High-reflection object imaging method based on polarization characteristic of light

Abstract

The invention relates to a high-reflection object imaging method based on polarization characteristics of light, and solves the problems that an existing method may damage the surface of an object, wastes time and labor and is low in efficiency. A system used for conducting circular polarized light illumination and imaging on a target highly-reflective object is built, an illumination subsystem comprises an annular white light source, a linear polaroid and a first quarter-wave plate, light emitted from the wave plate is circular polarized light, and an imaging subsystem comprises a polarization camera and a second quarter-wave plate; stokes vectors of the specular reflection light and the diffuse reflection light which are incident to the polarization camera are calculated; the polarization camera collects polarization images at four different polarization angles, the Stokes vector of the diffuse reflection light is calculated according to the intensity of each polarization image and a diffuse reflection light intensity formula, the pixel value of the created image is determined according to the Stokes vector of the diffuse reflection light, and a target high-reflection object image after the specular reflection light is removed is obtained. The method is easy to implement, does not damage a target high-reflection object, and is simple in process and high in efficiency.

Description

Technical Field

[0001] The present invention relates to the field of imaging technology, and in particular to a method for imaging a highly reflective object based on the polarization characteristics of light. Background Art

[0002] When capturing images of certain objects, since the objects may have high reflective properties, resulting in highly reflective areas in the captured images, it is necessary to use the difference in optical properties between the reflected specular light and diffuse light to suppress the specular reflection light. Xiangyu G et al. proposed a method for suppressing specular reflection in stroboscopic stereoscopic full-field imaging (Xiangyu G, Cha Bum L. Fluorescence strobo-stereoscopy for specular reflection-suppressed full field of view imaging [J]. Measurement, 2022, 192: 110907.), which uses laser to excite fluorescence and uses the difference in laser and fluorescence wavelengths to suppress the reflection of the metal surface, such as Figure 1 As shown, a pair of cameras are used for image acquisition, 365nm ultraviolet light is used as the light source for exciting fluorescence, the wavelength of light emitted from the fluorescent liquid is 520nm, and a pair of high-pass filters (filter cutoff frequency is greater than 370nm) are selected to filter out the specular reflection light. However, this method requires the target object to be placed in a container filled with fluorescent liquid, and fluorescent liquid is a chemical substance. Metals placed in fluorescent liquid may undergo certain chemical reactions, damage the metal surface, and affect subsequent tasks.

[0003] Lee S et al. proposed a method for reducing specular reflection in fluorescence-induced diagnostic evaluation based on linear polarization control (Lee S, Yoon K, Kim J, et al. Reduction of Specular Reflection Based on Linear Polarization Control for Fluorescence-Induced Diagnostic Evaluation [J]. Diagnostics, 2022, 12 (8): 1990.), which selected two linear polarizers F1 and F2. After the light source passes through the linear polarizer F1 and irradiates the target surface, the angle of the linear polarizer F2 is rotated so that its direction is perpendicular to the polarization direction of the linear polarizer F1, thereby removing the specular reflection light. Although this method can achieve the purpose of removing specular reflection light, when the polarization direction of the linear polarizer F1 is uncertain, it is necessary to manually adjust the direction of the linear polarizer F2 to make it perpendicular to the polarization direction of the linear polarizer F1, which is time-consuming and labor-intensive and has low efficiency. Summary of the invention

[0004] In order to solve the problems of existing mirror reflection light removal imaging methods that may damage the surface of the object and require multiple adjustments of the angle of the optical device, resulting in time-consuming, labor-intensive and low efficiency, the present invention proposes a high-reflective object imaging method based on the polarization characteristics of light. The method can effectively suppress mirror reflection light without damaging the surface of the object and can avoid the problem of decreased efficiency caused by multiple adjustments of the angle of the optical device.

[0005] To solve the above problems, the present invention adopts the following technical solutions:

[0006] A method for imaging a highly reflective object based on the polarization characteristics of light, the method comprising the following steps:

[0007] Step 1: construct a system for circularly polarized light illumination and imaging of a target highly reflective object, the system comprising a bracket and an illumination subsystem and an imaging subsystem respectively fixed on the bracket; wherein the illumination subsystem comprises an annular white light source, a linear polarizer and a first quarter wave plate arranged in sequence, and the light emitted from the first quarter wave plate is circularly polarized light; the imaging subsystem comprises a polarization camera and a second quarter wave plate located in front of the polarization camera, the circularly polarized light emitted from the first quarter wave plate is irradiated to the target highly reflective object, and then reflected by the target highly reflective object and enters the second quarter wave plate and the polarization camera in sequence, the light reflected by the target highly reflective object comprises specular reflection light and diffuse reflection light, the specular reflection light is circularly polarized light and has a rotation direction opposite to that of the incident circularly polarized light;

[0008] Step 2: Calculate the Stokes vector T5 of the mirror-reflected light incident on the polarization camera. When the light emitted from the first quarter-wave plate is right-handed circularly polarized light, the calculation formula is as follows:

[0009]

[0011] in, is the Stokes vector of the specular light reflected from the target highly reflective object, is the Mueller matrix of the second quarter-wave plate, R specular is the proportion of the specular reflected light reflected by the target highly reflective object in the total reflected light, P is the light intensity, and U is the angle between the fast axis of the second quarter-wave plate and the x-axis;

[0012] Calculate the Stokes vector T6 of the diffusely reflected light incident on the polarization camera. When the light emitted from the first quarter-wave plate is right-handed circularly polarized light, the calculation formula is as follows:

[0013]

[0014] in, is the Stokes vector of the diffuse light reflected by the target highly reflective object, R diffuse is the proportion of diffuse light reflected by the target highly reflective object in the total reflected light, and R specular +R diffuse =1;

[0015] Step 3: The polarization camera collects the original polarization image of the target highly reflective object (1) and polarization images with polarization angles of 0°, 45°, 90°, and 135° at one time, and calculates the parameters S0, S1, S2, S3=0 in the Stokes vector T7 of the original polarization image according to the intensity of each polarization image. The calculation formula is as follows:

[0016] S0=I(0°)+I(90°) (4)

[0017] S1=I(0°)-I(90°) (5)

[0018] S2=I(45°)-I(135°) (6)

[0019] Step 4: The Stokes vector T7 of the original polarization image is equal to the sum of the Stokes vector T5 of the specular reflection light incident to the polarization camera and the Stokes vector T6 of the diffuse reflection light incident to the polarization camera. The calculation formula is as follows:

[0020]

[0021] and

[0022] According to formula (7), the formula for diffuse reflected light intensity is derived: According to the calculated diffuse reflected light intensity Find the Stokes vector T6 of the diffuse reflected light;

[0023] Step 5: Create an image of the same size as the original polarization image, and determine the pixel values ​​of the image according to the Stokes vector T6 to obtain an image of the target highly reflective object after removing the specular reflection light.

[0024] The present invention utilizes the characteristic that mirror reflected light can change the hand rotation of circularly polarized light and its difference from the polarization characteristics of diffuse reflected light to build a highly reflective object lighting and imaging system, and derives the formula for the intensity of diffuse reflected light based on the difference in the relationship between the Stokes vectors of mirror reflected light and diffuse reflected light. By using the images of four polarization angles collected by a polarization camera, after calculating the parameter values ​​in the formula through a program, an image is created, whose pixel value is equal to the value calculated by the formula for the intensity of diffuse reflected light, and finally an image of the target highly reflective object after removing the mirror reflected light is obtained. The present invention has the following beneficial effects:

[0025] The imaging method for highly reflective objects proposed by the present invention is simple to implement. Without damaging the target highly reflective object, it only needs to rotate the angle between the polarization direction of the linear polarizer and the fast axis of the quarter-wave plate once to take a picture of any highly reflective object and perform image processing to remove the specular reflected light. The process is simple and efficient. And from the image processing experimental results, the contrast of the processed image is more than twice that of the original image, and the experimental effect is significant. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 A schematic diagram of the background technology;

[0027] Figure 2 It is a flow chart of a method for imaging a highly reflective object based on polarization characteristics of light according to an embodiment of the present invention;

[0028] Figure 3 Schematic diagram of the structure of the constructed imaging system.

[0029] Explanation of reference numerals: 1. target highly reflective object; 2. bracket; 3. annular white light source; 4. linear polarizer; 5. first quarter wave plate; 6. polarization camera; 7. second quarter wave plate. DETAILED DESCRIPTION

[0030] The technical solution of the present invention will be described in detail below in conjunction with the accompanying drawings and preferred embodiments.

[0031] The present invention provides a method for imaging a highly reflective object based on the polarization characteristics of light, such as Figure 2 As shown, it specifically includes the following steps 1 to 5.

[0032] Step 1: First, a system for circularly polarized light illumination and imaging of a target highly reflective object 1 is constructed. The system mainly includes a bracket 2 and an illumination subsystem and an imaging subsystem respectively fixed on the bracket 2, wherein the illumination subsystem is used to generate circularly polarized light to provide illumination for imaging of the target highly reflective object 1; the imaging subsystem is mainly composed of a polarization camera 6 with a second quarter wave plate 7 in front, and is used to collect a polarized image of the target highly reflective object 1. The bracket 2 is used to support the illumination subsystem and the imaging subsystem, and can be implemented using an existing optical bracket or an experimental bracket, which is not limited here.

[0033] like Figure 3As shown, the illumination subsystem includes an annular white light source 3, a linear polarizer 4, and a first quarter wave plate 5, which are arranged in sequence, and the light emitted from the first quarter wave plate 5 is circularly polarized light, which can be left-handed circularly polarized light or right-handed circularly polarized light. When the slow axis of the first quarter wave plate 5 is 45° to the direction of the incident linear polarized light, right-handed circularly polarized light will be generated, and when the fast axis of the first quarter wave plate 5 is 45° to the direction of the incident linear polarized light, left-handed circularly polarized light will be generated. Both left-handed circularly polarized light and right-handed circularly polarized light are suitable for the imaging method of the present invention.

[0034] When setting up the illumination subsystem, firstly, the laser light source, the linear polarizer 4, the first quarter wave plate 5, and the polarization meter are sequentially set up. The laser light emitted by the laser light source passes through the linear polarizer 4 and the first quarter wave plate 5 in sequence and then enters the incident end of the polarization meter. The polarization meter is connected to the computer, and the state of the Poincare sphere is displayed in real time on the computer. Next, the angle of the first quarter wave plate 5 is adjusted, and circularly polarized light is generated when the state of the Poincare sphere on the computer is circular. Then, the polarization meter is removed, and the laser light source is replaced by an ordinary annular white light source 3. The illumination subsystem for generating circularly polarized light is constructed in the order of the annular white light source 3, the linear polarizer 4, and the first quarter wave plate 5.

[0035] Taking the illumination subsystem generating right-handed circularly polarized light as an example, the calculation formula of the Stokes vector T2 of right-handed circularly polarized light is:

[0036]

[0037] in, T0 is the Stokes vector of the ordinary annular white light source 3, M1 is the Mueller matrix of the linear polarizer 4, M2 is the Mueller matrix of the first quarter wave plate 5, P is the light intensity, θ is the angle between the polarization direction of the linear polarizer 4 and the x-axis (the defined reference direction is the horizontal direction), and U is the angle between the fast axis of the first quarter wave plate 5 and the x-axis.

[0038] Still see Figure 3 An imaging subsystem is installed at another angle of the bracket 2, and the imaging subsystem includes a polarization camera 6 and a second quarter wave plate 7 located in front of the polarization camera 6 and placed at an arbitrary angle. After the circularly polarized light emitted from the first quarter wave plate 5 is irradiated to the target highly reflective object 1, it is reflected by the target highly reflective object 1 and then enters the second quarter wave plate 7 and the polarization camera 6 in sequence. The light reflected by the target highly reflective object 1 includes specular reflection light and diffuse reflection light, wherein the specular reflection light is circularly polarized light, and the rotation direction of the specular reflection light is opposite to the rotation direction of the circularly polarized light incident on the target highly reflective object 1.

[0039] Still taking the illumination subsystem generating right-handed circularly polarized light as an example, the light reflected from the target highly reflective object 1 includes specular reflection light and diffuse reflection light. At this time, the specular reflection light becomes left-handed circularly polarized light, and its Stokes vector is Diffuse reflected light is depolarized light, and its Stokes vector is Where R specular and R diffuse are the proportions of specular reflected light and diffuse reflected light in the total reflected light, which are two unknown quantities, and R specular +R diffuse =1.

[0040] Step 2: Calculate the Stokes vector T5 of the mirror-reflected light incident on the polarization camera 6. When the light emitted from the first quarter-wave plate 5 is right-handed circularly polarized light, the calculation formula is as follows:

[0041]

[0042] in, M3 is the Mueller matrix of the second quarter wave plate 7 , P is the light intensity, and U is the angle between the fast axis of the second quarter wave plate 7 and the x-axis.

[0043] According to formula (2), the mirror reflected light becomes linearly polarized light after passing through the second quarter wave plate 7.

[0044] The Stokes vector T6 of the diffusely reflected light incident on the polarization camera 6 is calculated. When the light emitted from the first quarter-wave plate 5 is right-handed circularly polarized light, the calculation formula is as follows:

[0045]

[0046] According to formula (3), the polarization state of the diffuse reflected light remains unchanged after passing through the second quarter wave plate 7. Therefore, the linearly polarized light (mirror reflected light) and the depolarized light (diffuse reflected light) are combined into partially polarized light and enter the polarization camera 6.

[0047] Step 3: The polarization camera 6 can collect the original polarization image of the target highly reflective object 1 and polarization images with polarization angles of 0°, 45°, 90°, and 135° at one time, and calculate the parameters S0, S1, S2 in the Stokes vector of the original polarization image according to the intensities of the four polarization images collected by the polarization camera 6, S3=0, and the calculation formula of S0, S1, S2 is expressed as:

[0048] S0=I(0°)+I(90°) (4)

[0049] S1=I(0°)-I(90°) (5)

[0050] S2=I(45°)-I(135°) (6)

[0051] Among them, I(0°) is the 0° polarization image intensity, I(45°) is the 45° polarization image intensity, I(90°) is the 90° polarization image intensity, and I(135°) is the 135° polarization image intensity.

[0052] Step 4: The Stokes vector of the partially polarized light is the sum of the Stokes vector of the linearly polarized light (mirror reflected light) incident to the polarization camera 6 and the Stokes vector of the depolarized light (diffuse reflected light) incident to the polarization camera 6, that is, the Stokes vector T7 of the original polarization image is equal to the sum of the Stokes vector T5 of the mirror reflected light incident to the polarization camera 6 and the Stokes vector T6 of the diffuse reflected light incident to the polarization camera 6, which can be expressed by the formula:

[0053]

[0054] and

[0055] According to formula (7), the formula for diffuse reflected light intensity can be derived: in, is the diffuse reflection light intensity, according to the calculated diffuse reflection light intensity The Stokes vector T6 of the diffuse reflected light can be calculated according to formula (3). The image corresponding to the Stokes vector T6 is the result image of the original polarized image of the target highly reflective object 1 after removing the specular reflected light.

[0056] Step 5: Create an image of the same size as the original polarization image captured by the polarization camera, and determine the pixel values ​​of the created image according to the Stokes vector T6, that is, make the pixel values ​​of the created image equal to the corresponding diffuse reflection light intensity values ​​calculated according to the diffuse reflection light intensity formula, and finally obtain the image of the target highly reflective object 1 after removing the mirror reflection light.

[0057] The method for imaging highly reflective objects based on the polarization characteristics of light proposed in the present invention is simple to implement. Without damaging the target highly reflective object, it is only necessary to rotate the angle between the polarization direction of the linear polarizer and the fast axis of the first quarter-wave plate once to photograph any highly reflective object and perform image processing to remove the specular reflected light. The process is simple and efficient. And from the results of the image processing experiment, the contrast of the processed image is more than twice that of the original image, and the experimental effect is significant.

[0058] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0059] The above-mentioned embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A method for imaging a highly reflective object based on the polarization characteristics of light, characterized in that: The following steps are involved: Step 1: constructing a system for circularly polarized light illumination and imaging of a target highly reflective object (1), the system comprising a support (2) and an illumination subsystem and an imaging subsystem respectively fixed on the support (2); wherein the illumination subsystem comprises an annular white light source (3), a linear polarizer (4) and a first quarter wave plate (5) arranged in sequence, and the light emitted from the first quarter wave plate (5) is circularly polarized light; the imaging subsystem comprises a polarization camera (6) and a second quarter wave plate (7) located in front of the polarization camera (6); after the circularly polarized light emitted from the first quarter wave plate (5) is irradiated onto the target highly reflective object (1), it is reflected by the target highly reflective object (1) and then enters the second quarter wave plate (7) and the polarization camera (6) in sequence; the light reflected by the target highly reflective object (1) comprises specular reflection light and diffuse reflection light, and the specular reflection light is circularly polarized light and has a rotation direction opposite to that of the incident circularly polarized light; Step 2: Calculate the Stokes vector T5 of the mirror-reflected light incident on the polarization camera (6). When the light emitted from the first quarter-wave plate (5) is right-handed circularly polarized light, the calculation formula is as follows: in, is the Stokes vector of the specular light reflected from the target highly reflective object (1), is the Mueller matrix of the second quarter-wave plate (7), R specular is the proportion of the specular reflected light reflected by the target highly reflective object (1) in the total reflected light, P is the light intensity, and U is the angle between the fast axis of the second quarter wave plate (7) and the x-axis; The Stokes vector T6 of the diffusely reflected light incident on the polarization camera (6) is calculated. When the light emitted from the first quarter wave plate (5) is right-handed circularly polarized light, the calculation formula is as follows: in, is the Stokes vector of the diffuse light reflected by the target highly reflective object (1), R diffuse is the proportion of diffuse light reflected by the target highly reflective object (1) in the total reflected light, and R specular +R diffuse =1; Step 3: The polarization camera (6) collects the original polarization image of the target highly reflective object (1) and polarization images with polarization angles of 0°, 45°, 90°, and 135° at one time, and calculates the parameters S0, S1, S2, S3=0 in the Stokes vector T7 of the original polarization image according to the intensity of each polarization image. The calculation formula is as follows: S0=I(0°)+I(90°) (4) S1=I(0°)-I(90°) (5) S2=I(45°)-I(135°) (6) Step 4: The Stokes vector T7 of the original polarization image is equal to the sum of the Stokes vector T5 of the specular reflection light incident to the polarization camera (6) and the Stokes vector T6 of the diffuse reflection light incident to the polarization camera (6). The calculation formula is as follows: and According to formula (7), the formula for diffuse reflected light intensity is derived: According to the calculated diffuse reflected light intensity Find the Stokes vector T6 of the diffuse reflected light; Step 5: Create an image of the same size as the original polarization image, and determine the pixel values ​​of the created image according to the Stokes vector T6 to obtain an image of the target highly reflective object (1) after removing the specular reflection light.

2. The method for imaging a highly reflective object based on the polarization characteristics of light according to claim 1, characterized in that: In step 1, set up the lighting subsystem by: First, a laser light source, a linear polarizing plate (4), a first quarter wave plate (5) and a polarization measuring instrument are sequentially arranged, and laser light emitted by the laser light source sequentially passes through the linear polarizing plate (4) and the first quarter wave plate (5) before being incident on the polarization measuring instrument, and the polarization measuring instrument is connected to a computer, and the computer is used to display the state of the Poincare sphere in real time; The angle of the first quarter wave plate (5) is adjusted until the Poincare sphere is in a circular state, the polarization meter is removed, and the laser light source is replaced by a ring-shaped white light source (3).

3. The method for imaging a highly reflective object based on the polarization characteristics of light according to claim 1 or 2, characterized in that: When the light emitted from the first quarter wave plate (5) is right-handed circularly polarized light, the calculation formula of the Stokes vector T2 of the right-handed circularly polarized light is: in, T0 is the Stokes vector of the annular white light source (3), M1 is the Mueller matrix of the linear polarizer (4), M2 is the Mueller matrix of the first quarter wave plate (5), P is the light intensity, θ is the angle between the polarization direction and the x-axis, and U is the angle between the fast axis of the first quarter wave plate (5) and the x-axis.

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