A rotational diffraction multispectral imaging system with switchable optical paths

Abstract

The present invention discloses a rotational diffraction multi-spectral imaging system with switchable optical paths. The system of the present invention is composed of a transmissive liquid crystal spatial light modulator, a polarizing film, a specially designed refractive-diffractive integrated lens, and a detector. The system can switch the imaging optical path by combining a transmissive liquid crystal spatial light modulator and a polarizing film. When the rotational diffraction encoding part at the center of the refractive-diffractive integrated lens is used for imaging, a rotational diffraction blurred image containing spectral features can be obtained; when the refractive lens part at the periphery of the refractive-diffractive integrated lens is used for imaging, an image containing rich spatial detail information can be obtained. These two types of images can effectively improve the quality of reconstructed spectral images of the rotational diffraction multi-spectral imaging system. Compared with other systems, the system of the present invention has a higher efficiency in collecting images, does not require a precision moving device, and the collected images do not need to be cropped, scaled, aligned, and other operations.

Description

Technical Field

[0001] The invention belongs to a spectral coding imaging system in spectral imaging technology, and in particular relates to a rotational diffraction multi-spectral imaging system with switchable optical paths. Background Art

[0002] Spectral imaging technology is a new type of technology that uses single or multiple spectral channels for spectral data collection and processing, image display and analysis and interpretation. Since spectral imaging combines spectroscopy and imaging technology, it can simultaneously obtain the two-dimensional spatial information and one-dimensional spectral information of the target to be measured, so it has the advantages of both spectroscopy and imaging. At present, it plays a vital role in agriculture, resource exploration, remote sensing detection, military reconnaissance and other fields.

[0003] Traditional spectral imaging systems generally acquire spectral data cubes through scanning and other methods. Such systems usually have relatively complex optical systems and large volumes, which limits the application of spectral imaging technology in many fields. Some application scenarios that require real-time multispectral imaging have an urgent need for lightweight, snapshot spectral imaging systems. In 2019, Daniel S. Jeon et al. proposed a diffractive optical element with a rotating PSF characteristic, which combined with deep neural network image restoration technology to achieve high-spectral imaging with a spectral interval of 10nm in the visible light band of 420-660nm. Since the system only uses a diffractive optical element to replace the complex optical system in the traditional spectral imaging system, it has the advantages of lightweight and snapshot imaging. However, the system also has the disadvantages of large initial image blur, limited spatial resolution of the reconstruction result, large consumption of computing resources, and long network training time.

[0004] Fusion reconstruction using clear RGB images and rotational diffraction blurred images can effectively improve the quality of reconstructed spectral images. However, it is not easy to obtain these two types of images. The traditional acquisition method has a complex optical system and requires precise displacement. The two imaging results must undergo tedious image preprocessing steps such as registration, cropping, and scaling. Summary of the invention

[0005] In order to solve the technical problems existing in the background technology, the method of obtaining clear images and rotational diffraction blurred images in a multispectral imaging system based on the rotational diffraction mechanism is not simple and flexible enough. The present invention proposes a rotational diffraction multispectral imaging system with switchable optical paths. The optical imaging system is composed of a specially designed lens integrating a rotational diffraction encoding module and a refractive lens, a transmissive liquid crystal spatial light modulator, and a polarizing film. The switching of the imaging optical path can be achieved by controlling the polarization state of the transmissive liquid crystal spatial light modulator. Using the central rotational diffraction encoding imaging optical path will obtain a rotational diffraction blurred image with spectral characteristics, and using the peripheral refractive lens imaging optical path will obtain an image with relatively richer spatial detail information. The fusion reconstruction of these two images can effectively improve the spectral image reconstruction quality of the rotational diffraction spectral imaging system.

[0006] The technical solution to achieve the purpose of the present invention is:

[0007] The present invention comprises a transmissive spatial light modulator, a polarizing film, a refractive-diffractive integrated lens and a detector; the transmissive spatial light modulator, the polarizing film and the refractive-diffractive integrated lens are placed in sequence from left to right along an optical axis, the optical axis is perpendicular to the plane of the detector and the optical axis passes through the center of the detector; the transmissive spatial light modulator modulates the polarization state of an incident light field, so that the incident light of the transmissive spatial light modulator is in a first or second polarization state, and the incident light passes through an optical path in the polarizing film with the same polarization state as the incident light and a corresponding area of ​​the refractive-diffractive integrated lens in sequence, thereby realizing switching of the optical path.

[0008] The polarizing film includes a circular polarizing film and an annular polarizing film, the polarization states of the circular polarizing film and the annular polarizing film are respectively the first and second polarization states, the diameter of the circular polarizing film is D2, the inner diameter and the outer diameter of the annular polarizing film are respectively D2 and D1, the circular polarizing film and the annular polarizing film are concentric and are in the same plane, and the first and second polarization states of the circular polarizing film and the annular polarizing film are orthogonal.

[0009] The refractive-diffractive integrated lens is a plano-convex mirror, the plano-convex mirror's planar end face is arranged close to the polarizing film, the plano-convex mirror's convex end face is arranged close to the detector, the area of ​​the convex end face of the plano-convex mirror with the optical axis as the center and a diameter of D2 is arranged as a plane parallel to the planar end face, and the area of ​​the plano-convex mirror's planar end face with the optical axis as the center and a diameter of D2 is arranged as a rotating diffraction encoding module.

[0010] The rotation diffraction encoding module is set by the following formula:

[0011]

[0012]

[0013] fDOE =f lens

[0014] Where λ(θ) is the operating wavelength at angle θ, Angle The working wavelength at , h0 is the thickness between the plane end face and the convex end face of the refractive-diffractive integrated lens, n λ and n0 are the refractive indices of light with wavelength λ in the plano-convex mirror and in air, respectively. DOE is the focal length of the rotating diffraction encoding module, m is the diffraction order, N is the number of repetition periods of the rotating diffraction encoding module, and f lens is the focal length of the plano-convex mirror, and the working wavelength of the rotating diffraction encoding module is [λ min ,λ max ],λ min and λ max are the minimum and maximum operating wavelengths respectively, and h(r,θ) is the height of the rotating diffraction encoding module at an angle of θ and a radius of r.

[0015] The transmissive spatial light modulator modulates the polarization state of the incident light field, so that the light field passing through the transmissive spatial light modulator is in a first or second polarization state that is orthogonal to each other.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] (1) The present invention can switch the imaging optical path by only adjusting the polarization state of the incident light field through a transmissive spatial light modulator, thereby achieving the acquisition of a clear image and a rotational diffraction blurred image;

[0018] (2) The method of the present invention does not require a complex optical system or precise displacement;

[0019] (3) The method of the present invention uses only one sensor to collect two images, without the need for image post-processing steps such as scaling, registration, and cropping. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Schematic diagram of a rotational diffraction multispectral imaging system with switchable optical paths.

[0021] Figure 2 Schematic diagram of polarization state distribution of polarization film.

[0022] Figure 3 Schematic diagram of a refractive-diffractive lens.

[0023] Figure 4 It is the working state of the system when the transmissive spatial light modulator is in the first polarization state.

[0024] Figure 5Schematic diagram of the design of the rotating diffraction encoding module.

[0025] Figure 6 This is the point spread function of the system in each spectral band when the transmissive spatial light modulator is in the first polarization state. The resolution of the single spectral band image in the figure is 151×151 pixels.

[0026] Figure 7 It is the working state of the system when the transmissive spatial light modulator is in the second polarization state.

[0027] Figure 8 This is the point spread function of the system in each spectral band when the transmissive spatial light modulator is in the second polarization state. The resolution of the single spectral band image in the figure is 151×151 pixels.

[0028] In the figure: 1. Transmissive spatial light modulator, 2. Polarization film, 3. Refractive and diffractive integrated lens, 4. Detector. DETAILED DESCRIPTION

[0029] The present invention is described in further detail below in conjunction with the accompanying drawings.

[0030] According to the inventive content of the present invention, the complete implementation example and its implementation process are as follows:

[0031] like Figure 1 As shown, the present invention includes a transmissive spatial light modulator 1, a polarization film 2, a refractive-diffractive integrated lens 3 and a detector 4; the transmissive spatial light modulator 1, the polarization film 2, and the refractive-diffractive integrated lens 3 are placed in sequence from left to right along the optical axis, and the optical axes are kept coincident, the optical axis is perpendicular to the plane of the detector 4 and the optical axis passes through the center of the detector 4; the transmissive spatial light modulator 1 modulates the polarization state of the incident light field so that the incident light is in the first or second polarization state, and the incident light passes through the optical path in the polarization film 2 with the same polarization state as the incident light and the corresponding area of ​​the refractive-diffractive integrated lens 3 in sequence to achieve switching of the optical path.

[0032] like Figure 2 As shown, the polarizing film 2 includes a circular polarizing film and an annular polarizing film, the polarization states of the circular polarizing film and the annular polarizing film are respectively the first and second polarization states, the diameter of the circular polarizing film is D2, the inner diameter and the outer diameter of the annular polarizing film are respectively D2 and D1, the circular polarizing film and the annular polarizing film are concentric and are in the same plane, and the first and second polarization states of the circular polarizing film and the annular polarizing film are orthogonal.

[0033] like Figure 3 As shown, Figure 3(a) and (b) are respectively the front view and the side view of the refractive-diffractive integrated lens 3. The refractive-diffractive integrated lens 3 is a plano-convex mirror, the plane end face of the plano-convex mirror is arranged close to the polarizing film 2, the convex end face of the plano-convex mirror is arranged close to the detector 4, and the area of ​​the convex end face of the plano-convex mirror with the optical axis as the center and the diameter D2 is arranged as a plane parallel to the plane end face, that is, the same as the diameter of the circular polarizing film. The area of ​​the plane end face of the plano-convex mirror with the optical axis as the center and the diameter D2 is arranged as a rotating diffraction encoding module.

[0034] like Figure 5 As shown, the rotation diffraction encoding module is set by the following formula:

[0035]

[0036]

[0037] In order to ensure that the focal length of the diffraction coding pattern is consistent with the focal length of the refractive lens and that the image plane does not need to move, the focal length of the diffraction coding pattern is f DOE The following conditions must be met:

[0038] f DOE =f lens

[0039] Where λ(θ) is the operating wavelength at angle θ, Angle The working wavelength at , h0 is the thickness between the plane end face and the convex end face of the refractive-diffractive integrated lens, n λ and n0 are the refractive indices of light with wavelength λ in the plano-convex mirror and in air, respectively. DOE is the focal length of the rotating diffraction encoding module, m is the diffraction order, N is the number of repetition periods of the rotating diffraction encoding module, and f lens is the focal length of the plano-convex mirror, and the working wavelength of the rotating diffraction encoding module is [λ min ,λ max ],λ min and λ max are the minimum and maximum operating wavelengths respectively, h(r,θ) is the height of the rotating diffraction encoding module at an angle of θ and a radius of r, that is, the distance between the rotating diffraction encoding surface at an angle of θ and a radius of r and the plane of the convex end face of the plano-convex mirror.

[0040] The transmissive spatial light modulator 1 modulates the polarization state of the incident light field, so that the light field passing through the transmissive spatial light modulator 1 is in a first or second polarization state orthogonal to each other. The transmissive spatial light modulator 1 combined with the polarization state of the polarizing film 2 can screen the aperture of the imaging system, realize the switching of the imaging light path, and thus realize the acquisition of a clear image and a rotation diffraction blurred image.

[0041] like Figure 4 As shown, the working state of the imaging system for collecting the rotational diffraction blurred image is: the polarization state of the transmitted light field of the transmissive spatial light modulator 1 is adjusted to the first polarization state. At this time, since the first and second polarization states are orthogonal to each other, only the area with a central diameter of D2 in the polarizing film 2 has light passing through, and the imaging light path is the rotational diffraction encoding module at the center of the refractive-diffractive integrated lens 3. The rotational diffraction encoding module encodes the incident light field, and the detector 4 collects a color image with the rotational diffraction spectrum blur feature.

[0042] The imaging model is detailed as follows:

[0043] Assuming the complex amplitude of the input light field is u0(x',y'), it can be expressed as:

[0044]

[0045] Where A(x',y') is the amplitude and φ0(x',y') is the phase. After the phase modulation of the diffraction coding pattern, the complex amplitude of the light field becomes:

[0046]

[0047] where φ h (x', y') is the phase modulation term of the diffraction coding pattern. The light field on the rear surface of the diffraction coding pattern propagates through the Fresnel diffraction distance z to the sensor plane. The complex amplitude of the light field on the sensor plane is:

[0048]

[0049] Where k = 2π / λ is the wave vector, and the incident light field is taken as a plane wave. After simplification, the complex amplitude of the light field on the sensor plane can be re-expressed as:

[0050]

[0051] in Represents Fourier transform. The intensity image collected by the sensor is the point spread function of the system, which can be described as:

[0052]

[0053] Take the diameter D2 = 5mm, the period number of the rotating diffraction encoding module N = 2, and design the focal length f DOE =50mm, the working spectrum is 420-660nm, and the designed rotating diffraction encoding module is as follows Figure 5 At this time, the point spread function of the corresponding imaging light path in each spectral band is as follows Figure 6As shown, it can be seen that the point spread function shows obvious rotational diffraction blur characteristics in different spectral bands, and the detector collects the rotational diffraction blur image.

[0054] like Figure 7 As shown, the working state of the imaging system for collecting clear images with rich spatial detail information is: the polarization state of the transmitted light field of the transmissive spatial light modulator 1 is adjusted to the second polarization state. At this time, since the first and second polarization states are orthogonal to each other, only the annular area with an inner diameter of D2 and an outer diameter of D1 in the polarizing film 2 has light passing through, and the imaging light path is the refractive lens part outside the refractive-diffractive integrated lens 3. The refractive lens converges and images the incident light, and the detector 4 collects a clear color image with rich spatial detail information.

[0055] The imaging model is detailed as follows:

[0056] Assuming the incident light field is parallel light, the incident light field can be described as:

[0057]

[0058] Where A is the amplitude of the incident light field, φ0 is the phase constant term, and after phase modulation by the refractive lens, the light field on the rear surface of the refractive-diffractive integrated lens can be described as:

[0059]

[0060] The phase modulation caused by the refractive lens can be expressed as:

[0061]

[0062] After the light field on the back surface of the refractive-diffractive integrated lens propagates through Fresnel diffraction at a distance of z, the complex amplitude of the light field reaching the sensor surface can be expressed as:

[0063]

[0064] After simplification, the complex amplitude of the light field on the sensor plane can be re-expressed as:

[0065]

[0066] in The intensity image collected by the sensor is the point spread function of the system, which can be described as:

[0067]

[0068] Take diameter D1 = 15mm, lens focal length f lens =50mm, the point spread function of the corresponding imaging light path in each spectral band is as follows Figure 8As shown, it can be seen that the scales of the point spread function in different spectral bands are very small. At this time, the blur of the imaging light path is small, and the detector collects a clear image containing more spatial detail information.

[0069] The present invention proposes a rotational diffraction multi-spectral imaging system with switchable optical paths. During operation, the imaging optical path can be switched by simply switching the working state of the transmissive spatial light modulator, and a rotational diffraction blurred image with spectral blur characteristics and a clear image with more spatial detail information can be obtained. During the entire process, no precise movement is required, and no operations such as cropping, scaling, and registration are required for the image collected by the detector. It is simple and convenient, and can effectively improve the stability and portability of the system.

Claims

1. A rotational diffraction multispectral imaging system with switchable optical paths, characterized in that: The invention comprises a transmissive spatial light modulator (1), a polarizing film (2), a refractive-diffractive integrated lens (3) and a detector (4); the transmissive spatial light modulator (1), the polarizing film (2) and the refractive-diffractive integrated lens (3) are placed in sequence from left to right along the optical axis, the optical axis is perpendicular to the plane of the detector (4) and the optical axis passes through the center of the detector (4); the transmissive spatial light modulator (1) modulates the polarization state of the incident light field, so that the incident light of the transmissive spatial light modulator (1) is in a first or second polarization state, and the incident light passes through the optical path in the polarizing film (2) with the same polarization state as the incident light and the corresponding area of ​​the refractive-diffractive integrated lens (3) in sequence, thereby realizing the switching of the optical path; The polarizing film (2) comprises a circular polarizing film and an annular polarizing film, the polarization states of the circular polarizing film and the annular polarizing film are respectively the first and second polarization states, the diameter of the circular polarizing film is D2, the inner diameter and the outer diameter of the annular polarizing film are respectively D2 and D1, the circular polarizing film and the annular polarizing film are concentric and in the same plane, and the first and second polarization states of the circular polarizing film and the annular polarizing film are orthogonal; The refractive-diffractive integrated lens (3) is a plano-convex mirror, the plano-convex mirror has a planar end face arranged close to the polarizing film (2), the convex end face of the plano-convex mirror is arranged close to the detector (4), the area of ​​the convex end face of the plano-convex mirror with the optical axis as the center and a diameter of D2 is arranged as a plane parallel to the planar end face, and the area of ​​the plano-convex mirror with the optical axis as the center and a diameter of D2 is arranged as a rotating diffraction encoding module; The rotation diffraction encoding module is set by the following formula: Where λ(θ) is the operating wavelength at angle θ, Angle The working wavelength at , h0 is the thickness between the plane end face and the convex end face of the refractive-diffractive integrated lens, n λ and n0 are the refractive indices of light with wavelength λ in the plano-convex mirror and in air, respectively. DOE is the focal length of the rotating diffraction encoding module, m is the diffraction order, N is the number of repetition periods of the rotating diffraction encoding module, and f lens is the focal length of the plano-convex mirror, and the working wavelength of the rotating diffraction encoding module is [λ min ,λ max ],λ min and λ max are the minimum and maximum operating wavelengths respectively, and h(r,θ) is the height of the rotating diffraction encoding module at an angle of θ and a radius of r.

2. The rotational diffraction multi-spectral imaging system with switchable optical paths according to claim 1, characterized in that: The transmissive spatial light modulator (1) modulates the polarization state of the incident light field, so that the light field passing through the transmissive spatial light modulator (1) is in a first or second polarization state that is orthogonal to each other.

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