Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
  • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
  • G01J3/02—Details
  • G01J3/0205—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows
  • G01J3/0208—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows using focussing or collimating elements, e.g. lenses or mirrors; performing aberration correction
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
  • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
  • G01J3/02—Details
  • G01J3/0205—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows
  • G01J3/0229—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows using masks, aperture plates, spatial light modulators or spatial filters, e.g. reflective filters
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
  • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
  • G01J3/28—Investigating the spectrum
  • G01J3/2823—Imaging spectrometer

Definitions

• the invention relates to a pupil imaging spectrometer.
• Spectrometers are instruments for spectral analysis of electromagnetic radiation, which are used for a large number of applications. Among these applications, spectral analyzes of radiation originating from areas of the Earth's surface are commonly performed, using spectrometers that are carried on board satellites orbiting the Earth.
• a spectrometer comprises the following elements:
• a radiation collecting optic which is adapted to form an image of a scene in an intermediate image plane, from radiation from the scene;
• an assembly with a spectral spread function comprising a collimator, an imager and a spectrally active element which is adapted to deflect the radiation differently depending on a wavelength of this radiation, the spectrally active element being located between the collimator and the imager along a path of the radiation from the scene in the spectrometer;
• the spectrally active element is a prism made of a dispersive material which is effective in the spectral range of analysis, or a grating which is suitable for this spectral range.
• the optics for collecting radiation can be constituted by a telescope.
• the spectrometer can be combined with a device for variable field orientation input optics of the spectrometer, or a device for scanning a scene, such as a single or dual axis steerable mirror, a steerable spectrometer stand, or an appropriate control function of the attitude control system and orbit of the satellite.
• the mask most often has a single opening, in the form of a slit, the width of which helps to determine, along with other parameters of the spectrometer, its spatial and spectral resolutions.
• the spectral dispersal power of the spectrally active element also helps determine the spectral resolution of the spectrometer.
• the photodetector can be a matrix structure image sensor, with radiation intensity detection pixels which are located at intersections of rows and columns in a detection matrix. It can be such a CMOS type sensor.
• the spectrometer is arranged so that the intermediate image plane is optically conjugated with the photosensitive surface of the photodetector through the assembly with a spectral spread function.
• the spectrometer thus forms an image of a scene band on the photodetector, this scene band being limited by the slit of the mask in the intermediate image plane.
• the slit of the mask thus has the function of a field diaphragm. It is oriented inside the intermediate image plane so that the scene strip which is thus imaged on the photodetector is perpendicular to the direction of the spectral spread which is produced by the spectrally active element.
• a scan of the terrestrial area is performed so that the scene strip which is imaged moves on the photodetector parallel to the direction of the image.
• spectral spread during the scan.
• all the intensities which are captured by the pixels of the photodetector aligned along the direction of the spectral spread are relative to the same spatial sampling portion of the scene, for spectral sampling intervals - ie in length waveform - which are determined by the position of each pixel along the spectral spread direction.
• the direction of the lines corresponds to a transverse direction in the scanning swath
• the direction of the columns simultaneously corresponds to the direction of the scanning of the terrestrial area and to the direction of spectral spread.
• the roles of the rows and the columns can be exchanged depending on the implementations of the matrix image sensor.
• the luminosity of the spectra which are thus obtained for spatial sampling segments of the scene strip, successive in the transverse direction of the swath, depends on the size of the entrance pupil of the spectrometer.
• This entrance pupil is generally determined by the radiation collection optics, and the spectrometer is arranged so that its exit pupil is located substantially at the level of the spectrally active element.
• FIG. 1 a and [Fig. 1 b] show such a spectrometer as known from the prior art.
• the references which are indicated in these figures have the following meanings:
• collimator which may be a converging lens or a combination of several lenses or mirrors
• spectrally active element for example a prism, it being understood that a deviation of the optical axis of the spectrometer which may be due to this spectrally active element is not shown, for the sake of clarity of the figures
• imager which may consist of a converging lens or a combination of several lenses or mirrors
• FIG. 1 a shows the formation of the spectrum which is captured for a spatial sampling segment in the length of the scene band delimited by the slit of the mask 2.
• FIG. 1 b shows the formation of the images of three points of the scene strip, which are located at different places along the transverse direction of swath X, by rays of the same wavelength which pass against two edges opposites of the entrance pupil PE. These images are denoted ⁇ ⁇ , l 2 and l 3 .
• the spectral response of the spectrometer for a scene sample depends not only on the characteristics of the spectrometer, but also on the illumination distribution in the scene sample.
• the detection signal which is produced by each pixel of the photodetector may vary as the luminance distribution varies spatially within the scene sample which is conjugated with that pixel. Because of this, a reconstruction of the luminance profile of the scene from the signals which are produced by the pixels of the photodetector, using a theoretical spectral response established for a uniform distribution of luminance, generates radiometric errors.
• mirror slot scrambler To avoid such errors, it is known to use a system called mirror slot scrambler. Such a system consists of two mirrors which are arranged facing each other from the intermediate image plane, these mirrors being parallel to one another and parallel to the optical axis of the spectrometer. Space between the two mirrors at the level of the intermediate image plane replaces the slit, and the mirrors reflect one or more times parts of the radiation beam which comes from the stage strip. These reflections have the effect of blurring the spatial content of the scene strip along the direction of spectral spreading, and therefore of homogenizing the spectral response of the spectrometer.
• the mirror slit scrambling system generates an anamorphosis because the intermediate image plane is located at the entrance of the equivalent slit according to the y direction of spectral spreading, and at the exit of the equivalent slit according to the x d direction imagery.
• An aim of the present invention is therefore to provide a new spectrometer, for which the spectral response is less or is not sensitive to the distribution of luminance within a spatial sample of the scene, and for which at least some of the drawbacks cited above are reduced.
• a first aspect of the invention provides a spectrometer with collection optics, mask, assembly with spectral spreading function and photodetector as indicated above, but in which the mask comprises several openings which are offset within the intermediate image plane.
• the spectrometer further comprises:
• a pupillary imaging system which is arranged between the optics for collecting radiation and the assembly having a spectral spreading function along the path of the radiation coming from the scene in the spectrometer, the pupil imaging system being adapted to simultaneously form in an object plane of the spectral spread function array, which is optically conjugated with the photodetector, multiple intermediate images of the entrance pupil of the spectrometer with distinct beams of the radiation from the scene which pass through the apertures of the mask, a separate intermediate image of the entrance pupil being formed from the beam of each aperture of the mask, so that the spectral spread function assembly then forms a separate image of the pupil input of the spectrometer to the photodetector from each intermediate image of the entrance pupil, for each wavelength of an operating spectral range of the spec trometer.
• the pupillary imaging system is such that any two images of the entrance pupil of the spectrometer formed from beams which pass through distinct openings of the mask, have between they have an offset component which is perpendicular to the direction of spectral spread of the spectrally active element.
• the pupil imaging system produces the slit scrambler function. For this, it replaces, vis-à-vis the assembly having a spectral spread function and the photodetector, the slit used in the prior art by images of the entrance pupil of the spectrometer which are produced by beams. from separate sample areas of the stage. Simultaneously, for each spectrum which is formed on the photodetector corresponding to a distinct sampling zone of the scene, the opening of the mask in which this zone is imaged by the collection optics, serves as a pupil. A total or quasi-total interference function is thus obtained, with respect to inhomogeneities of luminance distribution that may exist within each spatial sample of the scene.
• the pupil imaging system is also suitable for forming an image of each opening of the mask, through the collimator, at the level of the spectrally active element.
• the spectrometer may further comprise a pupillary mask which is arranged in the object plane of the assembly with a spectral spread function, this pupil mask comprising several openings, with a separate opening which is separately dedicated to each opening of the mask of the intermediate image plane, the openings of the pupil mask determining the entrance pupil of the spectrometer as a common object which is conjugated by the pupil imaging system with all the openings of the pupil mask.
• a pupillary mask which is arranged in the object plane of the assembly with a spectral spread function, this pupil mask comprising several openings, with a separate opening which is separately dedicated to each opening of the mask of the intermediate image plane, the openings of the pupil mask determining the entrance pupil of the spectrometer as a common object which is conjugated by the pupil imaging system with all the openings of the pupil mask.
• the pupillary imaging system can comprise several optical subsystems which are dedicated one by one to the apertures of the mask, each optical subsystem being offset parallel to the plane of intermediate image so that a beam of the radiation which comes from the scene and which passes through one of the openings, then passes through the optical subsystem which is dedicated to this opening.
• each optical subsystem can comprise two converging lenses which have equal focal lengths and a common optical axis, with a first of these two converging lenses which is substantially superimposed on the plane of intermediate image, and the second of the two converging lenses which is located downstream of the first in the direction of propagation of the radiation which comes from the scene, and at a distance from this first lens which is substantially equal to the focal length of the two lenses .
• the common optical axis of the two converging lenses of each of the optical subsystems is parallel to the optical axis of the spectrometer.
• the first converging lenses of all the optical subsystems can be microlenses formed in a first transparent plate which is common to all the optical subsystems.
• the second converging lenses of all optical subsystems can be other microlenses formed in a second transparent plate which is also common to all optical subsystems.
• the Pupillary imaging system can thus be simple to assemble and align within the spectrometer.
• a further improvement may in addition consist, for such embodiments of the invention, in that the mask is at least partly formed on an entry face of the first transparent plate, facing towards the optics for collecting radiation. For this, one of the openings is located on each microlens which constitutes one of the first converging lenses.
• each optical subsystem it is also possible to use a single converging lens, of the thick lens type, instead of the two lenses described above.
• the optical subsystem is then formed by this single converging lens.
• each converging lens of optical subsystem may be a microlens formed in a transparent plate which is common to all optical subsystems.
• the photodetector can be a matrix image sensor, with a direction of columns or rows of this sensor which is parallel to the direction of spectral spread of the spectrally active element;
• the pupillary imaging system can be such that the images of the entrance pupil of the spectrometer which are formed on the photodetector by the beams of radiation coming from the scene which pass through different apertures of the mask, do not overlap;
• the radiation collection optics can be telecentric, so that the optical subsystems of the pupil imaging system can then be identical;
• each opening of the mask can be rectangular, with a length and a width of this opening which are less than 3 mm (millimeter), preferably less than 1 mm.
• a second aspect of the invention proposes a method of spectrometric analysis of a zone on the surface of the Earth, called an analysis zone, according to wherein a spectrometer which conforms to the first aspect of the invention is on board a satellite orbiting the Earth.
• the method then comprises the following steps, performed on board the satellite:
• each captured image containing, separately for each of several spatial sampling portions of the analysis area which are delimited by the openings of the mask, a spectrum of the part of the radiation which selectively comes from this spatial sampling portion of the analysis area.
• FIG. 1 a and [Fig. 1 b], already described, are optical diagrams of a spectrometer as known from the prior art;
• FIG. 2a] and FIG. 2b] correspond respectively to [Fig. 1 a] and [Fig. 1 b], for a spectrometer according to the invention
• FIG. 3 shows a mask which can be used in the spectrometer of [Fig. 2a] and [Fig. 2b];
• FIG. 4a symbolically shows a pupillary imaging system that can be used in the spectrometer of [Fig. 2a] and [Fig. 2b];
• FIG. 4b corresponds to [Fig. 4a] for a particular embodiment of the pupillary imaging system
• - [Fig. 5] shows an image as captured by a photodetector of a spectrometer according to [Fig. 2a], [Fig. 2b], [Fig. 3] and [Fig. 4b].
• the photodetector 7 is a matrix image sensor, for example of the CMOS type.
• a photodetector comprises radiation intensity detection pixels which are independent, and located at the intersections of rows and columns forming a matrix.
• the photodetector 7 is oriented so that its row direction is parallel to the x direction, and its column direction is parallel to the y direction.
• the terms “perpendicular” and “parallel” must be understood in the optical sense, that is to say that two directions are considered perpendicular (resp. Parallel) if the image of one through some of the optical components of the spectrometer is perpendicular (resp. parallel) to the other direction.
• the spectrally active element 5 can be a prism.
• the collimator 4 and the imager 6 can each be a converging lens or a combination of several lenses or mirrors.
• the photosensitive surface of the image sensor 7 is located in the image focal plane of the imager 6.
• the radiation collection optics 1 can be a telescope of a type known to those skilled in the art. It forms an image of a scene to be analyzed in the intermediate image plane PI. In a known manner, this collection optic 1, in particular a primary mirror of the telescope used, can determine the entrance pupil PE of the spectrometer 10.
• a mask 2 with multiple apertures is used in the intermediate image plane PI, and a pupillary imaging optic is used so that images of the entrance pupil PE are formed on the photodetector 7, each image being formed with a part of the radiation which has passed through one of the openings of the mask 2.
• each opening of the mask 2 has a pupil function for the radiation which has passed through this opening and which reaches the photodetector 7, and this radiation comes selectively from a spatial sampling portion of the scene, delimited by the corresponding opening of the mask 2.
• the mask 2 can have a configuration of openings as shown in [Fig. 3].
• the plurality of openings Oi, 0 2 , 0 3 , ... can be distributed so that no strip parallel to the Y axis exists between two successive openings in the X direction, without being cut by the one of those openings.
• a swath is traveled, which appears continuous in the X direction between two lateral swath edges.
• the openings Oi, 0 2 , 0 3 , ... can be distributed so that their projections on the direction X, parallel to the direction Y, do not overlap.
• each of the openings Oi, 0 2 , 0 3 , ... can have a length L, parallel to the X direction, of about 500 ⁇ m (micrometer), and a width I, parallel to the Y direction, of 350 ⁇ m, and all openings Oi, 0 2 , 0 3 , ... can belong alternately to one of two rows each parallel to the X direction.
• the pupillary imaging system 3 can comprise an optical subsystem which is constituted by two converging lenses 3i and 3 2 , these can be identical.
• the two converging lenses 3i and 3 2 can be arranged along a common optical axis, which is further parallel to the optical axis Z of the spectrometer 10 and can pass through a center of the corresponding opening Oi, 0 2 , ... of the mask 2.
• the converging lenses 3i and 3 2 of the same optical subsystem can have respective focal lengths which are equal, and equal to the separation distance d between the two lenses, in the thin lens approximation. Furthermore, the converging lens 3i of each optical subsystem, closest to the radiation collecting optics 1, can be superimposed or substantially superimposed on the intermediate image plane PI. It is then against the mask 2. Under these conditions, each optical sub-system forms an image of the entrance pupil PE of the spectrometer 10 which is located substantially at the level of the converging lens 3 2 of this optical sub-system.
• the plane which contains all the images of the entrance pupil PE of the spectrometer 10 which are formed by all the optical subsystems of the pupil imaging system 3, is denoted PE '.
• all the images thus formed in the plane PE ', of the entry pupil PE of the spectrometer 10, are distributed in the plane PE' according to a distribution which corresponds to that of the openings Oi, 0 2 , 0 3 ,. .. of mask 2 in the intermediate image plane PI.
• FIG. 4b shows a possible embodiment of the pupillary imaging system 3, which corresponds to the optical principle illustrated by [FIG. 4a].
• All the converging lenses 3-i, 3 2 can be identical, of the plano-convex type with their plane faces turned towards the outside of the system 3. Then, all the converging lenses 3i, belonging one-to-one to the subsystems system 3 optics, can be produced in the form of microlenses in a first plate transparent 3a, and all the convergent lenses 3 2 , also belonging one to one to the optical subsystems of the system 3, can be produced in the same way in the form of microlenses in a second transparent plate 3b.
• the entry face FE of the microlens plate 3a which is flat
• the exit face FS of the microlens plate 3b which is also flat, can be superimposed on the plane PE 'of the images of the entrance pupil PE.
• the microlens plates 3a and 3b can be made of silica (Si0 2 ). Each microlens forming one of the converging lenses 3i or 3 2 can have a radius of curvature of 1.15 mm for its convex face and a thickness of 720 ⁇ m, corresponding to a focal length of 1.95 mm. The distance d between the plates 3a and 3b can then be 1.90 mm, and each microlens can have a diameter of 800 ⁇ m, parallel to the planes PI and PE '. Under these conditions, the entrance pupil PE can be chosen so that each of its images, as formed by any one of the optical subsystems of the pupil imaging system 3, has a length of about 420 ⁇ m depending on the size. x direction, and a width of about 130 ⁇ m in the y direction, in the PE 'plane.
• the mask 2 as shown in [Fig. 3] and with the dimensions indicated above for the openings Oi, 0 2 , 0 3 , ... of the mask 2, can be separated from the converging lenses 3-
• Such an embodiment for which all the openings Oi, 0 2 , 0 3 , ... of the mask 2 are identical, and all the optical subsystems of the pupillary imaging system 3 are identical, is particularly suitable when the radiation collection optic 1 is telecentric.
• Those skilled in the art are familiar with such telecentric collection optics, so that it is not necessary to describe examples of them here.
• the assembly with a spectral spreading function comprising the collimator 4, the spectrally active element 5 and the imager 6, listed in order according to the direction of propagation of the radiation inside the spectrometer 10, is arranged so that the object focal plane of the collimator 4 coincides with the PE 'plane of the images of the entrance pupil PE.
• each image of the entrance pupil PE as it exists in the plane PE ', is re-imaged for each wavelength of radiation on the surface of the photodetector 7 through the assembly with a spread function spectral.
• these pupillary images on the photodetector 7 are denoted PE-i, PE 2 , PE 3 , ... in [Fig. 2b] and [Fig. 5].
• the spectrum Si extends from the pupillary image PE-i on the photodetector 7, and corresponds to a spatial sampling portion of the scene area located in the input optical field of the spectrometer 10, such as that delimited by the opening Oi of the mask 2.
• the spectrum S 2 extends from the pupillary image PE 2 on the photodetector 7, and corresponds to another spatial sampling portion of the scene area as delimited by the opening 0 2 of the mask 2. Ditto for the spectrum S 3 with respect to the pupillary image PE 3 and the opening 0 3 of the mask 2, etc.
• Each operating sequence of the photodetector 7 thus makes it possible to simultaneously acquire the spectra of several spatial samples of the scene, which are distinct. Such a faculty is called co-registration in the jargon of those skilled in the art.
• the replacement which is carried out by the pupillary imaging system 3 on the photodetector 7, of the respective images of the openings Oi, 0 2 , 0 3 , ... of the mask 2 by the images PE-i, PE 2 , PE 3 , ... of the entrance pupil PE of the spectrometer 10, ensures homogenization of the distribution of luminance inside each spatial sampling portion of the scene which is delimited by one of the openings Ch, 0 2 , 0 3 , ... of mask 2. In this way, a luminance distribution within the scene can be reconstructed from images captured by photodetector 7, using a response function spectral set for uniform luminance conditions in the scene.
• the pupillary imaging system 3 provides a function of total or almost total scrambling of luminance inhomogeneities which may exist within each spatial scene sample which is delimited by one of the openings Oi, 0 2 , 0 3 , ... of mask 2.
• the focal length of the converging lenses 3i and 3 2 , and the position of the spectrally active element 5 between the collimator 4 and the imager 6, can be chosen so that the pupil imaging system 3 forms images of the apertures Oi, 0 2 , 0 3 , ... of the mask 2, through the collimator 4, in a plane PI 'which is roughly superimposed on the spectrally active element 5.
• the invention can be reproduced by modifying secondary aspects of the embodiments which have been described in detail above, while retaining at least some of the cited advantages.
• the apertures of the mask may have offsets along the Y axis which are different from those shown in [Fig. 3], and the pupillary imaging system may have different constitutions from that of [Fig. 4a] and [Fig. 4b].
• all the numerical values which have been quoted are only for illustration, and can be changed depending on the application considered.

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• Physics & Mathematics (AREA)
• Spectroscopy & Molecular Physics (AREA)
• General Physics & Mathematics (AREA)
• Spectrometry And Color Measurement (AREA)

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• 2019
  • 2019-02-19 FR FR1901658A patent/FR3092912A1/fr active Pending
• 2020
  • 2020-02-14 WO PCT/FR2020/050279 patent/WO2020169905A1/fr unknown
  • 2020-02-14 EP EP20710227.8A patent/EP3784998A1/de not_active Withdrawn

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