Hyperspectral camera

Abstract

Hyperspectral camera. A camera includes a first lens configured to focus incident light onto a reflective slit assembly. The reflective slit assembly includes an elongated strip of reflective material configured to reflect some, but not all, of the incident light as return light. The first lens is configured to at least partially collimate the return light from the elongated strip of reflective material. A first mirror is configured to reflect the return light from the first lens. A second mirror is configured to reflect the return light from the first mirror. An optical element is configured to separate the return light from the first mirror by wavelength. A second lens is configured to focus the return light from the optical element onto a first detector. The first detector is configured to measure an intensity of the return light as a function of two-dimensional position on the first detector.

Description

Technical Field

[0001] This invention relates to spectral imaging, and more particularly to a compact optical system for push-broom hyperspectral imaging. Background Technology

[0002] Hyperspectral imaging involves collecting spectral data for each pixel in an image of a scene. The combination of spectral data and its spatial location (i.e., pixel) can be used to locate objects, identify materials, or detect processes. There are three general types of spectral imagers: pushbroom (line) scanners and whiskbroom (point) scanners, which involve spatial scanning over time; sequential scanners, which involve acquiring images of regions at different wavelengths; and snapshot hyperspectral imaging, which uses an array to generate images instantly. Hyperspectral imaging uses most of the electromagnetic spectrum to observe objects. Some objects leave unique “fingerprints” in the electromagnetic spectrum, especially when combined with their spatial location within the image. These are called spectral signatures, and they enable the identification of the materials or objects that make up the scanned scene.

[0003] Pushbroom hyperspectral imaging involves capturing strips of a scene and spectrally dispersing the slit image using a prism or grating to collect spectral data from the strips. Figure 1 The concept of pushbroom imaging is illustrated, where light 1 from strip 2 of scene 3 is captured by camera 4. In this example, light 1 originates from the entire dimension of scene 3 in the X direction and only from a narrow portion of the dimension of scene 3 in the orthogonal Y direction. However, it is also possible to perform multiple scans only on portions of the scene dimension in the X direction. Light 1 is collimated by one or more lenses 5, separated by wavelength using one or more optical elements 6 (e.g., diffraction gratings, prisms, etc.), and focused onto detector 7 by one or more lenses 8, such as... Figure 2This is conceptually illustrated. To capture only the strip of light 1 from the scene at any given time, the light from the scene passes through a transmissive strip (not shown) placed in the optical path between lens 5 and detector 7, which transmits only the portion of light 1 originating from scene 3. The remaining light from the scene is blocked, scattered, or reflected, preventing it from reaching detector 7. The transmissive strip can be, for example, an opaque sheet with an elongated aperture (i.e., a slit) through which the strip of light 1 passes. Detector 7 measures the amplitude of the wavelength component of the strip of light 1 at each position across the X-axis dimension of the image. This spatial and spectral data of strip 2 of scene 3 can be represented in a hyperspectral image, also called a hyperspectral data cube 9, having two spatial dimensions (X, Y) and one spectral dimension (λ), as shown below. Figure 3 As shown in the diagram. Strip 2 of scene 3 was scanned separately, and the spatial / spectral data from the strips were stitched together to create the full hyperspectral data cube 9 of scene 3, as shown in the diagram. Figure 4 As shown in the diagram. The data value of each pixel within the cube represents the value of a specific wavelength detected at a spatial location within Scene 3. Conventional pushbroom hyperspectral cameras are large and bulky due to the need for numerous optical components.

[0004] There is a need for a pushbroom hyperspectral imaging device with a simpler and more compact optical design. Summary of the Invention

[0005] The aforementioned problems and needs are addressed by a camera comprising a first lens configured to focus incoming light onto a reflective slit assembly. The reflective slit assembly includes an elongated strip of reflective material configured to reflect some, but not all, of the incident light as return light. The first lens is configured to at least partially collimate the return light from the elongated strip of reflective material. A first mirror is configured to reflect the return light from the first lens. A second mirror is configured to reflect the return light from the first mirror. Optical elements are configured to separate the return light from the first mirror according to wavelength. The second lens is configured to focus the return light from the optical elements onto a first detector. The first detector is configured to measure the intensity of the return light as a function of a two-dimensional position on the first detector.

[0006] Other objects and features of the invention will become clear upon review of the specification, claims, and drawings. Attached Figure Description

[0007] Figure 1 This is a perspective view from a conventional hyperspectral camera.

[0008] Figure 2 This is a schematic diagram of a conventional hyperspectral camera.

[0009] Figure 3 It is a graphical representation of a single row of pixels within a hyperspectral data cube.

[0010] Figure 4 It is a graphical representation of all pixels within the hyperspectral data cube.

[0011] Figure 5 This is a diagram of a camera.

[0012] Figure 6 It is a perspective view of the reflected slit.

[0013] Figure 7A This is a side view of a diffraction grating, which is an optical element that separates light according to wavelength.

[0014] Figure 7B This is a side view of a prism, which is an optical element that separates light according to wavelength.

[0015] Figure 8 It is a side cross-sectional view of the first or second lens.

[0016] Figure 9 This is a schematic diagram of a camera, showing the path of incident light entering the camera.

[0017] Figure 10 This is a schematic diagram of a camera showing the path of the returning light inside the camera.

[0018] Figure 11 This is a schematic diagram showing the separation of light according to wavelength within a camera.

[0019] Figure 12 This is a perspective view of an alternative example of a reflective slit assembly.

[0020] Figure 13 This is a perspective view of an alternative example of a reflective slit assembly.

[0021] Figure 14 This is a schematic diagram of an alternative example of a camera.

[0022] Figure 15A and 15B This is a schematic diagram of an alternative example of a camera.

[0023] Figure 16A and 16B This is a schematic diagram of an alternative example of a camera.

[0024] Figure 17 This is a side cross-sectional view of an alternative example of the first or second lens. Detailed Implementation

[0025] A pushbroom hyperspectral imaging device-type camera is disclosed, which utilizes a simple and compact optical configuration for capturing spatial and spectral data from bar images of a scene. Figure 5 An example of a camera 20 is shown, which includes an entrance pupil 22, a first mirror 24, a first lens 26, a reflective slit assembly 28, a second mirror 30, an optical element 32 for separating light according to wavelength, a second lens 34, and a (first) detector 36.

[0026] The entrance pupil 22 is optional and can be a transmissive material or aperture through which light from the scene passes. The first mirror 24 is semi-transparent, allowing at least some of the light entering through the entrance pupil 22 to pass through (i.e., be transmitted) the first mirror 24. A non-limiting example of the first mirror 24 is a 50% beam splitter cube or other type of 50% beam splitter or mirror that transmits 50% of the light and reflects 50% of the light incident on it from either direction. The first lens 26 focuses the light from the first mirror 24 onto the reflective slit assembly 28. The reflective slit assembly 28 (in...) Figure 6 (As better shown in the diagram) has a top surface made of or covered with a light-absorbing or scattering material 38, in addition to an elongated strip 40 of reflective material (also referred to herein as a mirror strip 40), which has a length L in the (first) X direction and a width W in the (second) Y direction (i.e., the (first) X direction is orthogonal to the (second) Y direction), wherein the length L is greater than the width W (i.e., the light-absorbing material 38 is adjacent to the mirror strip 40). The mirror strip 40 is preferably configured to match the full size of the image reaching the reflective slit assembly 28 in the X direction, but only the small size of the image reaching the reflective slit assembly 28 in the Y direction, which is achieved by making the length L greater than the width W. The light-absorbing or scattering material 38 surrounds the mirror strip 40 and is positioned to absorb or scatter light not reflected by the mirror strip 40. The light-absorbing or scattering material 38 can be, for example, a metal oxide such as black chromium (chromium oxide), silver oxide (Ag₂O), electroless nickel-phosphorus, iron oxide, black matrix, carbon, dielectric coating, copper selenide (CuSe₅), graphene, and commercially available black-absorbing materials such as Acktar Black, Vantablack, diamond-backed ADLC, and anodized surfaces. The mirror strip 40 can be, for example, any good light-reflecting material, such as TiO₂, SiO₂, Ta₂O₅, Cr, Al, Au, Ag, etc. As a non-limiting example, the width W can be, for example, 1.5 μm, and the length L can be, for example, 3-5 mm (e.g., similar to the X-axis dimension of detector 36).

[0027] The first mirror 24 is configured to reflect at least some of the light reflected by the mirror strip 40 (and passing through the first lens 26) toward the second mirror 30. The second mirror 30 reflects the light from the first mirror 24 toward an optical element 32, which separates the incident light into different directions based on wavelength (e.g., by diffraction or refraction). As a non-limiting example, the optical element 32 may be as follows: Figure 7A The transmission diffraction grating shown is an example. The transmission diffraction grating may include a transparent substrate 32a having periodic structures 32b formed thereon or in it, which diffract light passing through it at different angles according to the wavelength of the light. The periodic structures 32b may be, for example, 500 diffraction lines per millimeter extending longitudinally in the X direction (i.e., elongated ridges and / or valleys). As another non-limiting example, the optical element 32 may be as follows: Figure 7B The prism shown is illustrated. The prism may include a transparent substrate 32c having non-parallel sides 32d (as seen in the X direction). A second lens 34 focuses light from the optical element 32 onto a detector 36. The detector 36 may be a two-dimensional pixel sensor array (i.e., an image sensor) that can resolve a two-dimensional image created by the optical elements of the camera 20 as a function of two-dimensional position on the array by measuring light intensity. A non-limiting example of the detector 36 is a detector with 2800 µm square and a resolution (i.e., pixel size) of approximately 1 µm per pixel. The output signal from the detector 36 is provided to the processor 70.

[0028] The first and second lenses 26 / 34 can be identical to each other. A non-limiting example of each of the first and second lenses 26 / 34 can be a lens stack having two lens components 42 stacked together, such that there are four aspherical surfaces 44 for each lens 26 / 34, as shown below. Figure 8 As shown in the diagram. If more than four aspherical surfaces 44 are required, more than two lens components 42 can be stacked together. The two lens components 42 of each lens 26 / 34 can be different in size from each other, and / or can be made of different materials (as a non-limiting example, two different polymer materials such as acrylate-based polymers and epoxy-based polymers can be used for the two lens components 42 respectively to improve chromatic performance). Lenses 26 / 34 can be PIM (plastic injection molded lenses), molded glass, machined and polished glass, combinations of glass lenses (e.g., achromatic lenses, e.g., achromat), glass replica lenses, wafer-level optics, or any combination thereof.

[0029] The entrance pupil 22, the first mirror 24, the first lens 26, and the reflecting slit assembly 28 are arranged along (i.e., at least a portion of each optical element is located) the first optical axis OA1, and the second mirror 30, the optical element 32, the second lens 34, and the detector 36 are arranged along the second optical axis OA2, as shown below. Figure 9 and 10 As shown in the diagram. Preferably, but not necessarily, the optical axes OA1 and OA2 are parallel to each other. Doing so provides the following advantages: minimizing the space occupied by the optical elements (i.e., minimizing the footprint occupied by all optical elements). It also simplifies the design of the optical elements (e.g., they can be manufactured on a wafer scale because, for example, lenses in the wafer will have the same pitch).

[0030] In operation, incident light 50 from the scanned scene enters the camera 20 through the incident pupil 22 (if used), is transmitted through the first mirror 24, and is focused by the first lens 26 onto the reflective slit assembly 28, as shown. Figure 9 As shown in the diagram. For example, an image of the scene is focused onto a two-dimensional region of the reflective slit assembly. Most of the light focused onto the reflective slit assembly 28 is absorbed or scattered. However, as... Figure 10 As shown, a portion of the incident light 50 focused onto the mirror strip 40 is reflected as return light 52. The return light 52 from the mirror strip 40 (corresponding only to the strips of the scene image) passes through the first lens 26, where it is collimated or at least partially collimated. The return light from the first lens 26 is reflected by the first mirror 24 towards the second mirror 30. The return light from the first mirror 24 is reflected by the second mirror 30 towards the optical element 32. The optical element 32 separates the return light 52 from the first and second mirrors 26 / 30 based on wavelengths in a dimension orthogonal to the longitudinal direction of the mirror strip 40 (i.e., orthogonal to the longitudinal direction of the strips of the scene from which the light source originates), and the second lens 34 focuses the return light from the first / second mirrors 26 / 30 and the optical element 32 onto the detector 36. Specifically, the optical element 32 is configured to separate the return light 52 based on wavelengths in the Y direction, which is orthogonal to the X direction, over which the longitudinal direction of the mirror strip 40 extends (compare). Figure 6 and 10 Therefore, the optical configuration of camera 20 preserves the original spatial position of light within the image in the X direction (as reflected by the mirror strip 40), while separating the color components of light in the Y direction for any given position along the X direction. This result in Figure 11As shown, for each position along the X direction of the mirror strip 40, the blue wavelength component 52b of the returned light 52 is directed to the upper part of the detector 36 (relative to the Y direction), the green wavelength component 52g of the returned light 52 is directed to the central portion of the detector 36 (relative to the Y direction), and the red wavelength component 52r of the returned light 52 is directed to the lower part of the detector 36 (relative to the Y direction). Therefore, the portion of the image reaching the detector 36 is spatially preserved in the X direction and separated according to wavelength in the Y direction. The detector 36 captures the image by measuring the intensity of the light as a function of its two-dimensional position on the detector 36. Preferably, the detector 36 is tilted at an angle θ such that the portion of the detector 36 receiving the blue wavelength component 52b is closer to the second lens 34 than the portion of the detector 36 receiving the red wavelength component 52r, to accommodate the different focal lengths of the different wavelengths of light. As a non-limiting example, the tilt angle θ can be 4 degrees. It should also be noted that the angle of the mirror 30 can be selected to best match the diffraction / refraction angle of the optical element 32.

[0031] like Figure 9-11 As shown, at any given time, only a single narrow strip of light from within the image of the scanned scene is directed to detector 36. To scan the entire scene, camera 20 can be moved relative to the scene being scanned, or optics can be used to alter the incident light relative to the camera's optics, thereby sequentially capturing data for individual strips of the scanned scene. Camera 20 may include (or be connected to) processor 70 for processing signals from detector 36, such as processing spatial and spectral data represented by signals from detector 36. This data processing may include creating a hyperspectral data cube or other hyperspectral image representing the collected data and subsequently labelling the scanned scene, including piecing together data collected from individual strips of the sequentially scanned scene. Data processing may also include comparing spatial and spectral data with a library of known values ​​to identify the scene, one or more objects within the scene, and / or materials within the scene.

[0032] Figure 12 An alternative example is shown, in which the reflective slit assembly 28 includes a (second) detector 60 covered by a transmissive material 58 and a mirror strip 40, such that a full image of the scene focused onto the reflective slit assembly 28 by the first lens 26 can be captured and measured (i.e., by measuring the intensity of light as a function of the two-dimensional position on the detector 60). The image detected by the detector 60 will have thin stripes missing, corresponding to the positions of the mirror strip 40. If necessary, the missing stripes in the image can be filled by the processor 70 either by extrapolating from adjacent data or by using a signal from the detector 36 representing a portion of the missing image. Figure 13Another alternative example is shown, in which the transmissive material 58 is omitted.

[0033] Wavelength separation is not required after the returned light 52 is reflected by the second mirror 30. For example, as... Figure 14 As shown (but a simplified representation of the light with optical element 32), optical element 32 can be arranged between the first and second mirrors 24 / 30, rather than as shown... Figure 5 This is shown between the second mirror 30 and the second lens 34. In this example, the reflected light 52 from the first mirror 24 is separated by wavelength before being reflected by the second mirror 30.

[0034] Figure 15A and 15B An alternative example is shown where the first mirror 24 is positioned such that the incident light 50 bypasses the first mirror 24 upon reaching the first lens 26 (e.g., the first mirror 24 is not positioned directly between the incident pupil 22 and the first lens 26, so the incident light can proceed to the first lens 26 without encountering the first mirror 24). The first mirror 24 is positioned to reflect all the return light 52 from the reflective slit assembly 28. The advantage of this optical configuration is that there is no loss of a portion of the incident light 50 due to passing through the first mirror 24. The incident light 50 completely avoids the first mirror 24. Furthermore, the first mirror 24 can be a highly reflective element that reflects all (or almost all) of the return light 52. Avoiding light loss for both the incident light 50 (by not having to pass through the first mirror 24) and the return light 52 (by avoiding configuring the first mirror 24 to reflect only some of the return light 52) ​​will increase the level of light reaching the detector 36 (i.e., increase any signal-to-noise ratio). Figure 16A and 16B It shows Figure 15A and 15B The optical configuration (but a simplified representation of the light with optical element 32) is such that optical element 32 is positioned between the first and second mirrors 24 / 30, rather than between the second mirror 30 and the second lens 34.

[0035] Figure 17 Another non-limiting example of the first and second lenses 26 / 34 is shown, which includes a first lens component 80 and a second lens component 84. The first lens component 80 includes two opposing convex spherical surfaces 82. The first lens component may be a single element, or may be as follows: Figure 17The two elements shown are bonded together. As a non-limiting example, the first lens component 80 may be formed of N-BAK4 type glass. The spherical surface 82 may be uncoated or coated (as a non-limiting example, the coating may be an acrylic polymer approximately 0.03 mm thick, which can optimize resolution across the field of view). The second lens component 84 includes two opposing surfaces 86 and 88, wherein surface 86 is concave and spherical and faces the first lens component 80, and surface 88 is planar. As a non-limiting example, the second lens component 84 may be formed of N-SF11 type glass. Surfaces 86 and 88 may be uncoated or coated. As a non-limiting example, the total thickness of the first and second lens components 80 and 84 may be approximately 10 mm.

[0036] Camera 20 offers several advantages. It comprises only two lenses 26 / 34 (where lens 26 is used to bidirectionally focus light onto and from the reflective slit assembly 28), two mirrors, and a folding design, with the optical components arranged on two parallel optical axes. Therefore, the camera can be made relatively small, enabling it to be wearable or integrated into mobile devices such as cellular phones. The absence of moving parts simplifies operation, reduces power consumption, and provides increased reliability. This optical system offers increased resolution compared to other camera systems with larger and more numerous optical components. The use of the reflective slit assembly 28 takes into account the optional detector 60 positioned along the first optical axis OA1 without obstructing light reflected from the mirror strip 40 to the optical elements 32 and detector 36. Processor 70 can combine data from both detector 36 and detector 60 to create a superimposed image of a conventional image (from detector 60) and a hyperspectral image (from detector 36).

[0037] It should be understood that the invention is not limited to the description above and the examples(s) shown herein, but covers any and all variations falling within the scope of any claim. For example, references to the invention herein are not intended to limit the scope of any claim or claim terminology, but rather to refer only to one or more features that may be covered by one or more claims. The above examples of materials, processes, and numerical values ​​are merely exemplary and should not be construed as limiting the claims.

Claims

1. A camera, comprising: A first lens is configured to focus incident light onto a reflective slit assembly, wherein the reflective slit assembly includes an elongated strip of reflective material configured to reflect some, but not all, of the incident light as a return light. The first lens is configured to at least partially collimate the reflected light from the elongated strip of the reflective material; The first mirror is configured to reflect the reflected light from the first lens; The second mirror is configured to reflect the reflected light from the first mirror; Optical elements are configured to separate the reflected light from the first mirror according to wavelength; and A second lens is configured to focus the returned light from the optical element onto a first detector, wherein the first detector is configured to measure the intensity of the returned light as a function of a two-dimensional position on the first detector.

2. The camera according to claim 1, further comprising: The processor is configured to create a hyperspectral image from the intensity of light measured by the first detector.

3. The camera according to claim 1, wherein: The first mirror, the first lens, and the reflecting slit assembly are arranged along the first optical axis; and The second mirror, the second lens, and the first detector are arranged along a second optical axis parallel to the first optical axis.

4. The camera according to claim 3, wherein, The optical elements are arranged along the second optical axis.

5. The camera according to claim 1, wherein, The optical element is a transmission diffraction grating.

6. The camera according to claim 1, wherein, The optical element is a prism.

7. The camera according to claim 1, wherein: The elongated strip of the reflective material has a length L and a width W; The length L is greater than the width W; The length L extends in the first direction; and The optical element is configured to separate the returning light according to the wavelength in a second direction orthogonal to the first direction.

8. The camera according to claim 7, wherein, The optical element includes diffraction lines extending longitudinally in a first direction.

9. The camera according to claim 1, wherein, The optical element is positioned between the second mirror and the second lens.

10. The camera according to claim 1, wherein, The optical element is positioned between the first mirror and the second mirror.

11. The camera according to claim 1, wherein, The first mirror is configured to transmit incident light through the first lens.

12. The camera according to claim 11, wherein, The first lens is a 50 percent beam splitter.

13. The camera according to claim 1, wherein, The first lens comprises a stack of lenses having at least four aspherical surfaces.

14. The camera according to claim 1, wherein, The second lens comprises a stack of lenses having at least four aspherical surfaces.

15. The camera according to claim 1, wherein, The first lens includes: The first lens component includes two opposing convex spherical surfaces; and The second lens component includes a first concave surface and a second planar surface facing the first lens component.

16. The camera according to claim 1, wherein, The second lens includes: The first lens component includes two opposing convex spherical surfaces; and The second lens component includes a first concave surface and a second planar surface facing the first lens component.

17. The camera according to claim 1, wherein, The reflective slit assembly includes light-absorbing material in an elongated strip adjacent to the reflective material.

18. The camera of claim 1, wherein the reflective slit assembly further comprises: The second detector is configured to measure the intensity of the incident light focused by the first lens as a function of the two-dimensional position on the second detector.

19. The camera of claim 18, further comprising: The processor is configured to create an overlay image from the light intensity measured from the first detector and the light intensity measured from the second detector.

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