IL263035B2

IL263035B2 - A method for increasing the field of view in a hyperspectral system

Info

Publication number: IL263035B2
Application number: IL263035A
Authority: IL
Inventors: Yacoby Erez
Original assignee: Israel Aerospace Ind Ltd; Yacoby Erez
Priority date: 2018-11-14
Filing date: 2018-11-14
Publication date: 2023-10-01
Also published as: IL263035A; WO2020100139A1; IL263035B1

Description

AN OPTICAL MODULE AND METHOD FOR EXTENDING FIELD OF VIEW IN HYPERSPECTRAL IMAGING SYSTEMS TECHNOLOGICAL FIELDThe present invention relates to the field of hyperspectral imaging systems. More specifically, it relates to an optical module and method for extending field of view (FOV) in hyperspectral imaging systems.

BACKGROUND ART References considered to be relevant as background to the presently disclosed subject matter are listed below: - Chinese patent publication number 101201459; - US patent publication number 2016/286121; - Japanese patent publication number 56054372; - US patent publication number 2002/021446; Acknowledgement of the above references herein is not to be inferred as meaning that these are in any way relevant to the patentability of the presently disclosed subject matter.

BACKGROUND High performance imaging spectrometers images an entrance slit onto a 2D (two-dimensional) detector array where the length of the slit extends along a first of the dimensions of the 2D detector array and the dispersion created by a diffraction grating, a prism or any combination of the two, relatively displaces images of the slit in a second orthogonal dimension of the 2D detector array. Displacements of the slit images in the second dimension register the spectral content of light collected through the entrance slit. A hyperspectral image of a scene is captured by incorporating fore- optics for imaging a slice of a scene onto the slit and translating the spectrometer in a so-called "pushbroom" manner to capture contiguous images of the scene's spatial radiance distribution. Each pixel of the scene is associated with a substantially contiguous spectrum spread over the second orthogonal dimension of the 2D detector array. Performance objectives are set for imaging spectrometers such as increasing spectral range and spectral and spatial resolution, decreasing package size, and enlarging the field of view. However, the performance gains achieved for one objective are offset by performance losses encountered for other objectives. To improve the spatial field coverage at a particular resolution, multiple systems are incorporated, where the "linear fields of view" of the systems are aligned end to end. This solution is prohibitive for many applications due to the costs of the multiple detectors, coolers, spectrometers, or the space, weight, or power constraints. Additionally or alternatively, hyperspectral sensors having a greater number of pixels may be developed.

GENERAL DESCRIPTIONThe present invention meets the need to provide a hyperspectral imaging system having a large scan width. The width of the commercially available matrix detector that can be used by conventional hyperspectral imaging systems is in the order of 1,000-2,000 pixels (number of effective pixels) such that the scan width and also the field of view (FOV) is limited. There is a need in the art to provide a solution attaching at least two detectors at the focal plane in hyperspectral imaging systems. It should be understood that typically, hyperspectral imaging systems have an entrance slit, a dispersive element (e.g. grating), and a hyperspectral sensor, which is placed downstream and in proximity with the dispersive element. These hyperspectral imaging systems have typically two focal planes: the first focal plane being located at the entrance slit and the second focal plane being located at the final imaging plane of the imager. The sensor collects the intensity of detected signal for a specific wavelength band/range creating a special spectral signature (i.e. fingerprint) enabling identification of the materials that make up a scanned object. The precision of these sensors is typically measured in spectral resolution, which is the width of each band of the spectrum that is captured. If the scanner detects a large number of narrow frequency bands, it is possible to identify objects even if they are only captured in a handful of pixels. However, spatial resolution is a factor in addition to spectral resolution. If the pixels are too large, then multiple objects are captured in the same pixel and become difficult to identify. If the pixels are too small, then the energy captured by each sensor cell is low, and the decreased signal-to-noise ratio reduces the reliability of measured features. The present invention uses the two focal planes of the hyperspectral imaging systems (i.e. first and second focal plane) to extend the FOV of any hyperspectral imaging system by placing at the first focal plane of the hyperspectral imaging system located at the region of the entrance slit a splitting reflective unit. The present invention thus provides an extension of the dimension of the entrance slit (e.g. doubled in size) being located at the first (intermediate) focal plane. More specifically, the present invention enables to increase the FOV at least by a factor of 2, and by this enables to launch to the space one hyperspectral imaging system having a certain FOV being equivalent to two hyperspectral imaging systems in terms of covered swath. In other words, the covered swath of one hyperspectral aircraft or spacecraft having an extended FOV according to the teachings of the present invention is equivalent to the covered swath of two hyperspectral aircrafts or spacecrafts. Because a plurality of spaced-apart detectors are provided, the present invention enables to reduce significantly the involved expenses associated with the production of aircrafts as well as the involved expenses associated with launching of a plurality of aircrafts (e.g. satellites) in the space. According to a broad aspect of the present invention, there is provided an optical module to be used with a fore optic imager of a hyperspectral imaging system defining a certain field of view (FOV) and focusing at least one input beam onto an intermediate focal plane. The optical module comprises a splitting reflective unit for receiving at least one focused input beam and splitting each focused input beam into a plurality of N spatially separated light beams, each light beam defining a respective optical path; a dispersion assembly being configured and operable to spectrally separate each of the plurality of N spatially separated light beams into components of different wavelength bands; the dispersion assembly comprising a plurality of N dispersion units, each dispersion unit being accommodated at the output of the splitting reflective unit in each respective optical path; an imaging assembly comprising a plurality of N spaced-apart detectors, each detector being accommodated at a respective imaging conjugated focal plane of the splitting reflective unit; each detector being aligned with the output of each respective dispersion unit in each respective optical path, wherein N is an integer having a value of at least two. As described above, the present invention enables to extend the FOV in a hyperspectral imaging system by splitting the entrance slit being the image of the fore optics upstream to the dispersive element. In other words, the imaging assembly defines an extended FOV as compared to the certain FOV of the hyperspectral imaging system. This may be implemented by placing a splitting reflecting unit configured for extending the FOV at the entrance slit at which the light beam is focused and narrow. The splitting reflecting unit can be placed at the intermediate focal plane of the hyperspectral optics such that each input beam is focused onto the splitting reflective unit. The intermediate focal plane of the hyperspectral optics is located at the entrance slit location such that the entrance slit is then configured as a reflective slit instead of a transmissive slit. Alternatively, the optical module may comprise an entrance slit accommodated at the intermediate focal plane. The splitting reflective unit is then accommodated downstream to the entrance slit. The splitting reflective unit comprises at least two reflective surfaces (e.g. retro-reflector element or angled prism) such that a focused input light beam impinging upon the reflective unit is redirected towards different spatial locations (at which the dispersive elements are placed) and imaged onto at least two focal planes instead of one focal plane. This splitting reflective unit spatially separates between two portions of the beam incident onto the two facets of the splitting reflective unit, to propagate towards two light sensitive surfaces/sensors. Both sensors are located at two conjugated focal planes of the splitting reflective unit. The splitting reflective unit may comprise a plurality of tilted reflective surfaces having a certain angle between them or an angled prism. As for dispersive elements, they are placed between the respective facets of the splitting reflective unit and the sensors. Alternatively, the splitting reflecting unit may be a prism placed after (downstream) the entrance slit. In some embodiments, each detector is configured as a two dimensional pixel matrix, wherein each pixel is configured and operable to detect a light beam having a different wavelength bands. According to another broad aspect of the present invention, there is provided a hyperspectral imaging system defining a certain field of view (FOV). The hyperspectral imaging system comprises a fore optic imager configured and operable for collecting at least one input beam from a target object and focusing it onto an intermediate focal plane; an optical module as defined above. According to another broad aspect of the present invention, there is provided a method of extending the field of view FOV of a hyperspectral imaging system. The method comprises receiving at least one focused beam and splitting each focused beam into a plurality of spatially separated light beams; reflecting each of the plurality of spatially separated light beams towards a plurality of dispersive elements; spectrally separating each of the plurality of spatially separated light beams into components of different wavelength bands; and imaging each of the plurality of beams onto a plurality of spaced apart detectors, defining together an extended FOV as compared to the FOV of the hyperspectral imaging system. In some embodiments, the method further comprises focusing at least one input beam indicative of a target object onto an intermediate focal plane.

BRIEF DESCRIPTION OF THE DRAWINGS In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: Fig. 1 shows a typical conceptual hyperspectral satellite system for imaging the ground by using a push-broom scanner; Fig. 2 shows a block diagram illustrating a possible configuration of the optical module according some embodiments of the present invention; Fig. 3 shows an example of a hyperspectral imaging system according some embodiments of the present invention; Fig. 4 shows a block diagram illustrating another possible configuration of the optical module according some embodiments of the present invention; and Fig. 5 shows a flow chart illustrating main steps for implementing a method of extending the field of view of a hyperspectral imaging system, according some embodiments of the present invention.

Claims

1.- 13 - 263035/

2.CLAIMS: 1. An optical module to be used with a fore optic imager of an hyperspectral imaging system defining a certain field of view (FOV) and focusing at least one input beam onto an intermediate focal plane; said optical module comprising: a splitting reflective unit for receiving at least one focused input beam and splitting each focused input beam into a plurality of N spatially separated light beams, each light beam defining a respective optical path, wherein said splitting reflective unit is accommodated at the intermediate focal plane such that each input beam is focused thereon; a dispersion assembly being configured and operable to spectrally separate each of said plurality of N spatially separated light beams into components of different wavelength bands; said dispersion assembly comprising a plurality of N dispersion units, each dispersion unit being accommodated at the output of said splitting reflective unit in each respective optical path; an imaging assembly comprising a plurality of N spaced-apart detectors, each detector being accommodated at a respective imaging conjugated focal plane of the splitting reflective unit; each detector being aligned with the output of each respective dispersion unit in each respective optical path, wherein N is an integer having a value of at least two. 2. The optical module of claim 1, wherein said splitting reflective unit comprises at least two reflective surfaces, such that each focused input beam impinging upon the reflective unit is redirected towards different spatial locations and imaged onto at least two spaced-apart focal planes.

3. The optical module of claim 2, wherein said splitting reflective unit comprises a plurality of tilted reflective surfaces having a certain angle between them or an angled prism.

4. The optical module of any one of claims 1 to 3, wherein each detector is configured as a two dimensional pixel matrix, wherein each pixel is configured and operable to detect a light beam having a different wavelength bands. - 14 - 263035/

5. The optical module of any one of claims 1 to 4, wherein imaging assembly defines an extended FOV as compared to the certain FOV of the hyperspectral imaging system.

6. A hyperspectral imaging system defining a certain field of view (FOV), comprising: - a fore optic imager configured and operable for collecting at least one input beam from a target object and focusing it onto an intermediate focal plane; - an optical module as defined in any one of claims 1 to 5.

7. A method of extending the field of view FOV of an hyperspectral imaging system, comprising: receiving at least one beam being focused at an intermediate focal plane; utilizing a splitting unit accommodated at the intermediate focal plane for splitting each beam that is being focused at said intermediate focal plane into a plurality of spatially separated light beams and directing each of said plurality of spatially separated light beams towards a plurality of dispersive elements; spectrally separating each of said plurality of spatially separated light beams into components of different wavelength bands; and imaging each of said plurality of beams onto a plurality of spaced apart detectors, defining together an extended FOV as compared to the FOV of the hyperspectral imaging system.

8. The method of claim 7, further comprising focusing at least one input beam indicative of a target object onto an intermediate focal plane. 30