Programmable light source based active hyperspectral imaging optical system and method

Abstract

This invention relates to the field of active hyperspectral imaging technology, specifically to an active hyperspectral imaging optical system and method based on a programmable light source. The active hyperspectral imaging optical system based on a programmable light source includes: a broadband light source, a beam shaping module, a CT structure beam splitting module, a digital micromirror device (DMM), a homogenizing module, and an imaging detector, all connected sequentially by optical paths. Both the DMM and the imaging detector are connected to a synchronization control unit and are synchronously controlled by the control unit. Specifically, the switching frequency of the coded pattern loaded onto the DMM is synchronized with the exposure sequence of the imaging detector. This invention, through the synergy of optical path design, high-speed electronically controlled coding, and active homogenizing technology, effectively improves the system's light energy transmission efficiency while retaining the high-speed programmable spectral selection advantage of the DMM and ensuring the uniformity of the output illumination in both spatial and spectral dimensions.

Description

Technical Field

[0001] This invention relates to the field of active hyperspectral imaging technology, and more specifically to an active hyperspectral imaging optical system and method based on a programmable light source. Background Technology

[0002] Hyperspectral imaging systems, as a key technology in the field of spectral analysis, play an important role in agriculture, food, and medicine. The core of existing active hyperspectral imaging systems typically includes a light source, a spectral selection device, and a detector. Among these, the key components for achieving flexible spectral selection are usually filters or tunable filters. Band selection is achieved by rotating different bandpass filters, but this method suffers from problems such as high mechanical inertia, slow switching speed, and difficulty in flexible programming. Summary of the Invention

[0003] Therefore, the technical problem to be solved by the present invention is to overcome the defects existing in the prior art, thereby providing an active hyperspectral imaging optical system based on a programmable light source.

[0004] An active hyperspectral imaging optical system based on a programmable light source includes: a broadband light source, a beam shaping module, a CT structure beam splitting module for linearly expanding the beam into a continuous color band in space according to wavelength, a digital micromirror device, a light homogenizing module, and an imaging detector connected in sequence. The digital micromirror device and the imaging detector are both connected to the synchronization control unit and are synchronized based on the synchronization control unit. Specifically, the switching frequency of the coded pattern loaded onto the digital micromirror device is synchronized with the exposure timing of the imaging detector.

[0005] Preferably, the beam shaping module includes a collimating lens, a first beam expander, a second beam expander, a plano-convex cylindrical lens, and an adjustable slit, which are connected in sequence by optical paths and located on the same optical axis. In this configuration, the incident end of the collimating lens is attached to the exit end of the broadband light source. The first and second beam expanders constitute a Keplerian beam expander structure.

[0006] Preferably, the CT structure beam splitting module includes: a spherical collimating mirror, a reflective diffraction grating, and a concave cylindrical reflector connected in sequence by optical paths; A spherical collimating lens is connected to an adjustable slit optical path to receive a rectangular parallel beam of light.

[0007] Preferably, the adjustable slit is a rectangular entrance slit with a width of 0.1 mm and a numerical aperture of 0.22.

[0008] Preferably, the beam homogenizing module includes: a beam coupling module and a main beam homogenizing module connected in sequence by optical paths; The beam coupling module uses a first-order curved light guide cone; Among them, the first-order curved light guide cone is connected to the optical path of the digital micromirror device; Preferably, the main homogenizing module uses an integrating sphere; The inlet and outlet of the integrating ball are arranged at a 90-degree angle.

[0009] An active hyperspectral imaging method, implemented using an active hyperspectral imaging optical system based on a programmable light source and an image reconstruction system, specifically includes the following steps: S1. Acquire illumination light: Activate the active hyperspectral imaging optical system based on a programmable light source and output illumination light to illuminate the target scene; S2. Switch the encoding pattern by preset timing and simultaneously acquire the corresponding single-band image: When the digital micromirror device switches to the encoding pattern representing different spectral components based on the timing control of the synchronous control unit, the imaging detector simultaneously acquires the single-band image of the target under the corresponding band illumination in the target scene. S3. Input several spectral components and corresponding single-band images into the image reconstruction system to reconstruct a hyperspectral data cube containing two-dimensional spatial information and one-dimensional spectral information.

[0010] The technical solution of this invention has the following advantages: The proposed architecture of CT beam splitting + digital micromirror device encoding + light guide cone-integrating sphere homogenization integrates optical path design, high-speed electronic control encoding and active homogenization technology. By combining these, it effectively improves the system's light energy transmission efficiency while retaining the advantages of high-speed programmable spectral selection of digital micromirror devices, and ensures the uniformity of output illumination in both spatial and spectral dimensions. Attached Figure Description

[0011] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0012] Figure 1 This is a schematic diagram of the optical system structure in Embodiment 1 of the present invention; Figure 2 A schematic diagram of the original spectral distribution of a representative grid; Figure 3 A schematic diagram of the Gaussian fit spectral distribution of a representative grid; Figure 4 This is a schematic diagram illustrating the relationship between wavelength and position in the X direction. Figure 5 This is a schematic diagram illustrating the analysis of wavelength and position in the Y direction; Figure 6This is a schematic diagram of the fitted test curve; Figure 7 This is a schematic diagram of the illumination distribution at the output end of the uniform light module; Figure 8 A schematic diagram showing that the output spectrum of the homogenizing module covers the entire visible band of the imaging detector. Figure 9 This is a schematic diagram of the wavelength distribution when some micromirrors are turned on.

[0013] Explanation of reference numerals in the attached figures: 1-Broadband light source; 2-Collimating lens, 3-First beam expander, 4-Second beam expander, 5-Planar-convex cylindrical lens, 6-Adjustable slit; 7-Spherical collimating mirror, 8-Reflective diffraction grating, 9-Concave cylindrical mirror, 10-Digital micromirror device, 11-First-order curved light guide cone, 12-Integrating sphere, 13-Target under test, 14-Imaging detector. Detailed Implementation

[0014] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0015] Example 1 This embodiment discloses an active hyperspectral imaging optical system based on a programmable light source, including: a broadband light source 1, a beam shaping module, a CT structure beam splitting module, a digital micromirror device 10, a light homogenizing module, and an imaging detector 14 connected in sequence by optical paths; Working principle: A broadband light source 1 emits a beam, which passes through a beam shaping module and is then linearly expanded into a continuous color band in space according to wavelength by a CT structure beam splitting module. This band is then projected onto the target surface of the digital micromirror device 10, forming a spatial position-wavelength mapping relationship. Furthermore, by loading different binary coding patterns onto the digital micromirror device 10, the on / off state of each column of micromirrors in the digital micromirror device 10 is controlled, enabling real-time and dynamic reflection of a specific wavelength combination spectrum from the target surface of the digital micromirror device 10. The beam selected by the digital micromirror device 10 is then guided to a homogenizing module. Within the homogenizing module, the beam undergoes multiple diffuse reflections and thorough mixing, ultimately outputting uniform illumination light in both spatial intensity and spectral composition, for actively and uniformly illuminating the target 13 within the target scene.

[0016] The digital micromirror device 10 and the imaging detector 14 are both connected to the synchronization control unit and are synchronized based on the synchronization control unit. Specifically, the switching frequency of the coded pattern loaded onto the digital micromirror device 10 is synchronized with the exposure timing of the imaging detector 14.

[0017] Specifically: Broadband Light Source 1: In this embodiment, the broadband light source 1 uses an LDLS laser-driven light source as the illumination source of the optical system. Compared with traditional halogen tungsten lamps or xenon lamps, it has higher brightness, smaller size, and covers the entire visible light range, so as to ensure that the final output illuminance meets the preset imaging requirements after subsequent beam splitting, modulation and homogenization.

[0018] Beam shaping module: The beam shaping module provides the entire optical system with high-brightness, wide-spectrum, continuous and stable illumination light, and shapes the divergent light emitted by the broadband light source 1 into a rectangular parallel beam that meets the requirements of the subsequent CT structure beam splitter entrance.

[0019] like Figure 1 As shown, in this embodiment, the beam shaping module includes a collimating lens 2, a first beam expander 3, a second beam expander 4, a plano-convex cylindrical lens 5, and an adjustable slit 6, which are connected in sequence by optical paths and located on the same optical axis. Since the emitted light from the broadband light source 1 has a certain divergence angle, in this embodiment, the incident end of the collimating lens 2 is attached to the emitting end of the broadband light source 1 to collimate the light beam into parallel light. The first beam expander 3 and the second beam expander 4 form a Keplerian beam expander structure. Combined with the matching focal length, they achieve a 5x beam expansion, enlarging the beam diameter to the required size and reducing the divergence angle. Furthermore, the plano-convex cylindrical lens 5 is used to compress the beam, shaping the circular light spot into a narrow rectangular light spot to increase the light throughput entering the slit of the beam splitting system.

[0020] CT structural beam splitter module: In this embodiment, the CT structure beam splitting module spreads light into a high-quality, low-distortion linear color band on the target surface of the digital micromirror device 10 to establish a precise spatial position-wavelength mapping relationship.

[0021] The CT structure beam splitting module includes: a spherical collimating mirror 7, a reflective diffraction grating 8, and a concave cylindrical reflector 9 connected in sequence by optical paths; The spherical collimating mirror 7 and the adjustable slit 6 are connected in the optical path to receive a rectangular parallel beam. The spherical collimating mirror 7 further collimates the rectangular field beam incident on the spherical collimating mirror 7 through the adjustable slit 6; the reflective diffraction grating 8 performs dispersion processing; finally, a concave cylindrical mirror 9 is used to converge the light, effectively suppressing the spectral line bending and first-order chromatic aberration introduced by the width of the adjustable slit 6, ensuring that the color band is linearly and stably spread on the target surface of the digital micromirror device 10.

[0022] In this embodiment, the design wavelength of the CT structure beam splitter module is 400 to 700 nm. The object-side numerical aperture of the adjustable slit 6 is 0.22, and the entrance slit size is 0.1 × 12 mm. The digital micromirror device 10 has a target surface size of 20 mm × 11.6 mm and a micromirror size of 10.8 μm. The design parameters of the core components of the CT structure beam splitter module in this embodiment are shown in Table 1. Table 1 Component parameters of the CT structure beam splitter module

[0023] Based on the above design, by optimizing the curvature and off-axis eccentricity parameters of the concave cylindrical mirror 9, the CT structure beam splitting module achieves a good balance between spectral performance and energy efficiency while ensuring a compact structure and minimizing spectral shift and line curvature within the full rectangular field of view.

[0024] The beam homogenization module includes a beam coupling module and a main beam homogenization module connected in sequence by optical paths; The beam coupling module adopts a first-order curved light guide cone 11; The primary curved light guide cone 11 is optically connected to the digital micromirror device 10, and the digital micromirror device 10 reflects the light beam carrying spatial coding information; the primary curved light guide cone 11 compresses the received light beam.

[0025] The main homogenizing module uses an integrating sphere 12 to homogenize the light beam transmitted from the first-curved light guide cone 11 again to generate polychromatic light with uniform spatial and spectral properties as illumination light. The imaging detector 14 employs an EMCCD or sCMOS detector.

[0026] Example 2 This embodiment discloses an active hyperspectral imaging method, implemented using the active hyperspectral imaging optical system and image reconstruction system based on a programmable light source from Embodiment 1, specifically including the following steps: S1. Acquire illumination light: Activate the active hyperspectral imaging optical system based on a programmable light source and output illumination light to illuminate the target scene; S2. Switch the encoding pattern by preset timing and simultaneously acquire the corresponding single-band image: When the digital micromirror device 10 switches to the encoding pattern representing different spectral components based on the timing control of the synchronous control unit, the imaging detector 14 simultaneously acquires the single-band image of the target 13 under the corresponding band illumination in the target scene. S3. Input several spectral components and their corresponding single-band images into the image reconstruction system to reconstruct a hyperspectral data cube containing two-dimensional spatial information and one-dimensional spectral information. The image reconstruction system can be selected according to the actual needs of the work.

[0027] Specifically: Spectral modulation and synthesis: After the CT structure beam splitting module forms a high-quality linear color band on the target surface of the digital micromirror device 10, it controls the digital micromirror device 10 to achieve the selection, modulation and synthesis of the spectrum, thereby constructing a complete active hyperspectral imaging function.

[0028] When the color bands, after being dispersed by the reflective diffraction grating 8, are arranged on the surface of the micromirror array, each column of micromirrors corresponds to a spectral band Δλm (m=1,2,…,1080) of approximately monochromatic light; m represents the band number, and λ represents the wavelength. By programming and controlling the switching between the on and off states of each column of micromirrors, the selection of specific wavelength components can be achieved. When a micromirror is on, the spectral component corresponding to that column is introduced into the subsequent optical path; when it is off, it is discarded. By rapidly switching different combinations of columns according to a time sequence, or adjusting the on duty cycle of a specific column over a period of time, weighted modulation of the output spectral intensity distribution can be achieved in the time domain. The wavelength components spatially encoded by the digital micromirror device 10 are superimposed and mixed in the subsequent first-order curved light guide cone 11 and integrating sphere 12, ultimately outputting light with specific spectral lines, thereby completing the spectral reconstruction of the light source.

[0029] To verify the spectral selection and synthesis capabilities of the digital micromirror device 10, this embodiment performs optical path simulation on the discrete micromirrors in the digital micromirror device 10. Since traditional sequential optics modeling is difficult to accurately describe, this embodiment uses LightTools non-sequential ray tracing software to perform non-sequential analysis on the complete optical path.

[0030] After completing the non-sequential ray tracing, a discrete ray sample set on the surface of the imaging detector 14 is derived. Each ray data includes its spatial position in the coordinate system of the imaging detector 14, wavelength information, and parameters representing energy weights.

[0031] The specific process of synthesizing the spectrum is as follows: In the analysis, the target surface of the digital micromirror device 10 is divided into a two-dimensional grid array to achieve spatial region discretization. The wavelength distribution of light is statistically analyzed in each grid, and the peak wavelength, full width at half maximum (FWHM), and spectral purity characteristics of the local spectrum are extracted to complete the reconstruction of the local continuous spectrum and output the overall output spectral characteristics of the system. Finally, a mapping model between wavelength and spatial coordinates is established, and the overall performance of the digital micromirror device 10 is quantified by calculating the gradient and distribution correlation of wavelength along each spatial dimension.

[0032] During verification, multiple representative spatial grids were selected from the 10 target surfaces of the digital micromirror device: the central grid, the minimum wavelength grid, and the maximum wavelength grid, for local spectral analysis. The results are as follows: Figure 2-3 As shown, the spectra corresponding to each spatial location all exhibit narrow-band characteristics, with peak wavelengths strictly corresponding to their spatial locations, and the spectral shape approximating a single-peak distribution. After Gaussian fitting, the full width at half maximum (FWHM) of the spectra of each grid generally remains within the range of 3-5 nm, indicating that the digital micromirror device 10 possesses good spectral resolution in local space.

[0033] By performing correlation analysis between wavelength and spatial location on the full field of view ray data, such as... Figure 4-5 The diagram shows the correlation analysis of wavelength with position in the X and Y directions. The analysis results verify the mapping relationship between dispersion and the two-dimensional XY direction. The Y direction shows a highly strict linear mapping with a correlation coefficient of -0.9999, while the X direction remains basically constant with a correlation close to 0. This proves that the technical means of Example 1 has successfully achieved stable and linear spectral expansion along a single dimension.

[0034] To verify the effectiveness of the spectral synthesis method, typical maize seed hyperspectral reflectance data was selected as the target spectrum for testing and synthesizing the spectrum. For example... Figure 6 The synthesized spectra from 450 to 650 nm showed a coefficient of determination (R²) of 0.9723, a mean absolute error (MAE) of 0.0163, and a center mean square error (CMSE) of 0.0054. Furthermore, 87.75% of the data points fell within the ±5% relative error band. These results demonstrate that the synthesized model based on the ray tracing data from the optical system in Example 1 can reconstruct complex target spectra with high accuracy.

[0035] Coupled output system design: After being encoded by the digital micromirror device 10, the light beam reflected from the open-state micromirror still has spatially separated wavelength components. Direct projection of this light would result in non-uniform target illumination in terms of intensity and spectral composition. To address this, the optical system of this embodiment 1 integrates a homogenizing module at the rear end of the reflected light path of the digital micromirror device 10, aiming to mix the discrete spectral components and output illumination light that is uniform in both spatial and spectral dimensions.

[0036] The system works as follows: The light beam selected by the digital micromirror device 10 enters the primary curved light guide cone 11. The specific parameters of the primary curved light guide cone 11 are shown in Table 2. Table 2 Parameters of a First-Order Curved Surface Light Guide Cone

[0037] The size of the inlet end of the light guide cone needs to match the aperture of the beam emitted by the digital micromirror device 10 to ensure full-aperture reception and efficient coupling of light energy; its outlet end size is slightly smaller than the incident port of the downstream integrating sphere 12 to achieve compression of the beam aperture.

[0038] The integrating sphere 12 is designed to balance signal strength and uniform light performance. Its diameter is set at 100mm, and its incident port diameter of 25mm is larger than the exit light spot of the first-order curved light guide cone 11, ensuring shear-free light injection. The exit port diameter can be set as needed, resulting in an output light field with an approximate Lambertian distribution. The incident and exit ends of the integrating sphere 12 are arranged at a 90-degree angle, physically isolating the direct path of the incident light, which is crucial for achieving high uniformity. Furthermore, the optical system strictly controls the total aperture area to be less than 5% to maintain cavity sealing, ensuring that light achieves sufficient random mixing through multiple diffuse reflections from the highly reflective inner walls. The final output maintains a uniform illumination scene in terms of spatial intensity and spectral composition.

[0039] To verify the actual performance of the homogenizing module, the illuminance distribution at the output surface of the module was evaluated. The results are as follows: Figure 7 As shown, a uniform two-dimensional distribution is formed within the illumination area of ​​the output surface of the uniform light module. From Figure 7 It is easy to see that, after processing by the homogenizing module, the discrete spectral components and non-uniform light intensity distribution related to the original coded pattern of the digital micromirror device 10 have been effectively suppressed, and the output light field has achieved a high degree of consistency in spatial intensity.

[0040] To quantitatively verify the ability of the spatial encoding of the digital micromirror device 10 to modulate the final output spectrum, the mapping relationship between encoding and output was tested. The test method was as follows: multiple micromirrors were set in the test software, the micromirror representing the target band was selected and turned on, and its output spectral distribution was detected at the output end of the integrating sphere 12.

[0041] After all the micromirrors are opened, the ideal emission spectrum should cover the entire visible wavelength range, such as... Figure 8 As shown.

[0042] When only the micromirrors corresponding to the target wavelength on the target surface of the digital micromirror device 10 are enabled and all other micromirrors are disabled, the energy of the optical system's output spectrum is concentrated in the target wavelength range of 526-574 nm, with a peak appearing near 556 nm. Figure 9 As shown, the energy outside this band decreases sharply and approaches zero.

[0043] The results show that by controlling the switching state of a specific spatial region of the digital micromirror device 10, the composition of the final output spectrum of the system can be directly and accurately modulated, realizing the function of extracting the expected narrowband spectrum from a broadband light source. Based on this verified physical mechanism, by timing the switching of the encoding pattern of the digital micromirror device 10 and synchronously controlling the exposure timing of the imaging detector 14, this embodiment 1 can be directly used for the active acquisition of hyperspectral data cubes.

[0044] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. An active hyperspectral imaging optical system based on a programmable light source, characterized in that, include: The broadband light source (1), beam shaping module, CT structure beam splitting module for linearly expanding the beam into a continuous color band in space according to the wavelength, digital micromirror device (10), light homogenizing module and imaging detector (14) are connected in sequence. The digital micromirror device (10) and the imaging detector (14) are both connected to the synchronization control unit and are synchronized based on the synchronization control unit. Specifically, the switching frequency of the coded pattern loaded onto the digital micromirror device (10) is synchronized with the exposure timing of the imaging detector (14).

2. The active hyperspectral imaging optical system based on a programmable light source according to claim 1, characterized in that, The beam shaping module includes a collimating lens (2), a first beam expander (3), a second beam expander (4), a plano-convex cylindrical lens (5), and an adjustable slit (6), which are connected in sequence and located on the same optical axis. The incident end of the collimating lens (2) is attached to the exit end of the broadband light source (1); The first beam expander (3) and the second beam expander (4) constitute a Keplerian beam expander structure.

3. The active hyperspectral imaging optical system based on a programmable light source according to claim 2, characterized in that, The CT structure beam splitting module includes: a spherical collimating mirror (7), a reflective diffraction grating (8), and a concave cylindrical reflector (9) connected in sequence by optical paths. The spherical collimating lens (7) and the adjustable slit (6) are connected in the optical path to receive a rectangular parallel beam.

4. The active hyperspectral imaging optical system based on a programmable light source according to claim 3, characterized in that, The adjustable slit (6) is a rectangular entrance slit with a width of 0.1 mm and a material-space numerical aperture of 0.

22.

5. The active hyperspectral imaging optical system based on a programmable light source according to claim 4, characterized in that, The beam homogenization module includes a beam coupling module and a main beam homogenization module connected in sequence by optical paths; The beam coupling module adopts a first-order curved light guide cone (11); Among them, the first-order curved light guide cone (11) is optically connected to the digital micromirror device (10).

6. The active hyperspectral imaging optical system based on a programmable light source according to claim 5, characterized in that, The main homogenizing module uses an integrating sphere (12); The incident and exit ends of the integrating ball (12) are arranged at a 90-degree angle.

7. An active hyperspectral imaging method, characterized in that, The application of the active hyperspectral imaging optical system and image reconstruction system based on a programmable light source as described in claim 6 specifically includes the following steps: S1. Acquire illumination light: Activate the active hyperspectral imaging optical system based on a programmable light source and output illumination light to illuminate the target scene; S2. By switching the encoding pattern through preset timing and synchronously acquiring the corresponding single-band image: When the digital micromirror device (10) switches to the encoding pattern representing different spectral components based on the timing control of the synchronous control unit, the imaging detector (14) synchronously acquires the single-band image of the target (13) under the corresponding band illumination in the target scene. S3. Input several spectral components and corresponding single-band images into the image reconstruction system to reconstruct a hyperspectral data cube containing two-dimensional spatial information and one-dimensional spectral information.

Images