Hyperspectral camera

Abstract

The invention relates to an optical system (28) for providing an image (12) of an object of interest (1, 6), with the image (12) being shiftable (5, 13) in an image plane of the optical system (28) and with the object of interest (1, 6) being located in an object plane of the optical system (28) and with the image plane being conjugate with respect to the object plane. The optical system (28) comprises 1) a first lens module (7, 9, 30) facing the object plane, the first lens module (7, 9, 30) having a first optical axis and a first entrance pupil, which first entrance pupil is located outside the first lens module (7, 9, 30) and is, with respect to the first optical axis, facing the opposite side of the first lens module (7, 9, 30) compared to the side of the first lens module (7, 9, 30) facing the object plane, a second 2) a second optical axis and a second entrance pupil, which second entrance pupil is located outside the second scan lens module (11) and is, with respect to the second optical axis, facing the opposite side of the second scan lens module (11) compared to the side of the second scan lens module facing the image plane, wherein the second scan lens module (11) is arranged such that the first optical axis and the second optical axis are coplanar and orthogonal to one another, and 3) a at least one mirror (10), with each mirror (10) of the at least one mirror (10) being rotatable about a respective axis of rotation and with the at least one mirror (10) being arranged at the first entrance pupil and at the second entrance pupil, and with all axes of rotation of the at least one axis of rotation (i) being parallel to one another, (ii) being coplanar to one another and lying in a rotation plane, and (iii) being orthogonal to the first optical axis and to the second optical axis, wherein the at least one mirror (10) is arranged such that the rotation plane passes through an intersection of (i) the first optical axis and (ii) the second optical axis. The invention also relates to a hyperspectral camera (4) comprising the optical system (28) and an imaging spectrometer (29) and to a method for capturing spectrally resolved data using the hyperspectral camera (4).

Description

[0001] 500313 -W01

[0002] 1

[0003] Hyperspectral Camera

[0004] Technical Field

[0005] The invention relates to an optical system for providing a shiftable, in particular laterally shiftable, image of an object of interest, to a hyperspectral camera comprising the optical system and to a method for capturing spectral data and spatially resolved data using the hyperspectral camera.

[0006] Background Art

[0007] A hyperspectral camera is an imaging spectrometer that measures both the laterally resolved intensity distribution of an object scene as well as the spectrally resolved intensity distribution in a desired spectral range. In the absence of any external scanning mechanism, most working principles as known in the prior art measure the spectrum along a narrow strip of the object field. The width of this narrow region is determined, in general, by the width of the entrance slit of the imaging spectrometer and its optics. To measure the spectrum of the entire object field, the camera needs to be translated across the entire object field or vice-versa. This usually involves either using translation stages for object movement or attaching the hyperspectral camera to a moving base, e.g. drones or airplanes for the lateral mapping of the object field. The use of external movement mechanisms limits the size of the object of measurement, makes it less versatile and may not always be possible to realise. There is therefore a need to enable a hyperspectral camera to measure the spectral distribution of an entire object field without the need for any external movement mechanisms. While some techniques are known in the prior art which enable such spectral scanning without external movement mechanisms, these known techniques are in general not suitable for applications such a high-end colour measurements and chemical component analysis due to their limited spectral resolution.500313 -W01

[0008] 2

[0009] Summary of the Invention

[0010] It is the objective of the invention to provide an optical system for providing a shiftable image of an object of interest and to provide a hyperspectral camera for spectral scanning without any external movement mechanism which mitigate at least some of the disadvantages of solutions as known in the prior art.

[0011] The solution of the invention is specified by the features of claim 1. The invention relates to an optical system for providing an image of an object of interest, with the image being shiftable in an image plane of the optical system and with the object of interest being located in an object plane of the optical system and with the image plane being conjugate with respect to the object plane. The optical system comprises 1) a first lens module facing the object plane, the first lens module having a first optical axis and a first entrance pupil, which first entrance pupil is located outside the first lens module and is, with respect to the first optical axis, facing the opposite side of the first lens module compared to the side of the first lens module facing the object plane, 2) a second scan lens module facing the image plane, the second scan lens module having a second optical axis and a second entrance pupil, which second entrance pupil is located outside the second scan lens module and is, with respect to the second optical axis, facing the opposite side of the second scan lens module compared to the side of the second scan lens module facing the image plane, wherein the second scan lens module is arranged such that the first optical axis and the second optical axis are coplanar and orthogonal to one another, and 3) at least one mirror, with each mirror being rotatable about a respective axis of rotation and with the at least one mirror being arranged at the first entrance pupil and at the second entrance pupil, and with all axes of rotation of the at least one axis of rotation (i) being parallel to one another, (ii) being coplanar to one another and lying in a rotation plane, and (iii) being orthogonal to the first optical axis and to the second optical axis, wherein the at least one mirror is arranged such that the rotation plane passes through an intersection of (i) the first optical axis and (ii) the second optical axis.500313 -W01

[0012] 3

[0013] The first lens module may comprise compound lenses and / or the second scan lens module may comprise compound lenses, i.e. each of the first lens module and of the second scan lens module may respectively comprise a plurality of lenses. The first lens module and / or the second scan lens module may be designed such that monochromatic aberrations, in particular spherical aberration, coma, astigmatism, defocus, field curvature or image distortion, and / or chromatic aberrations may be reduced. The first lens module and / or the second scan lens module may comprise achromatic doublets, for example.

[0014] The first optical axis is the imaginary line that passes through the geometrical centre of the first lens module, and the second optical axis is the imaginary line that passes through the geometrical centre of the second scan lens module. In the inventive optical system, the first optical axis and the second optical axis are coplanar, i.e. lie in a common plane, and are orthogonal to one another: accordingly, the first optical axis and the second optical axis intersect one another at an intersection point.

[0015] The optical system of the invention is such that an object located in an object plane of the optical system is imaged onto an image plane of the optical system, which image plane is conjugate with respect to the object plane, i.e. disregarding optical aberrations, an object point in the object plane is imaged sharply by the optical system onto an image point in the image plane. The optical system may be such that besides said image plane, further intermediate image planes may exist within the optical system, onto which intermediate image planes the object point may be imaged sharply as well.

[0016] Viewed on their own and not as part of the overall optical system, the first entrance pupil of the first lens module and the second entrance pupil of the second scan lens module potentially may not correspond to the entrance pupil(s) of the entire optical system. The entrance pupil(s) of a lens system in general, for example of the inventive optical system, may be defined as being the optical image of the aperture stop(s) of the lens system as viewed from the front of the lens system, i.e. from the side of the lens system in which an object to be imaged lies, which object in general may be real or virtual. Similarly, the exit pupil(s) of a lens system in general, for example of the inventive optical system, may be defined as being the optical image of the aperture stop(s) of the lens system as viewed500313 -W01

[0017] 4

[0018] from the back of the lens system. Depending on the position of such aperture stop(s) with respect to the lenses of the lens system, the entrance pupil(s) and / or the exit pupil(s) may either be real, i.e. correspond to an aperture stop, or virtual. Depending on the definition of 'front' and 'back' of a lens system, an entrance pupil may become an exit pupil and vice versa. Other concepts such as object-side telecentricity and image-side telecentricity of lens systems as discussed below may similarly be exchanged depending on the definition of 'front' and 'back' of a lens system: an object-side telecentric lens system may be turned into an image-side telecentric lens system by reversing its direction and vice versa.

[0019] As the first lens module and the second scan lens module are only individual parts of the inventive optical system, front and back of respectively the first lens module or of the second scan lens module therefore may be defined differently from front and back of the entire optical system. For the inventive optical system, the first entrance pupil of the first lens module is outside the first lens module, and the second entrance pupil of the second scan lens module is outside the second scan lens module. The first entrance pupil may be oriented orthogonally with respect to the second entrance pupil. Through at least one mirror that is placed at the first entrance pupil and at the second entrance pupil, light may be guided between the first lens module and the second scan lens module. The first entrance pupil and / or the second entrance pupil may be formed by dedicated aperture stops, for example embodied as diaphragms, or the first entrance pupil and / or the second entrance pupil may alternatively effectively be based on the mirror.

[0020] The first entrance pupil faces the opposite side of the first lens module compared to the side of the first lens module that faces the object plane. The first lens module may be such that at its first entrance pupil, no sharp image of an object point in the object plane is formed. Instead, by the at least one mirror of the optical system, light rays that pass through the first entrance pupil are guided through the second entrance pupil to the second scan lens module: the second scan lens module is configured to form a sharp image on the image plane. By distancing the first entrance pupil from the first lens module and the second entrance pupil from the second scan lens module, angles at which light rays may enter or leave the first lens module respectively the second scan lens module500313 -W01

[0021] 5

[0022] may advantageously be restricted. The image plane of the optical system may coincide with a focal plane of the second scan lens module.

[0023] Each mirror of the at least one mirror is rotatable about a respective axis of rotation. All axes of rotation of the at least one axis of rotation are parallel to one another, coplanar to one another in a rotation plane and are orthogonal to both the first optical axis and the second optical axis. The at least one mirror is arranged such that the rotation plane passes through the intersection point between the first optical axis and the second optical axis.

[0024] Preferentially, each mirror of the at least one mirror is embodied as a plane mirror. In case the optical system comprises a plurality of mirrors, in particular plane mirrors, each mirror may be rotated independently around its respective axis of rotation, or the optical system may alternatively be configured such that all mirrors may only be rotated together, with each mirror of the plurality of mirrors being rotatable around its respective axis of rotation by a same rotation angle as the other mirrors of the plurality of mirrors. Even in case the plurality of mirrors are independently rotatable, however, preferentially all mirrors are rotated in unison around their respective axis of rotation so that at any time all mirrors have a same common rotation angle with respect to their respective axis of rotation.

[0025] Preferentially, the optical system may be embodied such that it only comprises one mirror that is arranged at the first entrance pupil and at the second entrance pupil. The axis of rotation of this one mirror may intersect the first optical axis and the second optical axis, i.e. the axis of rotation of this one mirror may pass through the intersection point between the first optical axis and the second optical axis. Having only one mirror advantageously may simplify control and construction of the optical system.

[0026] By suitably rotating the at least one mirror about its respective axis of rotation, the direction of light rays that leave the first entrance pupil at a first angle with respect to the first optical axis may be changed in such a way that the light rays enter the second entrance pupil at a desired second angle with respect to the second optical axis. A rotation of the at least one mirror about its respective axis of rotation may thereby cause a shift of500313 -W01

[0027] 6

[0028] the image in the image plane, which shift is advantageously provided with a minimal decrease in image quality.

[0029] In an embodiment of the optical system according to the invention, the at least one mirror comprises a plurality of mirrors, with each mirror of the plurality of mirrors being independently rotatable about its respective axis of rotation.

[0030] In a further embodiment of the optical system according to the invention, the at least one mirror comprises one mirror, wherein the axis of rotation of the one mirror intersects the first optical axis and the second optical axis.

[0031] In a further embodiment of the optical system according to the invention, the first lens module comprises a front lens module and a first scan lens module, with the front lens module facing the object plane and being configured to provide a reference image of the object of interest in a reference image plane, which reference image plane is conjugate with respect to the object plane and which reference image plane is located between the front lens module and the first scan lens module, and the first lens module is embodied such that a focal plane of the first scan lens module is coincident with the reference image plane and such that the first entrance pupil corresponds to an entrance pupil of the first scan lens module.

[0032] The reference image plane corresponds to an intermediate image plane. Both the front lens module and the first scan lens module respectively may each comprise a plurality of lenses. The front lens module may be configured to collect light from the object space in which the object plane lies and to form a sharp image of object points on the object plane at the reference image plane. The reference image that may be provided by the front lens module in the reference image plane may serve as a reference for the subsequent optical components of the optical system. In case the first scan lens module is positioned in such a way with respect to the front lens module that a focal plane of the first scan lens module is coincident with the reference image plane, the light rays that leave the first scan lens module towards the first entrance pupil and that correspond to a common point in the reference image plane may be parallel to one another. The first lens module may be500313 -W01

[0033] 7

[0034] dimensioned such that its first entrance pupil corresponds to the entrance pupil of the first scan lens module, i.e. the first scan lens module may restrict the size of the cone of light rays that the first lens module accepts. The first scan lens module and the second scan lens module are juxtaposed, that is mirrored. The scaling of the image in the image plane may be dependent on the ratio between the focal lengths of the first scan lens module and of the second scan lens module.

[0035] Preferentially, both the first scan lens module and the second scan lens module are flatfield scan lens modules. Focus positions of a simple spherical lens do not lie in a plane, for example, but on an approximately spherical surface and thereby cause blurriness when moving away from the optical axis and when capturing an image with a flat image sensor; flat-field scan lens modules may be designed such that focus positions lie on a plane.

[0036] For both the first scan lens module and the second scan lens module, image height as measured with respect to an optical axis through the respective module may be dependent on the angle of incidence of light with respect to the optical axis. The maximum amount of lateral shift in the image plane of the optical system may depend on maximum possible rotation angles of the at least one mirror and on the focal length of the second scan lens module.

[0037] In a further embodiment of the optical system according to the invention, the each mirror of the at least one mirror is embodied as a plane mirror, and the optical system is such that for a rotation angle of each plane mirror of the at least one plane mirror of 45 degrees with respect to the first entrance pupil and to the second entrance pupil, the first scan lens module, the at least one plane mirror and the second scan lens module are configured to relay the reference image to the image plane.

[0038] In case the optical system comprises a plurality of mirrors and the mirrors are plane mirrors, at a rotation angle of each plane mirror of 45 degrees with respect to the first entrance pupil and to the second entrance pupil, all plane mirrors are coplanar, i.e. the plurality of plane mirrors substantially appears a one plane mirror.500313 -W01

[0039] 8

[0040] For such an optical system in which each plane mirror is rotated at 45 degrees with respect to both the first entrance pupil and the second entrance pupil, disregarding optical aberrations, the image in the image plane may be the same as the reference image in the reference image plane, i.e. the first scan lens module, each mirror rotated at 45 degrees with respect to the first entrance pupil and the second entrance pupil, and the second scan lens module together may form a relay unit.

[0041] In a further embodiment of the optical system according to the invention, the first scan lens module and / or the second scan lens module are image-side telecentric.

[0042] In case the first scan lens module is image-side telecentric, the exit pupil of the first scan lens module is at infinity, and in case the second scan lens module is image-side telecentric, the exit pupil of the second scan lens module is at infinity. For the first scan lens module, 'image-side' may correspond to the side of the first scan lens module facing the reference image plane, and for the second scan lens module, 'image-side' may correspond to the side of the second scan lens module facing the image plane of the optical system. This way, chief rays of the first scan lens module and / or chief rays of the second scan lens module may be, on the respective 'image sides', parallel to the first optical axis and / or to the second optical axis, respectively. The first scan lens module and / or the second scan lens module may be made image-side telecentric by placing the aperture stop corresponding to the first entrance pupil and / or to the second entrance pupil in a respective focal plane of the first scan lens module and / or the second scan lens module.

[0043] Through suitable design of the first lens module and of the second scan lens module, the optical system as a whole may be image-side telecentric, with 'image-side' of the optical system referring to the side of the optical system in which the image plane lies. This way, chief rays corresponding to each image point advantageously impinge at the image plane in a mostly orthogonal manner at all points of the image plane.500313 -W01

[0044] 9

[0045] In a further embodiment of the optical system according to the invention, f-numbers of the front lens module, the first scan lens module and the second scan lens module are matched.

[0046] The term 'f-number' may refer to the ratio between focal length and diameter of an entrance pupil. By matching the f-numbers of the front lens module, the first scan lens module and the second scan lens module, in particular by setting the f-numbers to be equal to one another, optical throughput of the optical system may be improved.

[0047] By matching f-numbers and ensuring image-side telecentricity of the optical system, optimal optical performance, high image quality and maximum light throughput through the optical system may advantageously be achieved.

[0048] In a further embodiment of the optical system according to the invention, the first scan lens module is embodied as an f-theta scan lens and / or the second scan lens module is embodied as an f-theta scan lens.

[0049] An f-theta scan lens is a special type of flat-field scan lens. In case lights enters an f-theta scan lens at a beam angle with respect to the optical axis of the f-theta scan lens, the position of spot onto which the f-theta scan lens focusses the light linearly depends on the product between the beam angle and the focal length of the f-theta scan lens. For the inventive optical system, the shift of the image in the image plane may therefore linearly depend on the angular orientation of the at least one mirror. For more general flat-field scan lenses, the shift of the image in the image plane may depend on the angular orientation of the at least one mirror in a nonlinear manner.

[0050] In a further embodiment of the optical system according to the invention, each mirror of the at least one mirror is embodied as a galvanometer-driven mirror.

[0051] Galvanometer-driven mirrors advantageously enable high-speed rotation of each mirror around its respective axis of rotation and may therefore enable accurate rapid lateral shifting of the image in the image plane of the optical system. Other types of swivelling mirrors may be used as well instead of galvanometer-driven mirrors, however.500313 -W01

[0052] 10

[0053] According to a second aspect, the invention relates to a hyperspectral camera, comprising 1) an optical system according to any one of the preceding claims and 2) an imaging spectrometer, which imaging spectrometer comprises (i) an entrance slit, (ii) a collimator, (iii) a diffraction grating, (iv) a focussing lens module and (v) an imaging sensor, wherein the imaging spectrometer is embodied in such a way that light enters the imaging spectrometer through the entrance slit, passes next through the collimator to the diffraction grating and next from the diffraction grating through the focussing lens module to the imaging sensor, and wherein the imaging spectrometer and the optical system are positioned in such a way with respect to each other that the entrance slit lies in the image plane of the optical system.

[0054] The part of the image on the entrance slit serves as input for the imaging spectrometer. The entrance slit may be such that it may allow only light originating from a line in the image plane of the optical system to continue towards the collimator, which collimator may then provide a parallel bundle of light rays. The entrance slit may therefore be stripshaped. The diffraction grating is configured to separate light into its different frequencies, which different frequencies may be spatially separated by the diffraction grating. By rotating the at least one mirror of the optical system, the image in the image plane of the optical system may be shifted, in particular orthogonally with respect to a main direction of extent of the entrance slit, and the spectrum of a different part of the imaged object may be obtained.

[0055] The imaging spectrometer is configured to obtain spectrally resolved data of the part of the image formed on the entrance slit by the optical system. Said differently, the imaging spectrometer is configured to measure the spectrum of the spatial region corresponding to the entrance slit opening area. The spectrally resolved data may be determined in a defined spectral range, which defined spectral range may depend on the choice of optical components in the imaging spectrometer, specifically the configuration of the diffraction grating, the transmission range of the optical components of the optical system and the characteristics of the imaging sensor. By gradually shifting the image on the entrance slit, the spectrum of each region, in particular resolved in the form of pixels, of the image500313 -W01

[0056] 11

[0057] provided by the optical system may be determined, resulting in a three-dimensional data cube comprising spatial information and spectral intensity distribution data. Such a three-dimensional data cube can be successively captured by the imaging sensor, which imaging sensor is comprised by the imaging spectrometer.

[0058] Besides being used as part of a hyperspectral camera, the optical system according to the invention may also be used as part of an apparatus for multispectral imaging, or as part of an apparatus for confocal microscopy, or as part of an apparatus for electronic document scanning.

[0059] In an embodiment of the hyperspectral camera according to the second aspect, each mirror of the at least one mirror of the optical system is embodied as a plane mirror and the optical system is such that for a rotation angle of each plane mirror of the at least one plane mirror of 45 degrees with respect to the first entrance pupil and to the second entrance pupil, the first scan lens module, the at least one plane mirror and the second scan lens module are configured to relay the reference image to the image plane, and wherein the entrance slit is positioned in such a way in the image plane that for a rotation angle of each plane mirror of the at least one plane mirror of 45 degrees with respect to the first entrance pupil and to the second entrance pupil, an object point in the object plane lying on the first optical axis is imaged by the optical system onto the entrance slit.

[0060] Preferably, the optical system of the hyperspectral camera is embodied such that in case it comprises a plurality of mirrors, the plurality of mirrors are rotated in unison around their respective axis of rotation so that each mirror of the plurality of mirrors always has a same rotation angle with respect to the first entrance pupil and to the second entrance pupil. Preferably, the optical system of the hyperspectral camera is embodied such that it comprises one mirror.

[0061] In a further embodiment of the hyperspectral camera according to the second aspect, for a rotation angle of each mirror of the at least one plane mirror different than 45 degrees with respect to the first entrance pupil and to the second entrance pupil, with each mirror of the at least one mirror having a same rotation angle with respect to the first entrance500313 -W01

[0062] 12

[0063] pupil and to the second entrance pupil, an off-axis object point in the object plane is imaged by the optical system onto the entrance slit.

[0064] In a further embodiment of the hyperspectral camera according to the second aspect, the collimator is embodied as a collimator lens, and the entrance slit lies in a focal plane of the collimator lens.

[0065] In a further embodiment of the hyperspectral camera according to the second aspect, the diffraction grating is embodied as a reflective grating.

[0066] Instead of a reflective grating, a transmissive grating may be used as well.

[0067] In a further embodiment of the hyperspectral camera according to the second aspect, the focussing lens module is positioned such that first-order diffracted light diffracted by the diffraction grating passes through the focussing lens module, and / or the imaging sensor is positioned in a spectrometer image plane of the focussing lens module.

[0068] The focussing lens module may advantageously provide a sharp image at the imaging sensor.

[0069] For a given order of diffraction, different wavelengths of light typically exit the diffraction grating at different angles. The positioning of the focussing lens module and of the imaging sensor with respect to the diffraction grating may take into account this angular separation.

[0070] According to a third aspect, the invention relates to a method for capturing spectral data and spatially resolved data of at least two lines along an object of interest using a hyperspectral camera according to the second aspect of the invention, the method comprising the following steps: 1) For each line of the at least two lines, determining a corresponding common rotation angle each mirror of the at least one mirror around its respective axis of rotation based on a configuration of the first lens module and of the second scan module and on a position of the entrance slit in the image plane of the optical system, which corresponding common rotation angle optically shifts the image in the image plane such that the respective line in the object plane is imaged onto the500313 -W01

[0071] 13

[0072] entrance slit of the imaging spectrometer positioned in the image plane of the optical system, and 2) Consecutively positioning the at least one mirror at each determined common rotation angle of the at least two common rotation angles, and acquiring the spectral data and the spatially resolved data using the imaging spectrometer after each repositioning of the at least one mirror.

[0073] In case the optical system comprises a plurality of mirrors, in the method according to the third aspect of the invention, the mirrors are rotated in unison around their respective axis of rotation so that each mirror of the plurality of mirrors always has a same rotation angle with respect to the first entrance pupil and to the second entrance pupil. Consecutively positioning at least one mirror at each determined common rotation angle of the at least two common rotation angles may therefore be understood as follows: at a first common rotation angle of the at least two common rotation angles, all mirrors of the plurality of mirrors are positioned at the first common rotation angle; at a second common rotation angle of the at least two common rotation angles, all mirrors of the plurality of mirrors are positioned at the second common rotation angle; the plurality of mirrors are therefore consecutively positioned at the determined common rotation angles, with the plurality of mirrors being rotated in unison.

[0074] The common rotation angles of the at least one mirror around its respective axis of rotation are determined such that a desired shift of the image in the image plane is created which in turn enables consecutive spectral scanning of different lines of the object in the object plane. Using the hyperspectral camera according to the second aspect of the invention, scanning is based on laterally shifting the image in the image plane in a direction perpendicular to the main direction of extent of the entrance slit. In a neutral position of the mirror, the image may be created symmetrically around the entrance slit; by clockwise or counter-clockwise rotation of the at least one mirror, the image may be laterally shifted left or right. The method according to the third aspect of the invention may be encoded in the form of a computer programme, for example on a microcontroller of the hyperspectral camera.500313 -W01

[0075] 14

[0076] In an embodiment of the method according to the third aspect of the invention, the first scan lens module of the optical system of the hyperspectral camera is embodied as an f-theta scan lens and / or the second scan lens module of the optical system of the hyperspectral camera is embodied as an f-theta scan lens, and the at least two common rotation angles are determined based on a linear relationship between the shift of the image in the image plane, a rotation angle of a mirror of the at least one mirror and a focal length of the second scan module.

[0077] Using f-theta scan lenses advantageously may simplify control of the at least one rotatable mirror using the method according to the third aspect of the invention.

[0078] Other advantageous embodiments and combinations of features come out from the detailed description below and the entirety of the claims.

[0079] Brief Description of the Drawings

[0080] The drawings used to explain the embodiments show:

[0081] Fig. 1 shows an embodiment of a hyperspectral camera with two different angles of rotation of the mirror around its axis of rotation;

[0082] Fig. 2 shows the imaging geometry of an embodiment of a scan lens module;

[0083] Fig. 3 shows embodiments of spectral scanning; and

[0084] Fig. 4 shows another embodiment of a hyperspectral camera

[0085] In the figures, the same components are given the same reference signs.

[0086] Detailed Description of the Drawings

[0087] Fig. 1 shows an embodiment of a hyperspectral camera, with a first angle of rotation of the mirror 10 around its axis of rotation shown in Fig. 1a and a second angle of rotation of the mirror 10 around its axis of rotation shown in Fig. 1b.500313 -W01

[0088] 15

[0089] The hyperspectral camera comprises an optical system 28 and an imaging spectrometer 29. The optical system 28 comprises a front lens module 7, a first scan lens module 9, the mirror 10 and a second scan lens module 11. The front lens module 7 forms a sharp reference image 8 in a reference image plane of an object 6 lying in an object plane. The first scan lens module 9 is placed such with respect to the reference image plane of the front lens module 7 that a focal plane of the first scan lens module 9 coincides with the reference image plane. Light rays from a point in the reference image plane are thereby parallel to one another after passing through the first scan lens module 9. The entrance pupil of the first scan lens module 9 is close to or formed by the mirror 10, which mirror is rotatable around an axis of rotation and which axis of rotation passes orthogonally into the paper on which Fig. 1 is printed.

[0090] In Fig. 1a, the mirror 10 is oriented at an angle of rotation which is such that the first scan lens module 9, the mirror 10 and the second scan lens module 11 together provide relay functionality, i.e. the image 12 in the image plane of the optical system 28 substantially corresponds to the reference image 8 in the reference image plane. In Fig. 1b, the mirror 10 is oriented at a different angle of rotation compared to the angle of rotation in Fig. 1a, which different angle of rotation shifts the image 12 in the image plane of the optical system 28. Through rotation of the mirror 10 about its axis of rotation, the image may be laterally shifted 13.

[0091] The imaging spectrometer 29 of Fig. 1 comprises an entrance slit 14 through which light may enter the imaging spectrometer 29, a collimator 15, a diffraction grating 16, a focussing lens module 17 and an imaging sensor 18. The entrance slit 12 is line-shaped, and the collimator 15 is placed such that a focal plane of the collimator 15 passes through the entrance slit 12. The entrance slit 12 of the imaging spectrometer 29 is positioned closely or in the image plane of the optical system 28.

[0092] The parallel light rays provided by the collimator 15 next impinge on the diffraction grating 16 that is embodied as a reflective grating in Fig. 1. The focussing lens module 17 is positioned such with respect to the reflective grating 16 that first-order diffracted light500313 -W01

[0093] 16

[0094] passes through the focussing lens module 17. The focussing lens module 17 provides a sharp image that is captured by the imaging sensor 18.

[0095] Fig. 2 shows the imaging geometry of an embodiment of a scan lens module 22. The scan lens module 22 has a focal plane 19, in which focal plane a virtual or real object may be placed. The scan lens module 22 is placed at a working distance 20 from the focal plane, working distance 20 referring to the distance between the focal plane 19 and a first lens of the scan lens module 22.

[0096] The entrance pupil 24 of the scan lens module 22 is outside the scan lens module 22. The scan lens module 22 is such that different heights of in the focal plane with respect to the optical axis are mapped into different angles 23. The chief rays 21 pass through the centre of the entrance pupil 24. The entrance pupil 24 is positioned in a second focal plane of the scan lens module 22. In case the entrance pupil 24 of Fig. 2 has a sufficiently small diameter, the scan lens module 22 of Fig. 2 is image-side telecentric. The scan lens module 22 of Fig. 2 may be embodied as an f-theta scan lens that provides a linear relationship between the angle 23 and height in the focal plane 19 with respect to the optical axis of the f-theta scan lens.

[0097] Fig. 3 shows embodiments of spectral scanning. In Fig. 3a, an object of interest 1 is shown lying in an object plane. A hyperspectral camera 4 captures both spatially resolved line data 2 and spectral data 3 for each pixel of the line data 2 at a time. In Fig. 3a, the spectral data 3 are plotted as spectral power distribution S(A) over wavelength A. To scan the entire object of interest 1, the line for which spatial and spectral data are captured needs to be moved 5 over the object plane, in particular along the x direction of the two-dimensional coordinate system of Fig. 3a. Using an inventive optical system as part of the hyperspectral camera 4, the captured line may be moved 5 by rotating the mirror of the optical system about its axis of rotation.

[0098] The selection of the line to be captured is further shown in Fig. 3b. In the image plane of the inventive optical system, an image 26 is formed, which image may be shifted along direction 25. By suitably shifting the image 26, a different segment of the image 26 may500313 -W01

[0099] 17

[0100] come to coincide with the entrance slit 27 of the imaging spectrometer of the hyperspectral camera.

[0101] Fig.4 shows another embodiment of a hyperspectral camera. The embodiment of Fig. 4 substantially corresponds to the embodiment of Fig. 1. In Fig. 4, the angle of rotation of the mirror 10 around its axis of rotation corresponds to the angle of rotation of the mirror 10 around its axis of rotation of Fig. 1b.

[0102] The only difference between the embodiment of Fig. 4 and the embodiment of Fig. 1 is that in Fig. 4, the first lens module is embodied as a single unit 30 and is not split into a front lens module 7 and a first scan lens module 9 as in the embodiment of Fig. 1. The first lens module 30 of Fig. 4 faces the object plane in which the object 6 lies. The first lens module 30 has a first optical axis and a first entrance pupil, which first entrance pupil is located outside the first lens module 30 and is, with respect to the first optical axis, facing the opposite side of the first lens module 30 compared to the side of the first lens module 30 facing the object plane. The mirror 10 is arranged at the first entrance pupil. Optically, the first lens module 30 of Fig. 4 therefore corresponds to the front lens module 7 and the first scan lens module 9 of Fig. 1.

Claims

500313 -W0118Claims1. Optical system (28) for providing an image (12) of an object of interest (1, 6), with the image (12) being shiftable (5, 13) in an image plane of the optical system (28) and with the object of interest (1, 6) being located in an object plane of the optical system (28), with the image plane being conjugate with respect to the object plane, wherein the optical system (28) comprises• a first lens module (7, 9, 30) facing the object plane, the first lens module (7, 9, 30) having a first optical axis and a first entrance pupil, which first entrance pupil is located outside the first lens module (7, 9, 30) and is, with respect to the first optical axis, facing the opposite side of the first lens module (7, 9, 30) compared to the side of the first lens module (7, 9, 30) facing the object plane,• a second scan lens (11) module facing the image plane, the second scan lens module (11) having a second optical axis and a second entrance pupil, which second entrance pupil is located outside the second scan lens module (11) and is, with respect to the second optical axis, facing the opposite side of the second scan lens module (11) compared to the side of the second scan lens module facing the image plane, wherein the second scan lens module (11) is arranged such that the first optical axis and the second optical axis are coplanar and orthogonal to one another, and• at least one mirror (10), with each mirror of the at least one mirror (10) being rotatable about a respective axis of rotation and with the at least one mirror (10) being arranged at the first entrance pupil and at the second entrance pupil, and with all axes of rotation of the at least one axis of rotation (i) being parallel to one another, (ii) being coplanar to one another and lying in a rotation plane, and (iii) being orthogonal to the first optical axis and to the second optical axis, wherein the at least one mirror (10) is arranged such that the rotation plane passes through an intersection of (i) the first optical axis and (ii) the second optical axis.500313 -W01192. Optical system (27) according to claim 1, wherein the at least one mirror (10) comprises a plurality of mirrors (10), with each mirror of the plurality of mirrors (10) being independently rotatable about its respective axis of rotation.

3. Optical system (28) according to claim 1, wherein the at least one mirror (10) comprises one mirror (10), wherein the axis of rotation of the one mirror (10) intersects the first optical axis and the second optical axis.

4. Optical system (28) according to any one of the preceding claims, wherein the first lens module (7, 9) comprises a front lens module (7) and a first scan lens module (9), with the front lens module (7) facing the object plane and being configured to provide a reference image (8) of the object of interest (1, 6) in a reference image plane, which reference image plane is conjugate with respect to the object plane and which reference image plane is located between the front lens module (7) and the first scan lens module (9), and wherein the first lens module (7, 9) is embodied such that a focal plane of the first scan lens module (9) is coincident with the reference image plane and such that the first entrance pupil corresponds to an entrance pupil of the first scan lens module (9).

5. Optical system (28) according to claim 4, wherein each mirror of the at least one mirror (10) is embodied as a plane mirror (10), and wherein the optical system (28) is such that for a rotation angle of each plane mirror of the at least one plane mirror (10) of 45 degrees with respect to the first entrance pupil and to the second entrance pupil, the first scan lens module (9), the at least one plane mirror (10) and the second scan lens module (11) are configured to relay the reference image (8) to the image plane.

6. Optical system (28) according to claim 4 or 5, wherein the first scan lens module (9) and / or the second scan lens module (11) are image-side telecentric.

7. Optical system (28) according to any one of claims 4 to 6, wherein f-numbers of the front lens module (7), the first scan lens module (9) and the second scan lens module (11) are matched.500313 -W01208. Optical system (28) according to any one of claims 4 to 7, wherein the first scan lens module (9) is embodied as an f-theta scan lens and / or wherein the second scan lens module (11) is embodied as an f-theta scan lens.

9. Optical system (28) according to any one of the preceding claims, wherein each mirror of the at least one mirror (10) is embodied as a galvanometer-driven mirror.

10. Hyperspectral camera (4), comprising 1) an optical system (28) according to any one of the preceding claims and 2) an imaging spectrometer (29), which imaging spectrometer (29) comprises (i) an entrance slit (14, 27), (ii) a collimator (15), (iii) a diffraction grating (16), (iv) a focussing lens module (17) and (v) an imaging sensor (18), wherein the imaging spectrometer (29) is embodied in such a way that light enters the imaging spectrometer (29) through the entrance slit (14, 27), passes next through the collimator (15) to the diffraction grating (16) and next from the diffraction grating (16) through the focussing lens module (17) to the imaging sensor (18), and wherein the imaging spectrometer (29) and the optical system (28) are positioned in such a way with respect to each other that the entrance slit (14, 27) lies in the image plane of the optical system (28).

11. Hyperspectral camera (4) according to claim 10, with the optical system (28) being embodied according to claim 5, and wherein the entrance slit (14, 27) is positioned in such a way in the image plane that for a rotation angle of each mirror (10) of the at least one mirror (10) of 45 degrees with respect to the first entrance pupil and to the second entrance pupil, an object point in the object plane lying on the first optical axis is imaged by the optical system (28) onto the entrance slit (14, 27).

12. Hyperspectral camera (4) according to claim 11, wherein for a rotation angle of each mirror of the at least one mirror (10) different than 45 degrees with respect to the first entrance pupil and to the second entrance pupil, with each mirror of the at least one mirror (10) having a same rotation angle with respect to the first entrance pupil and to the second entrance pupil, an off-axis object point in the object plane is imaged by the optical system (28) onto the entrance slit (14, 27).500313 -W012113. Hyperspectral camera (4) according to any one of claims 10 to 12, wherein the collimator (15) is embodied as a collimator lens, and wherein the entrance slit (14, 27) lies in a focal plane of the collimator lens.

14. Hyperspectral camera (4) according to any one of claims 10 to 13, wherein the diffraction grating (16) is embodied as a reflective grating.

15. Hyperspectral camera (4) according to claim 14, wherein the focussing lens module (17) is positioned such that first-order diffracted light diffracted by the diffraction grating (16) passes through the focussing lens module (17), and / or wherein the imaging sensor (18) is positioned in a spectrometer image plane of the focussing lens module (17).

16. Method for capturing spectral data and spatially resolved data of at least two lines along an object of interest (1, 6) using a hyperspectral camera (4) according to any one of claims 10 to 15, the method comprising the following steps:• For each line of the at least two lines, determining a corresponding common rotation angle of each mirror of the at least one mirror (10) around its respective axis of rotation based on a configuration of the first lens module (7, 9, 30) and of the second scan module (11) and on a position of the entrance slit (14, 27) in the image plane of the optical system (28), which corresponding common rotation angle optically shifts the image (12) in the image plane such that the respective line in the object plane is imaged onto the entrance slit (14, 27) of the imaging spectrometer (29) positioned in the image plane of the optical system (28), and• Consecutively positioning the at least one mirror (10) at each determined common rotation angle of the at least two common rotation angles, and acquiring the spectral data and the spatially resolved data using the imaging spectrometer (29) after each re-positioning of the at least one mirror (10).

17. Method according to claim 16, with the optical system (28) of the hyperspectral camera (4) being embodied according to claim 8, wherein the at least two common rotation angles are determined based on a linear relationship between the shift of the image500313 -W0122(12) in the image plane, a rotation angle of a mirror of the at least one mirror (10) and a focal length of the second scan module (11).

Images