Abstract:
Optical device comprising: a spatial filter means for eliminating, from the light rays emanating from an observed scene those coming from a direction or restricted range of directions in space, while letting through most of the light rays coming from said scene; means for varying the direction or the restricted range of directions in space in correspondence with which the spatial filter means eliminates said light rays; a spectral dispersion means for imparting to the light rays coming from said spatial filter means a deviation that is dependent on their wavelength; and an image detector for recording the light rays dispersed by said spectral dispersion means, each point on said image detector receiving light rays coming from said scene and having a different wavelength depending on the direction in space from which they come.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a national stage application filed under 35 U.S.C. 371 of International Application No. PCT/FR2007/000220, filed Feb. 7, 2007, which claims priority from French patent application 06 02442, filed Mar. 21, 2006. 
     FIELD AND BACKGROUND OF THE INVENTION 
     The invention pertains to an optical device making it possible to obtain an image of an observed scene and, at the same time, the optical spectrum at any point of said image. 
     Such a device allows in particular the discrimination of elements present in a scene, whether gasses, liquids, solid materials or particles. 
     A first approach, known from the prior art, for obtaining optical image spectra (or spectral images) consists in disposing narrow passband filters in front of a camera, in order to acquire substantially monochromatic images, consisting of light rays having the same wavelength; by performing a plurality of image acquisitions of the same scene using different filters it is possible to reconstitute the optical spectrum of each point of said scene. Such an approach is not satisfactory for several reasons. Firstly, the luminous flux gathered by the camera is generally too weak to make it possible to achieve an acceptable signal-to-noise ratio; an increase in the flux can be obtained only by widening the passband of each filter, that is to say by degrading the spectral resolution. This problem is particularly serious in the case of commercial applications, in which it is desired to use uncooled radiation sensors so as to reduce the cost and complexity of the device. Another drawback is due to the fact that the luminous flux gathered varies greatly from one wavelength to another: consequently the sensitivity and the operating point of the radiation sensors vary from one acquisition to another. 
     A more promising approach, also known from the prior art, consists in using, in place of the narrow passband filters, band rejection filters, the rejected band also being narrow. A substantially monochromatic image at the wavelength λ 1  can be obtained by subtracting from a reference image acquired without any filter, an image acquired through a filter eliminating the spectral component at this same wavelength λ 1 . This technique makes it possible to obtain a better signal-to-noise ratio and more contained variations in luminous flux, but it nevertheless exhibits numerous drawbacks related to the use of filters: the wavelengths stopped by the filters cannot be varied in a continuous manner, this leading to a sub-sampling of the spectral space; each filter exhibits, outside the spectral rejection band, a different transmission curve; the use of filters of different thicknesses leads to geometric shifts between the various images; and the movement of the supports of the filters past the sensors tends to modify the operating point of the latter. 
     For these reasons, the present inventor has developed an optical device using a continuously tunable spectral rejection device not exhibiting the aforesaid drawbacks. This device is described in document FR 2 788 137 and in the article: 
     Yves Guern, Laurence Grenier and François Carpentier, “Uncooled IRFPA for low-cost multispectral/hyperspectral LWIR imaging device”, Spie, Vol. 5093, April 2003, page 126. 
     This device comprises in particular a first spectral dispersion means (grating, prism), a mask interposed in the path of the dispersed rays so as to intercept a narrow band of wavelengths and a second spectral dispersion means (grating, prism), acting as means for recombining the light rays dispersed by the first means. A continuous scan of the spectrum can be obtained by displacing the mask. 
     This device constitutes a considerable improvement with respect to the prior art, but it nevertheless exhibits certain drawbacks. 
     A first drawback is related to the presence of two spectral dispersion means: this gives rise to flux losses and a reduction in the optical passband. Furthermore, diffraction gratings and prisms are expensive devices. 
     Another drawback consists in the fact that the optics of the device are relatively difficult to design for reasons which will become clearer subsequently. 
     Another drawback is related to the fact that any modification of the scene or any movement of the apparatus in the course of a series of image acquisitions is liable to generate artifacts in the information acquired, which is compensated at the price of more complex information processing. 
     SUMMARY OF THE INVENTION 
     An object of the invention is therefore to attenuate or eliminate at least one of the aforesaid drawbacks. 
     This object is achieved by an optical device comprising:
         a spatial filtering means for eliminating, from among the light rays arising from an observed scene, those which originate from a direction or a restricted range of directions in space;   means for varying the direction or the restricted range of directions in space in correspondence with which the spatial filtering means eliminates said light rays;   a spectral dispersion means for imparting to the light rays arising from said spatial filtering means a deviation dependent on their wavelength; and   an image detector for recording the light rays dispersed by said spectral dispersion means, each point of said image detector receiving light rays arising from said scene and exhibiting a different wavelength as a function of the direction in space from which they originate.       

     According to particular embodiments of the invention:
         Said spectral dispersion means can exhibit a dispersion plane, said restricted direction or range of directions in space in correspondence with which the spatial filtering means eliminates said light rays being a direction in said plane.   The device can comprise a single spectral dispersion means.   The device can also comprise a data processing means for reconstituting the spectrum of an image of the scene observed on the basis of the signals recorded by said image detector for various choices of the direction or of the restricted range of directions in space in correspondence with which the spatial filtering means eliminates said light rays.   The device can also comprise a data processing means for reconstituting a wideband image of said scene on the basis of the signals recorded by said image detector. In particular, said data processing means for reconstituting said image can comprise means for calculating the deconvolution of said signals recorded by the image detector with respect to an apparatus function of said device. Preferably said data processing means performs said image reconstruction in real time.   Said spatial filtering means can comprise a first convergent optical system, a second convergent optical system and at least one mask disposed in an image plane of said first convergent optical system.   Said first and second convergent optical systems can form an afocal optical system.   At least said second convergent optical system can be a system with pupil conjugation on the spectral dispersion means.   At least one out of said first and second convergent optical systems can consist of lenses.   Said first convergent optical system can comprise an objective with variable focal length such as a zoom.   Said opaque mask can has a linear shape.   Said spatial filtering means can also comprise at least one, and preferably at least two, screens situated in said image plane so as to delimit a range of directions of the light rays arising from said observed scene and eliminate the rays arising from directions not belonging to said range.   Said means for varying the direction or the restricted range of directions in space in correspondence with which the spatial filtering means eliminates the light rays can comprise an oscillating or rotating mirror for imparting a variable deviation to said light rays before they enter said spatial filtering means; in this case said first convergent optical system can be a system with pupil conjugation on said oscillating or rotating mirror.   Said means for varying the direction or the restricted range of directions in space in correspondence with which the spatial filtering means eliminates the light rays can comprise means of linear displacement of said image detector with respect to said observed scene.   Said means for varying the direction or the restricted range of directions in space in correspondence with which the spatial filtering means eliminates the light rays can comprise means for displacing said mask in the image plane of said afocal system.   Said spectral dispersion means comprises a diffraction grating, which can be concave so as to simplify the optics.   Said grating can exhibit an angle of inclination with respect to an optical axis of said device such that, among the light rays diffracted by said diffraction grating, only those corresponding to a predetermined diffraction order (generally a first order) reach said image detector.   Said image detector can comprise a matrix of luminous radiation sensors, preferably exhibiting an axis perpendicular to said dispersion plane of the spectral dispersion means.   The device can also comprise an objective disposed between said spectral dispersion means and said image detector.   The device can be adapted for operating in any spectral region, such as the ultraviolet, the visible and the infrared, for example in the region of wavelengths lying between 7 and 14 μm.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other characteristics, details and advantages of the invention will emerge on reading the description given with reference to the appended drawings given by way of example and which represent, respectively: 
         FIG. 1 , an optical diagram of a spectral imaging device known to the prior art; and 
         FIG. 2 , an optical diagram of a spectral imaging device according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The spectral imaging device known to the prior art represented in  FIG. 1  comprises a spectral selection device  10 , an infrared camera  11  and a data processing device  12 . The spectral selection device  10  consists essentially of a first dispersive prism  3 , a first lens or objective  4 , an opaque mask  6 , a second lens  7  and a second dispersive prism  8 . Each objective can have its pupil on the grating which faces it. 
     A beam  2  of light rays arising from a scene  1  observed by way of the device crosses the first dispersive prism  3  which imparts to each ray a deviation dependent on its wavelength; the figure represents rays of a first wavelength λ 1  (dotted line), of a second wavelength λ 2  (dashed line) and of a third wavelength λ 3  (continuous line). For the sake of simplicity, only mutually parallel rays are represented, but in fact the device receives rays originating from various directions in space as input. The rays dispersed by the prism  3  are focused by the first lens  4 ; in the latter&#39;s focal plane  5 , each spectral component (λ 1 , λ 2 , λ 3 ) is focused in a distinct point (P 1 , P 2 , P 3  respectively). An opaque mask  6  is situated in said focal plane  5  so as to intercept all the rays having a determined wavelength, λ 2  in the case of the figure. The rays which are not intercepted by the mask  6  are collimated by the second lens  7 , a focal plane of which coincides with the focal plane  5  of the first lens  4 , and are recombined spectrally by the second dispersive prism  8 . The beam  9  of light rays exiting the spectral selector  10  is intercepted by the infrared camera  11  which forms an image of the scene  1 , from which the spectral component at the wavelength λ 2  has been removed. A series of acquisitions is performed for various positions of the mask  6  in the focal plane  5 , and the data processing device  12  reconstructs the complete spectrum of the image of the scene  1  on the basis of this series of acquisitions. 
     The spectrum of the beam of light rays at the input  2  is represented symbolically in the figure and identified by the reference SP 1 . The reference SP 2  identifies the spectrum of the output beam  9 , exhibiting a dark band (zero brightness) in correspondence with the spectral component λ 2  intercepted by the mask  6 . 
       FIG. 1  shows the spectral selector  10  in a plane parallel to the dispersion plane of the prisms  3  and  8 , for example vertical. It is assumed that the camera  11  is provided with a matrix of detectors exhibiting a vertical axis (columns) and a horizontal axis (rows). In the absence of dispersion introduced by the prisms  3  and  8 , the light rays arising from a spatial direction are focused in a point, also indicated by i, of the focal plane  5  and are imaged on a row i of the matrix of detectors of the camera. Because of the dispersion, the rays having a wavelength λ j , arising from this same spatial direction i, are not focused at the point i of the focal plane  5 , but at another point, dependent on the wavelength, which can be indicated by (i+j), assuming linear dispersion. As the lens  7 -prism  8  system exactly compensates the dispersion of the prism  3 -lens  4  system, these rays are still imaged on row i of the matrix of detectors, provided that they are not intercepted by the mask  6 . If the position of said mask is indicated by k, all the rays whose spatial direction of origin i and wavelength λ j  are such that i+j=k will be intercepted. It is therefore readily understood that, assuming the transmission and the spectral sensitivity of the device are constant, the signal sensed by the sensors of row i of the matrix of the camera  11  when the mask  6  is at the position k can be expressed by: 
     
       
         
           
             
               
                 
                   
                     S 
                     ik 
                   
                   = 
                   
                     
                       
                         ∑ 
                         
                           j 
                           = 
                           0 
                         
                         N 
                       
                       ⁢ 
                       
                         
                           L 
                           i 
                         
                         ⁡ 
                         
                           ( 
                           
                             λ 
                             j 
                           
                           ) 
                         
                       
                     
                     - 
                     
                       
                         L 
                         i 
                       
                       ⁡ 
                       
                         ( 
                         
                           λ 
                           
                             k 
                             - 
                             i 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   [ 
                   1 
                   ] 
                 
               
             
           
         
       
     
     where L i (λ i ) is the luminance at the wavelength λ j  for the direction i, assuming a discrete set of wavelengths λ j , j=0−N, a single one of which is eliminated by the mask  6  at each acquisition, and of directions in space i=0−M. Equation [1] shows that each row of the matrix of detectors of the camera  11  records a linear element of an image of the scene  1  of which a spectral component has been removed; the removed spectral component depends on the position of the mask  6  and the row considered. Let 
               S   i     =       ∑     j   =   0     N     ⁢       L   i     ⁡     (     λ   j     )               
be the signal obtained in the absence of any mask; a simple subtraction makes it possible to obtain the luminances for the direction i at the various wavelengths:
 
 S   i   −S   ik   =L   i (λ k-i )  [2]
 
     To obtain all the wavelength values of the i-th row, the mask  6  must scan all the positions k. The spectrum originating from the region i is therefore reconstructed gradually on the basis of several images (as many as possible values of k). Consequently, the processing of the raw data by the means  12  can be done only at the end of the series of acquisitions. 
     The origin of certain drawbacks of this device which were mentioned above can now be understood. Firstly, the lens  7 —prism  8  system must exactly compensate the dispersion of the prism  3 —lens  4  system; in particular, if the prisms  3  and  7  are equal, the lenses  4  and  7  must have the same focal length. This is a constraint which limits the freedom of the designer of the device. Still another drawback is related to the effects of the movements of the scene or of the device during acquisition. Specifically, these movements have the effect of “mixing” the spectra; stated otherwise, it may happen that the spectral component λ j  allocated to the direction i originates in reality from the direction i′≠i. This type of particularly detrimental artifact must be compensated by information processing, based on detecting the moving zones in the image. 
     A potentially advantageous variant of the device of  FIG. 1  comprises diffraction gratings in place of the prisms  3  and  8 . However, one encounters the difficulty of eliminating the spurious contributions of order zero (non-dispersive) and of higher orders of diffraction. 
     A basic diagram of a spectral imaging device  20  according to the invention is represented in  FIG. 2 , intended to show such a device seen from above. Such a device comprises a mirror  21 , optionally oscillating or rotating (double arrow r), a first convergent lens  22  having a focal plane  23 , a linear opaque mask  24  disposed in the focal plane  23  and optionally mobile in this plane (double arrow t), a second convergent lens  25 , a focal plane of which coincides with the focal plane  23  of the first lens  22 , and a spectral dispersion means such as a diffraction grating  26 . The grating  26  is observed by an image detector  28  (matrix of radiation sensors) through an objective  27 ; the objective  27  and the image detector  28  correspond essentially to the camera  11  of the device of  FIG. 1 . The raw signals acquired by the image detector  28  are processed by a data processing device  30 . 
     Optionally, two screens  29 ′ and  29 ″ disposed in the focal plane  23  delimit a useful region of the latter. If the mask  24  is mobile, the mirror  21  can be fixed or optionally absent. Here and subsequently, the term “lens” should be interpreted in the wide sense as a synonym of refractive optical system that may comprise a plurality of individual lenses. 
     Light rays arising from an observed scene  1  are incident on the mirror  21 , assumed fixed. These rays originate from various directions in space;  FIG. 2  shows by way of example a first ray d 1  originating from a first direction in space and a second ray d 2  originating from a second direction in space. If the scene  1  is situated at infinity, each direction in space is associated with a precise point of said scene. 
     The rays d 1  and d 2  are directed by the mirror  21  towards the afocal system formed by the lenses  22  and  25  and the mask  24 , identified as a unit by the reference FS. This unit FS constitutes a spatial filtering means: all the rays arising from one and the same direction in space (for example d 1  and the rays parallel to d 1 ) are focused in one and the same point of the focal plane  23 , also known as the “image plane” of the afocal system. The mask  24  disposed in this plane makes it possible to eliminate all the rays arising from the scene  1  which originate from a determined direction in space, or a restricted range of directions if the finite width of the mask is taken into account; in the case of  FIG. 2 , this is the direction from which the ray d 2  originates. The general principles of spatial filtering are known from the prior art and set out in particular in the work by J. W. Goodman “Introduction to Fourier Optics”, McGraw-Hill Book Company. It is understood that it is also possible to provide two or several masks to eliminate the rays originating from a corresponding number of directions. It is however necessary that only a small fraction of the rays arising from the scene  1  is eliminated by the spatial filtering means FS. 
     It is observed that the device of  FIG. 1  comprises a similar arrangement of optical elements (lenses  4  and  7 , mask  6 ) to the spatial filtering means FS of the invention, but whose function is completely different. In the case of  FIG. 1  a spectral selector is involved, which eliminates a spectral component of all the rays arising from the scene  1 , but does not entirely remove any “spatial component”. On the contrary, in the case of the invention, FS is a spatial filtering means within the proper sense of the term, which removes a spatial component of the radiation arising from the scene  1  without distinguishing among its various spectral components. 
     The rays arising from the spatial filter FS are directed towards the grating  26  which imparts to said rays a deviation dependent on their wavelength. Thus, in  FIG. 2 , the ray d 1  is decomposed into two monochromatic rays having different wavelengths, λ 1  and λ 2 . By way of the objective  27  these monochromatic rays are imaged at two distinct points of the image detector  28 . 
     In the device of  FIG. 2 , the grating  26  exhibits a horizontal dispersion plane and the linear mask  24  extends perpendicularly to said dispersion plane (therefore vertically). The person skilled in the art understands that, in such a configuration, the term “direction in space” in fact indicates a direction of the dispersion plane of the grating  26 . 
     Hereinafter it is considered that the detector  28  has a matrix structure and exhibits an axis parallel to the dispersion plane (rows) and an axis perpendicular to the latter (columns). This characteristic is not essential, but facilitates the understanding of the invention and simplifies the numerical processing of the data. For the sake of simplicity, consideration can be limited to a single row of the detector  28 : specifically, each of these rows of the detector receives only light rays originating from one and the same horizontal line of the scene  1  and can therefore be processed independently of the others. It is understood that in the limit the detector  28  may comprise just a single row. In this case, the device only allows the acquisition of a unidimensional image; a bidimensional image can be reconstructed on the basis of a plurality of such unidimensional images. 
     A direction in space from which light rays originate is indicated by i; in the absence of dispersion of the grating  26  and without considering the mask  24 , these rays would be focused on the i-th column of the matrix detector  28  (or on the i-th sensor of a horizontal linear detector). Now, the dispersion introduces a wavelength-dependent deviation, which implies that the rays originating from direction i and exhibiting a wavelength λ j  are in fact focused on sensor (i+j). 
     If all the rays entering the device originated from the same direction in space, the detector  28  would simply record a spectrum of these rays, as in a conventional spectrometer. Such a result would be obtained by stationing in the image plane  24  a screen provided with a slot. On the contrary, in the device of the invention, the detector  28  records a superposition of mutually shifted spectra corresponding to the various directions in space. Let L i (λ j ) be the luminance at the wavelength λ j  of the rays originating from direction i; when the mask  24  is disposed so as to block the rays arising from direction k, the signal gathered by the i-th sensor (or by the sensors of the i-th column) of the detector  28  is given by: 
     
       
         
           
             
               
                 
                   
                     S 
                     ik 
                   
                   = 
                   
                     
                       
                         ∑ 
                         
                           j 
                           = 
                           0 
                         
                         N 
                       
                       ⁢ 
                       
                         
                           L 
                           
                             i 
                             - 
                             j 
                           
                         
                         ⁡ 
                         
                           ( 
                           
                             λ 
                             j 
                           
                           ) 
                         
                       
                     
                     - 
                     
                       
                         L 
                         k 
                       
                       ⁡ 
                       
                         ( 
                         
                           λ 
                           
                             i 
                             - 
                             k 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   [ 
                   3 
                   ] 
                 
               
             
           
         
       
     
     where N is the total spectral width of the device and where, for simplicity, it has been considered that the transmission of the device and the sensitivity of the detector  28  are independent of wavelength. More realistically it may be considered that the mask  24 , having a finite width, intercepts all the rays originating from a restricted range of directions in space [k−n/2; k+n/2]; the expression “restricted range” is understood to mean a range of directions which is not wider than a tenth of the visual field of the device, and preferably of the order of a hundredth or less. Equation [3] therefore becomes: 
     
       
         
           
             
               
                 
                   
                     S 
                     ik 
                   
                   = 
                   
                     
                       
                         ∑ 
                         
                           j 
                           = 
                           0 
                         
                         N 
                       
                       ⁢ 
                       
                         
                           L 
                           
                             i 
                             - 
                             j 
                           
                         
                         ⁡ 
                         
                           ( 
                           
                             λ 
                             j 
                           
                           ) 
                         
                       
                     
                     - 
                     
                       
                         ∑ 
                         
                           j 
                           = 
                           
                             k 
                             - 
                             
                               n 
                               / 
                               2 
                             
                           
                         
                         
                           k 
                           + 
                           
                             n 
                             / 
                             2 
                           
                         
                       
                       ⁢ 
                       
                         
                           L 
                           j 
                         
                         ⁡ 
                         
                           ( 
                           
                             λ 
                             
                               i 
                               - 
                               j 
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   4 
                   ] 
                 
               
             
           
         
       
     
     The signal in the absence of the mask is indicated by S i , without the index k: 
     
       
         
           
             
               
                 
                   
                     S 
                     i 
                   
                   = 
                   
                     
                       ∑ 
                       
                         j 
                         = 
                         0 
                       
                       N 
                     
                     ⁢ 
                     
                       
                         L 
                         
                           i 
                           - 
                           j 
                         
                       
                       ⁡ 
                       
                         ( 
                         
                           λ 
                           j 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   [ 
                   5 
                   ] 
                 
               
             
           
         
       
     
     Performing the subtraction S i −S ik , we obtain: 
     
       
         
           
             
               
                 
                   
                     
                       S 
                       i 
                     
                     - 
                     
                       S 
                       ik 
                     
                   
                   = 
                   
                     
                       ∑ 
                       
                         j 
                         = 
                         
                           k 
                           - 
                           
                             n 
                             / 
                             2 
                           
                         
                       
                       
                         k 
                         + 
                         
                           n 
                           / 
                           2 
                         
                       
                     
                     ⁢ 
                     
                       
                         L 
                         j 
                       
                       ⁡ 
                       
                         ( 
                         
                           λ 
                           
                             i 
                             - 
                             j 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   [ 
                   6 
                   ] 
                 
               
             
           
         
       
     
     The meaning of equation [6] is more clearly apparent if one considers a mask that eliminates only the direction k in space. In this case:
 
 S   i   −S   ik   =L   k (λ i-k )  [7]
 
     The signal obtained after subtraction is nothing other than the spectrum of the rays arising from direction k. By displacing the mask  24  with the aid of an actuation means (not represented), such as a galvanometer, a scan of k is performed over an admissible range of directions in space, which enables the reconstitution of the spectrum of the complete image of the scene  1 . 
     It is desirable to be able to acquire, in the course of the scan of the mask  24 , a real-time wideband image of the observed scene  1 . The expression wideband image is understood to mean an image consisting of a plurality of spectral components which can be distinguished by the spectral imaging device; in an equivalent manner, a wideband image is an image whose spectrum exhibits a bandwidth which is greater than the spectral resolution of the device. Such an image acquisition makes it possible to verify that the device always observes the same scene and, if appropriate, to correct small displacements of the visual field. The data processing means  30  performs a real-time deconvolution of the signal acquired by the detector  28  and the apparatus function of the device, transmission of the device (which can be measured), comprising in particular its optical transmission function, then reconstructs the image of the scene  1 . In a conventional manner, the deconvolution can be calculated with the aid of Fourier transforms, using wavelets or matrix calculation. This method therefore exploits the fact that the flux originates each time from (almost) all of the scene, which would not be possible with a conventional slot spectrometer. 
     It is known that deconvolution is a potentially unstable operation which is very sensitive to the initial conditions. For this reason it is advantageous to provide, in the image plane  23  of the spatial filtering means FS, two fixed screens  29 ′,  29 ″ so as to delimit a range of directions of the light rays arising from said scene observed and to eliminate the rays arising from directions not belonging to said range. The mask  24  is situated or moves in the gap between the two screens. In this way it is known a priori that the signal is zero on the edges of the range delimited by the screens  29 ′,  29 ″ and in correspondence with the mask  24 , thereby limiting the risks of instability when reconstituting the spectral images. As a variant, a single fixed screen can be used. 
     Equation [7] shows that the finer the mask  24 , the better the spatial resolution of the device  20 , but more acquisitions are necessary to obtain a spectrum of the entire image. Furthermore, the use of too fine a mask gives rise to a degradation of the signal-to-noise ratio of the reconstituted image. In practice, n is chosen between 1 and 10 pixels (individual sensors of the matrix detector  28 ), thereby corresponding to a physical mask width given by n.PixelSize.f 27 /f 25 , where PixelSize is the size of each pixel of the detector  28  and f 27 , f 25  are the focal lengths of the objective  27  and of the lens  25 , respectively. The spectral resolution, on the other hand, is independent of the width of the mask and depends exclusively on the dispersion of the grating  26  and the spatial resolution of the objective  27 -detector  28  unit. 
     Numerous variants of the device of  FIG. 2  can be envisaged. Firstly, the mask  24  can be kept fixed, scanning possibly being obtained by rotating or oscillating the entrance mirror  21 . In this case it is advantageous that the entrance pupil of the device be situated on said mirror  21 , so as to prevent the rotation or oscillation of this mirror from giving rise to a lateral displacement of the images of close objects. Alternatively, the movements of the mirror and of the mask can be combined. 
     As was stated earlier, the “lenses”  22  and  25  can in fact be complex optical systems, consisting of several individual lenses. In particular, it is very advantageous that at least the second lens  25  effects the pupil conjugation of the grating, so as to limit the problems of vignetting and of non-uniformity of the reflectivity of the grating as a function of field angle. When the entrance mirror  21  is an oscillating or rotating one, it is advantageous that the first lens  22  has pupil conjugation too. 
     Said lenses  22  and  25  can also be replaced with systems comprising mirrors, for example concave mirrors. More generally, the spatial filter FS can be replaced with other spatial filtering means having a different physical structure. The spatial filter is rendered “tunable” by means making it possible to vary the direction or the restricted range of directions in space in correspondence with which the light rays are eliminated. The means for displacing the mask  24  and the oscillating or rotating mirror  21  merely constitute examples of such means. Alternatively, the same effect can be obtained by performing a linear displacement of the image detector  28  with respect to the observed scene, for example by mounting a device according to the invention on a moving vehicle, such as an airplane. In this way, the spectral imaging device need not comprise any moving part. The linear displacement of the detector  28  can also be combined with the use of a mobile mask and/or of an oscillating or rotating mirror. 
     The grating  26  can be replaced with another spectral dispersion means, such as a prism. 
     The grating can also be concave: it then also ensures all or part of the imaging functions thus eliminating the lens  25  and/or the objective  27 . 
     In all these cases, the optimal choice from among the various possibilities offered depends on the specific application considered, and in particular on the spectral domain for which the device is designed. 
     The device  20  of  FIG. 2  exhibits significant advantages with respect to the device of the prior art represented in  FIG. 1 . Firstly, the presence of a single spectral dispersion means makes it possible to substantially increase the transmission factor and the passband of the device and at the same time to reduce the cost thereof. Furthermore, the designer has greater freedom of choice when dimensioning the optical lenses or systems  22 ,  25 : specifically, it is not necessary for said lenses to have the same focal length, or for them to form an afocal system. It is thus possible to replace the lens  22  with an objective with variable focal length such as a zoom, preferably motorized, which would not have been conceivable in the case of the device of  FIG. 1 , since any modification of the focal length of the lens  4  would have prevented the spectral recombination of the dispersed rays. 
     Another advantage consists in the elimination of spectral artifacts due to the movements of the scene  1  or of the device in the course of the series of acquisitions. Specifically, since the complete spectrum of a part of the scene is obtained at each acquisition of an image of the matrix, the temporal variations of the scene which produce artifacts in the case of the device of  FIG. 1  are of markedly lesser consequence. 
     Yet another advantage is that the processing of the raw data making it possible to obtain the spectral images, as well as the wideband image, can be done in real time, that is to say as these data are acquired, without having to wait for the end of the scan. 
     Yet another advantage is that it is possible to perform a reduced scan so as to obtain complete spectra of just a part of the scene  1 . In the case of the device of  FIG. 1 , a partial scan on the contrary provides partial spectra of the entire image. 
     Yet another advantage consists in the fact that it is relatively simple to remove the spurious images due to the higher diffraction orders of the grating  26 , simply by suitably choosing the inclination of said grating with respect to the optical axis of the device. On the contrary, this is not possible in the case of a spectral selector of the type of  FIG. 1 , in which the prisms would be replaced with two gratings. In such a device, spurious images are due, for example, to rays which undergo specular reflection (order 0) on the first grating and a 2 nd -order diffraction on the second grating. These rays pass through the plane of the mask substantially in the same place as the “useful” rays, undergoing two 1 st -order diffractions, and consequently they cannot be eliminated by screens or apertures. 
     On the other hand, the use of a mask  24  having a finite width introduces blur into the image, but this undesirable effect can easily be compensated by performing a deconvolution with a rectangular “gate” signal.