Off-axis reflective split-aperture snapshot spectral imaging optical system

Abstract

The present application belongs to the technical field of multispectral imaging, and discloses an off-axis reflection split-aperture snapshot spectral imaging optical system, which comprises a front mirror, a field diaphragm, a collimating mirror, a diaphragm array, a filter array, a lenslet array and a large area array detector. The front mirror, the field diaphragm and the collimating mirror constitute a telescope system, the diaphragm array and the filter array split the aperture and the light, and the lenslet array collects the energy of multiple channels to generate multiple spectral images on the large area array detector at the same time. The present application adopts an off-axis reflection telescope system to correct the system chromatic aberration and secondary spectrum, splits the aperture in the parallel light path provided by the telescope system, solves the parallax problem, and adopts a structure type of multiple lens combinations for each independent lenslet of the lenslet array to improve the image quality by designing the spectrum segments. The present application meets the requirements of aberration correction and image calibration of a large aperture optical system, and has simple system structure and high implementability.

Description

Technical Field

[0001] This invention belongs to the field of multispectral imaging technology and relates to an off-axis reflective aperture snapshot spectral imaging optical system. Background Technology

[0002] As a new generation of photoelectric detection technology, spectral imaging technology has been developing for over 40 years. Due to its imaging and spectral detection characteristics, it has been widely applied in various civilian and civilian fields, including meteorology, agriculture, medicine, environmental monitoring, resource surveys, and target reconnaissance and surveillance. Based on data acquisition methods, spectral imaging technology can be divided into scanning spectral imaging and snapshot spectral imaging. Scanning spectral imaging requires acquiring spectral data cubes through temporal or spatial scanning. Snapshot spectral imaging can acquire spectral data cubes within a single integration cycle of the detector. When acquiring dynamic scenes with significant spatial and spectral variations, scanning spectral imaging sensors cannot acquire complete data cubes due to the scanning process. In contrast, snapshot spectral imaging, which does not require time-series point scans, line scans, or spectral scans to acquire data cubes, can acquire complete data cubes in a single exposure cycle, thus meeting the detection and identification needs of dynamic scenes.

[0003] Aperture-splitting snapshot spectral imagers are a type of channel-based snapshot spectral imager, widely studied due to their simple principle, ease of implementation, and high reliability. Aperture-splitting snapshot spectral imagers solve the problem that two-dimensional imaging systems cannot acquire three-dimensional data simultaneously through multi-channel parallel acquisition. Each channel consists of an optical system, filters, and a detector. Each channel is equipped with a narrowband filter with a different center wavelength, and one channel acquires one spectral image, forming a spectral data cube.

[0004] The optical system of a snapshot spectrometer can be either reflective or transmissive. It not only needs to meet the requirements of color difference correction and image calibration for large-aperture optical systems, but also needs to be easy to implement in terms of manufacturing process.

[0005] Patent CN 107271039 B describes a transmissive aperture-splitting snapshot optical system, consisting of a telescope objective, a field stop, and a microlens array. The field stop is placed on the image plane of the objective, and the image plane of the objective coincides with the object plane of the microlens array. This is a secondary imaging system, and the microlens array is a near-field imaging system. The zero-field principal rays of the microlenses in each channel are not parallel, resulting in parallax, i.e., inconsistency in imaging caused by the non-parallel optical axes of each microlens in the microlens array. Patent CN 21077 435U and the doctoral dissertation "Research on Compact Aperture-Splitting Snapshot Spectral Imaging System" from the Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, describe another transmissive aperture-splitting snapshot optical system. This system integrates the imaging objective and the lens array into a single design to achieve imaging functionality. The system's lens array is located in the converging optical path, making it difficult to achieve strict parallelism of the principal rays in each channel, thus still resulting in some parallax. In patent CN107271039 B and the paper titled "Compact snapshot optically replicating and remapping imaging spectrometer (ORRIS) using a focal plane continuous variable filter" published in the journal Optical Letters, a transmissive aperture-splitting snapshot optical system is introduced. The optical system is a structure of a telescope system plus a microlens array. The telescope system consists of a telescope objective and a collimating eyepiece. For transmissive systems, wide-spectrum chromatic aberration correction is difficult and the structure is complex. The system in the paper is composed of off-the-shelf products and cannot achieve the connection and matching of pupils when multiple systems are combined, thus affecting the consistency of image and energy in each channel.

[0006] Patent CN 107271039 B describes a total internal reflection aperture-splitting snapshot optical system. The system has a total internal reflection structure, and the aperture splitting and beam splitting also use a mirror array. The entire system uses 13 off-axis parabolic mirrors, which is difficult to assemble and adjust, and the lateral size is large, which cannot meet the size requirements of small optoelectronic platforms for spectral imaging systems. Summary of the Invention

[0007] (a) Purpose of the invention The purpose of this invention is to provide an off-axis reflective aperture snapshot spectral imaging optical system, which must satisfy the requirement of long-distance n (n=k) 2 The system requires the detection of k≥2) spectral channels, and each channel image must be strictly registered, with small color difference, easy to implement, and small size.

[0008] (II) Technical Solution To address the aforementioned technical problems, this invention provides an off-axis reflective snapshot spectral imaging optical system, comprising: a front mirror 1, a field stop 2, a collimating mirror 3, an aperture array 4, a filter array 5, a small lens array 6, and a large-area detector 7 arranged sequentially along the optical path; the front mirror 1, the field stop 2, and the collimating mirror 3 constitute an off-axis reflective telescope system, with the field stop 2 located at the primary image plane between two off-axis mirrors to limit the imaging range; the aperture array 4, the filter array 5, and the small lens array 6... Lens array 6 forms n spectral channels. Aperture array 4 and filter array 5 are located in the parallel optical path between the off-axis reflective telescope system and small lens array 6 for aperture and beam splitting. Aperture array 4, located in front of filter array 5 and small lens array 6, includes n apertures and serves as the aperture stop of the imaging optical system, as well as the entrance pupil of small lens array 6. Small lens array 6 includes n independent small lenses, collecting energy from multiple channels and simultaneously generating multiple spectral images on the large array detector 7, ultimately obtaining a spectral data cube; where n=k 2 , k≥2.

[0009] The front reflector 1 and the collimating reflector 3 are both off-axis parabolic reflectors, both of which are concave aspherical surfaces and coated with a visible-near-infrared broadband high reflectivity film.

[0010] The optical axes of the n spectral channels are parallel and the spacing is consistent.

[0011] The filter array 5 is composed of n small-sized narrow-band filters, a substrate, and n small-sized cutoff filters bonded together.

[0012] Among them, the n small-sized narrowband filters of the filter array 5 are coated with narrowband filter films with a set spectrum, and the n cutoff filters are coated with bandpass filter films corresponding to the set spectrum.

[0013] The microlens array 6 includes n independent microlenses, each of which is parallel to the optical axis of the off-axis reflective telescope system and is symmetrically distributed on a plane perpendicular to the optical axis of the off-axis reflective telescope system. The distance between each optical axis ensures that the images formed by each channel do not interfere with each other and are consistent.

[0014] In the small lens array 6, if n is even, each of the n small lenses is not coaxial with the off-axis reflective telescope system and is symmetrically distributed on a plane perpendicular to the optical axis of the off-axis reflective telescope system, and has different off-axis amounts relative to the off-axis reflective telescope system; if n is odd, one of the small lenses is coaxial with the off-axis reflective telescope system, and n-1 small lenses are not coaxial with the off-axis reflective telescope system and are symmetrically distributed on a plane perpendicular to the optical axis of the off-axis reflective telescope system, and have different off-axis amounts relative to the off-axis reflective telescope system.

[0015] In the small lens array 6, each independent small lens adopts a structure of multiple lens combinations to correct broadband chromatic aberration and secondary spectrum. The structure is a cemented doublet lens plus a cemented doublet lens, a single lens plus a cemented doublet lens, or a cemented triplet lens. The cemented doublet lens is formed by bonding a positive power lens 6-1 and a negative power lens 6-2 together with optical adhesive. The cemented triplet lens is formed by bonding a positive power lens 6-1, a negative power lens 6-2, and a positive power lens 6-3 together with optical adhesive.

[0016] The microlens array 6 has two different structural parameters, namely, different radii of curvature and thickness, and its surface is coated with high anti-reflection films for visible light and near-infrared light, respectively.

[0017] In the microlens array 6, each individual microlens has the same optical material. Each individual microlens is made of special glass and crown glass or fluorite (CAF2) and crown glass with corrected secondary spectrum, and is composed of a structure of separate lens group or cemented lens group.

[0018] (III) Beneficial Effects The off-axis reflection split-aperture snapshot spectral imaging optical system provided by the above technical solution has the following beneficial effects: (1) The present invention adopts an off-axis parabolic telescope system with a small lens array. The small lens array is placed in a parallel optical path, and the optical axes of each channel are strictly parallel, which solves the parallax problem. The multi-channel, multi-structure integrated design ensures the matching of the pupil of the entire system.

[0019] (2) The telescope system of the present invention adopts an off-axis reflector structure, which is unobstructed and has no chromatic aberration, and can meet the image quality requirements of large aperture wide spectrum system, and simplifies the system and reduces the lateral size of the system.

[0020] (3) The small lens array of the present invention adopts a structure of multiple lens combinations for each independent small lens and a spectral segment design, which solves the problem of correction of wide spectral color difference and secondary spectrum of the system. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the structural composition of the off-axis reflective aperture snapshot spectral imaging optical system of the present invention.

[0022] Figure 2 This is a schematic diagram showing the distribution of the 16 channels in a preferred embodiment of the present invention.

[0023] Figure 3 This is a modulation transfer function diagram of channel 16-1 in the preferred embodiment of the present invention at a speed of 0.4μm to 0.65μm.

[0024] Figure 4 This is a modulation transfer function diagram of channel 16-2 in the preferred embodiment of the present invention at a speed of 0.4μm to 0.65μm.

[0025] Figure 5 This is a modulation transfer function diagram of channels 16-5 in the preferred embodiment of the present invention at a speed of 0.4μm to 0.65μm.

[0026] Figure 6 This is a modulation transfer function diagram of channels 16-6 in the preferred embodiment of the present invention at a speed of 0.4μm to 0.65μm.

[0027] Figure 7 This is a modulation transfer function diagram of channel 16-1 in the preferred embodiment of the present invention at a speed of 0.65μm to 0.9μm.

[0028] Figure 8 This is a modulation transfer function diagram of channel 16-2 in the preferred embodiment of the present invention at a speed of 0.65μm to 0.9μm.

[0029] Figure 9 This is a modulation transfer function diagram of channels 16-5 in the preferred embodiment of the present invention at a speed of 0.65μm to 0.9μm.

[0030] Figure 10 This is a modulation transfer function diagram of channels 16-6 in the preferred embodiment of the present invention at a speed of 0.65μm to 0.9μm. Detailed Implementation

[0031] To make the objectives, contents, and advantages of the present invention clearer, the specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples.

[0032] The preferred embodiment of this invention is for a broad visible-near infrared spectrum of 0.4μm to 0.9μm, with a field of view of ±2.486°, 16 (4 2 =16) optical channels provided by an optical system.

[0033] like Figure 1 As shown, the off-axis reflective aperture snapshot spectral imaging optical system of this embodiment includes a front reflector 1, a field stop 2, a collimating reflector 3, an aperture array 4, a filter array 5, a small lens array 6, and a large-area detector 7 arranged sequentially along the optical path.

[0034] The scene at infinity is imaged onto the primary image plane by the front mirror 1. After being collimated by the collimating mirror 3, the primary image plane is divided into 16 aperture channels by the aperture array 4. The 16 aperture channels are then transformed into 16 spectral channels by the filter array 5 coated with narrow-band filter film. Finally, the same scene is imaged into 16 regions of the large array detector 7 by the small lens array 6, and each region has different spectral characteristics.

[0035] A front-facing mirror 1, a field stop 2, and a collimating mirror 3 constitute an off-axis reflective telescope system. Both the front-facing mirror 1 and the collimating mirror 3 are off-axis parabolic mirrors, employing concave aspherical surfaces coated with a broad-spectrum, high-reflectivity visible-near-infrared coating. Because off-axis reflective telescope systems eliminate chromatic aberration, they effectively improve image quality and avoid the central obstruction problem common in coaxial dual-mirror systems. The field stop 2 is designed as a square to limit the imaging range.

[0036] The aperture array 4, filter array 5, and microlens array 6 constitute 16 spectral channels. Aperture array 4 and filter array 5 are located in the parallel optical path between the off-axis reflective telescope system and microlens array 6. The optical axes of each channel, i.e., the principal rays of the 0-field of view, are strictly parallel and their spacing is strictly consistent. The edge field rays are symmetrically distributed on both sides of the 0-field of view, thus eliminating parallax problems. Aperture array 4 performs the function of dividing the system's apertures. It includes 16 apertures, serving as both the aperture stop of the entire system and the entrance pupil of the microlens array. The 16 channels of the entire system, with their multiple structures, simultaneously perform optical path tracing, thus solving the problem of pupil connection and matching when combining multiple optical systems and avoiding inconsistencies in image and energy between channels. The filter array 5 is the system's beam splitter. It consists of 16 small narrowband filters and cutoff filters bonded together on a large substrate, all made of K9 glass. The narrowband filters are coated with narrowband filter films with specific spectra, and the cutoff filters are coated with bandpass filter films corresponding to specific spectra to narrow the spectral range. The three sets of optical plates have high-precision surface shapes and each surface is strictly parallel, eliminating ghosting caused by internal reflection from the flat glass surface.

[0037] The small lens array 6 is the imaging component of the entire system. To fully utilize the pixels of the large-area detector 7, this preferred embodiment employs 16 channels. Therefore, the small lens array also consists of 16 independent small lenses. These 16 independent small lenses have completely identical fields of view, enabling the generation of 16 images of the same scene of the same size in different areas of the same detector 7. The optical axes of each independent small lens are parallel to the optical axis of the off-axis reflective telescope system and must be at a certain distance and strictly consistent to ensure that the scene images of each channel do not interfere with each other and to fully utilize the large-area detector 7. Figure 2 As shown, each of the 16 independent microlenses is not coaxial with the off-axis reflective telescope system, but is symmetrically distributed on a plane perpendicular to the optical axis of the telescope system. Each independent microlens has different off-axis values ​​in the X and Y directions.

[0038] Since off-axis reflective telescope systems do not have chromatic aberration, the chromatic aberration of the entire system is generated by the microlens array 6. In addition, each individual microlens in the microlens array 6 is off-axis. In order to improve the image quality of the system and correct chromatic aberration, in this preferred embodiment, the working spectrum is divided into two shorter spectrums, the visible spectrum and the near-infrared spectrum, for optimization. Channels 1 to 8 of the 16 individual microlenses are in the first group, and channels 9 to 16 are in the second group. The two groups have different structural parameters such as radius of curvature and thickness, and are coated with two different high-reflection coatings for different spectrums, namely 0.4μm~0.65μm and 0.65μm~0.9μm. The structural type of each individual microlens in the microlens array 6 can be selected from multiple lens combination structures, such as cemented doublet lens plus cemented doublet lens, single lens plus cemented doublet lens, and triplet lens. The materials used are a combination of special glass and crown glass that can correct the second-order spectrum, or a combination of fluorite (CAF2) and crown glass. In this preferred embodiment, to facilitate assembly and adjustment and ensure the coaxiality of each individual small lens, a three-layer cemented lens is adopted, consisting of lens 6-1 (made of CAF2), lens 6-2 (made of ZK7), and lens 6-3 (made of CAF2) bonded together with optical adhesive. Figure 2 As shown, due to the different positions of the 16 channels, there are differences in aberrations. The 16 channels are symmetrically distributed in groups of four on both sides of the X and Y axes. The modulation transfer functions of channels 16-1, 16-2, 16-5, and 16-6 are as follows: Figure 3 , Figure 4 , Figure 5 , Figure 6 , Figure 7 , Figure 8 , Figure 9 , Figure 10 As shown, the system achieves good image quality in both the edge channel and the middle channel near the optical axis. The optical component parameters of the two sets of small lenses in this preferred embodiment are shown in Table 1.

[0039] Table 1. Optical Component Parameter Table (Unit: mm)

[0040] The optical system described in this invention includes, but is not limited to, the design parameters described herein.

[0041] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. An off-axis reflective aperture snapshot spectral imaging optical system, characterized in that, include: It consists of a front reflector (1), a field stop (2), a collimating reflector (3), an aperture array (4), a filter array (5), a small lens array (6), and a large-area detector (7) arranged sequentially along the optical path. The front reflector (1), the field stop (2), and the collimating reflector (3) constitute an off-axis reflective telescope system. The field stop (2) is located at the primary image plane between two off-axis reflectors, limiting the imaging range. The aperture array (4), the filter array (5), and the small lens array (6) constitute n spectral channels. The aperture array (4) and filter array (5) are located in the parallel optical path between the off-axis reflective telescope system and the small lens array (6) for aperture and beam splitting. The aperture array (4) is located in front of the filter array (5) and the small lens array (6), and includes n apertures. It serves as the aperture stop of the imaging optical system and is also the entrance pupil of the small lens array (6). The small lens array (6) includes n independent small lenses, which collect the energy of multiple channels and generate multiple spectral images simultaneously on the large array detector (7), ultimately obtaining a spectral data cube; where n=k 2 , k≥2; Each independent microlens in the microlens array (6) adopts a structure of multiple lens combinations to correct broadband chromatic aberration and secondary spectrum. The structure is a cemented doublet lens plus a cemented doublet lens, a single lens plus a cemented doublet lens, or a cemented triplet lens. The cemented doublet lens is formed by bonding a positive power lens (6-1) and a negative power lens (6-2) together with optical adhesive. The cemented triplet lens is formed by bonding a positive power lens (6-1), a negative power lens (6-2), and a positive power lens (6-3) together with optical adhesive.

2. The off-axis reflective aperture snapshot spectral imaging optical system as described in claim 1, characterized in that, The front reflector (1) and collimating reflector (3) are both off-axis parabolic reflectors, both of which are concave aspherical and coated with a visible-near-infrared broadband high reflectivity film.

3. The off-axis reflective aperture snapshot spectral imaging optical system as described in claim 2, characterized in that, The optical axes of the n spectral channels are parallel and the spacing is consistent.

4. The off-axis reflective aperture snapshot spectral imaging optical system as described in claim 3, characterized in that, The filter array (5) is composed of n small-sized narrowband filters, a substrate, and n small-sized cutoff filters bonded together.

5. The off-axis reflective aperture snapshot spectral imaging optical system as described in claim 4, characterized in that, The filter array (5) has n small-sized narrowband filters coated with narrowband filter films of a set spectrum, and n cutoff filters coated with bandpass filter films corresponding to the set spectrum.

6. The off-axis reflective aperture snapshot spectral imaging optical system as described in claim 5, characterized in that, The microlens array (6) includes n independent microlenses, each of which is parallel to the optical axis of the off-axis reflective telescope system and is symmetrically distributed on a plane perpendicular to the optical axis of the off-axis reflective telescope system. The distance between each optical axis ensures that the images formed by each channel do not interfere with each other and are consistent.

7. The off-axis reflective aperture snapshot spectral imaging optical system as described in claim 6, characterized in that, In the small lens array (6), if n is even, each of the n small lenses is not coaxial with the off-axis reflective telescope system and is symmetrically distributed on a plane perpendicular to the optical axis of the off-axis reflective telescope system, and has a different amount of off-axis relative to the off-axis reflective telescope system; if n is odd, one of the small lenses is coaxial with the off-axis reflective telescope system, and n-1 small lenses are not coaxial with the off-axis reflective telescope system and are symmetrically distributed on a plane perpendicular to the optical axis of the off-axis reflective telescope system, and have a different amount of off-axis relative to the off-axis reflective telescope system.

8. The off-axis reflective aperture snapshot spectral imaging optical system as described in claim 7, characterized in that, The microlens array (6) has two different structural parameters, namely different radii of curvature and thickness, and its surface is coated with high-reflection coatings for visible light and near-infrared light, respectively.

9. The off-axis reflective aperture snapshot spectral imaging optical system as described in claim 8, characterized in that, Each individual microlens in the microlens array (6) has the same optical material. Each individual microlens is made of special glass and crown glass or fluorite and crown glass with corrected secondary spectrum. They are combined in the form of separate lens groups or cemented lens groups.

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