Patent Publication Number: US-2023145952-A1

Title: Hyperspectral Imaging Device

Description:
FIELD 
     The present invention relates to spectral imaging devices. 
     BACKGROUND 
     Referring to the comparative example shown in  FIG.  1   , a spectral camera CAM 1  may comprise a focusing lens FLNS, a Fabry-Perot interferometer FPI, and an image sensor SEN 1 . The Fabry-Perot interferometer FPI may operate as an adjustable optical passband filter. The lens FLNS may form an image IMG 2  of an object on the image sensor SEN 1 , by focusing light LB 1  received from the object to the image sensor SEN 1  through the Fabry-Perot interferometer FPI. 
     The focusing distance L IMG2  between the lens FLNS and the image sensor SEN 1  may represent a significant proportion of the total length L CAM1  of the spectral camera CAM 1 . The focusing distance L IMG2  may cause e.g. that the size of the camera CAM 1  is too large for mobile applications. An attempt to reduce the focusing distance L IMG2  may increase the divergence of light beams transmitted through the Fabry-Perot interferometer FPI, which in turn may have an adverse effect on the spectral resolution of the Fabry-Perot interferometer FPI. 
     SUMMARY 
     An object is to provide a spectral imaging device. An object is to provide a method for spectral imaging. An object is to provide an imaging spectrometer. 
     According to an aspect, there is provided a device of claim  1 . 
     Further aspects are defined in the other claims. 
     The scope of protection sought for various embodiments of the invention is set out by the independent claims. The embodiments, if any, described in this specification that do not fall under the scope of the independent claims are to be interpreted as examples useful for understanding various embodiments of the invention. 
     Optical micro-structures may be utilized to provide a compact size of the imaging device. In particular, the imaging device may comprise a micro lens array to reduce the length of the imaging device. The imaging device may comprise the micro lens array to provide compact size. 
     The imaging device may be used for multispectral imaging. The Fabry-Perot interferometer may operate as a tunable band-pass filter of the imaging device. The imaging device may simultaneously capture all input fields of a viewing sector at a single wavelength. The spectral position of the passband of the Fabry-Perot interferometer may be scanned to obtain spectral narrowband images of an object at several different wavelengths. The imaging device may scan the optical input spectrally to generate band-pass image data set of a scenery to an image sensor. 
     The imaging device may be arranged to operate such that the divergence of light transmitted through the Fabry-Perot interferometer is smaller than a predetermined limit. Light received from each field angle of the viewing sector may simultaneously pass through the Fabry-Perot interferometer, so as to provide a spectral image of an object. A single spectral image may represent a narrow spectral band of the total spectrum of the object. Several spectral images may be combined to provide a multi-wavelength spectral image, if desired. 
     Using the micro lens array may allow substantially reduction of the size of the imaging device. The length of the imaging device may be e.g. in the range of 3 mm to 15 mm. 
     In an embodiment, the imaging device may comprise a telecentric system to form axial light beams from light beams received from different field angles of the viewing sector. 
     In an embodiment, the imaging device may comprise an afocal system to reduce the length of the imaging device. The afocal system may comprise a combination of a negative lens and a limiter unit. The limiter unit may prevent propagation of light rays which are outside an acceptance cone. 
     In an embodiment, the imaging device may comprise a combination of a modulator array and a filter array e.g. in order to enable using one of the several transmittance peaks of the Fabry-Perot interferometer. The modulator array may comprise e.g. a plurality of first modulable regions and a plurality of second modulable regions. The transmittance of the modulable regions may be changed e.g. by an external control signal. The filter array may comprise e.g. a plurality of first optical spectral filter regions, and a plurality of second optical spectral filter regions. The spectral transmittance of the first filter regions may be different from the spectral transmittance of the second filter regions. The transverse positions of the first modulable regions may match the transverse positions of the first filter regions. A first transmittance peak of the interferometer may be at a first wavelength, and a second transmittance peak of the interferometer may be at a second wavelength. The modulator array may be first controlled to allow light at the first wavelength to propagate to the image sensor, wherein the modulator array may prevent propagation of light at the second wavelength. Next, the modulator array may be controlled to allow light at the second wavelength to propagate to the image sensor, wherein the modulator array may prevent propagation of light at the first wavelength. 
     The imaging device may be used e.g. for hyperspectral imaging. The imaging device may also be called e.g. as a hyperspectral camera device. 
     The imaging device may be e.g. a portable device. The imaging device may be e.g. a wearable device. The imaging device may be a pocketable device (i.e. may be easily carried in a pocket). The imaging device may be implemented e.g. in a smartphone. The imaging device may be implemented e.g. in a vehicle. The imaging device may be implemented e.g. in an unmanned aerial vehicle (drone). 
     The imaging device may be easily integrated as a part of an optical apparatus. The imaging device may be implemented e.g. in an industrial measuring device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following examples, several variations will be described in more detail with reference to the appended drawings, in which 
         FIG.  1    shows, by way of example, in a cross-sectional side view, a comparative example of a device, which comprises a Fabry-Perot interferometer, 
         FIG.  2    shows, by way of example, in a cross-sectional side view, an imaging device, which comprises a Fabry-Perot interferometer, and a microlens array, 
         FIG.  3   a    shows, by way of example, in a cross-sectional side view, forming a plurality of sub-images by using the microlens array, 
         FIG.  3   b    shows, by way of example, in an axial view, the microlens array, 
         FIG.  3   c    shows, by way of example, in an axial view, a plurality of sub-images formed by using the microlens array, 
         FIG.  4   a    shows, by way of example, forming a plurality of sub-images from light received from an object, 
         FIG.  4   b    shows, by way of example, in an axial view, a plurality of sub-images formed by using the microlens array, 
         FIG.  5   a    shows, by way of example, spectral transmittance peaks of the Fabry-Perot interferometer, 
         FIG.  5   b    shows, by way of example, a filter array superposed on an image sensor, 
         FIG.  5   c    shows, by way of example, a filter array superposed on an image sensor, 
         FIG.  5   d    shows, by way of example, spectral sensitivities of detector pixels of an image sensor, and spectral transmittance peaks of a Fabry-Perot interferometer, 
         FIG.  6    shows, by way of example, forming a composite multi-wavelength image by stitching and combining, 
         FIG.  7   a    shows, by way of example, in a cross-sectional side view, an imaging device, which comprises telecentric system, a Fabry-Perot interferometer, and a microlens array, 
         FIG.  7   b    shows, by way of example, in a three-dimensional view, forming an axial light beam from a received light beam, 
         FIG.  8   a    shows, by way of example, in a cross-sectional side view, forming image points by focusing light with the microlenses, 
         FIG.  8   b    shows, by way of example, in a cross-sectional side view, forming a first image point in a situation where the center of a first axial beam coincides with the center of a first microlens, 
         FIG.  8   c    shows, by way of example, in a cross-sectional side view, forming a second image point in a situation where the center of a second axial beam coincides with the center of a second microlens, 
         FIG.  8   d    shows, by way of example, in a cross-sectional side view, forming a first image point and a second image point in a situation where the third axial beam overlaps the first microlens and the second microlens, 
         FIG.  9    shows, by way of example, in a cross-sectional side view, an imaging device which comprises a modulator array and a filter array, 
         FIG.  10   a    shows, by way of example, in a cross-sectional side view, an imaging device which comprises an afocal system, 
         FIG.  10   b    shows, by way of example, in a cross-sectional side view, an imaging device, which comprises a Fresnel lens, 
         FIG.  10   c    shows, by way of example, in a cross-sectional side view, a limiter unit of the afocal system, 
         FIG.  11    shows, by way of example, in a cross-sectional side view, a Fabry-Perot interferometer, and 
         FIG.  12    shows, by way of example, a spectral imaging device. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG.  2   , the imaging device  500  may comprise a light beam modifier system SYS 1 , a Fabry-Perot interferometer FPI, a microlens array ARR 1 , and an image sensor SEN1. 
     The light beam modifier system SYS 1  may form axial light beams LB 2  from received light beams LB 1  such that the radial position (r) of each formed axial beam LB 2  is substantially proportional to the field angle (φ) of the corresponding received beam. The modifier system SYS 1  may be e.g. a telecentric system or an afocal system ( FIG.  10   a   ). 
     The imaging device  500  may receive light LB 1  from an object OBJ 1 . The imaging device  500  may be arranged to form a spectral image of the object OBJ 1  by filtering the light LB 1  with the Fabry-Perot interferometer FPI. The object OBJ 1  may be located in the viewing sector VIEW 1  of the device  500 . Spectral images may be formed at several different wavelengths, and the spectral images of the different wavelengths may subsequently be combined to form multi-wavelength spectral image (CIMG) of the object OBJ 1 , if desired. 
     The object OBJ 1  may reflect, emit and/or transmit light LB 1 , which may be received to the imaging device  500 . The device  500  may be used e.g. for measuring reflection, transmission (absorption) and/or emission of the light LB 1  of the object OBJ 1 . 
     The object OBJ 1  may comprise a plurality of object points P 1   a , P 1   b , P 1   c , P 1   d , P 1   e . The imaging device  500  may receive light LB 1   a  from the point P 1   a , light LB 1   b  from the point P 1   b , and light LB 1   c  from the point P 1   c , respectively. 
     The imaging device  500  may have an optical axis AX 1 . The modifier system SYS 1  may form an axial light beam from a received light beam such that the angular orientation (α,φ) of the received light beam is mapped into a transverse position (α,r) of the centerline of said axial light beam. For example, a field angle φ b  of the light beam LB 1   b  may be mapped into a radial position r b  of the centerline of the axial beam LB 2   b . For example, a field angle φ c  of the light beam LB 1   c  may be mapped into a radial position r c  of the centerline of the axial beam LB 2   c . The modifier system SYS 1  may also be called e.g. as an optical mapping system SYS 1 . The modifier system may convert light of inclined beams into axial beams. The modifier system SYS 1  may also be called e.g. as a conversion system SYS 1 . 
     Each axial beam may be substantially parallel with the optical axis AX 1  of the device  500 . Each light beam (LB 1   a , LB 1   b , LB 1   c ) received from an object point may correspond to an axial beam, which has a different transverse position (α,r). The transverse position of each axial beam may be specified e.g. by an angle α and a radial distance r. The transverse position (α,r) of each axial beam may be a function of the angular orientation (α,φ) of the corresponding received light beam LB 1 . The modifier system SYS 1  may comprise e.g. a telecentric system. The modifier system SYS 1  may comprise e.g. a combination of a negative lens and a limiter unit ( FIG.  10   a   ). The limiter unit may be arranged to block light rays which are outside a predetermined acceptance cone ( FIG.  10   b   ). The negative lens means a lens which has a negative focal length. 
     The imaging device  500  may from an image point P 4   a  by modifying, filtering and focusing the light LB 1   a  received from the object point P 1   a . The imaging device  500  may from an image point P 4   b  by modifying, filtering and focusing the light LB 1   b  received from the object point P 1   b . The imaging device  500  may from an image point P 4   c  by modifying, filtering and focusing the light LB 1   c  received from the object point P 1   c . 
     The Fabry-Perot interferometer FPI comprises a pair of semi-transparent mirrors M 1 , M 2 , which are arranged to operate as an optical cavity. The spectral position of the transmittance peak (PEAK 1 ) of the Fabry-Perot interferometer FPI may be changed by changing the distance (d F ) between the mirrors M 1 , M 2  ( FIG.  5   a   ). 
     SX, SY and SZ may denote orthogonal directions. The direction SZ may be parallel with the optical axis AX 1  of the device  500 . The mirrors M 1 , M 2  of the Fabry-Perot interferometer may be perpendicular to the optical axis AX 1 . The mirrors M 1 , M 2  of the Fabry-Perot interferometer may be parallel with a plane defined by the directions SX and SY. 
     L 0  may denote a distance between the object OBJ 1  and the device  500 . L 500  may denote the external length of the device  500  in the direction of the axis AX1. L SEN  may denote a distance between a principal plane of the modifier system SYS1 and the image sensor SEN 1 . 
     Using the microlens array ARR1 together with the modifier system SYS 1  may allow reducing the distance L SEN . Reducing the distance L SEN  may allow reducing the total length L 500  of the imaging device  500 . 
     Referring to  FIG.  3   a   , the modifier system SYS 1  may form axial light LB 2  from light LB 1  received from the viewing sector VIEW 1  of the imaging device  500 . The interferometer FPI may form transmitted light LB 3  from the axial light LB 2 . The lens array ARR 1  may form focused light LB 4  by focusing light of the transmitted light LB 3 . The focused light LB 4  may impinge on the image sensor SEN 1  in order to form an optical image IMG 4  on the image sensor SEN 1 . The lens array ARR 1  may form the optical image IMG 4  on the image sensor SEN 1  by focusing light of the transmitted light LB 3 . 
     The lens array ARR 1  may form a plurality of spatially separate optical sub-images S -6,-6 .., S 0,0 , .. S 6,6 . The optical image IMG 4  may consist of a plurality of spatially separate sub-images S -6,-6 .., S 0,0 , .. S 6,6 . The sub-images may also be called e.g. as partial images. 
     The light LB 3  for forming the plurality of sub-images may be transmitted simultaneously through the mirrors M 1 , M 2  of the interferometer FPI. The light LB 3  for forming the plurality of sub-images may be transmitted through the same single interferometer FPI. 
     The image sensor SEN 1  may capture the sub-images S -6,-6 .., S 0,0 , .. S 6 , 6 . The image sensor SEN 1  covert the optical sub-images S -6,-6 .., S 0,0 , .. S 6,6  into digital form. The image sensor SEN 1  may provide image data of the sub-images S -   6,-6 .., S 0,0 , .. S 6,6  to one or more data processors. 
     The sub-images S -6,-6 .., S 0,0 , .. S 6,6  may be stitched together to form a single continuous image (IMGλ1) of the object OBJ 1 . The device  500  may comprise a data processor (CNT1) for performing stitching. The stitching may also be performed e.g. in an internet server. 
     In an embodiment, the stitching may be carried out as a device-specific image processing operation, without a need to analyze the captured sub-images to find image points corresponding to common object points. 
     Referring to  FIG.  3   b   , the lens array ARR 1  may comprise an array of lenses LNS -6,-6 , ...LNS 0,0 , ... LNS 6,6 . The lenses may be arranged e.g. in a rectangular array, which may comprise e.g. M columns and N rows. The lenses may also be arranged e.g. in a staggered array and/or in a hexagonal array. The lens array ARR1 may be formed e.g. by molding, etching or joining lenses together. The lenses may be e.g. spherical or aspherical lenses. The lenses may also be e.g. GRIN lenses (GRIN means gradient index). The lenses may also be Fresnel lenses or diffractive lenses. d 50  may denote a distance between centers of adjacent lenses. The dimension d 50  may also be called e.g. as the pitch of the lens array ARR1. w 50  may denote a transverse dimension of clear aperture of a single lens. 
     Referring to  FIG.  3   c   , the image IMG 4  formed on the image sensor SEN 1  may comprise the plurality of spatially separate sub-images S -6,-6 .., S 0,0 , .. S 6,6 . The transverse positions of the sub-images may match the transverse positions of the lenses of the array ARR 1 . The center of each sub-image may coincide with the center of the corresponding microlens. 
     Referring to  FIGS.  4   a  and  4   b   , the sub-images S -1,0 , ..S 1,1  may be partial images of the object OBJ 1 . (The object OBJ 1  may be e.g. a printed paper). 
     Referring to  FIG.  4   b   , a first sub-image S 0,0  may comprise a first image F 1 ′ 0,0  of a feature F 1  of the object OBJ 1 , and a second adjacent sub-image S 0,1  may comprise a second image F 1 ′ 0,1  of the same feature F 1  of the object OBJ 1 . The feature F 1  may be e.g. the point where the horizontal line of the character “H” meets the vertical line of the character “H”. The adjacent sub-images S 0,0 , S 0,1  may comprise images F 1 ′ 0,0 , F 1 ′ 0,1  of the same feature F 1  of the object OBJ 1 , so as to allow forming a continuous spectral image IMGλ1 by stitching the sub-images S 0,0 , S 0,1 . 
     In particular, four or more adjacent sub-images (S 0,0 , S 0,1 , S -1,0 , S -1,1 ) may comprise images F 1 ′ of the same object point, so as to allow forming a continuous larger image by stitching. For example, the vertical neighbor S 0,1  of the first sub-image S 0,0  may comprise the second image F1′ 0,1  of the feature F 1 . A horizontal neighbor S -1,0  of the first sub-image S 0,0  may comprise a third image F 1 ′ -1,0  of the feature F 1 . A diagonal neighbor S -1,1  of the first sub-image S 0,0  may comprise a fourth image F 1 ′ -1,0  of the feature F 1 . 
     A spectral transmittance peak (PEAK1) of the Fabry-Perot interferometer FPI may be adjusted e.g. to a first wavelength λ1 to capture a first set of sub-images S. The sub-images S of the first set may be stitched together to form a continuous spectral image IMGλ1 of the object OBJ 1 . 
       FIG.  5   a    shows, by way of example, the spectral transmittance T F (λ) of the Fabry-Perot interferometer FPI. The spectral transmittance T F (λ) may refer to the ratio I LB3 (λ)/I LB2 (λ), wherein I LB2 (λ) may denote the intensity of an axial light beam LB 2  impinging on the interferometer FPI, and I LB3 (λ) may denote the intensity of the corresponding light beam LB 3  transmitted through the interferometer FPI. 
     The spectral width Δλ FWHM  of a transmittance peak PEAK 1  may be e.g. in the range of 5 nm to 30 nm. FWHM denotes full width at half maximum. 
     The spectral transmittance T F (λ) may have one or more adjacent transmittance peaks PEAK 1 , PEAK 2 , PEAK 3  of the Fabry-Perot interferometer FPI. For example, a first transmittance peak PEAK 1  may be at a wavelength λ 1 , a second transmittance peak PEAK 2  may be at a wavelength λ 2 , and a third transmittance peak PEAK 3  may be at a wavelength λ 3 , in a situation where the mirror distance d F  is equal to a first value d F,1 . The interferometer FPI may be scanned by changing the mirror distance d F . 
     The spectral positions λ 1 , λ 2 , λ 3  of the transmission peaks PEAK 1 , PEAK 2 , PEAK 3  may depend on the mirror distance d F  according to the Fabry-Perot transmission function. The spectral positions of the transmission peaks may be changed by changing the mirror gap d F . The transmission peaks PEAK 1 , PEAK 2 , PEAK 3  may also be called passbands of the Fabry-Perot interferometer. 
     Changing the mirror distance d F  may move the spectral position of the transmittance peaks PEAK 1 , PEAK 2 , PEAK 3 . For example, the first transmittance peak PEAK 1 ′ may be at a wavelength λ 1b , a second transmittance peak PEAK 2 ′ may be at a wavelength λ 2b , and a third transmittance peak PEAK 3 ′ may be at a wavelength λ 3b , in a situation where the mirror distance d F  is equal to a second value d F,2 . 
     The device  500  may optionally comprise one or more optical filters (e.g. CFA 1 , FIL 1 , FIL 2 ) to limit the spectral response of the device  500 . The one or more filters may together provide a spectral transmittance. For example, the one or more filters may allow using a single selected transmittance peak of the Fabry-Perot interferometer (e.g. PEAK 1 , PEAK 2 , or PEAK 3 ), by preventing transmission of light at the wavelengths of the other transmittance peaks. 
     For example, the device  500  may comprise one or more filters (CFA 1 , FIL 1 , FIL 2 ) to provide a first band pass region PB 1  defined e.g. by cut-off wavelengths λ 11  and λ 12 . For example, the device  500  may comprise one or more filters (CFA 1 , FIL 1 , FIL 2 ) to provide a second band pass region PB 2  defined e.g. by cut-off wavelengths λ 21 , and λ 22 . For example, the device  500  may comprise one or more filters (CFA 1 , FIL 1 , FIL 2 ) to provide a third band pass region PB 3  defined e.g. by cut-off wavelengths λ 31 , and λ 32 . 
     In an embodiment, the device  500  may comprise a modulator (MOD 1 ) and a filter array (FIL 1 ) to alternately enable transmission of light via a first passband PB 1  or via a second passband PB 2  ( FIG.  14   ). The first passband PB 1  may be defined by cut-off wavelengths λ 11  and λ 12 . The second passband PB 1  may be defined by cut-off wavelengths λ 21  and λ 22 . The filter array may comprise a plurality of first filter regions to provide the first passband PB 1 , and a plurality of second filter regions to provide the second passband PB 2 . 
     Referring to  FIG.  5   b   , the device  500  may comprise a filter array CFA 1 , which may be superposed on the image sensor SEN 1 . The filter array CFA 1  may be e.g. an RGB Bayer matrix. The filter array CFA 1  may comprise first filter regions (R) to provide a first spectral transmittance. The filter array CFA 1  may comprise second filter regions (G) to provide a second spectral transmittance. The filter array CFA 1  may comprise third filter regions (B) to provide a third spectral transmittance. The first filter regions (R) may provide e.g. a first spectral sensitivity for first detector pixels DPX 1  of the image sensor SEN 1 . The second filter regions (G) may provide e.g. a second spectral sensitivity for second detector pixels DPX 2  of the image sensor SEN 1 . The third filter regions (G) may provide e.g. a third spectral sensitivity for third detector pixels DPX 3  of the image sensor SEN 1 . For example, the first detector pixels DPX 1  may selectively detect light transmitted at the first transmittance peak PEAK 1  of the interferometer, wherein the first detector pixels DPX 1  may be insensitive to light transmitted at the other transmittance peaks (PEAK 2 , PEAK 3 ). For example, the second detector pixels DPX 2  may selectively detect light transmitted at the second transmittance peak PEAK 2 . For example, the third detector pixels DPX 3  may selectively detect light transmitted at the third transmittance peak PEAK 3 . 
     Referring to  FIGS.  5   c  and  5   d   , the filter array CFA 1  may comprise first filter regions (R) to provide a first spectral transmittance, second filter regions (G) to provide second spectral transmittance, third filter regions (B) to provide third spectral transmittance, and fourth filter regions (IR) to provide fourth spectral transmittance. 
     The first filter regions (R) may provide e.g. a first spectral sensitivity for first detector pixels DPX 1  of the image sensor SEN 1 . The second filter regions (G) may provide e.g. a second spectral sensitivity for second detector pixels DPX 2  of the image sensor SEN 1 . The third filter regions (G) may provide e.g. a third spectral sensitivity for third detector pixels DPX 3  of the image sensor SEN 1 . The fourth filter regions (IR) may provide e.g. a fourth spectral sensitivity for fourth detector pixels DPX 4  of the image sensor SEN 1 . 
     The first detector pixels DPX 1  may detect light e.g. at the wavelength (λ1) of the first transmittance peak PEAK 1  of the interferometer. The second detector pixels DPX 2  may detect light e.g. at the wavelength (λ2) of the second transmittance peak PEAK 2 . The third detector pixels DPX 3  may detect light e.g. at the wavelength (λ3) of the third transmittance peak PEAK 3 . The fourth detector pixels DPX 4  may detect light e.g. at the wavelength (λ4) of the fourth transmittance peak PEAK 4 . 
     The first detector pixels DPX 1  may spectrally selectively detect e.g. red light (R). The second detector pixels DPX 2  may spectrally selectively detect e.g. green light (G). The third detector pixels DPX 3  may spectrally selectively detect e.g. blue light (B). The fourth detector pixels DPX 4  may spectrally selectively detect e.g. infrared light (IR). 
     The filter regions (R, G, B, IR) of the filter array CFA 1  do not need to reject all spectral components which are outside the primary passband of each filter region. For example, the first filter regions (R) may allow transmission of light at the wavelengths λ1 and λ4. For example, spectral components of light LB 1  at the wavelengths λ1, λ2, λ3, λ4 may be determined from the detected signals of the detector pixels (DPX 1 , DPX 2 , DPX 3 , DPX 4 ) and from the known spectral sensitivity functions of the detector pixels, by solving a system of equations. 
     Referring to  FIG.  6   , a spectral transmittance peak (e.g. PEAK 1 ) of the Fabry-Perot interferometer FPI may be adjusted to a first wavelength λ1 to capture a first set of sub-images S. The sub-images S of the first set may be stitched together to form a first spectral image IMGλ1 of the object OBJ 1 . 
     A spectral transmittance peak (e.g. PEAK 1 ) of the Fabry-Perot interferometer FPI may be adjusted to a second wavelength λ2 to capture a second set of sub-images S. The sub-images S of the second set may be stitched together to form a second spectral image IMGλ2 of the object OBJ1. 
     A spectral transmittance peak (e.g. PEAK 1  or PEAK 2 ) of the Fabry-Perot interferometer FPI may be adjusted to a third wavelength λ3 to capture a third set of sub-images S. The sub-images S of the third set may be stitched together to form a third spectral image IMGλ3 of the object OBJ 1 . 
     A spectral transmittance peak (e.g. PEAK 1  or PEAK 2 ) of the Fabry-Perot interferometer FPI may be adjusted to a fourth wavelength λ4 to capture a fourth set of sub-images S. The sub-images S of the fourth set may be stitched together to form a fourth spectral image IMGλ4 of the object OBJ 1 . 
     The spectral images IMGλ1, IMGλ2, IMGλ3, IMGλ4 may be combined to form a multi-spectral image CIMG. The multi-spectral image CIMG may also be called e.g. as hyperspectral cube. The image CIMG may comprise a three-dimensional array of pixel values, wherein each pixel value may represent a measured intensity value associated with transverse position coordinates (x,y) of said pixel and with a wavelength value of said pixel (λ1, λ2, λ3, or λ4). 
     The number of spectral positions (λ1, λ2, λ3, or λ4) used for capturing the image data for a single image CIMG may be e.g. in the range of 2 to 100. 
     In an embodiment, it may also be sufficient to form a single spectral image IMGλ1 without forming a multi-spectral image CIMG. 
     In an embodiment, the image data of the captured sub-images S may be used without stitching the sub-images S together. For example, change of an object OBJ 1  may be detected by comparing captured sub-images S with reference data also without stitching the sub-images S together. A change of an optical property of the object OBJ 1  may be detected by comparing the captured sub-images S with reference data. 
     In an embodiment, the interferometer FPI may be adjusted to a selected wavelength to capture a plurality of sub-images S, wherein the image data of the captured sub-images S may be used e.g. for background correction. The method may comprise capturing sub-images S without stitching the sub-images S together. 
     Referring to  FIG.  7   a   , the modifier system SYS 1  may comprise a telecentric system. The modifier system SYS 1  may be an image-space telecentric lens system, which comprises an aperture APE 1  and a lens LNS 1 . The distance between the aperture APE 1  and the lens LNS 1  may be selected such that the light beams LB 2  formed by the modifier system SYS 1  may be substantially parallel with the optical axis AX 1  of the device  500 . 
     The device  500  may receive a first light beam LB 1   a  from a first object point P 1   a . The modifier system SYS 1  may form a first axial beam AX 1   a  from light of a the first received light beam LB 1   a . The angular orientation (φ a ) of the received beam LB 1   a  may be mapped into a radial position (r a ) of the first axial light beam LB 2   a . 
     The modifier system SYS 1  may form a substantially axial beam LB 2  from light of each light beam LB 1  received from the viewing sector VIEW 1  of the device  500 , wherein the radial position r of the formed axial beam AX 2  may depend on the field angle φ of said received light beam LB 1 . The field angle φ may denote the angle between the centerline of the received beam LB 1  and the optical axis AX 1  of the device  500 . The radial position r may indicate the distance between the centerline of the formed axial beam LB 2  and the optical axis AX 1  of the device  500 . To the first approximation, the radial position (r) may be substantially proportional to the field angle (φ). For example, the modifier system SYS 1  may form the axial beams LB 2  such that r=k SYS1 ▪φ, where k SYS1  may denote a proportionality constant. 
     The device  500  may receive a second light beam LB 1   b  from a second object point P 1   b . The modifier system SYS 1  may form a second axial beam AX 1   b  from light of a the second received light beam LB 1   b . The angular orientation (φ b ) of the received beam LB 1   b  may be mapped into a radial position (r b ) of the second axial light beam LB 2   b . 
     The device  500  may receive a third light beam LB 1   c  from a third object point P 1   c . The modifier system SYS 1  may form a third axial beam AX 1   c  from light of a the third received light beam LB 1   c . The angular orientation (φ c ) of the received beam LB 1   c  may be mapped into a radial position (r c ) of the third axial light beam LB 2   b . 
     The Fabry-Perot interferometer FPI may form transmitted light beams LB 3   a , LB 3   b , LB 3   c  by filtering light of the axial light beams LB 2   a , LB 2   b , LB 2   c . 
     The lens array ARR 1  may form a plurality of sub-images S by focusing light of the transmitted light beams LB 3   a , LB 3   b , LB 3   c  to the image sensor SEN 1 . 
     The distance L 1  between the aperture APE 1  and principal plane of the lens LNS 1  may be e.g. substantially equal to the focal length f LNS1  of the lens LNS 1  of the telecentric system SYS 1 . The focal length of the lens LNS 1  may be e.g. in the range of 2 mm to 20 mm, advantageously in the range of 4 mm to 8 mm. 
     The aperture APE 1  may be e.g. circular or rectangular. The diameter or width w APE1  of the aperture APE 1  may be e.g. in the range of 0.2 mm to 2 mm. 
     The diameter or width w APE1  of the aperture APE 1  may be selected to provide a desired spectral resolution of the Fabry-Perot interferometer FPI. Selecting a smaller aperture APE 1  may improve spectral resolution. The device  500  may comprise a diaphragm DIA 1  to define the aperture APE 1 . 
     L4 may denote the distance between the image sensor SEN 1  and the principal plane of the lenses of the lens array ARR 1 . The distance L4 may be selected such that the lenses of the lens array ARR 1  may form substantially sharp sub-images of the object OBJ 1  on the image sensor SEN 1 . For example, the distance L4 may be smaller than the focal length of the lenses LNS of the lens array ARR 1 . The device  500  may be arranged to operate such that the lens array ARR 1  does not form a sharp image of the input aperture APE 1  on the image sensor SEN 1 . 
     The distance L4 may be selected such that at least one of lens of the array ARR 1  may form a sharp image F 1 ′ of a feature F 1  of the object OBJ 1  on the image sensor SEN 1 . The distance L4 may be selected such that at least one of lens of the array ARR 1  may form a sharp image point (P 4 ) of an object point (P 1 ) on the image sensor SEN 1 . In an embodiment, the distance L 0  between the object OBJ 1  may be e.g. greater than or equal to 100 times the length L 500  of the device  500 . In an embodiment, the object OBJ 1  may be at infinite distance. The distance L4 may be selected to provide a sharp image point for an object point located at infinite distance. 
     L SEN  may denote the distance between the image sensor SEN 1  and the principal plane of the lens LNS 1  of the telecentric system SYS 1 . Using the lens array ARR 1  may substantially reduce the distance L SEN . Using the lens array ARR 1  may substantially reduce the total external length L 500  of the spectral imaging device  500 , in the direction of the optical axis AX 1 . 
     Referring to  FIG.  7   b   , the modifier system SYS 1  may convert each received light beam LB 1  into a corresponding axial light beam LB 2 . Each received beam LB 1  has a centerline CEN 1 . Each axial beam has a centerline CEN 2 . The direction of each received light beam LB 1  may be specified e.g. by angles (α,φ). The angle α may be called e.g. as the azimuth angle. The azimuth angle α may denote the angle between the direction SY and the projection (PRJ) of the centerline CEN 1  on the plane defined by the directions SY and SX. The angle φ may be called e.g. as the field angle. The field angle may denote the angle between the optical axis AX 1  and the centerline CEN 1  of the light beam LB 1 . The transverse position of each corresponding axial beam LB 2  may be specified e.g. by the radial position (r) and by the azimuth angle (α). The radial position r may denote the distance between the centerline CEN 2  and the optical axis AX 1 . The transverse position of the centerline CEN 2  may also be defined by cartesian coordinates (x,y). The coordinate x may define a position in the direction SX, and the coordinate y may define a position in the direction SY. 
     The modifier system may form the axial beam LB 2  from light of the received beam LB 1  such that the radial position (r) of the axial beam LB 2  is substantially proportional to the field angle φ of the received beam. 
     The modifier system SYS 1  may form an axial beam LB 2   k  from light of a received input beam LB 1   k . The input beam LB 1   k  has a direction (α k ,φ k ). The axial beam has a transverse position (α k , r k ). 
     Referring to  FIG.  8   a   , the transmitted beams LB 3  may be converging or diverging. The (half) divergence angle Δθ LB3  may denote the maximum angle between light rays of the beam LB 3  and the optical axis AX 1 . Each transmitted beam LB 3  propagating via the lens array ARR 1  to the image sensor SEN 1  may have a divergence Δθ LB3 . The divergence Δθ LB3  may have an effect on the spectral resolution of the Fabry-Perot interferometer FPI. Reducing the divergence Δθ LB3  may improve resolution. 
     The transmitted light beam LB 3  may have a width w LB3  at the input surface of the lens array ARR 1 . d 50  may denote the distance between centers (AX 0,0 , AX 0,1 ) of adjacent lenses (LNS 0,0 , LNS 0,1 ) of the array ARR 1 . The pitch distance d 50  of the array ARR 1  may be e.g. in the range of 25% to 100% of the width w LB3  so as to provide sufficient spatial resolution and to facilitate stitching of the sub-images S. 
     The lens array ARR 1  may comprise a plurality of lenses arranged in a rectangular MxN array. The number (N) of rows of the lens array ARR 1  may be e.g. greater than or equal to 8, and the number (M) of columns of the lens array may be e.g. greater than or equal to 8. 
     The number (N) of rows of the lens array ARR 1  may be e.g. greater than or equal to 2, and the number (M) of columns of the lens array may be e.g. greater than or equal to 2. Using a 2 ×2 lens array may already provide significant reduction of the length of the device. 
     The device  500  may form an image point P 4   a  from light of a light beam LB 1   a  received from an object point P 1   a . The device  500  may form an image point P 4   b  from light of a light beam LB 1   b  received from an object point P 1   b . The device  500  may form an image point P 4   c  from light of a light beam LB 1   c  received from an object point P1c. 
       FIG.  8   b    illustrates how light of a light beam LB 1   a  received from an object point P 1   a  may form an image point P 4   a  of a first sub-image S 0,0 . A first lens LNS 0,0  of the array ARR 1  may form the first sub-image S 0,0 . 
       FIG.  8   c    illustrates how light of a light beam LB 1   e  received from an object point P 1   e  may form an image point P 4   e  of a second sub-image S 0,1 . A second adjacent lens LNS 0,1  of the array ARR 1  may form a second sub-image S 0,1 . 
       FIG.  8   d    illustrates how light of a light beam LB 1   d  received from the same object point P 1   d  may form two different image points (P4d 0,0 , P4d 0,1 ) appearing in two adjacent sub-images (S 0,0 , S 0,1 ). This feature may allow forming a continuous larger image (IMGλ1) by stitching the sub-images (S 0,0 , S 0,1 ) together. 
     A first lens LNS 0,0  of the array ARR 1  may form a first sub-image S 0,0 . A second adjacent lens LNS 0,1  of the array ARR 1  may form a second adjacent sub-image S 0,1 . 
     The system SYS 1  may form an axial light beam LB 2   d  from light of the received light beam LB 1   d . The interferometer FPI may form an axial filtered light beam LB 3   d  by filtering light of the axial light beam LB 2   d . The transmitted light beam LB 3   d  may overlap a first lens LNS 0,0  and a second adjacent lens LNS 0,1  of the array ARR 1 . The first lens LNS 0,0  may form a first focused beam LB 4   d   0,0  by focusing a first part of the transmitted light beam LB 3   d . The focused beam LB 4   d   0,0  may impinge on the image sensor SEN 1  to form the first image point P 4   d   0,0 . The first sub-image S 0,0  may comprise the first image point P 4   d   0,0 . The second lens LNS 0,1  may form a second focused beam LB 4   d   0,1  by focusing a second part of the transmitted light beam LB 3   d . The focused beam LB 4   d   0,1  may impinge on the image sensor SEN 1  to form the second image point P 4   d   0,1 . The second sub-image S 0,1  may comprise the second image point P 4   d   0,1 . 
     In an embodiment, the transverse position of the first image point P 4   d   0,0  with respect to the transverse position of the second image point P 4   d   0,1  may depend on the distance (L 0 ) between the object point P 1   d  and the spectral imaging device  500 . This phenomenon may be significant at small distances L 0 , for example at distances L 0  where the ratio w APE1 /L 0  is greater than 1%. Consequently, the method may comprise determining a distance (L 0 ) between an object point (P 1   d ) and the spectral imaging device  500  (by triangulation) from the relative position of the second image point (P 4   d   0,1 ) with respect to the first image point (P 4   d   0,0 ). The device may be arranged to determine distance values for a plurality of different object points, e.g. for measuring three-dimensional geometric form of the object. The determined distance may also be used e.g. for autofocusing. The determined distance may also be used e.g. for verifying a distance which has been determined by another method. 
     Referring to  FIG.  9   , the spectral imaging device  500  may comprise a combination of a modulator array MOD 1  and a filter array FIL 1  e.g. in order to enable using one of the several transmittance peaks (PEAK 1 , PEAK 2 ) of the Fabry-Perot interferometer FPI. The modulator array MOD 1  may comprise e.g. a plurality of first modulable regions and a plurality of second modulable regions. The transmittance of the modulable regions may be changed e.g. by a control signal. The filter array FIL 1  may comprise e.g. a plurality of first optical spectral filter regions, and a plurality of second optical spectral filter regions. The spectral transmittance of the first filter regions may be different from the spectral transmittance of the second filter regions. The transverse positions of the first modulable regions may match the transverse positions of the first filter regions. A first transmittance peak PEAK 1  of the interferometer may be at a first wavelength (λ1), and a second transmittance peak of the interferometer may be at a second wavelength (λ2). The modulator array MOD 1  may be first controlled to allow light at the first wavelength (λ1) to propagate to the image sensor SEN 1 , wherein the modulator array MOD 1  may prevent propagation of light at the second wavelength (λ2). Next, the modulator array ARR 1  may be controlled to allow light at the second wavelength (λ2) to propagate to the image sensor SEN 1 , wherein the modulator array MOD 1  may prevent propagation of light at the first wavelength (λ1). The modulator array MOD 1  may be e.g. a liquid crystal modulator. 
     Referring to  FIG.  10   a   , the modifier system SYS 1  may comprise a combination of a negative lens LNS 2  and a limiter unit NAL 2 . The lens LNS 2  may have a negative focal length. The limiter unit NAL 2  may prevent propagation of light rays which are outside a predetermined acceptance cone (θ LIM ). The negative lens LNS 2  and the limiter unit NAL 2  may together form an afocal system. The afocal system of  FIG.  10   a    may further reduce the axial length of the device  500 . 
     In an embodiment, the limiter unit NAL 2  of the afocal system SYS 1  may also be positioned between the Fabry-Perot interferometer FPI and the lens array ARR 1 . The limiter unit NAL 2  may allow propagation of axial filtered light beams LB 3  to the lens array ARR 1 , wherein the limiter unit NAL 2  may eliminate unwanted light rays which are outside the acceptance cone. The limiter unit NAL 2  may be e.g. a stack of aperture arrays. The limiter unit NAL 2  may be e.g. a fiber optic array. 
     The system SYS 1  may form axial light beams (LB 2   a , LB 2   b , LB 2   c ) from received light beams (LB 1   a , LB 1   b , LB 1   c ) such that the radial position (r) of each axial beam depends on the field angle (φ) of the corresponding received beam. The system SYS 1  may form axial light beams (LB 2   a , LB 2   b , LB 2   c ) from received light beams (LB 1   a , LB 1   b , LB 1   c ) such that the radial position (r) of each axial beam may be substantially proportional to the field angle (φ) of the corresponding received beam. 
     Referring to  FIG.  10   b   , the modifier system SYS 1  may comprise one or more Fresnel lenses and/or diffractive lenses. Usinga Fresnel lens or a diffractive lens may allow further reduction of the length L500 of the device  500 . 
     For example, an afocal system may comprise a combination of a Fresnel lens LNS 2  and a limiter unit NAL 2 . The limiter unit NAL 2  may be e.g. a stack of aperture arrays. The limiter unit NAL 2  may be e.g. a fiber optic array. 
     Referring to  FIG.  10   c   , θ2 denotes an angle between a light ray LR 2  and the optical axis AX 1 . θ3 denotes an angle between a light ray LR 3  and the optical axis AX 1 . The angles θ2, θ3 may be called e.g. as input angles. θ LIM  denotes an acceptance angle of the limiter unit NAL 2 . The limiter unit NAL 2  may allow propagation of a light ray through the limiter unit NAL 2  if the input angle θ of said light ray is smaller than or equal to the acceptance angle θ LIM . The limiter unit NAL 2  may prevent propagation of a light ray through the limiter unit NAL 2  if the input angle θ of said light ray is greater than the acceptance angle θ LIM . The limiter unit NAL 2  may be implemented e.g. by an array of optical fibers. The limiter unit NAL 2  may be e.g. a fiber optic array. The limiter unit NAL 2  may be e.g. a stack of aperture arrays ( FIG.  10   b   ). The acceptance angle θ LIM  may be e.g. in the range of 1 to 10°. 
     Referring to  FIG.  11   , The interferometer FPI may comprise a first semi-transparent mirror M 1  implemented on a first mirror plate  100 , and a second semi-transparent mirror M 2  implemented on a second mirror plate  200 . The interferometer may comprise one or more actuators ACU 1  to change the distance d F  between the first mirror M 1  and the second mirror M 2 . 
     The width of the mirrors M 1 , M 2  of the interferometer may be e.g. in the range of 2 mm to 50 mm. The semi-transparent mirrors M 1 , M 2  of the interferometer may be produced with a high degree of accuracy. The deviations of the semi-transparent mirror from the perfect planar shape may initially be e.g. smaller than λ/200. The flatness of the mirror M 1 , M 2  may be e.g. better λ N /200, in order to provide a suitable finesse (i.e. the ratio of the free spectral range to the spectral width of a transmission peak). λ N  denotes a predetermined operating wavelength. The predetermined operating wavelength λ N  may be e.g. in the range of 500 nm to 4000 nm. The distance d F  between the semi-transparent mirrors M 1 , M 2  may be e.g. in the range of 0.2 µm to 1 mm, depending on the desired spectral resolution and depending on the desired free spectral range. 
     The width of the light-detecting area of the image sensor SEN 1  may be e.g. greater than or equal to the width of the mirrors M 1 , M 2 . The width of the lens array ARR 1  may be e.g. greater than or equal to the width of the mirrors M 1 , M 2 . 
     The second mirror M 1  may be substantially parallel with the first mirror M 1  during operation. The mirrors M 1 , M 2  may have e.g. a substantially circular form or a substantially rectangular form. 
     The distance d F  between the mirrors M 1 , M 2  may be adjusted to provide constructive interference for transmitted light at one or more given wavelengths so that the interferometer FPI may transmit light. The distance d F  may also be adjusted to provide destructive interference for transmitted light at the given wavelength so that the interferometer FPI may reflect light. 
     The mirror distance d F  may be adjusted by one or more actuators ACU 1 , ACU 2 . One or more actuators may be arranged to move the second mirror plate  200  with respect to the first mirror plate  100 . The actuator ACU 1 , ACU 2  may be e.g. a piezoelectric actuator, an electrostatic actuator, an electrostrictive actuator, or a flexoelectric actuator. 
     The semi-transparent mirrors M 1 , M 2  may be e.g. dielectric multilayer coatings deposited on a transparent substrate. The semi-transparent mirrors M 1 , M 2  may be e.g. metallic coatings deposited on a transparent substrate. The substrate material of the mirror plates  100 ,  200  may be transparent in the operating wavelength range of the interferometer  300 . The material of the mirror plates  100 ,  200  may be e.g. glass, silica, silicon or sapphire. The mirror plates  100 ,  200  may comprise ceramic material. The mirror plates  100 ,  200  may comprise dimensionally stable material, which is transparent in the operating range of wavelengths of the spectral imaging device  500 . 
     The interferometer FPI may optionally comprise capacitive sensor electrodes G 1   a , G 1   b , G 2  for capacitively monitoring mirror distance d F . Sensor electrodes G 1   a , G 1   b , G 2  may together form a sensor capacitor C 1 , wherein the capacitance value of the sensor capacitor C 1  may depend on the mirror distance d F . Consequently, the mirror distance d F  may be monitored by monitoring the capacitance value of the sensor capacitor C 1 . The sensor capacitor C 1  may be connected to a capacitance monitoring unit  410  e.g. by conductors CONa, CONb ( FIG.  12   ). 
     Referring to  FIG.  12   , the imaging device  500  may comprise a control unit CNT 1 . The control unit CNT 1  may be arranged to send a control signal SET D  to the interferometer FPI in order to adjust the mirror gap d F . The interferometer FPI may comprise a driver unit  420 . The driver unit  420  may e.g. convert a digital control signal SET D  into an analog signal suitable for driving one or more actuators. The driver unit  420  may provide a signal HV 1  for driving an actuator. The driver unit  420  may provide e.g. a high voltage signal HV 1  for driving a piezoelectric actuator. 
     The interferometer FPI may optionally comprise means for monitoring the distance d F  between the mirrors and/or the mirror plates. The interferometer FPI may comprise e.g. capacitive means for monitoring the distance. The interferometer FPI may comprise e.g. inductive means for monitoring the distance. The interferometer FPI may comprise e.g. interferometric means for monitoring the distance. 
     The interferometer FPI may comprise e.g. capacitive sensor electrodes for capacitively monitoring mirror distance d F . Sensor electrodes may together form a sensor capacitor C 1 , wherein the capacitance value of the sensor capacitor C 1  may depend on the mirror distance d F . Consequently, the mirror distance d F  may be monitored by monitoring the capacitance value of the sensor capacitor C 1 . The sensor capacitor C 1  may be connected to a capacitance monitoring unit  410  e.g. by conductors CONa, CONb. The capacitance monitoring unit  410  may provide a sensor signal S d  indicative of the mirror distance d F . 
     The capacitance monitoring unit  410  may provide a sensor signal S d . The sensor signal may be used for monitoring the mirror gap d F . The spectral response of the interferometer FPI may be calibrated e.g. as a function of the mirror gap d F . The device  500  may comprise a memory MEM 2  for storing spectral calibration parameters DPAR 2 . The mirror gap d F  and/or a spectral position λ may be determined from the sensor signal S d  e.g. by using the spectral calibration parameters DPAR 2 . 
     The image sensor SEN 1  may provide image data, which may be communicated as an image data signal S SEN . The image data signal S SEN  may comprise e.g. the pixel values of an image frame captured at a selected wavelength. 
     The device  500  may optionally comprise a memory MEM 1  for storing intensity calibration parameters CALPAR 1 . The device  500  may be arranged to obtain pixel values from the image sensor SEN 1 , and to determine intensity values X(λ) from the pixel values by using one or more intensity calibration parameters CALPAR 1 . An intensity value X(λ) of the light LB 1  may be determined from a pixel value of a captured image frame as a function of the position (x,y) of the pixel and/or as a function of the mirror distance value d F , by using the one or more intensity calibration parameters CALPAR 1 . Calibrated intensity value may be determined for each pixel of a captured wavelength image, respectively. 
     The image sensor SEN 1  may be e.g. a CMOS sensor or a CCD sensor. CMOS means complementary metal oxide. CCD means charge coupled device. 
     The device  500  may optionally comprise a memory MEM 3  for storing output OUT 1 . The output OUT 1  may comprise e.g. pixel values of one or more captured images IMGλ1, IMGλ2, one or more calibrated intensity values, and/or one or more combined images CIMG. 
     The device  500  may optionally comprise one or more filters FIL 2  to at least partly define one or more passbands PB 1 . 
     The device  500  may optionally comprise a modulator array MOD 1 , a filter array FIL 1 , and a driver unit  430  for changing the state of the modulator array MOD 1 . The driver unit  430  may change the state of the modulator array MOD 1  according to a modulator control signal SET MOD  received from the control unit CNT 1 . 
     The device  500  may comprise a memory MEM 4  for storing a computer program PROG 1 . The computer program PROG 1  may be configured, when executed by one or more data processors (e.g. CNT 1 ), cause the apparatus  500 , FPI to perform one or more of the following:
     measure a distance d F  between the mirrors M 1 , M 2 ,   adjust parallelism (tilt angle) of the mirrors M 1 , M 2 .   set a transmittance peak of the interferometer FPI to a selected position (e.g. λ1),   control an optical modulator (MOD1) to enable or disable a passband (PB 1 ),   cause spectral scanning of the interferometer FPI (e.g. from λ1 to λ2),   capture a plurality of sub-images S 0,0 , S 0,1 , ...   form a spectral image IMGλ1 by stitching the sub-images S 0,0 , S 0,1 , ...   form a combined image CIMG,   form a calibrated spectral image from captured pixel values.   

     The device  500  may optionally comprise a user interface USR 1  e.g. for displaying information to a user and/or for receiving commands from the user. The user interface USR 1  may comprise e.g. a display, a keypad and/or a touch screen. 
     The device  500  may optionally comprise a communication unit RXTX 1 . The communication unit RXTX 1  may transmit and/or receive a signal COM 1  e.g. in order to receive commands, to receive calibration data, and/or to send output data OUT 1 . The communication unit RXTX 1  may have e.g. wired and/or wireless communication capabilities. The communication unit RXTX 1  may be arranged to communicate e.g. with a local wireless network (Bluetooth, WLAN), with the Internet and/or with a mobile communications network (4G, 5G). 
     The object OBJ 1  may be e.g. a real object or a virtual object. A real object OBJ 1  may be e.g. in solid, liquid, or gaseous form. The real object OBJ 1  may be a cuvette filled with a gas. The real object OBJ 1  may be e.g. a plant (e.g. tree or a flower), a combustion flame, or an oil spill floating on water. The real object OBJ 1  may be e.g. the sun or a star observed through a layer of absorbing gas. The real object may be e.g. an image printed on a paper. A virtual object OBJ 1  may be e.g. an optical image formed by another optical device. 
     The object may be e.g. a biological object, e.g. human body, animal body, tissue sample, or a plant. The object may be e.g. an inorganic object, e.g. a mineral sample or a gaseous sample. The formed spectral image (CIMG) may be compared with reference data e.g. in order to identify the object OBJ 1 . The formed spectral image (CIMG) may be compared with reference data e.g. in order to determine whether the object belongs to a given category or not. The formed spectral image (CIMG) may be compared with reference data e.g. in order to determine whether the state of the object is normal or abnormal. 
     The device  500  may be arranged to capture spectral images, which represent two or more wavelengths (λ1, λ2, λ3, λ4) selected e.g. from the range of 600 nm to 1050 nm. The device  500  may be arranged to capture spectral images, which represent several wavelengths (λ1, λ2, λ3, λ4) selected e.g. from the visible and/or near infrared range. 
     The device  500  may be arranged to capture spectral images, which represent two or more wavelengths (λ1, λ2, λ3, λ4) selected e.g. from the range of 950 nm to 1700 nm. The device  500  may be arranged to capture spectral images, which represent two or more wavelengths (λ1, λ2, λ3, λ4) selected e.g. from the shortwave infrared (SWIR) range. The image sensor SEN 1  may be e.g. an InGaAs image sensor. 
     The dimensions of the spectral imaging device  500  may be selected e.g. such that angular distribution (Δθ LB3 ) of light rays transmitted through the Fabry-Perot interferometer is as narrow as possible. 
     The F-number of the lenses of the lens array may be e.g. as small as possible in order to minimize the length of the device  500 . The F-number of a lens is equal to the ratio f/D, where f denotes the focal length of said lens and D denotes the diameter of said lens. 
     The one or more dimensions of the device  500  may be selected to optimize performance. Said dimensions may include e.g. the width w APE1  of the input aperture APE 1 , the focal length of the lens (LNS 1  or LNS 2 ) of the light beam modifier system SYS 1 , the focal length of the lenses of the lens array ARR 1 , and/or the pitch dimension d 50  between centers of adjacent lenses of the lens array ARR 1 . 
     Selecting a small aperture size (w APE1 ) may improve spectral resolution of the Fabry-Perot interferometer. The aperture size (w APE1 ) may be selected to be large enough so as to enable stitching of the sub-images. 
     By way of example, the width w APE1  of the aperture APE 1  of the telecentric system SYS 1  may be e.g. substantially equal to 1.2 mm. The focal length of the lens LNS 1  of the telecentric system SYS 1  may be e.g. substantially equal to 6 mm. The width of the mirror M 1 , M 2  may be e.g. substantially equal to 5 mm. The length L 500  of the spectral imaging device  500  may be e.g. substantially equal to 9 mm. The lens array ARR 1  may comprise e.g. a rectangular 15 ×15 array of microlenses LNS. The pitch dimension d 50  of the lens array ARR 1  may be e.g. substantially equal to 0.25 mm. The focal length of the lenses of the lens array ARR 1  may be e.g. substantially equal to 1 mm. The image sensor SEN 1  may comprise e.g. a rectangular 640 ×480 array of detector pixels. The diagonal field of view (VIEW1) may be e.g. substantially equal to 40°. The distance L 0  between the object OBJ 1  and the device  500  may be e.g. substantially equal to 500 mm. 
     For the person skilled in the art, it will be clear that modifications and variations of the devices and methods according to the present invention are perceivable. The figures are schematic. The particular embodiments described above with reference to the accompanying drawings are illustrative only and not meant to limit the scope of the invention, which is defined by the appended claims.