG .

[0139] (Steps C3 - C5):

[0140] If the mean calibration method is adopted to obtain A PSG , when the sample or the sampling distance changes, the above steps do not need to be repeated.

[0141] If the distribution calibration method is adopted to obtain A PSG , when the sample or the sampling distance changes, it is necessary to re - perform the mean calibration method to update A PSG .

[0142] (Step C6):

[0143] When the sample is in the position to be measured, the four regions of the illumination unit are sequentially switched according to the time sequence (i.e., sequentially switched between incident lights of different polarization states modulated by the illumination unit and the polarization modulation unit). When switching to the i-th region, the analyzer modulation unit is used to detect the matrix S composed of the outgoing Stokes vectors of each pixel point of the sample. outi . Stack the four outgoing Stokes corresponding to these four regions together column by column to form the matrix of the outgoing light:

[0144]

[0145] (Step C9):

[0146] Then the expression of the Mueller matrix of the sample is:

[0147] M sample = S out ·(A PSG ) -1 .

[0148] (4) Method for removing background noise:

[0149] Take the four-zone division of the illumination unit and the polarization modulation unit as an example.

[0150] (Steps C7 - C8):

[0151] Before the illumination unit is lit or after it is completely turned off, the analyzer takes a set of sample images under only background light irradiation on the premise that the system spatial position remains unchanged, and converts them into a matrix S composed of the outgoing Stokes vectors of each pixel point. out0

[0152] Then the expression of the sample outgoing light corresponding to each region can be reconstructed as:

[0153] S outi = S out - S out0

[0154] Stack the new Stokes of the four regions together column by column to obtain S out , and substitute it into M sample = S out ·(A PSG ) -1 for calculation to obtain the Mueller matrix imaging result with background light interference eliminated.

[0155] Figure 11 The result demonstration of the method for removing background noise of the spatially divided polarization modulation illumination Mueller matrix imaging system is shown. It can be verified that this method is correct both theoretically and experimentally.

[0156] The background section of the present invention may include background information about the problems or circumstances of the present invention, rather than necessarily describing the prior art. Therefore, the content included in the background section is not an admission by the applicant of the prior art.

[0157] The above content is a further detailed description of the present invention in combination with specific / preferred embodiments, and it cannot be determined that the specific implementation of the present invention is only limited to these descriptions. For those of ordinary skill in the technical field to which the present invention pertains, without departing from the concept of the present invention, several alternatives or modifications can be made to these described embodiments, and these alternative or modified forms should all be regarded as falling within the protection scope of the present invention.

Claims

1. A spatially partitioned polarization modulation light source, comprising an illumination unit and a polarization modulation unit, characterized in that, It is used to cooperate with any polarization analysis modulation unit to form a Mueller matrix imaging system. The illumination unit cooperates with the polarization modulation unit. By switching the illumination area of the illumination unit to switch the illuminated range, rapid switching of incident polarized light between different polarization states is achieved. Through spatial partitioning and timing modulation, multiple incident polarized lights with specific polarization states are generated to illuminate the sample. Among them, the illumination unit is directly connected to the polarization modulation unit. The unit partitions at different spatial positions of the polarization modulation unit can modulate different incident polarization states. The illumination area division of the illumination unit corresponds one-to-one with the unit partitions, so that when different illumination areas are illuminated, incident polarized lights with different polarization states can be generated.

2. The spatially partitioned polarization modulation light source according to claim 1, wherein The illumination unit includes a light source, a control circuit, a power supply, and a host computer. Among them, the light source is spatially divided as a whole to obtain multiple illumination areas. The voltage required by the light source is provided by connecting the power supply through the control circuit. The host computer controls the lighting or extinguishing of different illumination areas through the control circuit.

3. The spatial partition polarization modulation light source according to claim 2, characterized in that, The polarization modulation unit includes a linear polarizer, a wave plate, and a fixing element. Among them, the polarizer and the wave plate are divided in a corresponding manner in the same area according to the multiple illumination areas obtained by spatially dividing the light source, resulting in multiple unit partitions. Each unit partition corresponds to an area of the polarizer and an area of the wave plate. The polarizer is located between the light source and the wave plate. The axes of the light source, the polarizer, and the wave plate coincide and are parallel to each other, and are cooperated by the fixing element to ensure tight combination without misalignment.

4. The spatially partitioned polarization modulation light source according to claim 3, wherein The angles of the regions of the polarizer and the wave plate can be selected and optimized according to the condition number of the instrument matrix or the equal-weight variance index of the polarization modulation unit instrument.

5. A Mueller matrix imaging system based on a spatially partitioned polarization modulation light source, characterized in that, It includes a spatially partitioned polarization modulation light source, a polarization analysis modulation unit, an acquisition control unit, and a data processing unit according to any one of claims 1-4. Among them, the polarization analysis modulation unit receives the reflected or scattered light from the sample under the illumination of different incident polarized lights and detects its polarization state. The acquisition control unit is used to control the illumination unit to switch its illuminated range. At the same time, the acquisition control system is also used to control the polarization analysis modulation unit to synchronously acquire the sample image in cooperation with the polarization modulation unit. The data processing unit is used to process the sample image to obtain the Mueller matrix.

6. The imaging system according to claim 5, wherein The acquisition control unit is connected to the illumination unit and the polarization analysis modulation unit through the control circuit and the data line respectively, and is used to control the illuminated range of the illumination unit, control the polarization analysis modulation unit to acquire images, and control the two to work alternately and cooperate with each other.

7. The imaging system according to claim 5, characterized in that, The data processing unit is connected to the polarization analysis modulation unit through the data line and calculates the Mueller matrix according to the acquired images.

8. A calibration method, characterized in that, It is used to calibrate the Mueller matrix imaging system based on the spatially partitioned polarization modulation light source according to any one of claims 5-7, including the following steps: S1: Calibrate the mean values of the intensity and polarization state of the incident light from each region received by the sample plane to perform mean calibration on its instrument matrix. S2: Calibrate the intensity distribution error introduced by the spatially partitioned polarization modulation light source.

9. The calibration method according to claim 8, characterized in that, Step S1 specifically includes the following steps: S1-1: Place the analyzer modulation unit at the position of the sample point to be calibrated, and image at the analyzer modulation unit end after each switching of the illumination area. S1-2: Integrate the image points of the images formed at the analyzer modulation unit end after each switching of the illumination area to obtain the Stokes vector in digital quantity form representing the actual intensity and polarization state of the incident light at this sample point position for each area. S1-3: Stack the Stokes vectors of the incident light corresponding to all areas traversed in sequence column by column to form the instrument matrix of the spatially partitioned polarization modulation light source for the incident light at this sample point position.

10. The calibration method according to claim 8, characterized in that, Step S2 specifically includes the following steps: S2-1: Perform the mean calibration in step S1 once and normalize the mean calibration result. S2-2: Remove the polarizer modulation unit of the spatially partitioned polarization modulation light source and perform a mean calibration without the polarizer modulation unit once using the calibration method in step S1. S2-3: Calculate the light intensity transfer coefficients of different illumination areas based on the mean calibration results of steps S2-1 and S2-2.

11. The calibration method according to claim 10, wherein Step S2 also includes the following steps performed after changing the sampling distance. S2-4: Before measuring the Mueller matrix of the sample, remove the polarizer modulation unit, and obtain the light intensity distribution on the sample surface corresponding to different illumination areas at the current sampling distance through the analyzer modulation unit to obtain the intensity distribution matrix of the incident light on the sample at the current distance. S2-5: Calculate the distribution of the incident light instrument matrix in the entire imaging plane based on the normalized result of the mean calibration in step S2-1, the light intensity transfer coefficient, and the light intensity distribution matrix.

12. A method for calculating the Mueller matrix of a sample, characterized in that, For the Mueller matrix imaging system based on a spatially partitioned polarization modulation light source according to any one of claims 5-7, it includes the following steps: A1: According to the structure of the analyzer modulation unit selected during the actual setup of the system, use a mature method to calibrate the analyzer modulation unit separately to obtain its instrument matrix. A2: An instrument matrix S of the sample incident light is obtained by using the corresponding calibration method according to any one of claims 8-11 in ; A3: The lighting unit sequentially switches and lights up each lighting area in turn to ensure that all incident polarization states are traversed. A measurement is performed at the polarization analysis and modulation unit before each lighting area switch. After the traversal, the sample outgoing light matrix S is calculated based on the image obtained by the polarization analysis and modulation unit out ; A4: Obtain the Mueller matrix M of the sample through calculation sample = S out ·pinv(S in ), where pinv(S in ) represents the pseudo-inverse matrix of the instrument matrix S of the incident light of the sample in of the sample 13. The calculation method according to claim 12, characterized in that, Step A2 includes the following steps: A2-1: Determine whether to use the instrument matrix of the polarizer modulation unit obtained by the mean calibration method or the instrument matrix obtained by the distribution calibration method according to the sample structure or accuracy requirements. A2-2: Obtain the instrument matrix of the sample incident light according to the calibration method determined in step A2-1; if the instrument matrix obtained by the distribution calibration method is used, it is necessary to re-perform the calibration using the distribution calibration method when the sample or sampling distance changes.

14. A method for removing background noise, characterized in that, For the Mueller matrix imaging system based on a spatially partitioned polarization modulation light source according to any one of claims 5-7, specifically: Before the illumination unit is turned on or after it is completely turned off, with the system spatial position unchanged, the analyzer modulation unit takes a set of sample images under only background light illumination, and this image is used as the background noise image. Subtract each pixel of the image obtained by the analyzer modulation unit after each switching of the illumination unit from this background noise image, and use it as the theoretical true value in this measurement and substitute it into the Mueller matrix calculation.

Images