Patent Description:
An intended application of the invention is for example the observation of earth, in orbit or airborne, in a spectral range from visible to infra-red.

A hyperspectral imager is an optical assembly that makes it possible to produce three-dimensional images of a scene: namely the two classical spatial dimensions (X, Y), plus a spectral dimension λ, corresponding to the decomposition in the wavelength domain of the light radiated by each point of the object.

There are several types of hyperspectral imagers distinguished by the scanning method of each point in the scene (point-by-point scanning, line-by-line scanning, or full-field acquisition) and by the process used to decompose light in the different wavelengths (diffraction, refractive dispersion, interferometry). The present invention relates to "pushbroom" type instruments using a diffraction grating to decompose light. This configuration is one of the most commonly used in earth observation in space (orbit) or airborne, in a spectral range from visible to infra-red.

Known hyperspectral imagers are either too bulky or too restrictive regarding the observed wavelength range. There is no compact hyperspectral imager providing a broad spectral range, for example from UV to short wavelength infrared (SWIR), without compromising hyperspectral image quality.

According to a first aspect, one of the objects of the invention is to provide a spectral imager with correction of in-field aberrations (typically astigmatism) over a broad spectral range. Another aspect of the invention is to provide a compact spectral imager with a broad range of wavelength detection. To this end, the inventors suggest a spectral imaging device with the features of claim <NUM>.

Even more than in a conventional imaging system, i.e. Offner spectrometer, the amount of collected photons is a key factor in the field of the invention, for keeping the measurement noise as low as possible. Indeed, the light energy radiated by the object is divided into a very large number of spectral bands (from <NUM> to <NUM>). To increase the amount of photons collected, hyperspectral sensors often use larger pixels than conventional sensors. But the solution of using larger pixels does not allow to achieve a compact design with good resolution which requires a high density of pixels per unit area. The claimed invention allows to modify the optical design of the Offner relay, so as to realize a magnification different from <NUM> (example <NUM>:<NUM> or <NUM>:<NUM>). This optical condenser makes it possible to use a wider slit, which therefore collects more light for a given opening. A reduced image of this wide slit is then produced on the sensor, whose pixels can then be smaller while maintaining a good signal to noise ratio. The present invention thus allows to provide a high signal to noise ratio and to use relatively small detectors without optical aberrations. Thanks to the high signal to noise ratio and correction of in-field aberrations the present invention allows to provide clear hyperspectral images with a high contrast.

The inventors propose a new design for a family of imaging spectrometers composed of two concave mirrors and a convex diffraction grating which surface has a complex invariant shape class (without symmetry of rotation or translation). This arrangement is distinguished from the conventional spectro-imager by introducing an additional demagnification function between the object (usually an input slit) and the image on the detection means. The invention allows to produce on a sensor, the image of an object, generally a narrow slit, which selects the unidimensional field of view of the spectro-imager. A diffraction grating, deposited or etched on the convex mirror, performs the spectral dispersion function. The dispersed image will represent a succession of lines, each being an image of the entrance slit in a different wavelength. A progressive scan of the scene makes it possible to reconstruct the second spatial dimension.

The claimed invention relates to the improvement of the optical design of a spectral imager using a convex diffraction grating and two concave mirrors (known as Offner-Chrisp). The invention is to introduce an important demagnification function (higher than <NUM>) by the judicious choice of the radii of curvature of the two concave mirrors such that the ratio between these two radii of curvature is approximately equal to the desired demagnification factor. This is made possible by the replacement of the spherical convex grating by a convex grating of complex shape (without rotation invariance).

For a given spatial and spectral resolution, these modifications lead to:.

Several optical relay designs exist, which can be classified according to the type of surface on which the grating is engraved, flat, concave or convex. An "Offner" montage uses two concentric surfaces: a convex spherical lattice and a concave spherical mirror. These two surfaces constitute a <NUM>:<NUM> magnification optical relay.

Convex diffraction gratings are used in an Offner spectral imaging device. This configuration offers a relatively large field of view and relatively low aberrations. The advantage of an Offner type spectral imaging device are that it operates with a relatively low F-number (≥ f/<NUM>), it accepts a long slit length while maintaining a compact size. Offner type spectral imaging device have a central position between the two concave mirrors which is the pupil stop of the system and where it is the most suitable place to correct in-field aberrations and/or distortion. The use of different first and second concave mirrors allow to further reduce smile and keystone distortions. Using a convex grating with a complex shape belonging to the complex invariant shape class at the pupil stop is advantageous for correction of in-field aberrations over a broad spectral range.

Hyperspectral imaging offers extremely diverse and varied application prospects in environmental and land use management for agriculture, forestry, limnology, geology and urbanology. The claimed invention allows to significantly reduce the volume and mass of hyperspectral instruments and make them compatible with small satellites (SmallSat) much cheaper to develop and to put into orbit. Such small satellite platforms are often foreseen in constellation.

The luminous energy received on the image plane per unit area in a given spectral band, called spectral illumination, increases with the square of the demagnification factor. The signal-to-noise ratio is improved proportionally to this demagnification factor. The use of a convex reflective diffractive grating with a reflective surface defined by a complex invariant shape class provides a better correction of field aberrations, chromatic aberrations and essentially third- and fifth-order astigmatism. This allows, for a given dimension of the input slit, to reduce significantly the size of the instrument without loss of image quality. This also makes it possible, for a given resolving power, to cover a wider spectral range, and in particular all visible near-infrared (VNIR) and short-wave infrared (SWIR) wavelengths.

The image plane is located at the first matrix detection means. The object plane is located at the slit. The claims invention is about modifying the design of the concentric spectro-imagers with convex gratings (so-called Offner-Chrisp) so as to add to them a function of demagnification (reduction of size between the object and its image) in order to improve their radiometric performance and reduce their volume (and consequently their weight). This evolution is not easy to obtain because it requires modifying the "form class" of the convex reflective diffractive grating, otherwise the resolution of the image is strongly degraded regarding the spectral range of the instrument (unless the factor of demagnification is kept at values sufficiently close to <NUM>).

Generally, one can define three classes of shapes for non-planar optical surfaces (according to ISO Standard <NUM>-<NUM>: <NUM>):.

In terms of manufacturing and metrology, the difficulty is also growing as the class complexity becomes higher. The solution to the problem of demagnification can be partially solved by replacing the spherical convex grating with an anamorphic (solution for a moderate spectral range) or radial polynomial convex grating (solution for an extended spectral range).

The invariant complex shape class of the convex grating of the invention allows to reach higher demagnification factor with lower chromatic aberration and field aberration, and with a relatively large spectral range than with an aspherical grating (revolution invariance) or with a toroidal grating (invariance of revolution). The invention consists of a spectro-imager design including a demagnification function and an extended corrected field with the use of a complex invariant grating.

The invention can be applied to different variants of the spectrometer, such as those described below, including multi-blaze grating or using several diffraction orders simultaneously, but also spectrometers with high spectral resolution (R = <NUM> or more).

The inventors propose several preferred embodiments comprising optional features, where some of them can be combined.

The spectral imaging device is preferably coupled to an input telescope. The spectral imaging device is aligned in the focal plane of the telescope. The entrance slit of the spectral imaging device allows to select a single line of the image formed by the telescope. The slit width (I) of the spectral imaging device, the telescope focal length (FTel) and the distance to the observed scene (D) define the spatial sampling of the instrument (SSD: Spatial Sampling Distance): <MAT>.

The imaging spectrometer (or spectro-imaging device) allows to measure the spectral radiance at each of the points of the entrance slit. The spectro imaging device of the invention is an optical relay including a diffraction grating, capable of angularly separating the different wavelengths. This optical relay forms an image of the slit on a sensitive detector in the target wavelength range, and converts the angular separation of the spectrum into a spatial separation along an axis perpendicular to that of the slit. An image in two dimensions is thus obtained: a spatial dimension parallel to the slit and a spectral dimension perpendicular to it.

Spectral Sampling Interval (SSI) depends on the angular dispersion of the diffraction grating ζ (rad / nm), the focal length of the second concave mirror (Fim) and the size of the pixel (Pix): <MAT>.

Spectral Sampling Interval (SSI) is also expressed in terms of resolving power (R) around the average wavelength λ: <MAT>.

The number of resolved bands (NB), often corresponding to the number of useful pixel lines on the detector, is the ratio between the spectral range covered by the instrument and the Spectral Sampling Interval (SSI): <MAT>.

Preferably said demagnification is higher than <NUM>, more preferably higher than <NUM> and even more preferably higher than <NUM>. Preferably the demagnification is lower than <NUM> and preferably lower than <NUM>. For example, the demagnification is <NUM>.

Preferably, said reflective surface having a complex invariant shape class is defined in a three dimensional space with the equation: <MAT> where z is the surface sag (z-coordinate) at the point of polar coordinate (r, ϕ), c is the curvature (the reciprocal of the radius of curvature) and k is the conic constant, αi is the ith even aspherical coefficients, Ai is the coefficient on the ith Zernike polynomial, zi is the ith Zernike polynomial, ρ is the normalized radial coordinate such that <MAT> where R is the useful radius of the surface, preferably M being an integer comprised between <NUM> and <NUM>.

The two first terms of the equation define an aspherical surface (rotationally invariant), and the third term add a invariant complex shape contribution defined by a series of Zernike polynomials Zi. More precisely, the first term defines the equation of a sphere and the second term defines aspheric coefficients. Preferably said aspheric coefficients are given by an even-order polynomial. Preferably, said Zernike polynomials allow to break the spherical symmetry of said reflective surface.

The Zernike polynomials are a sequence of polynomials, with two polar variables (r, ϕ), that are continuous and orthogonal over a unit circle. As such there can be used to define any invariant complex surface with a circular boundary. Preferably, M is an integer comprised between <NUM> and <NUM>. Preferably N is equal to <NUM> or <NUM>. Preferably, α<NUM> = <NUM>.

Preferably, said diffractive grating is a blazed grating.

Preferably, said blazed grating has a constant line spacing d, said line spacing being comprised between <NUM> and <NUM> and more preferably comprised between <NUM> and <NUM>.

A blazed grating has a constant line spacing , determining the magnitude of the wavelength splitting caused by the grating. The grating lines possess a triangular, saw tooth-shaped cross section, forming a step structure. The steps are tilted at the so-called blaze angle θB with respect to the grating surface. Accordingly, the angle between step normal and grating normal is θB. Commonly blazed gratings are manufactured in the Littrow configuration. The blaze angle is optimized to maximize efficiency for the wavelength λb of the used light.

Descriptively, this means θB is chosen such that the beam diffracted at the grating and the beam reflected at the steps are both deflected into the same direction.

The advantage of a constant line spacing grating is to allow maximum optical power in one desired diffraction order for a given wavelength range.

Preferably, said blazed grating has a step height h comprised between <NUM> and <NUM>, more preferably comprised between <NUM> and <NUM>, and even more preferably between <NUM> and <NUM>.

Preferably, said line spacing d being comprised between <NUM> and <NUM>, more preferably between <NUM> and <NUM>, and even more preferably d being equal to <NUM>.

Such line spacing is particularly well suited for having a spectral imaging device with very high spectral resolution but for a relatively narrow range of wavelength. Thus the spectral imaging device allows for very high resolution spectroscopy with bands from <NUM> to <NUM>, over a limited spectral range, for example between <NUM> to <NUM>. Preferably this embodiment allows to discretize bands with a spectral width of <NUM> to <NUM> in a wavelength range of about <NUM> in the range of wavelength <NUM> to <NUM>. In this embodiment, a demagnification factor of <NUM>:<NUM> can be utilized by putting the constitutive elements with the relative distance allowing <NUM>:<NUM>. To make such a design (low line spacing on the grating, demagnification <NUM>:<NUM>), the slit can be positioned such that in incoming light beam from the slit <NUM> toward the first concave mirror <NUM> passes between the second concave mirror <NUM> and the convex reflective diffractive grating <NUM> making the design of this embodiment compact. This embodiment can achieves a resolution of <NUM> for <NUM> pixels.

In a preferred embodiment of the invention, several gratings having different blaze angles are engraved on the convex surface of said reflective diffractive grating. Preferably, said blazed grating comprises a first grating portion with a first blaze angle θB<NUM> and a second grating portion with a second blaze angle θB<NUM>, said first blaze angle θB<NUM> being different than said second blaze angle θB<NUM>. More preferably said blazed grating further comprises a third grating portion with a third blaze angle, and even more preferably said blazed grating further comprises a fourth grating portion with a fourth blaze angle. Preferably said gratings with different blaze angles are best suited for spectral imaging devices with a band width resolution above <NUM> per pixel and more preferably of <NUM> per pixel. Preferably, the first, second, and third blaze angle having different values. Preferably, the first, second, third and fourth blaze angle having different values.

For example, θB<NUM> are comprised between <NUM>° and <NUM>° and more preferably between <NUM>° and <NUM>°.

The advantage of a two grating portion with the same line spacing but with a different step height is to allow a maximized distribution of the optical power in one desired diffraction order or more diffraction orders for a given wavelength range.

Preferably, said first grating portion and said second grating portion covering essentially said reflective surface, said first grating portion covering a surface S<NUM> and said second grating covering a surface S<NUM> such that the distribution of S<NUM> and S<NUM> over said reflective surface is such that: <MAT>.

For example, said first grating portion and said second grating portion covering essentially said reflective surface have a distribution over said reflective surface of <NUM>/<NUM> for said first grating portion and <NUM>/<NUM> for said second grating portion.

The advantage of having gratings with different blaze angles is to allow a better distribution of the diffracted light aver the wavelength range to be covered. The sum of the efficiency curves of each of these gratings allows to cover a wavelength range from <NUM> to <NUM> and more preferably a wavelength range from <NUM> to <NUM>, for example a wavelength range of <NUM> to <NUM>. The advantage of using these gratings is to cover mentioned wavelength ranges more effectively, either by using a single diffraction order or even more effectively, by exploiting, for example, the diffraction orders +<NUM> or -<NUM> in the SWIR and +<NUM> or -<NUM> in the VNIR.

For example, said second grating surface portion is surrounded by said first grating surface portion on said reflective surface.

For example, when using the diffraction orders +<NUM> or -<NUM> and +<NUM> or - <NUM>, these two orders can be partially superimposed spatially, so a dichroic separator is necessary to address them separately with said first and second matrix detection means.

Preferably, said spectral imaging device further comprises a second matrix detection means and a dichroic filter positioned between said second concave mirror and said first matrix detection means such that:.

Here the term reflected means that part of the light beam is directed on the same side of the filter.

For example, the first fraction of said light beam transmitted by the dichroic filter toward the first matrix detection means for being detected by this one has a wavelength range of at least <NUM> to <NUM>, preferably of at least <NUM> to <NUM> corresponding to the first order diffraction. For example the second fraction of the light beam reflected by the dichroic filter toward the second matrix detection means for being detected by this one has a wavelength range of at least <NUM> to <NUM>, preferably of at least <NUM> to <NUM> corresponding to the second order diffraction.

Preferably, the first spectral range fraction of said light beam is comprised in the SWIR wavelength range, in particular from <NUM> to <NUM>, and originating from a diffracted order -<NUM> or +<NUM> and the second spectral range fraction of said light beam is comprised in the VNIR wavelength range, in particular from <NUM> to <NUM>, and originating from a diffracted order -<NUM> or +<NUM>, said diffracted orders being diffracted orders generated by the convex diffraction grating.

Preferably, said spectral imaging device further comprises a mirror such that it is positioned to reflect said light beam from said slit to said first concave mirror. Such mirror allows a more compact spectral imaging device.

Preferably, said slit having a slit F-number and said first or second detection means having a detection means F-number such that: <MAT> said slit F-number being comprised between <NUM> and <NUM>, preferably between <NUM> and <NUM> and said detection means F-number being comprised between <NUM> and <NUM>, preferably between <NUM> and <NUM>, for said demagnification factor being larger than <NUM>.

Preferably, said matrix detection means comprises a matrix of pixels defining pixel lines and pixel columns, pixel columns defining spatial position and pixel lines defining spectral information, said matrix detection means comprising at least <NUM> pixel lines, preferably at least <NUM>, and even more preferably at least <NUM>.

Preferably, said spectral imaging device can be included in a rectangular parallelepiped having main dimensions of <NUM> x <NUM> x <NUM>, preferably of <NUM> x <NUM> x <NUM>.

These aspects of the invention as well as others will be explained in the detailed description of specified embodiments of the invention, with reference to the drawings in the figures, in which:.

The drawings in the figures are not to scale. Generally, similar elements are designated by similar reference signs in the figures. The presence of reference numbers in the drawings is not to be considered limiting, even when such numbers are also included in the claims.

<FIG> shows an example of a spectral imaging device <NUM>. The system spectral imaging device <NUM> comprises a slit <NUM> with an aperture width. The aperture width can be modified depending on the demagnification of the spectral imaging device <NUM>, signal to noise ratio or type of observed object. For example the slit has a length of comprised between <NUM> and <NUM>, more preferably comprised between <NUM> and <NUM> and for example of <NUM> and a width comprised between <NUM> and <NUM>, more preferably comprised between <NUM> and <NUM> and for example a width of <NUM>.

Light enters the spectral imaging device <NUM> from the slit <NUM>. Preferably, an optical system is placed in upstream of the spectral imaging device <NUM> such as a telescope in order to form an image of the observed scene on the entrance slit. The light beam entering the spectral imaging device <NUM> via the slit is directed onto the first concave mirror <NUM>. The light beam is then reflected by the first concave mirror <NUM> towards the convex diffractive reflective grating <NUM>. The light beam is then diffracted and reflected by the convex diffractive reflective grating <NUM> in the direction of the second concave mirror <NUM>.

For the reflective grating <NUM> of the invention the light beam is diffracted by the saw tooth-shaped cross section, forming a step structure and then back-reflected into the direction of the second concave mirror <NUM>. The diffracted light back-reflected into the direction of the second concave mirror <NUM> is reflected by the second concave mirror <NUM> and focused onto a first matrix detection means <NUM>.

First <NUM> and second <NUM> concave mirrors make possible to reflect and focus light with wavelengths in the range <NUM> to <NUM>. First <NUM> and second <NUM> concave mirror are preferably non-symmetrical. For example their focal lengths are different, their aperture are offset with respect to their axis of symmetry represented by a dot-dashed line, and their distance to the convex diffractive reflecting grating <NUM> are different.

The spectral imaging device <NUM> is configured such that it allows a demagnification higher than <NUM> and preferably higher than <NUM>. For example starting from a known Offner-Crisp type spectrometer, a demagnification is obtained by considering a significantly different radius of curvature for the first concave mirror <NUM> and for the second concave mirror <NUM>, and moving away the slit <NUM>. The demagnification can be defined by considering the length A of the entrance slit <NUM> and the length B of its image on the first matrix detection means <NUM>, such that the demagnification is given by the ratio A/B. For example A is in the range <NUM> to <NUM>, preferably in the range <NUM> to <NUM>, for example B is in the range <NUM> to <NUM> and preferably in the range <NUM> to <NUM>. Preferably the demagnification achievable with the spectral imaging device <NUM> is comprised between <NUM> and <NUM>, preferably comprised between <NUM> and <NUM> and more preferably comprised between <NUM> and <NUM>.

The spectral imaging device of <FIG> allows to obtain a demagnification higher than <NUM> for detecting a range of wavelengths preferably comprised between <NUM> and <NUM> and more preferably comprised between <NUM> and <NUM>. The advantage of the embodiment of <FIG> is to allow reduced field aberration and chromatic aberration such as to obtain a keystone and smile as low as possible, preferably both lower than <NUM>. <FIG> is a robust embodiment for allowing detection of a wide range of wavelengths with a relatively good spectral resolution, for example with a spectral resolution of <NUM> over a range of wavelength of <NUM> to <NUM> and more preferably of <NUM> over a range of <NUM> to <NUM>.

For the spectral imaging device of <FIG>, the slit F-number is <NUM> and the detection means F-number is <NUM>.

<FIG> shows another example of a spectral imaging device <NUM>. <FIG> shows a design for a spectro imager <NUM> with very high spectral resolution but for a relatively narrow range of wavelength. The spectro imaging device <NUM> of <FIG> allows for very high resolution spectroscopy with bands from <NUM> to <NUM>, over a limited spectral range, for example between <NUM> to <NUM>. <FIG> preferably allows to discretize bands with a spectral width of <NUM> to <NUM> in a wavelength range of about <NUM> in the range of wavelength <NUM> to <NUM>.

For the spectral imaging device of <FIG>, to achieve such a high spectral resolution, the grating has a much higher spatial frequency than for the embodiment of <FIG>. The grating <NUM> frequency for <FIG> is up to <NUM> lines / mm. For such a grating <NUM>, the diffraction angle of the order -<NUM> is much larger than in <FIG>. For example the diffraction angle of the order -<NUM> is of about <NUM>°. It is then more advantageous to work with the order <NUM>, in a so-called "Littrow" configuration. In the spectral imaging device of <FIG>, a demagnification factor of <NUM>:<NUM> is represented with the relative distance of the constitutive elements. The peculiarity of this design is that it is naturally compact. It achieves a resolution of <NUM> for <NUM> pixels. In the spectral imaging device of <FIG>, the slit is positioned such that in incoming light beam from the slit <NUM> toward the first concave mirror <NUM> passes between the second concave mirror <NUM> and the convex reflective diffractive grating <NUM>. For the spectral imaging device of <FIG>, the slit F-number is <NUM> and the detection means F-number is <NUM>.

<FIG> shows an exemplary embodiment of the spectral imaging device <NUM> of the invention. This embodiment of the spectral imaging device <NUM> has been designed to cover a wider spectral range than design of <FIG>, between <NUM> and <NUM>. In <FIG>, the optical condensation factor or demagnification factor is <NUM>:<NUM> and the convex grating shows variable step height h which are adjustable locally to optimize diffraction efficiency in visible (VIS) and short-infrared wavelengths (SWIR: Short Wavelength Infra-Red) for orders -<NUM> and -<NUM> respectively. At the focal plane these two orders are separated by means of a dichroic plate or dichroic filter, towards two different detectors, namely a first matrix detection means <NUM> and a second matrix detection means <NUM>. The spectral resolution is <NUM> with a wide wavelength range. The convex grating in this embodiment of <FIG> has a line spacing in the range of <NUM> to <NUM> and preferably <NUM>. According to the invention, the slit F-number is <NUM> and the detection means F-number is <NUM>.

<FIG> shows an exemplary use of the spectral imaging device <NUM> of the invention on-board a satellite being in orbit around a planet. The spectral imaging device <NUM> is able to detect line by line the planet surface. Following an orbital motion, the spectral imaging device <NUM> of the invention is able to detect a strip at the planet surface. Thanks to the relative motion of the satellite with the planet, successive strips can be analysed by the spectral imaging device of the invention and stitched together to form an image in two dimensions plus a spectral dimension. The image in two dimension is represented by the square image representing a detail of the planet surface. The spectral dimension is represented by the successive images, each in a different wavelength and reprinted one behind the other. Basically for a spectral resolution of <NUM>, there exist on two dimensional image every <NUM> of the spectral range covered by the spectral imaging device <NUM>. The diagram showed along the axis representing the successive images one behind the other represents for example an average value of the two dimensional image for the spectral range covered by the spectral imaging device <NUM>. In this configuration, the spectral imaging device creates a line image which is swept relative to the planet in "pushbroom" fashion so as to create a three-dimensional data structure (X, Y, λ) represented schematically successive images one behind the other. The main advantage of the spectral imaging device of the invention for such a "pushbroom" type of imager, is to allow a better signal to noise ratio thus allowing sharper images because of the reduced exposure time for each line. A too long exposure time introducing blurring in the three-dimensional data images. The spectral imaging device <NUM> of the invention allowing a decrease of the exposure time thanks to the high demagnification allowed by the convex grating with a invariant class shape surface <NUM>.

<FIG> shows a preferred embodiment of the convex reflective diffractive grating <NUM> according to the invention. The grating <NUM> is a blazed grating engraved on a convex surface <NUM>. The engraving forms saw tooth-shaped cross section, forming a step <NUM> structure. The steps are tilted at the blaze angle θB with respect to the grating surface <NUM>. Accordingly, the angle between step normal and grating normal <NUM> is θB. The blazed grating <NUM> has a constant line spacing d, determining the magnitude of the wavelength splitting caused by the grating. Accordingly a step height h can be derived from the blaze angle θB and the line spacing d. Preferably, a step comprises the rising feature and the step jump having a height h. The step edges, namely the start of the rising feature and the bottom of the step jump are as represented on <FIG> aligned with the surface <NUM>. The steps cross section is preferably triangular. The grating surface <NUM> defines the overall surface of the grating <NUM>. The surface <NUM> could also be taken parallel to the actual surface <NUM> but being secant with the top of each step jump. Such a surface would actually be the original surface of the grating before engraving of the grating in a saw tooth-shaped fashion.

<FIG> shows a preferred embodiment of the convex reflective diffractive grating <NUM> according to the invention. The grating <NUM> is a blazed grating engraved on a convex surface <NUM>. The engraving forms saw tooth-shaped cross section, forming a step <NUM> structure with a first grating portion <NUM> having a first blaze angle θB<NUM> and a second grating portion <NUM> having a second blaze angle θB<NUM>, said first blaze angle θB<NUM> being different than said second blaze angle θB<NUM>. The steps of the first <NUM> and second <NUM> portions are tilted at the blaze angle θB<NUM>, θB<NUM> with respect to the grating surface <NUM>. The blazed grating <NUM> has a constant line spacing d, determining the magnitude of the wavelength splitting caused by the grating. Accordingly, two step heights h<NUM> and h<NUM> can be derived from the two blaze angle θB<NUM>, θB<NUM> and the line spacing d. Preferably, a step comprises the rising feature and the step jump having a height h<NUM> or h<NUM>. The step edges, namely the start of the rising feature and the bottom of the step jump are as represented on <FIG> aligned with the surface <NUM>. The steps cross section is preferably triangular. The grating surface <NUM> preferably defines the overall surface of the grating <NUM>. In other embodiment such a dual blaze grating has the top of the steps aligned at the grating surface <NUM>. In a preferred embodiment such a dual blaze grating the steps have equal average heights.

<FIG> show two embodiments of the convex reflective diffractive grating <NUM> described in <FIG> having a first grating portion <NUM> with a first blaze angle θB<NUM> and a second grating portion <NUM> with a second blaze angle θB<NUM>. In <FIG>, the first grating portion <NUM> and the second grating portion <NUM> cover essentially the reflective surface <NUM>. The first grating portion <NUM> covers a surface S<NUM> and the second grating covering a surface S<NUM> such that the distribution of S<NUM> and S<NUM> over said reflective surface is such that <MAT>. In <FIG>, this ratio is achieve with the first <NUM> and second <NUM> grating portion being placed next to each other on the reflective surface <NUM>. The demarcation line could be of any shape: straight, curved, free-form. In <FIG>, the ratio S1/S2 is achieved with the and second <NUM> grating portion being surrounded by the first grating portion <NUM>. The demarcation line could be of any shape: round, ellipsoid, polygonal or even free-form. Any other distribution of the first <NUM> and second <NUM> grating portion on the reflective surface <NUM> are possible, for example alternating several first <NUM> and second <NUM> portion with several demarcation line could foresee. Ultimately a grating with alternating blaze angle θB<NUM>, θB<NUM>, θB<NUM>, θB<NUM> could be used.

The grating substrate is made of Kanigen (NiP) plated aluminium 6061T6 alloy and diamond machined. The pattern of the grating is engraved on a complex invariant shape class substrate. The diamond machining of the optical surface of the grating is performed on <NUM> axes ultra-precision lathe, in two steps:.

The present invention has been described with reference to a specific embodiments, the purpose of which is purely illustrative, and they are not to be considered limiting in any way. In general, the present invention is not limited to the examples illustrated and/or described in the preceding text. Use of the verbs "comprise", "include", "consist of", or any other variation thereof, including the conjugated forms thereof, shall not be construed in any way to exclude the presence of elements other than those stated. Use of the indefinite article, "a" or "an", or the definite article "the" to introduce an element does not preclude the presence of a plurality of such elements. The reference numbers cited in the claims are not limiting of the scope thereof.

Claim 1:
Spectral imaging device (<NUM>) comprising:
- a slit (<NUM>) for directing part of a source light beam inside said spectral imaging device (<NUM>), said slit having a slit length A;
- a first concave mirror (<NUM>);
- a convex reflective diffraction grating (<NUM>) comprising a reflective surface (<NUM>) on which a diffractive grating (<NUM>) is formed;
- a second concave mirror (<NUM>);
- first matrix detection means (<NUM>);
said slit (<NUM>) and said first concave mirror (<NUM>) being configured such that said part of a source light entering the spectral imaging device (<NUM>) from said slit (<NUM>) is directed towards said first concave mirror (<NUM>);
said first concave mirror (<NUM>) and said convex reflective diffraction grating (<NUM>) being configured such that said first concave mirror (<NUM>) is able to reflect a light beam originating from said slit (<NUM>) towards said convex reflective diffractive grating (<NUM>);
said convex reflective diffraction grating (<NUM>) and said second concave mirror (<NUM>) being configured such that said convex reflective diffraction grating (<NUM>) is able to reflect and diffract incoming light from said convex reflective diffraction grating (<NUM>) towards said second concave mirror (<NUM>);
said second concave mirror (<NUM>) and said matrix detection means (<NUM>) being configured such that said second concave mirror (<NUM>) is able to reflect incoming light from said second concave mirror (<NUM>) towards said first matrix detection means (<NUM>);
wherein said reflective surface (<NUM>) of said convex reflective diffraction grating (<NUM>) is defined by a complex invariant shape class;
wherein
said first concave mirror (<NUM>), said convex reflective diffraction grating (<NUM>), and said second concave mirror (<NUM>) are configured such that a spectrally dispersed image of the slit (<NUM>) with said slit length A can be produced on the detection means (<NUM>) with an image length B, said slit length A and image length B defining a demagnification such that : <MAT>
said demagnification factor being higher than <NUM>, characterized in that:
said slit having a slit F-number of <NUM> and
said first matrix detection means (<NUM>) having a detection means F-number equal to <NUM>.