Spectral modulator and computational hyperspectral imaging system and method

Abstract

Provided are a spectral modulator, a hyperspectral imaging system and method, the spectral modulator comprising: a first linear polarizer for receiving incident light; a second linear polarizer; at least one birefringent dispersion plate and at least one achromatic quarter-wave plate between the first linear polarizer and the second linear polarizer; and a rotating device for quantitatively rotating the second linear polarizer; wherein the at least one birefringent dispersion plate and the at least one achromatic quarter-wave plate comprise linear birefringent material; and wherein the included angle between the at least one birefringent dispersion plate and the in-plane orientation of one of the first linear polarizer and the second linear polarizer is fixed. The present application uses low-cost linear birefringent material to realize a spectral modulator in a limited thickness, while the spectral coding performance is improved compared with existing birefringent dispersion technology, thereby realizing a low-cost computational spectrometer.

Description

Technical Field

[0001] This invention relates to the field of imaging, and more particularly to a rotating spectral modulator and a computational hyperspectral imaging system and method based on the rotating spectral modulator. Background Technology

[0002] The statements in this section are merely to provide background information in relation to the present invention to aid in understanding the invention, and such background information does not necessarily constitute prior art.

[0003] Hyperspectral imaging is a type of imaging technology with spectral resolution, playing an important role in fields such as agricultural remote sensing, climate, astronomy, food safety, medicine, and industrial production. However, traditional hyperspectral imaging typically requires complex beam splitting and imaging mechanisms with gratings, as well as push-broom or point-scan mechanical scanning structures, which leads to drawbacks such as low light throughput and long imaging time.

[0004] To overcome the above shortcomings, computational hyperspectral imaging technology utilizes digital camera signals and computer computing power and algorithms to reconstruct hyperspectral images from grayscale images that have undergone spatial spectral encoding. Specifically, the spectral manipulation methods in computational hyperspectral imaging technology can be divided into amplitude-encoded, phase-encoded, and filter-encoded types. Amplitude-encoded computational hyperspectral imaging technology captures spectral information from many spatial points at once using an encoding aperture and a dispersive prism; phase-encoded computational hyperspectral imaging technology encodes point light sources of different wavelengths into patterns of different shapes and distributions; filter-encoded computational hyperspectral imaging modulates the wavelength distribution of transmitted light intensity; all of them require reconstruction algorithms to obtain hyperspectral images from one or a series of grayscale images.

[0005] Among these technologies, filter-coded computational hyperspectral imaging has gained widespread attention in the past two years. Currently, the mainstream technologies are divided into two types: filter array type and tunable filter type. The former uses miniature filters with different filter curves in front of pixels at different locations on the image sensor, obtaining a hyperspectral image through de-mosaic and other reconstruction algorithms. The latter obtains a spectrally modulated grayscale image by adjusting the transmission or reflection spectral curve of the tunable filter, and then combines the grayscale images with different filter curves through reconstruction algorithms to obtain a hyperspectral image. The main implementation paths for the former include organic dye lithography, Fabry-Perot interferometer array technology, and metasurface and micro / nano structured color technology. These technologies share the characteristics of complex fabrication, high cost, and the need to sacrifice spatial resolution. Furthermore, once fabricated, they cannot be adjusted, making them unsuitable for different scenarios. While the latter can adjust the spectral curve according to the needs of the scenario, it often suffers from slow response time, high cost, and poor modulation coding efficiency. Summary of the Invention

[0006] In view of the above-mentioned problems in the prior art, the present invention provides a spectral modulator, comprising:

[0007] The first linear polarizer is used to receive incident light;

[0008] Second linear polarizer;

[0009] At least one birefringent dispersion plate and at least one achromatic quarter-wave plate located between the first linear polarizer and the second linear polarizer; and

[0010] A rotating device for quantitatively rotating the second linear polarizer;

[0011] Wherein, the at least one birefringent dispersion plate and the at least one achromatic quarter-wave plate comprise linear birefringent materials; and

[0012] The angle between the at least one birefringent dispersion plate and the in-plane orientation of one of the first linear polarizer and the second linear polarizer is fixed.

[0013] In one embodiment, the spectral modulator includes a birefringent dispersion plate and an achromatic quarter-wave plate, wherein the first linear polarizer, the birefringent dispersion plate, and the achromatic quarter-wave plate are fixed together, and the birefringent dispersion plate is located between the first linear polarizer and the achromatic quarter-wave plate.

[0014] In one embodiment, the birefringent dispersion plate is composed of four layers of ethylene-tetrafluoroethylene copolymer polymer film material, each polymer film material having a thickness of 25 micrometers. The optical axes of the four layers of material are kept in the same direction, and the fast axis of the birefringent dispersion plate makes an angle of 45 degrees with the transmission axis of the first linear polarizer.

[0015] In one embodiment, the spectral modulator includes a birefringent dispersion plate and an achromatic quarter-wave plate, the first linear polarizer and the achromatic quarter-wave plate are fixed together, the birefringent dispersion plate and the second linear polarizer are fixed together, and the birefringent dispersion plate is tilted at an angle to the vertical direction.

[0016] In one embodiment, the tilt angle is 5-30 degrees.

[0017] In one embodiment, the rotating device further includes an encoder for outputting a position signal and a rotation angle.

[0018] The present invention also provides a computational hyperspectral imaging system, comprising:

[0019] The light source module is used to provide illumination to the target object;

[0020] An imaging module is used to receive reflected light from the target object and to image the spectrally modulated scene.

[0021] The aforementioned spectral modulator, located between the light source module and the target object, or between the target object and the imaging module, is used to spectrally modulate the light it receives; and

[0022] The control and calculation module is used to control the rotating device to adjust the spectral modulator to different polarizer angles and spectral transmittances, control the imaging module to acquire images after receiving the positioning signal from the rotating device, and obtain hyperspectral images by processing the acquired images through a hyperspectral image reconstruction algorithm.

[0023] In one embodiment, the spectral modulator is located between the light source module and the target object, and the light source module includes:

[0024] White light source, used to provide full-spectrum illumination covering the target wavelength range;

[0025] A pinhole for receiving light from the white light source to provide a point light source; and

[0026] A coupling lens is used to couple light emitted through a pinhole into parallel light.

[0027] The present invention also provides a computational hyperspectral imaging method for the above-mentioned computational hyperspectral imaging system, the method comprising:

[0028] Select the image acquisition angle from the calibrated sampling angles to acquire images of the target object;

[0029] Row vectors consistent with the image acquisition angle are selected from the calibrated spectral sensing matrix to form a sub-spectral sensing matrix; and

[0030] The acquired image and the sub-spectral sensing matrix are input into a pre-trained hyperspectral reconstruction model to reconstruct a hyperspectral image.

[0031] In one embodiment, the method further includes:

[0032] The spectral intensity matrix of the modulated spectrum of a point light source after passing through a spectral modulator is obtained at different rotation angles and wavelengths. This matrix is ​​then interpolated by angle and spectral dimension and transposed to obtain the spectral sensing matrix.

[0033] The hyperspectral reconstruction model is constructed using the acquired spectral sensing matrix and publicly available hyperspectral image datasets through a reconstruction algorithm.

[0034] This invention utilizes a low-cost material with linear birefringence to realize a spectral modulator with limited thickness, while simultaneously improving spectral coding performance compared to existing birefringence dispersion techniques, thereby achieving a low-cost computational spectrometer. This invention has potential applications in food safety testing, agricultural production, and other scenarios. Attached Figure Description

[0035] The embodiments of the present invention will be further described below with reference to the accompanying drawings, wherein:

[0036] Figure 1 A schematic diagram of a rotary spectral modulator according to an embodiment of the present invention is shown.

[0037] Figure 2 A schematic diagram of a rotary spectral modulator according to another embodiment of the present invention is shown.

[0038] Figure 3 A schematic diagram of a rotating spectral modulator according to yet another embodiment of the present invention is shown.

[0039] Figure 4 A schematic diagram of a computational hyperspectral imaging system according to an embodiment of the present invention is shown.

[0040] Figure 5 A schematic diagram of a computational hyperspectral imaging system according to another embodiment of the present invention is shown. Detailed Implementation

[0041] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that the embodiments given in this invention are for illustrative purposes only and do not limit the scope of protection of this invention.

[0042] In this invention, the inventors discovered through theoretical calculations that a low-cost material with linear birefringence can realize a spectral modulator with limited thickness, while its spectral coding performance is improved compared with existing birefringence dispersion technology, thereby realizing a low-cost computational spectrometer.

[0043] Figure 1 A schematic diagram of a rotating spectral modulator according to an embodiment of the present invention is shown. Figure 1As shown, the rotating spectral modulator 10 includes a first linear polarizer 11, a birefringent dispersion plate 12, an achromatic quarter-wave plate 13, a second linear polarizer 14, and a rotating device 15. The rotating device 15 is used to quantitatively rotate the second linear polarizer 14. The birefringent dispersion plate 12 and the achromatic quarter-wave plate 13 are located between the first linear polarizer 11 and the second linear polarizer 14, and both are made of linear birefringent material. The angle between the birefringent dispersion plate 12 and one of the first and second linear polarizers 11 in-plane orientation is fixed.

[0044] In this invention, the in-plane orientation of the birefringent dispersion plate and the achromatic quarter-wave plate refers to their fast axis direction, and the in-plane orientation of the first linear polarizer and the second linear polarizer refers to their transmission axis direction.

[0045] The first linear polarizer 11 is used to receive the incident light IL and convert it into linearly polarized light. In one embodiment, the first linear polarizer 11 is a thin-film polarizer or a metal wire grid polarizer.

[0046] The birefringent dispersion plate 12 is used to disperse the polarization state of its incident light according to different wavelengths. In one embodiment, the birefringent dispersion plate 12 is an optical flat or a thin film, and the materials that can be used include quartz, sapphire, and various transparent polymer films including polyethylene terephthalate (PET) and ethylene-tetrafluoroethylene copolymer (ETFE).

[0047] The achromatic quarter-wave plate 13 is used to generate a quarter-wave delay in different bands of incident light, which can convert linearly polarized light into circularly polarized light or restore circularly polarized light to linearly polarized light, while eliminating the chromatic aberration problem of ordinary single-wave plate, thereby improving spectral coding efficiency while ensuring spectral modulation depth.

[0048] The second linear polarizer 14 is used to filter out different wavelengths by polarization state filtering, thereby achieving spectral modulation of the transmitted light. Since different wavelengths of light have different polarization states, the wavelength distribution of the transmitted light can be controlled by using the polarizer, and the wavelength distribution of the transmitted light can be changed by adjusting the angle of the polarizer.

[0049] The rotating device 15 is used to quantitatively rotate the second linear polarizer 14 and is configured to output a position signal and a rotation angle. In one embodiment, the rotating device 15 includes a rotating assembly 16, to which the second linear polarizer 14 is fixed. The rotating device 15 is used to quantitatively rotate the rotating assembly 16 to control the rotation angle of the second linear polarizer 14 and is capable of outputting a position signal and a rotation angle. In one embodiment, the rotating assembly 16 can be a hollow bearing, a bracket, a transparent glass plate, etc., as long as it can fix the second linear polarizer 14 and does not affect the optical path. In one embodiment, the rotating device 15 also includes an encoder 17 for outputting a position signal and a rotation angle. In one embodiment, the rotating device 15 includes a hollow bearing and an outer peripheral fixing component, which can be manually controlled or motor-controlled. The motor control can use a servo motor, stepper motor, etc., to drive the bearing to rotate via gear transmission, worm gear transmission, or belt transmission.

[0050] In this invention, both the birefringent dispersion plate 12 and the achromatic quarter-wave plate 13 comprise linear birefringent materials. The linear birefringent material thin film can be considered as a linear retardation plate, and its modulation of the polarization state of light can be expressed in Muller matrix form as follows:

[0051] (1)

[0052] The linear birefringent material thin film used in this invention has a retardation amount With wavelength Relevant, i.e., satisfying , among which angle It is the angle between the fast axis (normal refractive index direction) of the thin film and the horizontal direction, and d is the thickness of the thin film. denoted as the birefringence of the thin film.

[0053] Achromatic quarter-wave plates, as a type of linear delay plate, have a near-constant delay over the target wavelength range. Its Muller matrix can be simplified to:

[0054] (2)

[0055] The Muller matrix expression for a linear polarizer with polarization direction θ is:

[0056] (3)

[0057] Combining the above formulas, the spectral transmittance of the system consisting of the first linear polarizer 11, the birefringent dispersion plate 12 (linear birefringent material thin film), the achromatic quarter-wave plate 13, and the second linear polarizer 14 for unpolarized incident light can be expressed as:

[0058] (4)

[0059] Where α is the angle between the fast axis of the birefringent dispersion plate 12 and the horizontal direction. To determine the angle between the fast axis of the achromatic quarter-wave plate 13 and the horizontal direction, Let be the angle between the transmission axis of the second linear polarizer 14 and the horizontal direction. The angle between the transmission axis of the first linear polarizer 11 and the horizontal direction is set to 0 to eliminate the rotational invariance of the system's spectral transmittance. The final formula for the spectral transmittance is:

[0060] (5)

[0061] In one embodiment, to achieve the maximum modulation depth, the angle of the birefringent dispersive plate 12 is set to α = -π / 4, and the angle of the achromatic quarter-wave plate 13 is set to... At this point, the spectral transmittance of the system can be expressed as:

[0062] (6)

[0063] From this formula, we can see that when the angle of the second linear polarizer 14... When the birefringence of the thin film changes, the spectral transmittance of the system also changes, thereby modulating the spectral distribution of the incident light. Specifically, if the birefringence of the thin film is appropriately selected... and its thickness To ensure that it has a single transmittance peak within the target spectral range, the wavelength of the peak will vary with the angle of the second linear polarizer 14. The increase in wavelength shifts towards shorter wavelengths. Compared to previous systems that did not use the achromatic quarter-wave plate 13, this spectral modulation avoids the problem that the spectral transmittance at some node wavelengths is not affected by the angle of the second linear polarizer 14. This not only expands the wavelength modulation range but also reduces the correlation between different wavelength modulations, thereby improving the effect of subsequent spectral reconstruction.

[0064] As can be seen from the above theoretical analysis, the positions of the birefringent dispersion plate 12 and the achromatic quarter-wave plate 13 can be interchanged without affecting the theoretical calculation results. It is only necessary to ensure that the in-plane orientation angle between the birefringent dispersion plate 12 and one of the first linear polarizer 11 and the second linear polarizer 14 is fixed. Furthermore, the system can include multiple birefringent dispersion plates and multiple achromatic quarter-wave plates, and these multiple birefringent dispersion plates and multiple achromatic quarter-wave plates can be arranged arbitrarily, because optically they can be equivalent to one birefringent dispersion plate and one achromatic quarter-wave plate.

[0065] Figure 2 A schematic diagram of a rotating spectral modulator according to another embodiment of the present invention is shown. Figure 2 As shown, the rotating spectral modulator 20 includes a first linear polarizer 21, a birefringent dispersion plate 22, an achromatic quarter-wave plate 23, a second linear polarizer 24, and a rotating device 25. The first linear polarizer 21, the birefringent dispersion plate 22, and the achromatic quarter-wave plate 23 are fixed together, and the birefringent dispersion plate 22 is located between the first linear polarizer 21 and the achromatic quarter-wave plate 23.

[0066] The first linear polarizer 21 is used to receive the incident light IL and convert it into linearly polarized light. In this embodiment, the first linear polarizer 21 is a polymer thin film polarizer with a polarization angle (i.e., the angle between the transmission axis and the horizontal direction) of 0 degrees, i.e., the horizontal direction.

[0067] The birefringent dispersion plate 22 is used to receive linearly polarized light from the first linear polarizer 21 and disperse its polarization state according to different wavelengths. In this embodiment, the birefringent dispersion plate 22 is composed of four layers of ETFE polymer film material, each layer of which is 25 micrometers thick. The optical axes of the four layers are kept in the same direction, which is used to control the birefringence retardation in the visible light range (i.e., 400nm-700nm), and its transmittance curve has a maximum and a minimum value. The four layers are held and fixed by two quartz glass optical flats. In this embodiment, the fast axis of the birefringent dispersion plate 22 makes an angle of 45 degrees with the horizontal direction, that is, the fast axis of the birefringent dispersion plate 22 makes an angle of 45 degrees with the transmission axis of the first linear polarizer 21.

[0068] The achromatic quarter-wave plate 23 receives the dispersed polarized light from the birefringent dispersive plate 22 and transfers the polarization state distribution of the dispersed polarized light from the elliptically polarized range to the linearly polarized range, thereby improving spectral coding efficiency while maintaining spectral modulation depth. In this embodiment, the achromatic quarter-wave plate 23 is composed of multiple layers of linear birefringent materials with different orientations, used to achieve a birefringence delay close to 1 / 4 wavelength in the visible light range, so as to convert the birefringently dispersed light into linearly polarized light, making its fast axis coincide with the polarization angle of the first linear polarizer 21, that is, located in the horizontal direction. In this embodiment, the angle between the fast axis of the achromatic quarter-wave plate 23 and the horizontal direction is 0 degrees.

[0069] The second linear polarizer 24 filters the dispersed linearly polarized light. In this embodiment, the second linear polarizer 14 is composed of a polymer thin film polarizer, fixed in the through hole of the bearing by an adapter, and rigidly connected to the transmission gear. The angle of the second linear polarizer 24 can be adjusted by a transmission device. By filtering different wavelengths through polarization state filtering, spectral modulation of the transmitted light is achieved.

[0070] The rotating device 25 is preferably composed of a motor and a gear set, consisting of a stepper motor, a drive gear set, and a bearing, used to drive the second linear polarizer 24 to rotate. The motor is preferably a stepper motor, whose connecting shaft is connected to the bearing through the gear set to drive the second linear polarizer 24 to rotate. Its step angle is 1.8 degrees and supports 16 microsteps, the gear set reduction ratio is 10, and the final angle accuracy is 1 arcminute.

[0071] In this embodiment, the rotating device 25 further includes an encoder 27. The encoder 27 uses a radial magnet at the end of the motor shaft in the motor and gear set to position the rotation angle, and is used to control the rotation of the motor and send a positioning signal after reaching the designated position.

[0072] Figure 3 A schematic diagram of a rotating spectral modulator according to yet another embodiment of the present invention is shown. Figure 3 As shown, the rotating spectral modulator 30 includes a first linear polarizer 31, an achromatic quarter-wave plate 32, a birefringent dispersion plate 33, a second linear polarizer 34, and a rotating device 35. The first linear polarizer 31 and the achromatic quarter-wave plate 32 are fixed together, the birefringent dispersion plate 33 and the second linear polarizer 34 are fixed together, and the birefringent dispersion plate 33 forms an angle 38 with the vertical direction.

[0073] The first linear polarizer 31 is used to receive the incident light IL and convert it into linearly polarized light. In this embodiment, the first linear polarizer 31 is a polymer thin film polarizer with a polarization angle (i.e., the angle between the transmission axis and the horizontal direction) of 0 degrees, i.e., the horizontal direction.

[0074] The achromatic quarter-wave plate 32 is composed of multiple layers of birefringent materials with different orientations, used to achieve a birefringence delay of nearly 1 / 4 wavelength in the visible light range, so as to convert linearly polarized light from the first linear polarizer 31 into circularly polarized light. Its fast axis is 45 degrees different from the polarization angle of the first linear polarizer 31, that is, it is located in the 45-degree or 135-degree direction.

[0075] The birefringent dispersion plate 33 is composed of two layers of PET polymer film material with a total thickness of 75 micrometers. The optical axes of the two layers are aligned, and the material is attached to the quartz glass plate using optically transparent adhesive. The birefringent dispersion plate 33 is fixed in the through-hole of the bearing via an adapter and rigidly connected to the transmission gear. The fast axis angle can be adjusted via a transmission device. The normal direction of this component is consistent with the rotation axis direction of the bearing. The birefringent dispersion plate 33 is used to control the birefringence retardation within the visible light range. Its transmittance curve has approximately five maximum peaks, and the perceived retardation changes when rotated to different angles.

[0076] The second linear polarizer 34 is composed of a polymer thin-film polarizer, fixed in the through hole of the bearing via an adapter, and rigidly connected to the transmission gear. The angle of the polarizer can be adjusted by a transmission device. The second linear polarizer 34 is fixed to the birefringent dispersion plate 33, maintaining a 45-degree angle between its transmission axis and the fast axis of the birefringent dispersion plate 33, both rotating simultaneously. The normal direction of this component is consistent with the rotation axis of the bearing. By filtering polarization states to filter different wavelengths, spectral modulation of the transmitted light is achieved.

[0077] The rotating device 35 consists of a stepper motor, a drive gear set, and bearings, used to drive the birefringent dispersion plate 33 and the second linear polarizer 34, which are fixed on the bearings, to rotate. The motor is a 57 stepper motor, whose connecting shaft is connected to the birefringent dispersion plate 33 and the second linear polarizer 34 through the gear set to drive their rotation. Its step angle is 1.8 degrees and supports 16 microsteps. The gear set reduction ratio is 10, and the final angle accuracy can be guaranteed to be 1 arcminute.

[0078] The rotating device 35 also includes an encoder 37, which uses a radial magnet at the end of the motor shaft in the rotating device 35 to position the rotation angle, and is used to control the motor rotation and send a positioning signal after reaching the designated position.

[0079] In this embodiment, the tilt angle 38 affects the effective optical path of the birefringent dispersion plate 33. Increasing the tilt angle changes the rotation angle of the birefringent dispersion plate 33 and the second linear polarizer 34, causing changes in the effective birefringence experienced by different wavelengths. This increases the diversity of modulation at different wavelengths and decreases the correlation of the spectral modulation curves, resulting in higher image quality reconstructed by the reconstruction algorithm. Depending on the material properties of the birefringent dispersion plate and the achromatic quarter-wave plate, the tilt angle 38 is 5-30 degrees, preferably 20-30 degrees.

[0080] The present invention also provides a passive computational hyperspectral imaging system based on the aforementioned rotating spectral modulator. This computational hyperspectral imaging system includes: a light source module for providing illumination to a target object; an imaging module for receiving reflected light from the target object and imaging the spectrally modulated scene; a spectral modulator located between the light source module and the target object, or between the target object and the imaging module, for spectrally modulating the received light; and a control and calculation module for controlling the rotating device to adjust the spectral modulator to different polarizer angles and spectral transmittances, controlling the imaging module to acquire images after receiving a positioning signal from the rotating device, and processing the acquired images using a hyperspectral image reconstruction algorithm to obtain a hyperspectral image.

[0081] Figure 4 A schematic diagram of a computational hyperspectral imaging system according to an embodiment of the present invention is shown. Figure 4As shown, the computational hyperspectral imaging system 40 includes a light source module 41, a spectral modulator 42, an imaging module 43, and a control and calculation module 44.

[0082] The light source module 41 is a broadband white light source used to provide full-spectrum illumination covering the target wavelength range.

[0083] The spectral modulator 42 is used to control the spectral distribution of transmitted light by rotating its second polarizer. In this embodiment, the rotation device 421 of the rotary spectral modulator 42 consists of a stepper motor, a drive gear set, bearings, and an encoder. The motor is a NEMA23 stepper motor, and its connecting shaft is connected to the bearing via the gear set to drive the second linear polarizer. Its step angle is 1.8 degrees and supports 16 microsteps. The gear set has a reduction ratio of 1:10, and the final angle accuracy can be guaranteed to be 1 arcminute. The encoder, using a radial magnet at the end of the motor shaft, positions the rotation angle and drives the rotatable part of the rotary spectral modulator fixed on the bearing to rotate. Upon reaching a predetermined position, it sends a positioning signal.

[0084] Imaging module 43 is used to image the spectrally modulated scene. In one embodiment, imaging module 43 includes a lens 431 and an image sensor 432. Lens 431 is used to project the image of the target object AP onto image sensor 432. In one embodiment, lens 431 is a commercial lens with a focal length of 35mm and an aperture of f / 2.0. In one embodiment, image sensor 432 is a grayscale CMOS sensor with a resolution of 2048 by 2048 pixels and a sensor size of 1 inch.

[0085] The control and calculation module 44 is used to control the rotating device 421 to adjust the rotating spectral modulator 42 to different polarizer angles and spectral transmittances. After receiving the positioning signal from the rotating device 421, it controls the image sensor 432 to acquire images. By acquiring multiple images with different spectral distributions at different polarizer angles, these acquired images are processed by a hyperspectral image reconstruction algorithm to obtain a hyperspectral image.

[0086] In one embodiment, the control and calculation module 44 includes a mechanical control module for controlling the movement of the rotating device 421 and processing the positioning signal emitted by it; an acquisition control module for controlling the image sensor 432 to acquire and collect images after the rotating device 421 reaches the designated position; and an algorithm module for post-processing and hyperspectral reconstruction of the acquired images to obtain hyperspectral imaging results.

[0087] In this embodiment, the spectral modulator, imaging module, and control and computing module can be integrated together, making it more portable.

[0088] Figure 5A schematic diagram of a computational hyperspectral imaging system according to another embodiment of the present invention is shown. Figure 5 As shown, the computational hyperspectral imaging system 50 includes: a light source module 51, a spectral modulator 52, an imaging module 53, and a control and calculation module 54.

[0089] The light source module 51 is used to provide a coupled parallel illumination source. In one embodiment, the light source module 51 includes a broadband white light source 511 for providing full-spectrum illumination covering a target wavelength range; a pinhole 512 for limiting the light source area to provide a point light source; and a coupling lens 513 for coupling light emitted through the pinhole 512 into parallel light. In one embodiment, the pinhole 512 is a slit with a diameter of 200 micrometers. In one embodiment, the coupling lens 513 is an achromatic lens.

[0090] The spectral modulator 52 is used to control the spectral distribution of transmitted light by rotating its second polarizer. The rotation mechanism 521 of the spectral modulator 52 consists of a stepper motor, a drive gear set, bearings, and an encoder. The motor is a 57 stepper motor, whose connecting shaft is connected to the rotatable part of the spectral modulator via the gear set. Its step angle is 1.8 degrees and supports 16 microsteps. The gear set has a reduction ratio of 10, ensuring a final angle accuracy of 1 arcminute. The encoder uses a radial magnet at the end of the motor shaft to position the rotation angle. Its function is to drive the rotatable part of the rotary spectral modulator 52 to rotate and send a positioning signal after reaching a predetermined position.

[0091] Imaging module 53 is used to image the spectrally modulated scene. In one embodiment, imaging module 53 includes a lens 531 and an image sensor 532. Lens 531 is used to project the image of the target object AP onto image sensor 532. In one embodiment, lens 531 is a commercial lens with a focal length of 35mm and an aperture of f / 2.0. In one embodiment, image sensor 532 is a grayscale CMOS sensor with a resolution of 2048 by 2048 pixels and a sensor size of 1 inch.

[0092] The control and calculation module 54 is used to control the rotating device 521 to adjust the rotating spectral modulator 52 to different polarizer angles and spectral transmittances. After receiving the positioning signal from the rotating device 521, it controls the image sensor 532 to acquire images. By acquiring multiple images with different spectral distributions at different polarizer angles, these acquired images are processed by a hyperspectral image reconstruction algorithm to obtain a hyperspectral image.

[0093] In one embodiment, the control and calculation module 54 includes a mechanical control module for controlling the movement of the rotating device 521 and processing the positioning signal emitted by it; an acquisition control module for controlling the image sensor 532 to acquire and collect images after the rotating device 521 reaches the designated position; and an algorithm module for post-processing and hyperspectral reconstruction of the acquired images to obtain a hyperspectral image.

[0094] In this embodiment, the illumination path is fixed, resulting in higher spectral resolution and making it more suitable for fixed-use scenarios such as laboratories.

[0095] The present invention also provides a computational hyperspectral imaging method for the above-described computational hyperspectral imaging system. The method includes the following steps.

[0096] Step S1: System spectral calibration, obtaining the spectral intensity matrix of the modulated spectrum of the point light source at different rotation angles and wavelengths after passing through the rotary spectral modulator. The spectral sensing matrix is ​​obtained by interpolating its angle and spectral dimension and then transposing it.

[0097] The spectral calibration procedure for this system only needs to be repeated once for each system. For systems that have already been calibrated, this step can be omitted.

[0098] In one embodiment, for Figure 4 The system shown allows the target object AP to be replaced by a point light source output by a monochromator. This point light source has different wavelengths, for example, outputting points every 2 nm from 400 nm to 700 nm. For each wavelength, the control and calculation module 44 sets different rotation angles, for example, uniformly setting 200 sampling angles between 0 and 180 degrees. The control and calculation module 44 controls the rotating device 421 to rotate to a specific angle, causing the second linear polarizer in the spectral modulator 42 to rotate to a specific angle, thereby giving the spectral modulator 42 a specific spectral transmittance curve. After the rotating device 421 reaches a preset angle each time, the control and calculation module 44 sends a command to the image sensor 432 and acquires an image of the point light source. For example, when a point light source with a wavelength of 500 nm is used as the target object AP, the rotatable second linear polarizer in the spectral modulator 42 is uniformly set with 200 sampling angles between 0 and 180 degrees, and an image is acquired at each sampling angle. (i is the sampling angle index), and from the image Average intensity value extracted from the midpoint light source region This operation is repeated for point light sources of different wavelengths to finally obtain the spectral intensity matrix. The spectral sensing matrix is ​​obtained by interpolating the angle and spectral dimension of the spectral intensity matrix and then transposing it.

[0099] In one embodiment, for Figure 5 The system shown eliminates the need for an additional point light source because the pinhole 512 provides a point light source. When the xenon lamp is used as the light source 511, different rotation angles are set by the mechanical control module, for example, 180 sampling angles are uniformly set between 0 and 90 degrees. The mechanical control module controls the rotating device 521 to rotate to a specific angle, causing the rotatable part of the spectral modulator 52 to rotate to that specific angle, thus giving the rotary spectral modulator 52 a specific spectral transmittance curve. After the rotating device 521 reaches a preset angle each time, a spectrometer measures the spectral curve after the point light source couples through the rotary spectral modulator 52. The spectral intensity values ​​of the illumination source at the target object at each sampling angle are collected and recorded as a spectral intensity matrix. The spectral sensing matrix is ​​obtained by interpolating the angle and spectral dimension of the spectral intensity matrix and then transposing it.

[0100] Step S2: Spectral Reconstruction Algorithm. A hyperspectral reconstruction model is constructed using the acquired spectral sensing matrix and publicly available hyperspectral image datasets.

[0101] In one embodiment, a deep learning algorithm is used to train on a publicly available hyperspectral image dataset. Each hyperspectral image is simulated into a compressed grayscale image group through a spectral sensing matrix. The simulated compressed image and the hyperspectral image are fed into the reconstruction network for training to obtain a hyperspectral reconstruction model.

[0102] S3: Spectral Image Acquisition. Select the image acquisition angle from the calibrated sampling angles to acquire an image of the target object.

[0103] In one embodiment, the second linear polarizer in the rotating spectral modulator is uniformly set with 20 sampling angles between 0° and 180°. Each time the rotating device reaches a preset angle, the acquisition control module of the control and calculation module sends a command to the image sensor of the imaging module and acquires a grayscale image of the modulated target object. The acquired grayscale images are then arranged sequentially as follows: .

[0104] In one embodiment, the angle at which the rotating device stops is preset. If driven by a stepper motor, the rotating device will stop at each angle and send a positioning signal to the control and calculation module. The control and calculation module then issues an image acquisition command. If driven by a servo motor or a brushless DC motor, the rotating device will trigger a positioning signal at a specific angle, and the imaging module will synchronously acquire images based on the image acquisition command issued by the calculation module or the positioning signal issued by the rotating device. The acquired images are recorded sequentially. .

[0105] S4: Reconstruct the hyperspectral image. Select row vectors consistent with the image acquisition angle from the calibrated spectral sensing matrix to form a sub-spectral sensing matrix. The acquired images and sub-spectral sensing matrix The image is input into a pre-trained hyperspectral reconstruction model to reconstruct a hyperspectral image. .

[0106] In one embodiment, image registration and channel alignment are performed on the acquired images to obtain... After being fed into the hyperspectral reconstruction model, a hyperspectral image Hpqn is obtained, where p and q are image pixels and n is the spectral channel. Image registration ensures that the positions of the acquired images are aligned, thereby improving image resolution.

[0107] This invention utilizes a low-cost material with linear birefringence to realize a spectral modulator with limited thickness, while simultaneously improving spectral coding performance compared to existing birefringence dispersion techniques, thereby achieving a low-cost computational spectrometer. This invention has potential applications in food safety testing, agricultural production, and other scenarios.

[0108] While the present invention has been described through preferred embodiments, it is not limited to the embodiments described herein, and various changes and modifications are made without departing from the scope of the invention.

Claims

1. A spectral modulator, comprising: The first linear polarizer is used to receive incident light; Second linear polarizer; At least one birefringent dispersion plate and at least one achromatic quarter-wave plate are located between the first linear polarizer and the second linear polarizer; as well as A rotating device for quantitatively rotating the second linear polarizer; Wherein, the at least one birefringent dispersion plate and the at least one achromatic quarter-wave plate comprise linear birefringent materials; and The angle between the at least one birefringent dispersion plate and the in-plane orientation of one of the first linear polarizer and the second linear polarizer is fixed.

2. The spectral modulator according to claim 1, wherein, The spectral modulator includes a birefringent dispersion plate and an achromatic quarter-wave plate. The first linear polarizer, the birefringent dispersion plate, and the achromatic quarter-wave plate are fixed together, and the birefringent dispersion plate is located between the first linear polarizer and the achromatic quarter-wave plate.

3. The spectral modulator according to claim 2, wherein, The birefringent dispersion plate is composed of four layers of ethylene-tetrafluoroethylene copolymer polymer film material, each layer of polymer film material is 25 micrometers thick, the optical axes of the four layers of material are kept in the same direction, and the fast axis of the birefringent dispersion plate is at an angle of 45 degrees to the transmission axis of the first linear polarizer.

4. The spectral modulator according to claim 1, wherein, The spectral modulator includes a birefringent dispersion plate and an achromatic quarter-wave plate. The first linear polarizer and the achromatic quarter-wave plate are fixed together, and the birefringent dispersion plate and the second linear polarizer are fixed together. The birefringent dispersion plate is tilted at an angle to the vertical direction.

5. The spectral modulator according to claim 4, wherein, The tilt angle is 5-30 degrees.

6. The spectral modulator according to any one of claims 1-5, wherein, The rotating device also includes an encoder for outputting position signals and rotation angles.

7. A computational hyperspectral imaging system, comprising: The light source module is used to provide illumination to the target object; An imaging module is used to receive reflected light from the target object and to image the spectrally modulated scene. The spectral modulator according to any one of claims 1-6 is located between the light source module and the target object or between the target object and the imaging module, and is used to perform spectral modulation on the light it receives; as well as The control and calculation module is used to control the rotating device to adjust the spectral modulator to different polarizer angles and spectral transmittances, control the imaging module to acquire images after receiving the positioning signal from the rotating device, and obtain hyperspectral images by processing the acquired images through a hyperspectral image reconstruction algorithm.

8. The computational hyperspectral imaging system according to claim 7, wherein, The spectral modulator is located between the light source module and the target object. The light source module includes: White light source, used to provide full-spectrum illumination covering the target wavelength range; A pinhole for receiving light from the white light source to provide a point light source; and A coupling lens is used to couple light emitted through a pinhole into parallel light.

9. A computational hyperspectral imaging method, used in the computational hyperspectral imaging system of any one of claims 7-8, the method comprising: Select the image acquisition angle from the calibrated sampling angles to acquire images of the target object; Select row vectors from the calibrated spectral sensing matrix that are consistent with the image acquisition angle to form a sub-spectral sensing matrix; as well as The acquired image and the sub-spectral sensing matrix are input into a pre-trained hyperspectral reconstruction model to reconstruct a hyperspectral image.

10. The computational hyperspectral imaging method according to claim 9, wherein, The method further includes: The spectral intensity matrix of the modulated spectrum of a point light source after passing through a spectral modulator is obtained at different rotation angles and wavelengths. This matrix is ​​then interpolated by angle and spectral dimension and transposed to obtain the spectral sensing matrix. The hyperspectral reconstruction model is constructed using the acquired spectral sensing matrix and publicly available hyperspectral image datasets through a reconstruction algorithm.

Images