A spectroscopic imaging system and imaging method for snapshot imaging spectrometer

Abstract

The present invention relates to a spectroscopic imaging system and an imaging method thereof for a snapshot imaging spectrometer. The spectroscopic imaging system is a coaxial symmetrical structure, comprising a spectroscopic element and a collimating lens group and a focusing lens group symmetrically arranged on both sides of the spectroscopic element. The imaging method is that the incident light passes through the collimating lens group to obtain a complex color parallel light beam parallel to the optical axis, which is incident on the spectroscopic element, and after being split by the spectroscopic element composed of a holographic grating and a prism, a monochromatic parallel light beam of different wavelengths parallel to the optical axis is emitted, which is incident on the focusing lens group, and is imaged at a detector after being focused. The spectroscopic imaging system provided by the present invention has the characteristics of large relative aperture, high spectral resolution, high light energy utilization, simple and compact structure, etc., and has broad application prospects.

Description

Technical Field

[0001] The invention relates to the technical field of spectroscopic imaging systems of imaging spectrometers, and in particular to a spectroscopic imaging system for a snapshot imaging spectrometer. Background Art

[0002] Spectral imaging technology is a new type of multi-dimensional information acquisition technology that combines imaging technology and spectral technology. It can simultaneously obtain two-dimensional spatial information and one-dimensional spectral information of the target being measured. Based on this, researchers can accurately identify, detect and analyze the target object according to the "spectral fingerprint" characteristics of the target object. Therefore, this technology has broad application prospects in production and life, scientific research and military reconnaissance.

[0003] Most existing spectral imaging technologies use slits to divide the field of view, obtain the spectral image of the line field of view, and obtain the spectral image of the full field of view by scanning and imaging the target. This type of system has shortcomings such as low energy utilization and non-real-time imaging, and cannot meet the spectral imaging needs of some moving targets.

[0004] As a novel spectral imaging technology, snapshot spectral imaging technology can obtain the two-dimensional spatial information and spectral information of the target object within one detector integration time without push-scanning, which can meet the actual needs of some applications that require real-time acquisition of spatial information and spectral information.

[0005] See attached Figure 1 , which is a schematic diagram of the overall structure of a snapshot spectral imaging system provided by the prior art. According to the light transmission direction, it is sequentially an aperture stop 1, a front objective lens 2, a microlens array 3, a collimating lens group 4, a spectroscopic element 5, a focusing lens group 6 and a CCD detector 7; wherein, the collimating lens group 4, the spectroscopic element 5 and the focusing lens group 6 constitute a spectroscopic imaging system, and the incident light is transmitted to the front objective lens through the aperture stop 1 located at the object focal plane of the front objective lens 2, and the front objective lens images the distant scene at the primary image plane; the microlens array 3 located at the primary image plane divides and scales the image plane to form a pupil image array at the image focal plane of the microlens array; the light emitted by the pupil image is incident on the collimating lens group 4 to obtain a complex color parallel light beam, and the complex color parallel light is emitted as a monochromatic parallel light beam of different wavelengths after being combined with the spectroscopic element 5, and is incident on the focusing lens group 6, and the focusing lens group focuses the light beam on the monochromatic pupil image array of different wavelengths obtained at the CCD detector 7 to complete the imaging process. As the core component of snapshot imaging spectrometer, the imaging quality of the spectroscopic imaging system determines the spectral resolution and imaging performance of the system.

[0006] The common splitting methods in the prior art mainly include prisms and gratings. The prism has low dispersion and poor splitting ability, while the relief grating has low diffraction efficiency and low light energy utilization. Commonly used grating splitting imaging systems, such as Offner grating type and Czerny-Turner grating type, are all off-axis structures, with defects such as large size, difficult installation and adjustment, and poor stability.

[0007] Chinese invention patent CN 110081976 A discloses a large-field grating prism spectral imaging system, which includes a slit, a spherical reflector and a plane reflector, a grating prism module, a focusing lens group consisting of 8 lenses, a filter and a detector. The system has an operating band of 420-1000nm, a slit length of 29.4mm, and an F number of 2.4; the optical system uses a reflective structure as a collimating objective lens and a transmissive structure as a focusing lens group, realizing wide-band, large relative aperture, and long slit spectral imaging, achieving the design purpose; but it has the following shortcomings: first, due to the use of a spherical reflector and a plane mirror to form an off-axis collimation group, the spectral imaging system belongs to an off-axis system, so there is a certain difficulty in installation and adjustment and the system stability is poor; second, the spectroscopic imaging system uses a slit to divide the field of view, the system has low light energy utilization, and scanning is required to achieve the acquisition of full-field spectral images.

[0008] Therefore, designing a spectroscopic imaging system for snapshot imaging spectrometer to solve the problems existing in most current spectroscopic imaging systems has great practical significance for the promotion and application of spectral imaging technology. Summary of the invention

[0009] In view of the deficiencies in the prior art, the present invention provides a direct-view spectral imaging system and an imaging method thereof which have high spectral resolution, good imaging quality, simple and compact structure, and are easy to process and assemble.

[0010] The technical solution adopted by the present invention is to provide a spectroscopic imaging system for a snapshot imaging spectrometer, which is a coaxial symmetrical structure, including a spectroscopic element and a collimating lens group and a focusing lens group symmetrically arranged on both sides of the spectroscopic element; the collimating lens group and the focusing lens group have the same structure, each including five refractive lenses; the spectroscopic element includes a volume holographic grating and a prism; according to the incident direction of light, the optical elements of the spectroscopic imaging system are: a first meniscus lens bent away from the incident direction of light, a first biconvex lens, a second meniscus lens bent toward the incident direction of light, a second biconvex lens, a first biconcave lens, a volume holographic grating, a prism, a second biconcave lens, a third biconvex lens, a third meniscus lens bent away from the incident direction of light, a fourth biconvex lens, and a second meniscus lens bent toward the incident direction of light, wherein the second biconvex lens and the first biconcave lens form a double-cemented lens group, and the second biconcave lens and the third biconvex lens form another double-cemented lens group.

[0011] The present invention provides a spectroscopic imaging system for a snapshot imaging spectrometer, wherein the working F number has a value range of 2.4≤F / #≤2.65, an object field of view of Ф>3mm, a magnification of -1×, a distortion of less than 0.2%, and a total length L of the system of 85mm≤L≤90mm.

[0012] A preferred embodiment of the present invention is that the angle between the first surface of the prism and the vertical direction of the optical axis is β 1 , 0°<β 1 <90°, the angle between the second surface of the prism and the perpendicular direction of the optical axis is β 2 , 0°<β 2 <90°, and β 1 and β 2 The condition is satisfied: the main wavelength light incident parallel to the optical axis is still parallel to the optical axis after being split by the beam splitter. A better effect is: the β 1 Make the main wavelength light incident parallel to the optical axis meet the grating Bragg condition.

[0013] The technical solution of the present invention also includes an imaging method for a spectroscopic imaging system of a snapshot imaging spectrometer, comprising the following steps:

[0014] (1) The incident light passes through the collimating lens group to obtain a complex parallel light beam parallel to the optical axis;

[0015] (2) the complex parallel light obtained in step (1) is incident on a beam splitter element, and after being split by the combined beam splitter element, a monochromatic parallel light beam of different wavelengths parallel to the optical axis is emitted; the beam splitter element comprises a volume holographic grating and a prism, and the angle between the first surface of the prism and the perpendicular direction of the optical axis is such that the main wavelength light incident parallel to the optical axis satisfies the grating Bragg condition;

[0016] (3) The monochromatic parallel light beams of different wavelengths obtained in step (2) are incident on a focusing lens group, and the incident light beams are focused and imaged on a detector.

[0017] In step (2), the principal ray of the central wavelength incident parallel to the optical axis is θ 1 The incident light is incident on the grating substrate and refracted inside the grating substrate at an angle θ 2 The incident light is diffracted at an angle θ 3 After the light exits the grating, it enters the prism. 3 =θ 2 , after being refracted inside the prism at an angle θ 4 It is incident on the rear surface of the prism and then at an angle θ 5 Emitted from the beam splitter, θ 5 =β 2 , to achieve straight in and straight out of the light, where θ 1 ,θ 2 ,θ 3 ,θ 4 and θ 5 are the angles between the light ray and the incident or exiting surface normal, respectively.

[0018] The snapshot spectral imaging spectroscopic imaging system provided by the present invention increases the relative aperture of the system through a coaxial symmetrical structural design, thereby improving the light collecting capability and resolution of the system and expanding the application range of the spectroscopic imaging system.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] 1. The collimating lens group and focusing lens group of the spectroscopic imaging system provided by the present invention are designed to have the same structure, which is beneficial to saving processing costs; by reasonably selecting glass materials and designs, the relative aperture of the optical lens is increased while achieving aberration correction and balance, so that the light gathering ability and resolution of the system are significantly improved.

[0021] 2. The spectroscopic imaging method provided by the present invention is to glue a volume holographic transmission grating with higher diffraction efficiency to a prism as a spectroscopic element, and to control the angle β between the first surface of the prism and the direction perpendicular to the optical axis. 1 The incident angle of the light whose central wavelength is parallel to the optical axis satisfies the grating Bragg condition, which improves the energy utilization of the system and has better spectroscopic performance. By controlling the angle between the second surface of the prism and the direction perpendicular to the optical axis to be β 2 The light whose central wavelength is parallel to the optical axis is emitted parallel to the optical axis. Therefore, the spectroscopic imaging system is a coaxial optical system with compact structure, smaller volume, easy processing and adjustment, strong stability, which is conducive to the miniaturization and portability of imaging spectrometers.

[0022] 3. The spectroscopic imaging system provided by the present invention satisfies both the object-side telecentricity and image-side telecentricity structures, and has a relatively high telecentricity. Both the object-side telecentricity and the image-side telecentricity are less than 0.03°, which is beneficial to improving the measurement accuracy of spatial position information.

[0023] 4. The optical lens of the spectroscopic imaging system provided by the present invention is composed of only ten spherical refracting lenses, and all lenses are made of domestic glass materials, which reduces the processing difficulty and processing cost of the lens and has practical application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is the overall structural principle diagram of the snapshot spectral imaging system provided by the prior art;

[0025] Figure 2 Is a schematic diagram of the structure of the spectroscopic imaging system provided in an embodiment of the present invention;

[0026] Figure 3 Is a schematic diagram of the structure of the spectroscopic element of an embodiment of the present invention;

[0027] Figure 4 Is a distortion curve diagram of the spectroscopic imaging system provided in an embodiment of the present invention;

[0028] Figure 5 Is a field curvature, astigmatism curve diagram of the spectroscopic imaging system according to an embodiment of the present invention;

[0029] Figure 6 is a ray tracing point diagram of the spectroscopic imaging system according to an embodiment of the present invention;

[0030] Figure 7 is a transfer function curve MTF curve diagram of the spectroscopic imaging system according to an embodiment of the present invention;

[0031] Figure 8 is an energy concentration curve of the spectroscopic imaging system according to an embodiment of the present invention;

[0032] In the figure: 1. aperture diaphragm; 2. front objective lens; 3. microlens array; 4. collimating lens group; 5. spectrometer; 6. focusing lens group; 7. image plane; 41. first meniscus lens; 42. first biconvex lens; 43. second meniscus lens; 44. second biconvex lens; 45. first biconcave lens; 510. front surface of glass substrate (aperture stop); 51. volume holographic grating; 511. glass substrate; 512. grating; 513. protective glass; 52. prism; 61. second biconcave lens; 62. third biconvex lens; 63. third meniscus lens; 64. fourth biconvex lens; 65. fourth meniscus lens. Specific implementation plan

[0033] The specific embodiments of the present invention will be further described below in conjunction with the accompanying drawings and examples.

[0034] Embodiment 1: The technical solution of this embodiment is to provide a spectroscopic imaging system for a snapshot imaging spectrometer. The optical lens is composed of a volume holographic transmission grating and a prism combined spectroscopic element and 10 refractive spherical lenses, the optical system has a focal length of f=17mm, an F number of F / #=2.6, an object field of view Φ=3mm, and an operating wavelength of 400nm to 700nm.

[0035] See attached Figure 2 , which is a structural schematic diagram of the spectroscopic imaging system provided in this embodiment, which is composed of a collimating lens group 4 and a focusing lens group 6 symmetrically arranged on both sides of a spectroscopic element 5, and the two lens groups each include five refracting lenses. According to the incident direction of light, the optical elements are, in order, a first meniscus lens 41; a first biconvex lens 42; a second meniscus lens 43; a second biconvex lens 44; a first biconcave lens 45; a spectroscopic element 5; a second biconcave lens 61; a third biconvex lens 62; a second meniscus lens 63; a fourth biconvex lens 64; and a fourth meniscus lens 65. Among them, the first meniscus lens 42 is bent away from the incident direction of the light; the second meniscus lens 43 is bent towards the incident direction of the light; the second biconvex lens 43 and the first biconcave lens 44 form a double-cemented lens group; the second biconcave lens 61 and the third biconvex lens 62 form a double-cemented lens group; the third meniscus lens 63 is bent away from the incident direction of the light; the fourth meniscus lens 65 is bent towards the incident direction of the light. When the spectroscopic imaging system forms an image, the incident light is first collimated into a complex parallel light beam by the collimating lens group 4, and the complex parallel light is dispersed by the spectroscopic element 5 and then emitted as monochromatic parallel light beams of different wavelengths. The spectroscopic element mainly includes a volume holographic grating 51 and a prism 52; and then the monochromatic parallel light beams of different wavelengths are focused by the focusing lens group 6 and imaged at the detector, thereby completing the imaging process.

[0036] See attached Figure 3 , which is a schematic diagram of the structure of the spectroscopic element provided in this embodiment, the spectroscopic element mainly includes a volume holographic grating 51 and a prism 52, wherein the system aperture 512 is arranged on the front surface 510 of the glass substrate, and the glass substrate 511, the protective glass 513 and the prism 52 are made of the same material; the angle between the first surface of the prism of the spectroscopic element and the direction perpendicular to the optical axis OO' is β 1 The angle between the second surface of the prism and the direction perpendicular to the optical axis OO' is β 2 . β 1， 0°<β 1 <90°,0°<β 2 <90°, and β 1 and β 2The condition is satisfied: the main wavelength light incident parallel to the optical axis is still parallel to the optical axis after the light-splitting element 5 emits the light.

[0037] The principal ray of the central wavelength incident parallel to the optical axis OO' is θ 1 The incident light is incident on the grating substrate and refracted inside the grating substrate at an angle θ 2 The incident light is diffracted at an angle θ 3 (θ 3 =θ 2 ) is incident on the prism after exiting the grating, and is refracted inside the prism at an angle θ 4 It is incident on the rear surface of the prism and finally at an angle θ 5 (θ 5 =β 2 ) is emitted from the beam splitter, achieving straight-in and straight-out of the light, where θ 1 ,θ 2 ,θ 3 ,θ 4 and θ 5 is the angle between the light and the normal of the incident (or exiting) surface. In order to obtain high diffraction efficiency, the incident angle of the light incident on the volume holographic grating must meet the Bragg condition, that is, the diffraction angle is equal to the incident angle.

[0038]

[0039] in, is the central wavelength, d is the grating constant, n G is the refractive index of the grating material. At the same time, according to the law of refraction, the incident angle when the central wavelength enters the grating substrate can be calculated, that is,

[0040]

[0041] Among them, n P is the refractive index of the grating substrate. In order for the incident light of the grating substrate to satisfy the Bragg condition, the angle between the front surface of the prism and the direction perpendicular to the optical axis satisfies .

[0042] In order to achieve the straight-in and straight-out characteristics of the central wavelength, the angle of the rear surface of the prism satisfies the following relationship, that is,

[0043]

[0044]

[0045]

[0046] Solving the above equations together, we can get the angle between the rear surface of the prism and the vertical optical axis. As shown below

[0047]

[0048] In this embodiment, the angle β between the first surface of the prism and the perpendicular direction of the optical axis is 1 =1.57°, the angle between the second surface of the prism and the perpendicular direction of the optical axis is β 2 =5.9°, the line density of the volume holographic grating is 100 lines / mm, and the parameters of the optical elements in this embodiment are shown in Table 1.

[0049] Table 1:

[0050] .

[0051] The imaging method using the above-mentioned spectroscopic imaging system for snapshot imaging spectrometer comprises the following steps:

[0052] (1) The incident light passes through the collimating lens group to obtain a complex parallel light beam parallel to the optical axis;

[0053] (2) the complex color parallel light obtained in step (1) is incident on a beam splitter element, and after being split by the combined beam splitter element, a monochromatic parallel light beam of different wavelengths parallel to the optical axis is emitted;

[0054] (3) The monochromatic parallel light beams of different wavelengths obtained in step (2) are incident on a focusing lens group, and the incident light beams are focused and imaged on a detector.

[0055] See attached Figure 4 , which is a distortion curve diagram of the spectroscopic imaging system provided in this embodiment, in which the horizontal axis represents the distortion value relative to the image plane (unit %), and the vertical axis represents the normalized field of view. Figure 4 The results show that the distortion aberration of the spectroscopic imaging system has been fully corrected, and the distortion rate is less than 0.2%.

[0056] See attached Figure 5 , which is a field curvature and astigmatism curve diagram of the spectroscopic imaging system provided by this embodiment, the horizontal axis in the figure represents the field curvature astigmatism value, the vertical axis is the normalized field of view, and the two curves in the figure, the dotted line curve and the solid line curve in the figure respectively represent the field curvature in the sagittal and meridian planes, Figure 5 The results show that the spectroscopic imaging system effectively corrects astigmatism and field curvature, so that the difference between the two curves, that is, the astigmatism value, is within the aberration tolerance range.

[0057] See attached Figure 6 It is a ray tracing point diagram of light passing through the spectroscopic imaging system provided by this embodiment. The root mean square radius of the point diagram of each field of view corresponding to the three wavelengths in the figure is less than 2μm, and the geometric radius of the point diagram is less than 3μm. The imaging quality is good and meets the system use requirements.

[0058] See attached Figure 7, which is the transfer function MTF curve on the image plane corresponding to each field of view of the spectroscopic imaging system provided in this embodiment. Figure 7 It can be seen that at 70lp / mm, the MTF values ​​of each field of view are all greater than 0.7, close to the diffraction limit, and the curve is smooth and compact, indicating that the lens imaging is clear and uniform, and the system has good imaging quality in the full band and full field of view.

[0059] See attached Figure 8 , which is the energy concentration curve of the spectroscopic imaging system described in this embodiment, is Figure 8 It can be seen that more than 80% of the energy is concentrated within the range of the Airy spot.

[0060] The spectroscopic imaging system for snapshot spectral imaging provided by the technical solution of the present invention consists of only ten lenses. By increasing the relative aperture of the lens imaging to increase the light collecting ability and system resolution of the system, an image with uniform illumination distribution, concentrated energy and high resolution can be obtained.

[0061] The spectroscopic imaging system and imaging method provided by the technical solution of the present invention have undergone strict aberration correction and have the characteristics of large relative aperture, good imaging quality, high spectral resolution, and high light energy utilization rate. In addition, they have the advantages of simple and compact structure, easy processing and assembly, and strong stability. They can be used in the field of spectral imaging and have broad application prospects.

Claims

1. A spectroscopic imaging system for a snapshot imaging spectrometer, characterized in that: It is a coaxially symmetrical structure, comprising a beam splitter element (5) and a collimating lens group (4) and a focusing lens group (6) symmetrically arranged on both sides of the beam splitter element (5); the collimating lens group (4) and the focusing lens group (6) have the same structure, and each comprises five refractive lenses; the beam splitter element (5) comprises a volume holographic grating (51) and a prism (52); according to the incident direction of light, the optical elements of the beam splitting imaging system are, in order: a first meniscus lens (41) bent away from the incident direction of light, a first biconvex lens (42), a second meniscus lens (43) bent towards the incident direction of light; 3), a second biconvex lens (44), a first biconcave lens (45), a volume holographic grating (51), a prism (52), a second biconcave lens (61), a third biconvex lens (62), a third meniscus lens (63) bent away from the incident direction of the light, a fourth biconvex lens (64), and a second meniscus lens (65) bent toward the incident direction of the light, wherein the second biconvex lens (44) and the first biconcave lens (45) form a double-cemented lens group, and the second biconcave lens (61) and the third biconvex lens (62) form another double-cemented lens group; Its working F number ranges from 2.4≤F / #≤2.65, the object field of view is Ф>3mm, the magnification is -1×, the distortion is less than 0.2%, and the total length L of the system is 85mm≤L≤90mm; The angle between the first surface of the prism (52) and the direction perpendicular to the optical axis is β1, 0°<β1<90°, the angle between the second surface of the prism (52) and the direction perpendicular to the optical axis is β2, 0°<β2<90°, and β1 and β2 satisfy the condition that the main wavelength light incident parallel to the optical axis is still parallel to the optical axis after being split by the splitting element (5).

2. The spectroscopic imaging system for a snapshot imaging spectrometer according to claim 1, characterized in that: The β1 makes the main wavelength light incident parallel to the optical axis meet the grating Bragg condition.

3. An imaging method for a spectroscopic imaging system of a snapshot imaging spectrometer, characterized in that The steps include: (1) The incident light passes through the collimating lens group (4) to obtain a complex parallel light beam parallel to the optical axis; (2) the complex color parallel light obtained in step (1) is incident on a beam splitter element (5), and after being split by the combined beam splitter element, a monochromatic parallel light beam of different wavelengths parallel to the optical axis is emitted; the beam splitter element (5) comprises a volume holographic grating (51) and a prism (52), and the angle between the first surface of the prism (52) and the direction perpendicular to the optical axis is such that the main wavelength light incident parallel to the optical axis satisfies the grating Bragg condition; (3) The monochromatic parallel light beams of different wavelengths obtained in step (2) are incident on the focusing lens group (6), and the incident light beams are focused and imaged on the detector; In step (2), the main light of the central wavelength incident parallel to the optical axis is incident on the grating substrate at θ1, is refracted inside the grating substrate and is incident on the grating at an angle θ2, and is emitted from the grating at a diffraction angle θ3 and then incident on the prism, θ3=θ2, is refracted inside the prism and is incident on the rear surface of the prism at an angle θ4, and then is emitted from the beam splitter at an angle θ5, θ5=β2, so that the light enters and exits directly, wherein θ1, θ2, θ3, θ4 and θ5 are the angles between the light and the normal of the incident or exit surface, respectively.

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