Polarized spectral image capturing device and polarized spectral image capturing system

Abstract

To provide a polarized spectral image capturing device capable of capturing two spectral images with one of the Stokes parameters S1, S2, and S3 and the Stokes parameter S0, and acquiring a polarized spectral image specific to an object.SOLUTION: A polarized spectral image capturing device is provided, comprising a polarization diffraction grating for diffracting and separating orthogonal polarization components of light under measurement, a spectral camera configured to simultaneously capture spectral images of the light under measurement separated into orthogonal polarization components, and an information processing unit for information processing of the spectral images.SELECTED DRAWING: Figure 6

Description

[Technical field]

[0001] The present invention relates to a polarized spectroscopic imaging device and a polarized spectroscopic imaging system for measuring the wavelength dependency of the polarization distribution of light to be measured. [Background technology]

[0002] Polarization imaging technology is used in a wide range of fields because it can obtain polarized images according to the optical anisotropy and optical rotation of the object's material. In recent years, a proposal has been made to simultaneously measure polarized images for multiple bands by using color filters. Since the degree of freedom of light includes wavelength in addition to polarization, this proposal can be considered as a technology that extends the polarization imaging technology to the degree of freedom of wavelength.

[0003] Patent Document 1 discloses a polarization imaging device that combines a polarizer array camera with four color filters to obtain a color polarization image. The polarizer array camera has four polarizers with different transmission axis orientations arranged in four pixels, and can image the Stokes parameters S1 and S2, which represent the intensity information of linearly polarized light, and S0, which represents the total intensity, as color images using the four pixels.

[0004] Here, the Stokes parameters S0, S1, S2, and S3 are generally defined as follows: S0 is the total light intensity; S1 is the difference between the light intensity of the 0-degree linearly polarized component and the 90-degree linearly polarized component divided by the total light intensity; S2 is the difference between the light intensity of the 45-degree linearly polarized component and the 135-degree linearly polarized component divided by the total light intensity; and S3 is the difference between the light intensity of the right-handed circularly polarized component and the left-handed circularly polarized component divided by the total light intensity. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 6923897 Summary of the Invention [Problem to be solved by the invention]

[0006] In the polarization imaging device that combines the above-mentioned polarizer array camera with four color filters, there is a problem in that the color filters have a broad transmission spectrum, resulting in low wavelength resolution and making it difficult to use them for spectroscopic purposes.

[0007] Because it is difficult to use for spectroscopic purposes, it is difficult for this polarization imaging device to identify the material that makes up the subject Ob, even if the material has wavelength-dependent polarization optical properties such as birefringence.

[0008] In addition, because the polarizer array camera is combined with color filters of multiple colors, the polarization distribution image acquired for each color has a lower resolution than the original resolution of the polarizer array camera. Therefore, an interpolation process is performed using super-resolution technology to restore the polarization image to the original resolution of the polarizer array camera. Therefore, there is a problem in that it is difficult to guarantee the accuracy of the restored image obtained by interpolation.

[0009] Furthermore, in principle, a polarizer array camera cannot recover the Stokes parameter S3, which represents the intensity information of circularly polarized light, making it difficult to accurately measure, for example, the wavelength dependence of the spatial distribution of the optical activity of a subject.

[0010] The object of the present invention is to solve the above-mentioned problems and provide a polarized spectral imaging device and a polarized spectral imaging system that identifies the material that constitutes a subject Ob by measuring a polarized spectral image specific to the subject Ob from two spectral images, one of the Stokes parameters S1, S2, and S3 of the measured light and the Stokes parameter S0 of the measured light, while ensuring the accuracy of the measured light by using a spectral camera to capture the measured light whose orthogonal polarization components are diffracted and separated by a polarizing diffraction grating. [Means for solving the problem]

[0011] In order to achieve the above-mentioned object, the polarized spectral image capturing system of the present invention is characterized in that it comprises a polarized irradiation unit having a function of irradiating a subject with controlled polarization irradiation light, a polarized diffraction grating that diffracts and separates orthogonal polarization components of the measured light, a spectral camera that acquires a spectral image of the measured light separated into each orthogonal polarization component, and an information processing unit that processes the spectral image.

[0012] More specifically, it has the following configuration.

[0013] (Aspect 1) A polarized spectral image capturing device comprising a polarizing diffraction grating having a function of spatially separating orthogonal polarization components, an information processing unit, and a spectral camera, wherein the imaging element simultaneously captures spectral images of each of the orthogonal polarization components of the light to be measured. (Aspect 2) The polarized spectral image capturing device according to aspect 1, wherein the polarization diffraction grating has a wavelength band of 300 nm or more in which the diffraction efficiency η0 of zero-order light is 30% or less among wavelengths of 300 nm or more and 1700 nm or less. (Aspect 3) The polarization diffraction grating has a diffraction efficiency of −1st order light, η -1 and the diffraction efficiency of +1st order light η +1 The polarized spectral imaging device according to aspect 1, wherein the absolute value of the difference between the above values ​​is 20% or less at wavelengths of 300 nm or more and 1700 nm or less. (Aspect 4) The polarized spectral imaging device according to aspect 1, wherein the information processing unit calculates a polarized spectral image of at least one of the Stokes parameters S1, S2, and S3. (Aspect 5) 5. The polarized spectral imaging device according to claim 4, wherein the information processing unit calculates both the polarized spectral image and a spectral image. (Aspect 6) The polarized spectral imaging device according to aspect 4, wherein the information processing unit identifies materials constituting the subject by image processing of the polarized spectral images. (Aspect 7) 5. The polarized spectral image capturing device according to embodiment 4, further comprising an optical element having birefringence as a component for capturing an image of a polarization state. (Aspect 8) 8. The polarized spectral imaging device according to any one of aspects 1 to 7, wherein the absolute value of the angle between the spectroscopic axis of the spectroscopic optical system and the grating vector of the polarization diffraction grating is 45 degrees or more and 135 degrees or less. (Aspect 9) An image capturing system including the polarized spectral image capturing device according to any one of aspects 1 to 8, a polarized irradiation unit, and the subject, wherein the spectral bandwidth of the irradiated light from the polarized irradiation unit is 200 nm or more, the irradiated light is fully polarized or partially polarized with a polarization degree of 50% or more but less than 100%, and the ellipticity is 50% or more. Effect of the Invention

[0014] The polarized spectral image capturing device of the present invention can capture two spectral images, one of the Stokes parameters S1, S2, and S3, and the other of the Stokes parameter S0, and can obtain a polarized spectral image specific to a subject, thereby providing a polarized spectral image capturing device and a polarized spectral image capturing system that can identify the material that makes up the subject Ob. [Brief description of the drawings]

[0015] [Figure 1] FIG. 1 is a block diagram of an example embodiment of a polarized spectral imaging system. [Diagram 2] FIG. 2 is a diagram illustrating a configuration of an example of an embodiment of a polarized light irradiation unit. [Diagram 3] FIG. 1 is a diagram illustrating a configuration of an example of an embodiment of a polarized spectral image capturing device. [Figure 4] FIG. 2 is an explanatory diagram of a spectroscopic axis of a spectroscopic optical system. [Diagram 5]1 is a diagram illustrating the optical characteristics of a polarization diffraction grating. [Figure 6] FIG. 2 is an explanatory diagram of a measurement state of the polarized spectral image capturing device in the first embodiment. [Figure 7] These are polarized spectral images of three types of plastic plates. [Figure 8] FIG. 1 is an explanatory diagram of polarization spectra measured on three types of plastic plates. [Figure 9] FIG. 1 is an explanatory diagram of a measurement state by a spectroscopic camera in Comparative Example 1. [Figure 10] These are scattering spectrum images measured for three types of plastic plates. [Figure 11] FIG. 1 is an explanatory diagram of scattering spectra measured on three types of plastic plates. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail with reference to the accompanying drawings.

[0017] (Polarized Spectral Imaging System) FIG. 1 is a diagram showing an example of a configuration of an embodiment of a polarized spectral imaging system. The polarized spectral imaging system detects a position r i =(r i sinθ i cosφ i ,r i sinθ i ·sinφ i ,r i cosθ i ) and the position r m =(r m sinθ m cosφ m ,r m sinθ m ·sinφ m ,r m cosθ m) is equipped with a polarized spectral image capturing unit 20 that is movably installed at the center of the object Ob. Here, r represents the center position of the object Ob, i.e., the distance from the origin of the polar coordinate system, θ represents the deviation angle from the z axis, and φ represents the deviation angle from the x axis. The movable range of the polarized light irradiation unit 10 is 0≦r i , 0≦θ i <180°, 0≦φ i <360°, and the movable range of the polarized spectral image capturing unit 20 is 0≦r m , 0≦θ m <180°, 0≦φ m <360°.

[0018] (Polarized Spectral Imaging System: Functions of Each Component) The polarized light irradiation unit 10 has a function of irradiating the object Ob with the irradiation light LS with controlled polarization. The polarized spectral image capturing unit 20 has a function of capturing an image of the polarization state of the measurement light LR generated by scattering, reflection, absorption, etc. of the irradiation light LS by the object Ob, and calculating one of the Stokes parameters S1, S2, and S3 of the measurement light LR and the Stokes parameter S0. Furthermore, the polarized light irradiation unit 10 has a function of measuring a change in the polarization component generated between the irradiation light LS and the measurement light LR by the object Ob. Note that this change in the polarization component changes uniquely for each object Ob.

[0019] In this specification, the "measurement light LR" may be referred to as the "light to be measured", but both refer to the same light.

[0020] (Polarized Spectral Imaging Device) The polarized spectral image capturing section 20 is also called a polarized spectral image capturing device. The polarized spectral image capturing section 20 includes at least a polarizing diffraction grating, a spectral camera, and an information processing unit.

[0021] (Polarized Spectral Imaging System: Configuration) In FIG. 1, the measurement light LR reflected by the subject Ob of the irradiation light LS irradiated from the polarized irradiation unit 10 is imaged by the polarized spectral image capturing unit 20. However, the arrangement of the polarized irradiation unit 10 and the polarized spectral image capturing unit 20 is not limited to this and can be changed as desired.

[0022] (Polarized Spectral Imaging System: Polarized Irradiation Unit 1) FIG. 2 is a configuration diagram of the polarized light irradiation unit 10, where (a) shows the configuration of the first polarized light irradiation unit 10 and (b) shows the configuration of the second polarized light irradiation unit 10' using a magnifying irradiation optical system 13.

[0023] (Polarized light irradiation part: light source) In the first polarized light irradiation unit 10, a light source 11 and a polarization control element 12 are sequentially arranged on the same optical axis. The light source 11 is preferably a broadband light source such as a supercontinuum light source, an ASE light source, an SLD light source, an LED light source, a halogen lamp light source, or a xenon lamp light source, and it is preferable to use a light source having a spectral bandwidth of preferably 200 nm or more, more preferably 400 nm or more, and even more preferably 600 nm or more. If the bandwidth is less than 200 nm, the narrow bandwidth is insufficient for acquiring a polarized spectral image for spectroscopic purposes, and it becomes necessary to prepare a plurality of light sources 11 and acquire the polarized spectral image while exchanging the light sources 11. If the bandwidth is wide, a broadband polarized spectral image can be acquired without exchanging the light source 11.

[0024] (Irradiation light) Moreover, the illumination light LS from the light source 11 preferably penetrates into the interior of the subject Ob, and has a spectral band in which the polarization state changes depending on the internal structure and physical properties of the subject, and more preferably has a spectral band from visible light to infrared light. The polarization control element 12 is not particularly limited in shape or configuration as long as it has a function of being able to arbitrarily change the polarization state of the illumination light LS emitted from the light source 11, and may be configured, for example, by combining a polarizing plate and a quarter-wave plate, or an electro-optical modulator or the like may also be used.

[0025] In the first polarized light irradiation section 10 configured in this manner, the illumination light LS emitted from the light source 11 passes through the polarization control element 12 and becomes highly accurate circularly polarized light, for example, completely polarized light or partially polarized light with a polarization degree of 50% or more and less than 100%, and can illuminate the subject Ob. When the circularly polarized illumination light LS is completely polarized, it is desirable that the ellipticity is 50% or more and less than 100%, and when it is partially polarized, it is desirable that the ellipticity ε and the polarization degree p are ε≧50%, p≧50%, preferably ε≧80%, p≧80%, more preferably ε≧90%, p≧90%, and even more preferably ε≧98%, p≧98%.

[0026] (Polarized Spectral Imaging System: Polarized Irradiation Unit 2) Unlike linearly polarized light, circularly polarized light does not have a polarization direction, and therefore makes it possible to obtain physical property information of the subject Ob isotropically compared to linearly polarized light. From this perspective as well, it is desirable for the illumination light LS to be circularly polarized light.

[0027] A second polarized light irradiation unit 10' shown in FIG. 2(b) can be used instead of the first polarized light irradiation unit 10. The second polarized light irradiation unit 10' is configured by adding a magnified irradiation optical system 13, and the light source 11 and the polarization control element 12 are the same as those of the first polarized light irradiation unit 10. The magnified irradiation optical system 13 is composed of a first lens that magnifies the irradiation light LS' and a second lens that converts the irradiation light LS' into parallel light or diffused light. This is a preferable configuration when using a light source 11 that has high directivity of the irradiation light LS'. In addition, the magnified irradiation optical system 13 does not need to be composed of a first lens and a second lens as long as it has the function of illuminating the subject Ob as parallel light or diffused light.

[0028] (Polarized Spectral Imaging Device) FIG. 3 is a configuration diagram of the polarized spectral image capturing unit 20, where (a) is a configuration diagram of the first polarized spectral image capturing unit 20 using a polarizing diffraction grating and a spectral camera SP (embodiment 1), and (b) is a configuration diagram of the second polarized spectral image capturing unit 20′ using a polarizing diffraction grating, a retarder, and a spectral camera SP (embodiment 2).

[0029] A retarder is an optical element that has birefringence.

[0030] (Polarized Spectral Imaging Device 1) The polarized spectral image capturing section 20 of the first embodiment is configured by sequentially arranging a polarizing diffraction grating 22 and a spectral camera Sp on the same optical axis from the subject Ob side. The information processing unit is only required to be electrically connected to the spectral camera and to be able to receive an image signal from the spectral camera, and may be incorporated integrally with the spectral camera or provided separately.

[0031] (Polarized Spectral Imaging Device: Spectral Camera) The inside of the spectral camera Sp is composed of an imaging lens optical system 23, a slit 24, a spectroscopic optical system 25, an imaging lens optical system 26, and an imaging element 27 arranged in this order on the same optical axis from the subject Ob side, but this structure is merely a representative example of an embodiment, and any existing technology can be adopted as long as it is a configuration that can provide the functionality of a spectral camera.

[0032] (Polarized Spectral Imaging Device: Polarized Diffraction Grating) The polarization diffraction grating 22 may be attached inside the spectral camera Sp, or may be located between the imaging lens optical system 23 and the slit 24, between the slit 24 and the spectroscopic optical system 25, between the spectroscopic optical system 25 and the imaging lens optical system 26, or between the imaging lens optical system 26 and the imaging element 27. Furthermore, the polarization diffraction grating 22 may be provided as an attachment, as a mechanism that is detachable from the spectral camera Sp.

[0033] The polarization grating 22 has a function of spatially separating the orthogonal polarization components of the measurement light LR. The specific optical characteristics of the polarization grating 22 will be described later, but when the measurement light LR is incident, the polarization grating 22 has a function of diffracting the left and right circularly polarized components in individual directions of specific orders, and spatially separating them into a left circularly polarized component and a right circularly polarized component. The left and right circularly polarized components of the measurement light LR pass through the imaging lens optical system 23 while being spatially separated into two by the polarization grating 22. Furthermore, only a part of the measurement light LR is transmitted by the slit 24, and two spectral images of the measurement light LR at the part that passed through the slit 24 are formed on the imaging element 27 by the spectroscopic optical system 25 and the imaging lens optical system 26. The spectral images are simultaneously captured while the slit 24 is moved sequentially, and the image processing unit processes the images, thereby calculating the spectral image Im. In this way, the polarization spectral image capturing device of the embodiment can simultaneously capture the spectral images of the orthogonal polarization components of the measurement light LR (measured light).

[0034] (Spectral camera: Spectroscopic optical system) The spectroscopic optical system 25 only needs to have a function of splitting the measurement light LR, and it is preferable to use an optical element such as a prism, a diffraction grating, or a diffractive optical element (DOE).

[0035] The spectral axis of the spectroscopic optical system 25 is defined as shown in FIG. 4. The spectral axis is on the line where the x'y' plane intersects with the spectroscopic surface. The direction of the spectral axis is from the short wavelength side to the long wavelength side of the light dispersed by the spectroscopic optical system 25. The angle between the spectral axis and the y' axis is Φ 分光軸 It is defined as:

[0036] (Spectral camera: imaging element) The imaging element 27 is preferably capable of detecting the measurement light LR from inside the object Ob, and desirably capable of detecting light within a frequency band ranging from visible light to infrared light. By using the spatial separation function of the polarization diffraction grating 22 for the left and right circularly polarized components, the spectral images Im of the right and left circularly polarized components can be captured simultaneously, and the information processing unit can calculate the polarization spectral images for the Stokes parameters S0 and S3.

[0037] (Polarized Spectral Imaging Device 2) As another embodiment of the present invention, the configuration of the polarized spectral image capturing section 20' of the embodiment 2 shown in Fig. 3(b) may be used instead of the polarized spectral image capturing section 20 of the embodiment 1. The polarized spectral image capturing section 20' of the embodiment 2 is configured such that the retarder 21 is disposed closer to the subject Ob than the polarization diffraction grating 22, and is otherwise the same as the polarized spectral image capturing section 20 of the embodiment 1.

[0038] (Polarized Spectral Imaging Device: Retarder) If the direction in which the polarization diffraction grating 22 spatially separates the measurement light LR into a left-handed circularly polarized component and a right-handed circularly polarized component is defined as the 0 degree direction, the fast axis of the retarder 21 is set to face one of 0 degrees, 45 degrees, 90 degrees, and 135 degrees. When the retarder 21 is an optical element having birefringence called a quarter-wave plate and the fast axis is set at 45 degrees or 135 degrees, (1) the 0-degree and 90-degree linearly polarized components in the measurement light LR are converted into left-handed and right-handed circularly polarized components, and when it is set at 0 degrees or 90 degrees, (2) the 45-degree and 135-degree linearly polarized components in the measurement light LR are converted into left-handed and right-handed circularly polarized components.

[0039] In the polarized spectral image capturing device of the second embodiment, it is preferable to include an optical element having birefringence as a component for capturing an image of the polarization state of the measurement light LR.

[0040] In the polarized spectral image capturing section 20' of the second embodiment configured as described above, the measurement light LR from the subject is incident on the retarder 21. The retarder 21 converts the measurement light LR either by (1) converting the linearly polarized components at 0 degrees and 90 degrees into left- and right-handed circularly polarized light, or by (2) converting the linearly polarized components at 45 degrees and 135 degrees into left- and right-handed circularly polarized light. The left- and right-handed circularly polarized components of the measurement light LR are then spatially separated into two by the polarizing diffraction grating 22 and pass through the imaging lens optical system 23. Furthermore, only a portion of the measurement light LR is transmitted by the slit 24, and the imaging element 27 forms two spectral images of the measurement light LR at the portion that has passed through the slit 24 by the spectroscopic optical system 25 and the imaging lens optical system 26. The spectral images are simultaneously captured while the slit 24 is moved in sequence, and image processing is performed to obtain a spectral image Im'.

[0041] (Polarized Spectral Imaging Device: Information Processing Unit) As mentioned above, the measurement light LR Stokes parameter S0 = total light intensity Stokes parameter S1 = The difference in light intensity between the 0-degree linearly polarized component and the 90-degree linearly polarized component divided by the total light intensity Stokes parameter S2=the difference in light intensity between the 45-degree linearly polarized component and the 135-degree linearly polarized component divided by the total light intensity, Stokes parameter S3 = The difference in light intensity between the right-handed circularly polarized component and the left-handed circularly polarized component divided by the total light intensity and the information processing unit calculates a polarization spectrum image of at least one of the Stokes parameters S1, S2, and S3 as follows:

[0042] The spectral image Im' of the two separated images of the measurement light LR captured by the imaging element 27 can be calculated in the information processing unit by calculating the difference between the spectral images of either two images, (1) a 0-degree linearly polarized component and a 90-degree linearly polarized component converted into left-right circularly polarized light, or (2) a 45-degree linearly polarized component and a 135-degree linearly polarized component converted into left-right circularly polarized light, to calculate the polarized spectral image for the Stokes parameter S1 from (1) and the Stokes parameter S2 from (2), and the sum of the two images can be calculated to calculate the polarized spectral image for the Stokes parameter S0. The Stokes parameter S3 can be calculated by moving the retarder 21 so that the measurement light LR does not pass through the retarder 21, and then calculating the difference between the spectral images of the two images, the right-handed circularly polarized component and the left-handed circularly polarized component, in the information processing unit.

[0043] In the embodiment of the present invention, the information processing unit calculates both the spectral image and the polarization spectral image. When calculating the polarization spectral image, the information processing unit calculates the difference between the spectral images of the two images, but the resolution of the spectral image is maintained at the resolution of the imaging element 27. Therefore, the resolution of the polarization spectral image is also maintained at the resolution of the imaging element 27, and no reduction in resolution occurs. In other words, since there is no need to perform an interpolation process using a super-resolution technique as in the technique of Patent Document 1, the accuracy of the polarization spectral image can be ensured.

[0044] The information processing unit processes the polarization spectrum image using machine learning to identify the materials that make up the subject. In the image processing of the polarization spectrum image using machine learning, for example, principal component analysis is used to extract feature quantities of the spectral waveform of the polarization spectrum image, and then cluster analysis using the k-means method is used to identify the materials.

[0045] (Polarized Spectral Imaging Device: Polarized Diffraction Grating) FIG. 5 is an explanatory diagram of the optical characteristics of the polarization diffraction grating 22 in the polarized spectral image capturing section 20, where (a) shows the optical anisotropy, (b) shows the diffraction characteristics, and (c) shows the diffraction plane and grating vector.

[0046] As shown in Fig. 5(a), the polarization diffraction grating 22 is a diffractive optical element whose optical axes are distributed periodically within the element surface. As shown in Fig. 5(b), when the phase difference induced by birefringence with respect to the incident light is an odd multiple of π, the polarization diffraction grating 22 diffracts right circularly polarized light in the +1st order light direction when left circularly polarized light is incident on the polarization diffraction grating 22, and diffracts left circularly polarized light in the -1st order light direction when right circularly polarized light is incident on the polarization diffraction grating 22, thereby separating linearly polarized light into left and right circularly polarized light. Depending on the distribution of the optical axes of the polarization diffraction grating 22, it is also possible to exhibit a function of diffracting left circularly polarized light in the +1st order light direction when right circularly polarized light is incident on the polarization diffraction grating 22, and right circularly polarized light in the -1st order light direction when left circularly polarized light is incident on the polarization diffraction grating 22. In addition, the direction in which the left and right circularly polarized light are spatially separated and diffracted does not necessarily have to be ±1st order as long as it is a specific order.

[0047] In general, the phase difference of the polarization diffraction grating 22 may be any value. In this case, diffracted light also appears in the direction of the zero-order light. The phase difference induced by the polarization diffraction grating 22 due to birefringence for the incident light is defined as Δφ, and the Stokes parameter S3 of the incident light is defined as S 3in Then, the diffraction efficiency of the 0th order light is η0, and the diffraction efficiency of the -1st order light is η -1 , Diffraction efficiency of +1st order light η +1 can be expressed by the following equations (1) to (3), where the diffraction efficiency is normalized between 0 and 1.

[0048]

number

[0049]

number

[0050]

number

[0051] According to formulas (1) to (3), when the phase difference induced by birefringence is not an odd multiple of π, a part of the light incident as 0th order light is emitted from polarization grating 22, but the function of spatially separating and diffracting left and right circularly polarized light is maintained in polarization grating 22. In particular, any optical system that can sufficiently separate the images of 0th order light, -1st order light, and +1st order light can be used for polarized spectral imaging without any problems.

[0052] Equations (1) to (3) are results derived from the most basic theory regarding polarization diffraction gratings, and the diffraction efficiency of polarization diffraction grating 22 does not necessarily have to conform to equations (1) to (3).

[0053] (Polarization grating: recording material) The polarization diffraction grating 22 having the optical characteristics described above includes a polarization sensitive recording material and is fabricated by recording a polarization hologram. In this process, the polarization direction of the irradiated light and the direction of the optical anisotropy according to the polarization ellipticity, that is, the direction of the optical axis and the magnitude of birefringence, are recorded in the recording material.

[0054] The recording material contained in the polarization diffraction grating 22 is made of a photoreactive polymer film, and is preferably a liquid crystal polymer film. The polarization diffraction grating 22 may be a liquid crystal cell type. The polarization diffraction grating 22 is preferably a polarization diffraction grating formed by applying a liquid crystal polymer film onto a substrate, such as glass, and then photocuring the film. More preferably, the liquid crystal polymer film has a twist rotation structure, is made up of two or more layers of liquid crystal polymer film, and further, the twist rotation direction is different between the left and right layers. When the polarization diffraction grating 22 is the polarization diffraction grating having a twist rotation structure in the liquid crystal polymer film described above, the diffraction efficiency η of -1st order light is -1 , Diffraction efficiency of +1st order light η +1 The diffraction efficiency η0 of the zeroth-order light does not follow equations (1) to (3), respectively, and the diffracted light of the zeroth-order light can be suppressed over a wide wavelength band, which can contribute to reducing noise in the polarization spectrum image and expanding the field of view.

[0055] (Polarizing grating: Diffraction efficiency of 0th order light) It is desirable that the diffraction efficiency η0 of the 0th-order light of the polarization diffraction grating 22 is low. This is because, if the 0th-order light exists, it becomes difficult to obtain an accurate polarization spectrum image in the region where the -1st-order light and the 0th-order light or the +1st-order light and the 0th-order light overlap on the imaging element 27. It is preferable that the wavelength band in which the diffraction efficiency η0 of the 0th-order light of the polarization diffraction grating 22 is 30% or less of the wavelengths of 300 nm to 1700 nm is 300 nm or more, more preferably the wavelength band in which the diffraction efficiency η0 is 20% or less of the wavelengths of 300 nm to 1700 nm is 250 nm or more, and even more preferably the wavelength band in which the diffraction efficiency η0 is 10% or less of the wavelengths of 300 nm to 1700 nm is 150 nm or more. As a result, the amount of the 0th-order light is reduced, so that it is possible to obtain an accurate polarization spectrum image in the region where the -1st-order light and the 0th-order light or the +1st-order light and the 0th-order light overlap, and the field of view that can be captured can be expanded.

[0056] (Polarizing grating: birefringence distribution) Due to imperfections in the distribution of birefringence induced in the polarization grating 22, some light may be diffracted into diffraction orders of +2 or more and -2 or less. This phenomenon is called the diffraction efficiency η -1 and the diffraction efficiency of +1st order light, η +1 This reduces the measurement accuracy of the polarized spectral image, and therefore it is desirable that the birefringence distribution is as close to the ideal distribution as possible, and the sum of the diffraction efficiencies of +2nd and higher orders and -2nd and lower orders at wavelengths of 300 nm or more and 1700 nm or less is preferably 5% or less, more preferably 2% or less, and most preferably 0.5% or less.

[0057] (Polarizing grating: Diffraction efficiency of ±1st order light) Diffraction efficiency η of the −1st order light of the polarization diffraction grating 22 -1 and the diffraction efficiency of +1st order light η +1 The diffraction efficiency η of ±1st order light is the sum of ±1It is desirable that the diffraction efficiency η of the polarizing diffraction grating 22 for ±1st order light is high over a wide wavelength range. This is because, when capturing a polarized spectrum image, a sufficient amount of light can be focused on the imaging element 27, and noise that occurs during light detection can be reduced. ±1 It is preferable that the wavelength band in which the wavelengths of 300 nm to 1700 nm account for 50% or more is 500 nm or more, it is more preferable that the wavelength band in which the wavelengths of 300 nm to 1700 nm account for 70% or more is 300 nm or more, and it is even more preferable that the wavelength band in which the wavelengths of 300 nm to 1700 nm account for 80% or more is 250 nm or more. This makes it possible to capture a polarized spectral image in the visible range, near infrared range, or visible to near infrared range with high accuracy.

[0058] In order to accurately measure the polarized spectral image, the diffraction efficiency η of the −1st order light of the polarized diffraction grating 22 is -1 and the diffraction efficiency of +1st order light η +1 It is desirable that the absolute value of the difference in the diffraction efficiency of polarization grating 22 be low over a wide wavelength band. The absolute value of the difference in the diffraction efficiency of polarization grating 22 is preferably 20% or less, more preferably 10% or less, and even more preferably 5% or less, over a wavelength range of 300 nm or more and 1700 nm or less.

[0059] (Polarization grating: grating vector) The grating vector of the polarization diffraction grating 22 is defined as shown in FIG. 5(c). The grating vector is on the straight line where the x'y' plane intersects with the diffraction plane. The direction of the grating vector is the direction from the right-handed circularly polarized component to the left-handed circularly polarized component in the +1st-order light and -1st-order light diffracted by the polarization diffraction grating 22. The angle between the grating vector and the y' axis is defined as Φ 格子ベクトル It is defined as:

[0060] In order to simultaneously capture the spectral images Im of the right and left circularly polarized components and calculate the polarization spectral images for the Stokes parameters S0 and S3, the images of the left and right circularly polarized components of the measurement light LR must fit within the imaging plane of the imaging element 27. The absolute value of the angle |Φ格子ベクトル -Φ 分光軸 is preferably 45 degrees or more and 135 degrees or less, more preferably 70 degrees or more and 110 degrees or less, and even more preferably 85 degrees or more and 95 degrees or less. This makes it possible to prevent the images of the left circularly polarized component and the right circularly polarized component of the measurement light LR from protruding from the imaging surface of the imaging element 27. Furthermore, it is possible to reduce registration errors that occur when calculating the difference between the spectral images of the two images described above. Here, the direction in which the spectroscopic optical system 25 splits the measurement light LR is from the short wavelength side to the long wavelength side.

[0061] For example, by irradiating circularly polarized illumination light LS from the polarized illumination unit 10 onto the object Ob and capturing a polarized spectral image, it is possible to accurately measure the wavelength dependence of the spatial distribution of the optical activity of the object Ob.

[0062] In the polarized spectral image capturing device and polarized spectral image capturing system according to the embodiment configured as described above, the illumination light LS emitted from the polarized illumination unit 10 becomes circularly polarized light with high ellipticity and high degree of polarization, and illuminates the object Ob. The illumination light LS is scattered, reflected, absorbed, transmitted, etc. by the object Ob to generate measurement light LR, and the polarized spectral image capturing unit 20 captures the measurement light LR spatially separated according to the polarization state to calculate a polarized spectral image for the Stokes parameter S3. EXAMPLES

[0063] Example 1 FIG. 6 is an explanatory diagram of measurement using the polarized spectral image capturing system of the first embodiment. In order to measure an object Ob placed at the origin of the coordinate system, the polarized light irradiation unit 10 is aligned along the r i = 40 cm, θ i =90 degrees, φ i = 30 degrees, and the polarized spectral image capturing unit 20 is located at r m = 40 cm, θ m =90 degrees, φ m= 0 degrees. A halogen lamp was used as the light source 11 of the polarized light irradiation unit 10. Left circularly polarized LCP was irradiated onto the subject Ob as the irradiation light LS from the polarized light irradiation unit 10. Plates of polyvinyl chloride (PVC), polycarbonate (PC), and acrylic resin (PMMA) were used as the subject Ob. The polarized spectral image capturing unit 20 captured an image according to the polarization state of the measurement light LR, and the polarized spectral images for the Stokes parameters S0 and S3 were calculated from the spectral image Im.

[0064] The spectral image Im was captured at wavelength intervals of 10 nm in the near-infrared wavelength band from 1100 nm to 1600 nm. Polarized spectral images for the Stokes parameter S0 and the Stokes parameter S3 were calculated from the spectral image Im at wavelength intervals of 10 nm from 1100 nm to 1600 nm.

[0065] Figure 7 shows polarized spectral images at 1100 nm, 1200 nm, 1300 nm, 1400 nm, 1500 nm, and 1600 nm for the Stokes parameter S3 captured by a polarized spectral imaging system for polyvinyl chloride (PVC), polycarbonate (PC), and acrylic resin (PMMA) plates.

[0066] FIG. 8 shows the wavelength dependence of the Stokes parameter S3 at the center of the polarization spectrum image.

[0067] The polarized spectral images of the polyvinyl chloride (PVC), polycarbonate (PC), and acrylic resin (PMMA) plates show that they each have different wavelength dependences. This is thought to reflect the wavelength dependence of the polarization optical properties, including birefringence, of the materials that make up the object Ob.

[0068] The polarization spectrum image has the same resolution as the imaging element 27, and there is no degradation in the resolution of the polarization spectrum image. Furthermore, the polarization spectrum image is not interpolated, so the accuracy of the polarization spectrum image is guaranteed. Moreover, the polarization spectrum image is captured at wavelength intervals of 10 nm from 1100 nm to 1600 nm, which is sufficient for spectroscopic purposes.

[0069] Comparative Example 1 9 is an explanatory diagram of the measurement by the spectroscopic camera of Comparative Example 1, in which the polarized spectroscopic image capturing unit 20 in Example 1 is changed to a spectroscopic camera SP. Specifically, in order to measure the object Ob placed at the origin of the coordinate system, the polarized light emitting unit 10 is set at r i = 40 cm, θ i =90 degrees, φ i = 30 degrees, and the spectral camera Sp is aligned in the polar coordinate system with r m = 40 cm, θ m =90 degrees, φ m = 0 degrees. A halogen lamp was used as the light source 11 of the polarized light irradiation unit 10. Left-handed circularly polarized LCP was irradiated onto the subject Ob as the irradiation light LS from the polarized light irradiation unit 10. The subject Ob was a plate made of polyvinyl chloride (PVC), polycarbonate (PC), and acrylic resin (PMMA). A scattering spectrum image of the subject Ob was captured by the spectral camera Sp.

[0070] Here, the scattered light spectral image is an image of normalized scattered light intensity obtained by dividing a spectral image of an object Ob by a spectral image of a reference white board.

[0071] Figure 10 shows scattering spectral images of polyvinyl chloride (PVC), polycarbonate (PC), and acrylic resin (PMMA) plates taken with a spectral camera Sp at 1100 nm, 1200 nm, 1300 nm, 1400 nm, 1500 nm, and 1600 nm.

[0072] FIG. 11 shows the wavelength dependence of the scattering spectrum at the center of the scattering spectrum image, that is, the wavelength dependence of the normalized scattered light intensity.

[0073] In the scattering spectrum images of the polyvinyl chloride (PVC), polycarbonate (PC), and acrylic resin (PMMA) plates, no clear difference in wavelength dependency was observed between the images. The materials that make up the object Ob, polyvinyl chloride (PVC), polycarbonate (PC), and acrylic resin (PMMA), are all plastic materials, and their scattering spectra are similar, which is thought to be why no clear wavelength dependency could be confirmed.

[0074] Even if no clear difference in wavelength dependency between the constituent materials is observed in the scattering spectral image obtained when an object Ob composed of multiple materials is photographed using a spectral camera alone, a polarized spectral image having clearly different wavelength dependencies between the multiple materials constituting the object Ob can be obtained by photographing the object using the polarized spectral image capturing system of the present invention.

[0075] In other words, even if it is difficult to distinguish the substances that make up the subject Ob using spectral images that can be captured with conventional spectral cameras, such as reflection spectral images, absorption spectral images, transmission spectral images, and scattering spectral images, the polarized spectral image capturing system of the present invention can identify the substances that make up the subject Ob.

[0076] A scattering spectrum image or a reflection spectrum image can be obtained by performing image processing such as normalizing a polarized spectrum image of the object Ob with a polarized spectrum image with a Stokes parameter S0 of a reference white board, that is, a spectrum camera equipped with a polarization diffraction grating can simultaneously capture both a polarized spectrum image and a spectrum image that can be captured by a conventional spectrum camera. Even when a polarized spectrum image showing no clear difference in wavelength dependency between the constituent substances is captured when an object Ob composed of multiple substances is captured, the polarized spectrum image capturing system according to the present invention can calculate the polarized spectrum image to identify the substances that compose the object Ob. [Industrial Applicability]

[0077] By processing the polarized spectral image with a program using machine learning, it is also possible to automatically identify the material that constitutes the subject Ob. The polarized spectral image capturing system having an information processing unit that executes the program is particularly useful in applications such as a camera for detecting defective products in the manufacturing industry, a camera for detecting defective products in food factories, a camera for managing crop growth in the agriculture, forestry and fisheries industry, a camera for autonomous driving of automobiles, a visual information device for self-driving heavy machinery in civil engineering works, a ground-based camera for satellites or aircraft for confirming the damage situation during disasters, a visual information device for robots, and a camera for exploring extraterrestrial resources on the moon or asteroids. [Explanation of symbols]

[0078] 1. Polarized Spectral Imaging System 10, 10' Polarized light irradiation unit 11 Light source 12 Polarization control element 13. Magnified illumination optical system 20, 20' Polarized spectral image capturing device (polarized spectral image capturing unit) 21 Retarder 22 Polarization grating 23, 26 Imaging lens optical system 24 Slit 25 Spectroscopic optical system 27 Imaging element LS, LS' irradiation light LR measurement light (light to be measured) Ob Subject

Claims

1. A polarization diffraction grating that has a function of spatially separating orthogonal polarization components; An information processing unit; A spectral camera is provided. The spectral camera simultaneously captures spectral images of the orthogonal polarization components of the light under test. A polarized spectral image capturing device.

2. The polarization diffraction grating has a diffraction efficiency of 0th order light, η ０ However, among the wavelengths of 300 nm or more and 1700 nm or less, the wavelength band in which the wavelengths are 30% or less is 300 nm or more.

2. The polarized spectral image capturing device according to claim 1 .

3. The polarization diffraction grating has a diffraction efficiency of −1st order light, η －１ and the diffraction efficiency of +1st order light η ＋１ The absolute value of the difference is 20% or less at wavelengths of 300 nm or more and 1700 nm or less.

2. The polarized spectral image capturing device according to claim 1 .

4. The information processing unit calculates the Stokes parameters S １ , S ２ , S ３ Calculate at least one polarized spectral image of 2. The polarized spectral image capturing device according to claim 1 .

5. The information processing unit calculates both the polarized spectral image and the spectral image.

5. The polarized spectral image capturing device according to claim 4.

6. The information processing unit identifies materials constituting the subject by image processing of the polarized spectral image.

5. The polarized spectral image capturing device according to claim 4.

7. The component for imaging the polarization state includes an optical element having birefringence.

5. The polarized spectral image capturing device according to claim 4.

8. The absolute value of the angle between the spectroscopic axis of the spectroscopic optical system and the grating vector of the polarization diffraction grating is 45 degrees or more and 135 degrees or less.

8. The polarized spectral image capturing device according to claim 1, wherein the polarized spectral image capturing device is a polarized spectral image capturing device.

9. A polarized spectral image capturing device according to any one of claims 1 to 8, A polarized light irradiation unit; An image capturing system including the subject, The spectral bandwidth of the light irradiated from the polarized light irradiation unit is 200 nm or more, the light irradiated is completely polarized or partially polarized with a polarization degree of 50% or more but less than 100%, and the ellipticity is 50% or more. A polarized spectral imaging system comprising:

Images