Hyperspectral imaging system

Abstract

The invention provides a hyperspectral imaging system. The hyperspectral imaging system comprises a telescope group, a collimator objective, a prism, a volume holographic diffraction grating, an imaging objective and an imaging sensor which are arranged in sequence, the telescope group is composed of a primary reflector, a secondary reflector, a third reflector, a plane reflector and an entrance slit; the primary reflector, the secondary reflector and the third reflector form an off-axis three-reflector structure; the plane reflector is used for folding a light path; the collimating objective lens, the prism, the volume holographic diffraction grating, the imaging objective lens and the imaging sensor form an imaging spectrometer, and an entrance slit is used as object space input; the formed off-axis three-mirror and folded light path and an imaging spectrometer are in coaxial butt joint by taking an entrance slit as a node, and are used for joint correction and splicing to form a hyperspectral imaging system. According to the optical path architecture, optimal matching of the unobstructed total reflection front end and the transmission type rear end is realized, and the detection sensitivity of crop weak spectrum signals is greatly improved while a better modulation transfer function of a full view field is ensured.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of spectral imaging, and particularly relates to a hyperspectral imaging system. BACKGROUND

[0002] In the field of precision agriculture and plant disease monitoring, the 900nm-1700nm short-wave infrared band is concerned because it covers the key characteristic absorption peaks of plant biochemical components. However, the existing hyperspectral imagers still have significant technical bottlenecks in terms of image quality, volume and energy utilization: the traditional transmission telescope system has difficulty in correcting chromatic aberration in a wide band, and commercial short-wave infrared telescopes are expensive, and few of them can meet the requirements of telecentricity. The coaxial reflective system has no chromatic aberration, but the central obscuration effect leads to energy loss and contrast reduction, which is not conducive to weak signal detection; the conventional off-axis three-mirror system eliminates obscuration, but the light path structure is often loose and large in transverse size, which is difficult to meet the requirements of compactness and lightness of airborne equipment; in addition, if the rear-end spectrometer system uses a traditional ruled grating, there are problems of multi-order diffraction and polarization sensitivity, and the diffraction efficiency is low, which makes it difficult to maintain high signal-to-noise ratio imaging under normal large light flux. Therefore, there is an urgent need for a short-wave infrared imaging spectrometer with compact structure, high light energy utilization and excellent image quality. SUMMARY

[0003] The application provides a hyperspectral imaging system, which adopts a design method of independent optimization of a telescope system and an imaging spectrometer, and final pupil matching and splicing. The telescope uses an off-axis three-mirror structure to eliminate central obscuration and wide-band chromatic aberration, and folds the light path through a plane mirror to compress the volume; the imaging spectrometer system adopts a structure of volume holographic grating combined with a prism, and uses the complementary characteristics of dispersion to correct spectral line curvature and chromatic aberration. The system realizes large light flux and high spectral resolution in the 900nm-1700nm short-wave infrared band, has the characteristics of compact structure, high light energy utilization, excellent full-field image quality, etc., and is suitable for weak signal detection scenes such as precision agriculture and vegetation monitoring.

[0004] According to a first aspect of the application, one or more embodiments in the application provide a hyperspectral imaging system, in the order of light incidence direction, comprising a telescope group, a collimating objective, a prism, a volume holographic diffraction grating, an imaging objective and an imaging sensor; The telescope group is composed of a primary mirror, a secondary mirror, a tertiary mirror, a plane mirror and an entrance slit, and is used for correcting primary aberration and avoiding chromatic aberration; the primary mirror, the secondary mirror and the tertiary mirror form an off-axis three-mirror structure, and the plane mirror is used for folding the light path.

[0005] The collimating objective, the prism, the volume holographic diffraction grating, the imaging objective and the imaging sensor constitute an imaging spectrometer with an incident slit as an object side input for correcting spectral line curvature and color distortion. The off-axis three-mirror structure and the folded light path are coaxially connected with the imaging spectrometer with the incident slit as a node for jointly correcting and splicing to form a hyperspectral imaging system.

[0006] According to the above technical scheme of the present application, the following improvements can be made: Optionally, the parallel light reflected by the target object passes through the primary mirror, the secondary mirror, the tertiary mirror, the plane mirror and the incident slit in sequence according to the light incident direction. The primary mirror is used for receiving incident light and reflecting it to the secondary mirror, the secondary mirror is used for receiving the reflected light of the primary mirror and reflecting it to the tertiary mirror, the tertiary mirror is used for receiving the light of the secondary mirror and reflecting it to the plane mirror, and the plane mirror is used for receiving the reflected light of the tertiary mirror and folding it to the incident slit.

[0007] Optionally, the primary mirror, the secondary mirror and the tertiary mirror are all spherical in shape, the primary mirror and the tertiary mirror are concave mirrors, and the secondary mirror is a convex mirror.

[0008] Optionally, the primary mirror has a curvature radius of -227.2mm and a conic coefficient of -97.85, the secondary mirror has a curvature radius of -44.3mm and a conic coefficient of -0.43, the tertiary mirror has a curvature radius of -52mm and a conic coefficient of 0.146, the distance between the primary mirror and the secondary mirror is -18mm, the distance between the secondary mirror and the tertiary mirror is 23.04mm, the distance between the tertiary mirror and the plane mirror is -34.96mm, and the distance between the plane mirror and the incident slit is 18mm.

[0009] Optionally, the collimating objective and the imaging objective are respectively formed by six lenses combined in sequence according to the light incident direction. The collimating objective comprises a first lens, a second lens, a third lens, a fourth lens, a fifth lens and a sixth lens arranged in sequence. The imaging objective comprises a seventh lens, an eighth lens, a ninth lens, a tenth lens, an eleventh lens and a twelfth lens arranged in sequence.

[0010] Optionally, the first lens has positive focal power and meniscus structure, the second lens has positive focal power and biconvex structure, the third lens has negative focal power and biconcave structure, the fourth lens has negative focal power and plano-concave structure, the fifth lens has positive focal power and plano-convex structure, the sixth lens has positive focal power and meniscus structure, the seventh lens has positive focal power and meniscus structure, the eighth lens has positive focal power and plano-convex structure, the ninth lens has negative focal power and plano-concave structure, the tenth lens has negative focal power and biconcave structure, the eleventh lens has positive focal power and biconvex structure, and the twelfth lens has positive focal power and meniscus structure.

[0011] Optionally, the surface of the object side of the first lens is curved towards the entrance slit, and the surface of the image side is curved towards the entrance slit. The surface of the object side of the second lens is curved towards the image plane, and the surface of the image side is curved towards the entrance slit. The surface of the object side of the third lens is curved towards the entrance slit, and the surface of the image side is curved towards the image plane. The surface of the object side of the fourth lens is curved towards the entrance slit, and the surface of the image side is a plane. The surface of the object side of the fifth lens is a plane, and the surface of the image side is curved towards the entrance slit. The surface of the object side of the sixth lens is curved towards the entrance slit, and the surface of the image side is curved towards the entrance slit. The surface of the object side of the seventh lens is curved towards the image plane, and the surface of the image side is curved towards the image plane. The surface of the object side of the eighth lens is curved towards the image plane, and the surface of the image side is a plane. The surface of the object side of the ninth lens is a plane, and the surface of the image side is curved towards the image plane. The surface of the object side of the tenth lens is curved towards the entrance slit, and the surface of the image side is curved towards the image plane. The surface of the object side of the eleventh lens is curved towards the image plane, and the surface of the image side is curved towards the entrance slit. The surface of the object side of the twelfth lens is curved towards the image plane, and the surface of the image side is curved towards the image plane.

[0012] Optionally, the apertures of the first lens, the second lens, the third lens, the fourth lens, the fifth lens, the sixth lens, the seventh lens, the eighth lens, the ninth lens, the tenth lens, the eleventh lens, and the twelfth lens are all less than 24.5 mm.

[0013] Optionally, the prism is attached to the volume holographic diffraction grating, and the top angle of the prism is less than 15°.

[0014] Optionally, the convex surfaces of the first lens and the second lens are arranged back-to-back, and together share a large positive focal power, and the fourth lens and the fifth lens form a double cemented lens; the convex surfaces of the eleventh lens and the twelfth lens are arranged back-to-back, and the eighth lens and the ninth lens form another double cemented lens.

[0015] The beneficial effects of this invention are as follows: This invention provides a hyperspectral imaging system that completely eliminates wide-band chromatic aberration by employing an off-axis three-mirror telescope system. A plane mirror is introduced to fold the optical path, effectively compressing the longitudinal and lateral dimensions of the system and solving the problem of bulky size. This allows the system to maintain a compact configuration while possessing excellent image quality without chromatic aberration and balancing system light energy utilization and imaging range. The beam-splitting system adopts a component form combining a volume holographic grating and a prism. The high diffraction efficiency of the volume holographic grating significantly improves the system's light throughput in the 900nm-1700nm band. Combined with the dispersive complementarity of the prism, it effectively corrects spectral line bending and color distortion. This optical path architecture achieves optimized matching between an unobstructed total internal reflection front end and a transmissive back end, significantly improving the detection sensitivity for weak spectral signals from crops while ensuring a superior modulation transfer function across the entire field of view. Attached Figure Description

[0016] Figure 1 This is the optical path structure of the hyperspectral imaging system according to a preferred embodiment of the present invention; Figure 2 This is a front-facing telescope system according to a preferred embodiment of the present invention; Figure 3 This is a preferred embodiment of the post-imaging spectrometer system of the present invention; Figure 4 The MTF curve of the telescope in a preferred embodiment of the present invention; Figure 5 Points in a preferred embodiment of the present invention Figure 1 ; Figure 6 Points in a preferred embodiment of the present invention Figure 2 ; Figure 7 Points in a preferred embodiment of the present invention Figure 3 ; Figure 8 The MTF curve of a preferred embodiment of the present invention at the 900nm wavelength; Figure 9 The MTF curve of a preferred embodiment of the present invention at the 1300nm wavelength; Figure 10 The MTF curve of a preferred embodiment of the present invention at the 1700nm wavelength; Figure 11 This is a spectral bending diagram of a preferred embodiment of the present invention; Figure 12 This is a color distortion diagram of a preferred embodiment of the present invention.

[0017] Explanation of reference signs: telescope system 1, collimation system 2, intermediate system 3, focusing system 4, imaging sensor 5, main mirror 11, secondary mirror 12, three-mirror 13, plane mirror 14, entrance slit 15, first lens 21, second lens 22, third lens 23, fourth lens 24, fifth lens 25, sixth lens 26, prism 31, volume holographic diffraction grating 32, seventh lens 41, eighth lens 42, ninth lens 43, tenth lens 44, eleventh lens 45, twelfth lens 46, imaging sensor 51. DETAILED DESCRIPTION

[0018] In order to make the objectives, technical solutions and advantages of the present disclosure clearer, the present disclosure will be further described in detail below with reference to specific embodiments and with reference to the drawings.

[0019] It should be noted that, unless otherwise defined, the technical terms or scientific terms used in one or more embodiments of the present application should be understood as the usual meaning understood by a person skilled in the art to which the present disclosure belongs. The terms "first", "second" and the like used in one or more embodiments of the present application do not represent any order, number or importance, but are only used to distinguish different components. The terms "include" or "contain" and the like mean that the elements or objects before the terms cover the elements or objects listed after the terms and their equivalents, and do not exclude other elements or objects. The terms "connect" or "connected" and the like are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. The terms "up", "down", "left", "right" and the like are only used to represent relative positional relationships, and when the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0020] As shown in Figures 1-12 The present application provides a hyperspectral imaging system and a design method, and the optical system in the present application is shown in Figure 1 As shown in the figure, the incident direction from left to right is, in order, the main mirror 11, the secondary mirror 12, the three-mirror 13, the plane mirror 14, the entrance slit 15, the first lens 21, the second lens 22, the third lens 23, the fourth lens 24, the fifth lens 25, the sixth lens 26, the prism 31, the volume holographic diffraction grating 32, the seventh lens 41, the eighth lens 42, the ninth lens 43, the tenth lens 44, the eleventh lens 45, the twelfth lens 46, and the imaging sensor 51.

[0021] The hyperspectral imaging system provided by the present application is based on the characteristics of the present system working in the short-wave infrared spectrum of 900nm-1700nm, and the off-axis three-mirror telecentric structure is adopted for the front telescope system, as shown in Figure 2Compared with a refractive system, the reflective system has the advantage of natural achromatism. This design completely eliminates the axial chromatic aberration and the off-axis chromatic aberration commonly seen in wide-band imaging, avoids the design complexity and transmittance loss caused by the lack of high-transmittance and low-dispersion special glass materials in the short-wave infrared band, and ensures consistent imaging quality of the system in the full working waveband. Traditional coaxial reflective systems, such as Cassegrain, have central obstruction, which reduces the energy utilization and reduces the MTF of the middle and low frequencies. The off-axis design eliminates the obstruction and improves the signal-to-noise ratio, which is crucial for spectral imaging with limited energy. The telecentric design on the image side facilitates the matching with the pupil of the rear imaging spectrometer in the integrated design.

[0022] The hyperspectral imaging system provided by the application comprises a front-mounted telescope, wherein the primary mirror 11, the secondary mirror 12 and the tertiary mirror 13 of the front-mounted telescope are all spherical mirrors. The curvature radius of the primary mirror 11 is -227.2 mm, the conic coefficient is -97.85, and the maximum radius is 8-11 mm. The curvature radius of the secondary mirror 12 is -44.3 mm, the conic coefficient is -0.43, and the maximum radius is 5-6 mm. The curvature radius of the tertiary mirror 13 is -52 mm, the conic coefficient is 0.146, and the maximum radius is 10-12 mm. The distance from the primary mirror 11 to the secondary mirror 12 is -18 mm, the distance from the secondary mirror 12 to the tertiary mirror 13 is 23.04 mm, the distance from the tertiary mirror 13 to the plane mirror 14 is -34.96 mm, and the distance from the plane mirror 14 to the entrance slit 15 is 18 mm. The primary mirror 11 is eccentric to the Y direction by -4.72 mm, the secondary mirror 12 is eccentric to the Y direction by 2.73 mm, and the tertiary mirror 13 is eccentric to the Y direction by -0.7 mm. The off-axis angle of the Y direction field of view is 26°, the X direction field of view is 7°, the focal length is 35 mm, and F is 2.5. The primary mirror 11, the secondary mirror 12 and the tertiary mirror 13 are not inclined and are all eccentrically arranged, so that the machining and assembly are simple and the cost is low.

[0023] The MTF analytical curves of the hyperspectral imaging system provided by the application under different fields of view of the front-mounted telescope are shown in the figure. Figure 4 The MTF of the telescope under the full field of view is shown in the figure. The abscissa is the spatial frequency value, the unit is cycles / mm, and the ordinate is the OTF module value, that is, the MTF modulation transfer function value. The MTF value is used to evaluate the imaging quality of the lens, and the higher the MTF curve, the better the imaging quality of the lens and the stronger the real image restoration capability. The system cutoff frequency is 33.3 lp / mm, and the optical modulation transfer function of the system under each field of view is close to 0.7, which meets the design requirements of the optical system.

[0024] The hyperspectral imaging system provided by the application comprises an imaging spectrometer, and the light path structure of the imaging spectrometer adopts a body holographic grating combined with a prism, as shown in the figure. Figure 3As shown, the dispersion complementary characteristics are used to correct the spectral line bending and color distortion. The first lens 21 has positive focal power and meniscus structure, the second lens 22 has positive focal power and biconvex structure, the third lens 23 has negative focal power and biconcave structure, the fourth lens 24 has negative focal power and plano-concave structure, the fifth lens 25 has positive focal power and plano-convex structure, the sixth lens 26 has positive focal power and meniscus structure, the seventh lens 41 has positive focal power and meniscus structure, the eighth lens 42 has positive focal power and plano-convex structure, the ninth lens 43 has negative focal power and plano-concave structure, the tenth lens 44 has negative focal power and biconcave structure, the eleventh lens 45 has positive focal power and biconvex structure, and the twelfth lens 46 has positive focal power and meniscus structure. The fourth lens 24 and the fifth lens 25 form a doublet cemented lens. The eighth lens 42 and the ninth lens 43 form a doublet cemented lens 2, and the aperture stop is placed on the volume holographic diffraction grating 32, which is used to limit the aperture size.

[0025] In the rear imaging spectrometer, the first lens 21 is made of heavy lanthanum flint glass, the second lens 22 is made of heavy lanthanum flint glass, the third lens 23 is made of heavy flint glass, the fourth lens 24 is made of heavy flint glass, the fifth lens 25 is made of heavy lanthanum flint glass, the sixth lens 26 is made of heavy lanthanum flint glass, the prism 31 is made of lanthanum flint glass, the volume holographic diffraction grating 32 is made of Schott borosilicate crown glass, the seventh lens 41 has the same material as the sixth lens 26, the eighth lens 42 has the same material as the fifth lens 25, the ninth lens 43 has the same material as the fourth lens 24, the tenth lens 44 has the same material as the third lens 23, the eleventh lens 45 has the same material as the second lens 22, and the twelfth lens 46 has the same material as the first lens 21.

[0026] In the rear imaging spectrometer, the system lens material is designed to be global surface glass, and after coating, the transmission efficiency of the optical lens is more than 99%, which has the advantages of high production frequency, sufficient supply, and low cost. The lens material and parameters of the rear imaging objective lens are consistent with those of the front collimating objective lens, which improves the production efficiency and reduces the optical cold processing cost.

[0027] In the rear imaging spectrometer, The center thickness of the first lens 21 is 4.9-6.2 mm, the center thickness of the second lens 22 is 4.5-5.5 mm, the center thickness of the third lens 23 is 4.5-6 mm, the center thickness of the fourth lens 24 is 2-3 mm, the center thickness of the fifth lens 25 is 4-6 mm, the center thickness of the sixth lens 26 is 3.5-4.2 mm, the center thickness of the prism 31 is 3.2-4.2 mm, the center thickness of the volume holographic diffraction grating 32 is 3 mm, the center thickness of the seventh lens 41 is 4.5-4.2 mm, the center thickness of the eighth lens 42 is 4-6 mm, the center thickness of the ninth lens 43 is 2-3 mm, the center thickness of the tenth lens 44 is 4.5-6 mm, the center thickness of the eleventh lens 45 is 4.5-5.5 mm, and the center thickness of the twelfth lens 46 is 4.9-6.2 mm.

[0028] The air gap between the first lens 21 and the second lens 22 is 0.2 mm, the air gap between the second lens 22 and the third lens 23 is 2.1 mm, the air gap between the third lens 23 and the double cemented lens composed of the fourth lens 24 and the fifth lens 25 is 6.69 mm, the air gap between the double cemented lens and the sixth lens 26 is 0.1 mm, the air gap between the sixth lens 26 and the prism 31 is 6.72 mm, the air gap between the prism 31 and the volume holographic grating 32 is 0 mm, the air gap between the volume holographic grating 32 and the seventh lens 41 is 7.83 mm, the air gap between the seventh lens 41 and the other double cemented lens composed of the eighth lens 42 and the ninth lens 43 is 0.1 mm, the air gap between the other double cemented lens and the tenth lens 44 is 6.69 mm, the air gap between the tenth lens 44 and the eleventh lens 45 is 2.1 mm, and the air gap between the eleventh lens 45 and the twelfth lens 46 is 0.17 mm.

[0029] The focal power of the first lens 21 to the twelfth lens 46 satisfies the following relationship: The first lens 21 has a focal power controlled between 0.02 and 0.035, the second lens 22 has a focal power controlled between 0.01 and 0.1, the third lens 23 has a focal power controlled between -0.08 and -0.05, the fourth lens 24 has a focal power controlled between -0.08 and -0.05, the fifth lens 25 has a focal power controlled between 0.02 and 0.07, the sixth lens 26 has a focal power controlled between 0.01 and 0.02, the seventh lens 41 has a focal power controlled between 0.01 and 0.02, the eighth lens 42 has a focal power controlled between 0.02 and 0.07, the ninth lens 43 has a focal power controlled between -0.08 and -0.05, the tenth lens 44 has a focal power controlled between -0.08 and -0.05, the eleventh lens 45 has a focal power controlled between 0.01 and 0.1, and the twelfth lens 46 has a focal power controlled between 0.02 and 0.035.

[0030] The above-mentioned focal power configuration makes the focal power of each lens in the optical system have a reasonable distribution ratio, which can effectively reduce the lens length and lower the tolerance sensitivity and other problems.

[0031] In the embodiment, the Abbe number v21 of the first lens 21 satisfies 30 < v21 < 38, the Abbe number v22 of the second lens 22 satisfies 25 < v22 < 30, the Abbe number v23 of the third lens 23 satisfies 15 < v23 < 20, the Abbe number v24 of the fourth lens 24 satisfies 15 < v24 < 20, the Abbe number v25 of the fifth lens 25 satisfies 25 < v25 < 30, the Abbe number v26 of the sixth lens 26 satisfies 33 < v26 < 37, the Abbe number v41 of the seventh lens 41 satisfies 33 < v41 < 37, the Abbe number v42 of the eighth lens 42 satisfies 25 < v42 < 30, the Abbe number v43 of the ninth lens 43 satisfies 15 < v43 < 20, the Abbe number v44 of the tenth lens 44 satisfies 15 < v44 < 20, the Abbe number v45 of the eleventh lens 45 satisfies 25 < v45 < 30, and the Abbe number v46 of the twelfth lens 46 satisfies 30 < v46 < 38.

[0032] The refractive index n21 of the first lens 21 satisfies 1.85 < n21 < 2, the refractive index n22 of the second lens 22 satisfies 1.9 < n22 < 2, the refractive index n23 of the third lens 23 satisfies 1.83 < n23 < 1.9, the refractive index n24 of the fourth lens 24 satisfies 1.9 < n24 < 1.98, the refractive index n25 of the fifth lens 25 satisfies 1.98 < n25 < 2, the refractive index n26 of the sixth lens 26 satisfies 1.9 < n26 < 1.95, the refractive index n41 of the seventh lens 41 satisfies 1.9 < n41 < 1.95, the refractive index n42 of the eighth lens 42 satisfies 1.98 < n42 < 2, the refractive index n43 of the ninth lens 43 satisfies 1.9 < n43 < 1.98, the refractive index n44 of the tenth lens 44 satisfies 1.83 < n44 < 1.9, the refractive index n45 of the eleventh lens 45 satisfies 1.9 < n45 < 2, and the refractive index n46 of the twelfth lens 46 satisfies 1.85 < n46 < 2.

[0033] In the embodiment, the imaging spectrometer adopts a double-sided telecentric optical layout. In the pre-telescope system, the exit pupil is placed at infinity through image-side telecentric design, thereby ensuring the parallelism of the chief ray at the slit interface and the uniformity of the spot. In order to meet the strict pupil matching requirement, the collimating objective of the imaging spectrometer is designed as a object-side telecentric structure to achieve ideal matching with the exit pupil of the pre-telescope system. Meanwhile, in order to optimize the energy response at the detector end, the imaging unit introduces an image-side telecentric mechanism to ensure that the chief ray is perpendicular to the focal plane. This measure effectively avoids the spectral crosstalk and energy attenuation problems caused by non-perpendicular incidence, and significantly improves the imaging quality of the edge field. In the specific optical design, the entrance pupil distance and the exit pupil distance of the system are set to infinity to ensure the achievement of various telecentricity indicators.

[0034] In the design of the dispersion assembly, a lanthanum fluorite prism 31 with an apex angle less than 15° is adopted. For the common spectral line bending and chromatic distortion in the imaging spectral data, the embodiment analyzes that the spectral line bending trend caused by the prism 31 splitting is short-wave, while the bending trend caused by the grating splitting is opposite, i.e., long-wave. Based on this physical property, a prism-grating dispersion architecture is introduced to offset each other by using the difference in non-main section dispersion rate. After iterative optimization by the optical design software, the originally bent spectral image is successfully corrected to a collimated form. This improvement not only optimizes the imaging quality, but also provides great convenience for subsequent data processing and geometric correction steps, avoiding complex resampling errors.

[0035] The volume holographic diffraction grating 32 is manufactured based on holographic interference imaging technology, and its core structure is to record a periodic refractive index modulation in a dichromated gelatin matrix and to seal and protect it through an optical glass window. Unlike traditional mechanically ruled gratings, the diffraction mechanism of the VPH grating follows the Bragg diffraction condition, that is, only when the incident light meets the specific Bragg angle, can the first-order diffraction with high efficiency be excited.

[0036] In order to meet the strict Bragg incidence condition of the VPH grating and avoid complex physical tilt adjustment of the grating in system adjustment, the embodiment adopts the structure that the prism 31 is closely attached to the volume holographic diffraction grating 32. By accurately designing the top angle parameter of the prism 31, the pre-deflection effect of the prism 31 is used to make the incident chief ray automatically meet the Bragg angle requirement when entering the grating medium layer. This design strategy of replacing tilt with prism top angle not only simplifies the mechanical support structure, improves the integration and stability of the system, but also effectively avoids the additional aberration introduced by the tilt of the grating.

[0037] In the short-wave infrared working waveband of 900-1700 nm, the traditional surface relief transmission grating often faces performance bottlenecks, with the highest diffraction efficiency usually being less than 50%, and the average efficiency being only about 35%, and the surface relief structure being the main source of stray light in the system. In contrast, the VPH grating selected by the present system has significant advantages in this waveband: at the central wavelength, the peak diffraction efficiency is more than 80%, the average efficiency in the whole waveband is maintained at about 70%, and the light flux of the system is greatly improved.

[0038] In order to meet the high-sensitivity detection requirements in the waveband of 900-1700 nm, the embodiment of the present imaging spectrometer integrates a high-performance LD-SW6401715-UC detector. The device has a spatial resolution of 640x512 and a pixel pitch of 15µm, and can provide an effective imaging area of 9.6mmx7.68mm. Based on the dispersion model of the system, the physical length of the spectral bandwidth of 800nm in the whole waveband is 7mm on the target surface of the detector, which corresponds to 467 pixel units in the spectral dimension. After calculation, the spectral sampling rate under this configuration reaches 1.49nm / pixel, effectively ensuring the analytical accuracy of the spectral signal.

[0039] In the embodiment, the first lens 21 and the second lens 22 are designed in a back-to-back convex surface in the imaging spectrometer, and the first lens 21 and the second lens 22 use high refractive index materials, which is beneficial to improve the light deflection ability of the lens. The light beam has a large beam aperture at the surface of the second lens 22 with positive focal power, the third lens 23 is a meniscus lens with negative focal power, and the surface with negative focal power has a smaller beam aperture, so that the contribution of the positive lens to the focal power is greater than that of the negative lens, thereby reducing the field curvature. The fourth lens 24 and the fifth lens 25 form a double cemented lens, which is originally a meniscus lens with small focal power. According to the past optical design experience, the double cemented lens is more suitable here, and the cemented surface can correct the aperture aberration, axial chromatic aberration, and the shape of the double cemented lens in the optical structure diagram is obtained after continuous hammer type optimization. The sixth lens 26 uses high refractive index materials to avoid high-order spherical aberration in the system. The arrangement of the seventh lens 41 to the twelfth lens 46 is substantially symmetrical to the first lens 21 to the sixth lens 26 based on the prism 31 and the volume holographic diffraction grating 32, but there can be some differences in the actual spatial position.

[0040] In the embodiment, in the imaging spectrometer, in order to overcome the influence of large temperature difference on the stability of the double cemented lens structure, the system selects optical glass materials with similar thermal expansion coefficients for cementing. The glass materials are screened by using the automatic replacement function of the software, and the difference between the thermal expansion coefficients of the two glasses of the double cemented lens is strictly limited to less than 1.

[0041] In the embodiment, in the imaging spectrometer, the front group is a telescope system 1, the prism and the grating are designed to be attached to form an intermediate system 3 considering the difficulty of adjustment and the balance of the entire system aberration. The mirror group on the left of the prism and the grating is regarded as a collimating system 2, and the right is regarded as a focusing system 4, and finally an imaging sensor 5. The aperture stop is on the volume holographic diffraction grating, the collimating objective and the imaging objective are symmetrical about the aperture stop, considering that the symmetrical system is helpful to correct distortion, coma and the like, and the parameters of the collimating and converging system mirrors are consistent, reducing the cost of opening the mold.

[0042] Considering the resolution and signal-to-noise ratio requirements of the hyperspectral imaging system, the numerical aperture of the system reaches more than 0.25; under the premise of high spectral resolution and high numerical aperture, good optical imaging capability is realized. The spectral range is 900nm-1700nm, the spectral resolution is 22nm, the aperture value FNO is 2.5, and the total length of the system is less than 175mm. All lens apertures are less than 24.5m.

[0043] The point spread diagram of the hyperspectral imaging system under different fields of view in the embodiment is as follows: Figure 5 、 Figure 6 、 Figure 7The point list simulation results of the system at 900 nm and 902 nm, 1300 nm and 1302 nm, and 1698 nm and 1700 nm are shown in FIG. 9. It can be seen that the diffraction spots of the two adjacent wave bands can be clearly separated in the full field of view. It is shown that the spectral resolution of the system is about 2 nm in the 900 nm-1700 nm wave band range, which meets the design requirements of high precision.

[0044] The MTF analysis curves of the hyperspectral imaging system in different fields of view are shown in FIG. 10. Figure 8 , Figure 9 , Figure 10 The modulation transfer function (MTF) characteristic curves of the optical system in this embodiment at three typical wavelengths of 900 nm, 1300 nm and 1700 nm are shown in FIG. 10. The abscissa in the figure represents the spatial frequency (unit: line pairs per millimeter, lp / mm), and the ordinate represents the modulus value of the optical transfer function (i.e. MTF value). As a core index for evaluating the imaging quality of the optical system, the higher the MTF value, the stronger the ability of the optical system to restore the details and contrast of the object. According to the simulation results, the MTF values of the system in the full field of view are better than 0.5 at the cutoff frequency of 33.3 lp / mm. This result shows that the system has excellent imaging quality and fully meets the design index requirements.

[0045] The spectral line curvature and color distortion of the hyperspectral imaging system in this embodiment are shown in FIG. 11. Figure 11 , Figure 12 The spectral line curvature Smile and color distortion Keystone characteristic curves of the optical system in this embodiment are shown in FIG. 11. The spectral line curvature is manifested as the imaging of monochromatic light on the photosensitive surface of the detector being curved in an arc shape, which directly introduces calibration errors of the central wavelength. The color distortion is manifested as the non-uniformity of the spatial imaging positions of the same object point under different working wavelengths. This spatial misalignment not only destroys the spectral consistency at the pixel level, but also causes misjudgment of the classification of the edges of the ground objects. Since excessive distortion can significantly increase the algorithm difficulty of subsequent data processing, it is usually required in engineering to strictly control the Smile and Keystone within 0.5 pixels to ensure the spectral inversion accuracy. According to the simulation results, the aberration of the short-wave infrared imaging spectrometer in this embodiment is fully corrected, and the maximum deviation of the Smile and Keystone is better than 0.5 pixels, which meets the high-precision detection requirements.

[0046] Although the preferred embodiments of the present application have been described, those skilled in the art can make further changes and modifications to these embodiments once they know the basic inventive concept. Therefore, the appended claims are intended to be interpreted as including all changes and modifications falling within the scope of the present application.

[0047] Obviously, many modifications and variations of the present application are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims and their equivalents, the application can be practiced otherwise than as specifically described.

Claims

1. A hyperspectral imaging system, characterized by, In the light incident direction, the telescope group, the collimating objective, the prism, the volume holographic diffraction grating, the imaging objective and the imaging sensor are sequentially arranged. The telescope group is composed of a primary mirror, a secondary mirror, a tertiary mirror, a plane mirror and an entrance slit, and is used for correcting primary aberration and avoiding chromatic aberration; the primary mirror, the secondary mirror and the tertiary mirror form an off-axis three-mirror structure, and the plane mirror is used for folding the light path. The collimating objective, the prism, the volume holographic diffraction grating, the imaging objective and the imaging sensor form an imaging spectrometer, and the entrance slit is used as the object input, and is used for correcting spectral line curvature and color distortion. The off-axis three-mirror structure and the folded light path are coaxially connected with the imaging spectrometer through the entrance slit, and are used for jointly correcting and splicing to form a hyperspectral imaging system.

2. The hyperspectral imaging system of claim 1, wherein, In the light incident direction, the parallel light reflected by the target object sequentially passes through the primary mirror, the secondary mirror, the tertiary mirror, the plane mirror and the entrance slit. The primary mirror is used for receiving the incident light and reflecting it to the secondary mirror, the secondary mirror is used for receiving the reflected light of the primary mirror and reflecting it to the tertiary mirror, the tertiary mirror is used for receiving the light of the secondary mirror and reflecting it to the plane mirror, and the plane mirror is used for receiving the reflected light of the tertiary mirror and folding it to the entrance slit.

3. The hyperspectral imaging system of claim 2, wherein, The primary mirror, the secondary mirror and the tertiary mirror are all spherical structures, the primary mirror and the tertiary mirror are concave mirrors, and the secondary mirror is a convex mirror.

4. The hyperspectral imaging system of claim 3, wherein, The curvature radius of the primary mirror is -227.2 mm, and the conic coefficient is -97.85; the curvature radius of the secondary mirror is -44.3 mm, and the conic coefficient is -0.43; the curvature radius of the tertiary mirror is -52 mm, and the conic coefficient is 0.146; the distance between the primary mirror and the secondary mirror is -18 mm, the distance between the secondary mirror and the tertiary mirror is 23.04 mm, the distance between the tertiary mirror and the plane mirror is -34.96 mm, and the distance between the plane mirror and the entrance slit is 18 mm.

5. The hyperspectral imaging system of claim 1, wherein, The collimating objective and the imaging objective are respectively formed by six lenses, and in the light incident direction, The collimating objective includes a first lens, a second lens, a third lens, a fourth lens, a fifth lens and a sixth lens which are sequentially arranged. The imaging objective includes a seventh lens, an eighth lens, a ninth lens, a tenth lens, an eleventh lens and a twelfth lens which are sequentially arranged.

6. The hyperspectral imaging system of claim 5, wherein, The first lens has positive focal power and meniscus structure, the second lens has positive focal power and biconvex structure, the third lens has negative focal power and biconcave structure, the fourth lens has negative focal power and plane concave structure, the fifth lens has positive focal power and plane convex structure, the sixth lens has positive focal power and meniscus structure, the seventh lens has positive focal power and meniscus structure, the eighth lens has positive focal power and plane convex structure, the ninth lens has negative focal power and plane concave structure, the tenth lens has negative focal power and biconcave structure, the eleventh lens has positive focal power and biconvex structure, and the twelfth lens has positive focal power and meniscus structure.

7. The hyperspectral imaging system according to claim 6, wherein the surface of the first lens on the object side is curved towards the entrance slit, and the surface on the image side is curved towards the entrance slit. The object-side surface of the second lens bends toward the image plane, and the image-side surface bends toward the entrance slit. The object-side surface of the third lens bends toward the entrance slit, and the image-side surface bends toward the image plane. The object-side surface of the fourth lens bends toward the entrance slit, while the image-side surface is flat. The object-side surface of the fifth lens is flat, while the image-side surface curves toward the entrance slit. The object-side surface of the sixth lens bends toward the entrance slit, and the image-side surface bends toward the entrance slit. The object-side surface of the seventh lens bends toward the image plane, and the image-side surface bends toward the image plane; The object-side surface of the eighth lens curves toward the image-side surface, while the image-side surface is flat. The object-side surface of the ninth lens is flat, while the image-side surface is curved toward the image plane. The object-side surface of the tenth lens bends toward the entrance slit, and the image-side surface bends toward the image plane; The object-side surface of the eleventh lens bends toward the image plane, and the image-side surface bends toward the entrance slit. The object surface of the twelfth lens bends toward the image plane, and the image surface bends toward the image plane.

8. The hyperspectral imaging system of claim 6, wherein, The apertures of the first lens, second lens, third lens, fourth lens, fifth lens, sixth lens, seventh lens, eighth lens, ninth lens, tenth lens, eleventh lens, and twelfth lens are all less than 24.5mm.

9. The hyperspectral imaging system of claim 1, wherein, The prism is attached to the volume holographic diffraction grating, and the apex angle of the prism is less than 15°.

10. The hyperspectral imaging system of claim 7, wherein, The first and second lenses are arranged back-to-back with their convex surfaces, sharing the large positive optical power. The fourth and fifth lenses form a cemented doublet. The eleventh and twelfth lenses are arranged back-to-back with their convex surfaces, and the eighth and ninth lenses form another cemented doublet.

Images