WO2017042435A1

WO2017042435A1 - Imaging spectrograph

Info

Publication number: WO2017042435A1
Application number: PCT/FI2016/050622
Authority: WO
Inventors: Kai Ojala
Original assignee: Teknologian Tutkimuskeskus Vtt Oy
Priority date: 2015-09-08
Filing date: 2016-09-07
Publication date: 2017-03-16
Also published as: FI20155642A

Abstract

An imaging device (500) comprises: a first telecentric system (SYS1) to form a first image (IMG1) of an object (OBJ1),5 a second telecentric system (SYS2) to provide a bundle of output beams (LB5a,LB5b) for forming a second image (IMG2) such that the output beams (LB5a,LB5b) are formed from light received from the first image (IMG1), and a first dispersive element (G1) positioned between the first telecentric system (SYS1) and the second telecentric system (SYS2) to provide a bundle of deflected light beams (LB3a,LB3b) such that the direction of the deflected light beams (LB3a,LB3b) depends on the wavelength (κ), wherein the second telecentric system (SYS2) is arranged to form the output beams (LB5a,LB5b) from light of the deflected light beams (LB3a,LB3b), and the second telecentric system (SYS2) comprises an aperture stop (AP2) to reject light which is outside a selected spectral range (PB1).

Description

IMAGING SPECTROGRAPH

FIELD

Some versions may relate to an imaging spectrograph. Some versions may relate to capturing an image which represents an adjustable wavelength range.

BACKGROUND

A color image of an object may be captured by using a color camera, which comprises an RGB Bayer filter and an image sensor. The color image captured by using the RGB Bayer filter may be decomposed to form a first image representing red color, a second image representing green color, and a third image representing the blue color. The wavelength ranges of the Bayer filter cannot be typically adjusted. Consequently, the first, second, and third images represent a fixed range of wavelengths. The wavelength range of the first image may be rather broad. The wavelength range of the first image may overlap the wavelength range of the second image. Due to the Bayer filter, the spatial resolution of the first image may be lower than the spatial resolution of the image sensor of the color camera.

SUMMARY

Some versions may relate to a spectrally selective imaging device. Some versions may relate to a method for capturing an image in a spectrally selective manner. Some versions may relate to a method for measuring an intensity value by using the spectrally selective imaging device.

According to an aspect, there is provided an imaging device (500), comprising: - a first telecentric system (SYS1 ) to form a first image (IMG1 ) of an object (OBJ1 ),

- a second telecentric system (SYS2) to provide a bundle of output beams (LB5a, LB5b) for forming a second image (IMG2) such that the output beams (LB5a, LB5b) are formed from light received from the first image (IMG1 ), and

- a dispersive element (G1 ) positioned between the first telecentric system (SYS1 ) and the second telecentric system (SYS2) to provide a bundle of deflected light beams (LB3a, LB3b) such that the direction of the deflected light beams (LB3a, LB3b) depends on the wavelength (λ), wherein the second telecentric system (SYS2) is arranged to form the output beams from light of the deflected light beams (LB3a,LB3b), and the second telecentric system (SYS2) comprises an aperture stop (AP2) to reject light which is outside a selected spectral range (PB1 ).

According to an aspect, there is provided a method for capturing an image (IMG2), the method comprising:

- forming a first image (IMG1 ) of an object (OBJ1 ) by using a first telecentric system (SYS1 ) to form a first image (IMG1 ) of an object (OBJ1 ),

- forming a bundle of output beams (LB5a,LB5b) from light received from the first image (IMG1 ) by using a second telecentric system (SYS2), and

- capturing a second image (IMG2) formed from light of the output beams (LB5a, LB5b) by using an image sensor (SEN1 ),

wherein a dispersive element (G1 ) positioned between the first telecentric system (SYS1 ) and the second telecentric system (SYS2) provides a bundle of deflected light beams (LB3a, LB3b) such that the direction of the deflected light beams (LB3a,LB3b) depends on the wavelength (λ), and wherein the second telecentric system (SYS2) provides said bundle of output beams from light of the deflected light beams (LB3a,LB3b).

Further aspects are defined in the claims.

The imaging device may operate as a tunable band pass filter. The operation of the device is based on the use of telecentric optical systems. The central wavelength of the passband may be selected e.g. by adjusting orientation of a beam steering element, e.g. the orientation of a mirror. The spectral width of the passband may be e.g. smaller than or equal to 20 nm.

The imaging device comprises a dispersive element. The dispersive element may be e.g. a diffraction grating. The dispersive element may provide deflected beams such that the direction of the deflected beams depends on the wavelength of the deflected beams. The dispersive element may spectrally decompose input light such that spectral components having different wavelengths propagate in different directions. The device may comprise an aperture stop, which allows transmission of light only when the deflected beams propagate in a selected wavelength-dependent direction. The device may allow transmission of light which is in the selected spectral range, and the device may prevent transmission of light which is outside the selected spectral range.

The imaging device may be arranged to operate such that only light of deflected beams having a selected wavelength is transmitted to an image sensor. The imaging device may be arranged to form an image on an image sensor. All detector pixels of the image sensor may be exposed substantially simultaneously at the selected spectral band. The whole active area of the image sensor may be simultaneously exposed to light, which represents the same (narrow) spectral band. Consequently, the imaging device may be suitable for monitoring transient or rapidly changing situations. For example, the imaging device may be arranged to provide an image of turbulent heterogeneous flow. One or more of the capture images may be used e.g. for distinguishing gases or liquids that are not fully mixed with surroundings.

Thanks to using the telecentric optical systems, the imaging device may provide a substantially orthographic view of an object. Consequently, the image magnification may be substantially independent of the object's distance or position in the field of view.

Using the telecentric optical systems may provide high spectral transmittance and/or a bright spectral image at the selected spectral band. Thanks to the telecentric optical systems, the imaging device may be implemented by using relatively low number of optical and mechanical components. Consequently, the imaging device may be optically and mechanically stable. The mechanical construction of the device may be rugged and stable. The spectral scale of the device may be stable and highly reproducible.

The imaging device may be arranged to capture an image which represents a selected spectral band within the visible range of wavelengths (e.g. in the range of 380 nm to 760 nm). The imaging device may be arranged to capture an image which represents a selected spectral band within the infrared range. The spectral position of the passband may be adjustable e.g. within a range of 1000 nm to 2500 nm. The image sensor may be sensitive to infrared light (IR), i.e. the image sensor may detect infrared light. The image sensor may be selected such that the (same) image sensor may detect spectral components within the whole range from 1000 nm to 2500 nm.

The imaging device may be arranged to capture an image which represents a selected spectral band within a tuning range. The tuning range may overlap e.g. the ultraviolet regime, the visible regime, and/or the infrared regime. The imaging device may be used e.g. for remote sensing, medical diagnosis, product quality control on a production line.

The imaging device may be arranged to monitor an industrial process. The imaging device may be used e.g. as a part of machine vision system. The imaging device may be arranged to monitor e.g. the quality of food. The imaging device may be arranged to monitor a pharmaceutical process.

The imaging device may be arranged to provide spectral images related to an agricultural application. The imaging device may be arranged to monitor the state of plants in a greenhouse or in the field. The imaging device may be used as a hyperspectral imager. The imaging device may be a portable instrument. The imaging device may be carried by a car. The imaging device may be used as an airborne instrument. The imaging device may be carried by a flying vehicle, e.g. by a helicopter or by a quadrocopter.

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 shows, by way of example, in a side view, an imaging device, Fig. 2 shows, by way of example, in a three dimensional view, the imaging device, shows, by way of example, in a side view, the imaging device Fig. 1 when the passband has been adjusted to a different spectral position,

Fig. 3b shows, by way of example, adjusting the input angle of light beams coupled to the second telecentric system, Fig. 4 shows, by way of example, formation of a sensor signal obtained from the image sensor,

Fig. 5 shows, by way of example, an optical image formed on the image sensor, shows, by way of example, dimensions of the imaging device, shows, by way of example, a control system of the imaging device, Fig. 8 shows, by way of example, an imaging device which comprises a second grating,

Fig. 9 shows, by way of example, an imaging device which comprises two prism-grating-prism units,

Figs. 10a shows, by way of example, an imaging device comprising a n actuator arranged to change the orientation of the second telecentric system with respect to the first telecentric system,

Fig. 10b shows, by way of example, the imaging device of Fig. 10a when the passband has been adjusted to a second spectral position, Fig. 1 1 shows, by way of example, an imaging device which comprises a reflective diffraction grating, and

Fig. 12 shows by way of example, attaching the imaging device to an external camera.

DETAILED DESCRPTION

Referring to Fig. 1 , the imaging device 500 may be arranged to capture an image IMG2 of an object OBJ1 such that the image IMG2 represents a selected spectral band. The imaging device 500 may be arranged to operate such that the spectral position λο of the center of the selected spectral band may be e.g. at a wavelength λι . The imaging device 500 may comprise a first telecentric system SYS1 , a second telecentric system SYS2, and a dispersive element G1 positioned in the optical path between the systems SYS1 , SYS2. The dispersive element G1 may be e.g. a diffraction grating. The object OBJ1 may be e.g. a solid, liquid and/or gaseous object. The object OBJ1 may comprise points Pa, Pb, Pc. The object OBJ1 may provide input beams LB1 a, LB1 b, LB1 c e.g. by reflecting, transmitting, scattering and/or emitting. The beam LB1 a may be received from a point Pa. The beam LB1 b may be received from a point Pb. The beam LB1 c may be received from a point Pc. The light received from the object OBJ1 may simultaneously comprise spectral components at different wavelengths. The light received from the object OBJ1 may comprise a first spectral component at a first wavelength λι, and a second spectral component at a second different wavelength λ₂.

The first telecentric system SYS1 may comprise an input aperture stop AP1 and an input lens LNS1 1 . The aperture of the aperture stop AP1 may have a width w_Api (see Figs. 2 and 6). The aperture stop AP1 may define the width of the light beams LB1 a, LB1 b, LB1 c which impinge on the input lens LNS1 1 .

The first telecentric system SYS1 may be an image space telecentric system. The distance L1 between the aperture stop AP1 and the focusing input lens LNS1 1 may be substantially equal to the focal length fi of the first telecentric system SYS1 (the distance L1 is shown in Fig. 6).

The first telecentric system SYS1 may form an auxiliary image IMG1 to a position, which is optically between the first telecentric system SYS1 and the second telecentric system SYS2. The first telecentric system SYS1 may provide focused beams LB2a, LB2b, LB2c by focusing light of the input beams LB1 a, LB1 b, LB1 c. The first telecentric system SYS1 may form the auxiliary image IMG1 on the dispersive element G1 . In particular, the first telecentric system SYS1 may form the auxiliary image IMG1 on the diffraction grating G1 . The second image IMG2 may be an image of the auxiliary image IMG1 . The second image IMG2 may be formed on an image sensor SEN1 .

Using the telecentric systems SYS1 , SYS2 may provide a high spectral transmittance at the selected passband and/or a bright optical image IMG2. Light received from the object OBJ1 at the selected passband may be effectively transmitted to the image sensor SEN1 in order to form the optical image IMG2.

The dispersive element G1 may provide deflected light beams LB3a, LB3b, LB3c, LB3a\ LB3b\ LB3c' by dispersing light of focused beams LB2a, LB2b, LB2c. The deflected light beams LB3a, LB3b, LB3c may have a wavelength λι, and the deflected light beams LB3a\ LB3b', LB3c' may have a different wavelength λ₂. The dispersive element G1 may provide a bundle of deflected light beams LB3a, LB3b, LB3c such that the direction of the deflected light beams depends on the wavelength of said deflected light beams LB3a, LB3b, LB3c. For example first group of first deflected beams LB3a, LB3b, LB3c having a wavelength λι may propagate in a first direction, and a second group of second deflected beams LB3a\ LB3b', LB3c' having a wavelength λ₂ may propagate in a second different direction. The group of beams may also be called as a bundle of beams.

The light of the deflected light beams LB3a, LB3b, LB3c, LB3a\ LB3b\ LB3c' may be coupled to the second telecentric system SYS2. The second telecentric system SYS2 may be arranged to allow transmission of light, which is in the selected spectral range, and the second telecentric system SYS2 may be arranged to prevent transmission of light which is outside the selected spectral range. The selected spectral range may also be called as the passband PB1 (see Fig. 4).

The second telecentric system SYS2 may comprise a lens unit and an aperture stop AP2. The lens unit may comprise one or more converging lenses LNS21 , LNS22. The second telecentric system SYS2 may comprise one or more converging lenses LNS21 , LNS22. The distance L2 between the converging lens LNS22 and the aperture stop AP2 may be e.g. substantially equal to the 50% of the focal length f₂ of the second telecentric system SYS2. The lenses LNS21 , LNS22 may form a first bundle of beams LB5a, LB5b, LB5c from the light of the beams LB3a, LB3b, LB3c coupled to the second telecentric system SYS2. The lenses LNS21 , LNS22 may form a second bundle of beams LB5a', LB5b', LB5c' from the light of the beams LB3a\ LB3b', LB3c' coupled to the second telecentric system SYS2.

The light of the deflected light beams LB3a, LB3b, LB3c may be directly or indirectly coupled to the second telecentric system SYS2. The light of the deflected light beams LB3a, LB3b, LB3c may be coupled to the second telecentric system SYS2 e.g. by using a beam steering element M1 . The device 500 may comprise a beam steering element M1 . The beam steering element M1 may be e.g. a mirror. As an intermediate step, the beam steering element M1 may form steered beams LB4a, LB4b, LB4c from the light of the deflected light beams LB3a, LB3b, LB3c, and the steered beams LB4a, LB4b, LB4c may be subsequently coupled to the lens unit of the second telecentric system SYS2. The dispersive element G1 may provide the beams LB3a, LB3b, LB3c, LB3a\ LB3b\ LB3c' such that the beams LB3a, LB3b, LB3c propagate in a first direction, and such that the beams LB3a', LB3b', LB3c' propagate in a second different direction. Consequently, the second telecentric system SYS2 may form the beams LB5a, LB5a' such that the direction of the beam LB5a is different from the direction of the beam LB5a'. The direction of the beam LB5b may be different from the direction of the beam LB5b'. The direction of the beam LB5c may be different from the direction of the beam LB5c\ The beams LB5a, LB5b, LB5c may have the wavelength λι, and the beams LB5a', LB5b', LB5c' may have the wavelength λ₂. The device 500 may be arranged to operate such that the beams LB5a, LB5b, LB5c having the selected wavelength λι may pass through the aperture stop AP2, wherein the beams LB5a', LB5b', LB5c' having the different wavelength λ₂ may be blocked by the aperture stop AP2.

The dispersive element may provide a first group of deflected light beams LB3a, LB3b, LB3c, and a second group of deflected light beams LB3a', LB3b', LB3c' such that the direction of the deflected light beams depends on the wavelength λ, wherein the second telecentric system SYS2 may be arranged to form output beams LB5a, LB5b, LB5c from light of the deflected light beams LB3a, LB3b, LB3c, and the second telecentric system SYS2 comprises an aperture stop AP2 to reject light LB5a', LB5b\ LB5c' which is outside the selected spectral range PB1 .

The light beams LB5a, LB5b, LB5c formed from light of the deflected light beams LB3a, LB3b, LB3c and which a transmitted through the aperture stop AP2 may be called e.g. as output beams. The spectral band of the output beams LB5a, LB5b, LB5c may be selectable and/or adjustable.

The device 500 may comprise an adjustable beam steering element M1 to steer the direction of the deflected beams LB3 provided by the first dispersive element G1 . The beam steering element M1 may be e.g. a mechanically adjustable mirror. The central wavelength λο of a spectral band of input light transmitted to the image sensor SEN1 may be selected by changing the orientation of the beam steering element M1 with respect to the orientation of the first dispersive element G1 .

The beam steering element M1 may be positioned between the first telecentric system SYS1 and the second telecentric system SYS2 to adjust the direction of the output beams LB5a, LB5b, LB5c with respect to the axis AX2 of the second telecentric system SYS2. Adjusting the beam steering element M1 may collectively and simultaneously change the directions of substantially all output beams LB5a, LB5b, LB5c, LB5a\ LB5b\ LB5c' provided by the second telecentric system SYS2. The beam steering element M1 may be e.g. a mirror arranged to provide the steered beams LB4a, LB4b, LB4c by specular reflection, i.e. such that the angle of incidence is equal to the angle of reflection. Adjusting the beam steering element M1 may move the positions of substantially all output beams LB5a, LB5b, LB5c, LB5a', LB5b', LB5c' with respect to the aperture stop AP2. The beam steering element M1 may be positioned e.g. between the first dispersive element G1 and the second telecentric system SYS2. The beam steering element M1 may provide a bundle of steered beams LB4a, LB4b, LB4c from light of the deflected light beams LB3a, LB3b, LB3c. The steered beams may be coupled into the second telecentric system SYS2. The position of the beam steering element M1 may be adjustable in order to select the direction of the steered beams with respect to the axis AX2 of the second telecentric system SYS2. The beam steering element M1 may be e.g. a mirror. The central wavelength of the selected spectral band may be adjusted e.g. by turning the mirror M1 . For example, the mirror may be rotated about a pivot axis, which is perpendicular to the optical axis AX1 of the first telecentric system SYS1 and perpendicular to the optical axis AX2 of the second telecentric system SYS2. The position of the beam steering element M1 may be specified e.g. by an angle a. The angle a may denote e.g. the angle between the reflective plane of the mirror M1 and the optical axis AX1 . The central wavelength λο of the spectral band may be changed by changing the orientation anglea. For example, the orientation angle a may be set to value ai to set the central wavelength λο to a value λι . For example, the orientation angle a may be changed to value 012 in order to change the central wavelength λο to a value λ₂ (See Fig. 3a).

The beam steering element M1 , in particular a mirror M1 may fold the optical path of the imaging device 500. Using the folded optical path may reduce one or more outer dimensions of the device 500.

The position and/or orientation of the beam steering element M1 may be changed e.g. by using an actuator ACT1 . The actuator ACT1 may comprise e.g. one or more stepper motors to move the beam steering element M1 . The position of the pivot axis of the mirror M1 may be selected e.g. such that movement of the image IMG2 is minimized during turning of the mirror M1 .

The angular orientation a of the mirror M1 may also be adjusted so that the position of the pivot axis is not fixed. The device 500 may optionally comprise e.g. a (curved) rail, guide or groove for moving the mirror M1 along an optimized path. The device 500 may optionally comprise e.g. a lever mechanism for moving the mirror M1 along an optimized path. The device 500 may optionally comprise two or more actuators for moving the mirror M1 along an optimized path. The beam steering element M1 may provide steered beams LB4 e.g. by reflecting or refracting light of the deflected beams LB3. The central wavelength λο of a spectral band of input light transmitted to the image sensor SEN1 may be selected by changing the direction of the steered beams with respect to the optical axis AX2 of the second telecentric system SYS2.

The central wavelength λο of a spectral band of input light transmitted to the image sensor SEN1 may also be selected by adjusting the direction of the deflected beams LB3 with respect to the optical axis AX2 of the second telecentric system SYS2. The device 500 may comprise e.g. an actuator unit to mechanically turn the second telecentric system SYS2 with respect to the first telecentric system SYS1 (see Figs. 10a, 10b). The device 500 may comprise e.g. an actuator unit ACT1 to mechanically move the aperture stop AP2 of the second telecentric system SYS2.

The grating G1 may be blazed grating to increase spectral transmittance at the selected spectral band and/or in order to provide more effective blocking of spectral components which are outside the selected spectral band.

The first dispersive element G1 may be a blazed transmissive diffraction grating. The first dispersive element G1 may be a reflective diffraction grating (see Fig. 1 1 ).The first dispersive element G1 may be a blazed reflective diffraction grating.

The imaging device 500 may comprise an image sensor SEN1 to capture the second optical image IMG2. The imaging device 500 may comprise focusing optics LNS31 to form a second optical image IMG2 on the image sensor SEB1 by focusing light of the output beams LB5a, LB5b, LB5c. The focusing optics LNS31 may provide focused beams LB6a, LB6b, LB6c by focusing light of the output beams LB5a, LB5b, LB5c. Using the focusing optics LNS31 may provide a brighter and/or sharper image IMG2 on the image sensor SEN1 .

In an embodiment, the focusing lens LNS31 may be omitted. The converging lens LNS22 of the second telecentric system SYS2 may be arranged to provide focused output beams LB5a, LB5b, LB5c which may impinge on the image sensor SEN1 .

The imaging device 500 may comprise the image sensor SEN1 to capture an image. The image sensor SEN1 and/or the focusing lens LNS31 may be parts of the imaging device 500.

The imaging device 500 may have a maximum horizontal angular field of view ΔΘΜΑΧ- The maximum field of view ΔΘΜΑΧ may be e.g. greater than 5°, greater than 10°, or even greater than 12°. The maximum field of view ΔΘΜΑΧ may be e.g. substantially equal to 12°. The maximum field of view ΔΘΜΑΧ may be e.g. substantially equal to 14°. The maximum field of view ΔΘΜΑΧ may be e.g. in the range of 10° to 15°. Using the telecentric systems SYS1 , SYS2 may provide a relatively wide angular field of view ΔΘΜΑΧ-

The imaging device 500 may comprise:

- a first telecentric system SYS1 to form a first image IMG1 of an object OBJ1 ,

- a second telecentric system SYS2 to provide a bundle of output beams LB5a, LB5b, LB5c for forming a second image IMG2 such that the output beams LB5a, LB5b, LB5c are formed from light received from the first image IMG1 , and

- a first dispersive element G1 positioned between the first telecentric system SYS1 and the second telecentric system SYS2 to provide a bundle of deflected light beams LB3a, LB3b, LB3c such that the direction of the deflected light beams LB3a, LB3b, LB3c depends on the wavelength λ, wherein the second telecentric system SYS2 is arranged to form the output beams LB5a, LB5b, LB5c from light of the deflected light beams LB3a, LB3b, LB3c, and the second telecentric system SYS2 comprises an aperture stop AP2 to reject light which is outside a selected spectral range PB1 . The imaging device 500 may comprise:

- a first telecentric system SYS1 to form a first image IMG1 of an object OBJ1 substantially at the location of a first dispersive element G1 ,

- the first dispersive element G1 to provide a bundle of deflected light beams LB3a, LB3b, LB3c by diffracting light of the first image IMG1 such that the direction of the deflected light beams LB3a, LB3b, LB3c on the wavelength λ of said deflected light beams LB3a, LB3b, LB3c,

- a beam steering element M1 to provide a provide a bundle of steered beams LB4a, LB4b, LB4c by steering light of the deflected light beams

LB3a, LB3b, LB3c, wherein the steered beams LB4a, LB4b, LB4c are coupled into a second telecentric system SYS2, and

- the second telecentric system SYS2 to provide a bundle of output beams LB5a, LB5b, LB5c for forming a second image IMG2 such that the output beams LB5a, LB5b, LB5c are formed from the light of the steered beams LB4a, LB4b, LB4c.

The device 500 may comprise an adjustable mirror M1 to provide a bundle of reflected beams LB4a, LB4b, LB4c by reflecting light of diffracted light beams LB3a, LB3b, LB3c, wherein the reflected beams LB4a, LB4b, LB4c are coupled into the second telecentric system SYS2, and wherein the second telecentric system SYS2 is arranged to provide the output beams LB5a, LB5b, LB5c from the light of the diffracted beams LB4a, LB4b, LB4c. The imaging device 500 may further comprise focusing optics LNS31 to focus light of the output beams LB5a on the image sensor SEN1 . The focusing optics LNS31 may provide focused beams LB6a, LB6b, LB6c by focusing light of the output beams LB5a, LB5b, LB5c. The second telecentric system SYS2 and the focusing optics LNS31 may together form the second image IMG2 on the image sensor SENLThe second image IMG2 may be an image of the auxiliary image IMG1 .

Using the focusing optics LNS31 is not necessary. The second telecentric system SYS2 may form the second image IMG2 on the image sensor SEN1 . The imaging device 500 may further comprise the image sensor SEN1

Fig. 2 shows, in a three-dimensional view, the imaging device 500. The aperture of the aperture stop AP1 of the first telecentric system SYS1 may have a width w_Api and a height h_Api . The aperture of the aperture stop AP2 of the second telecentric system SYS2 may have a width w_Ap2 and a height h_AP2- Referring to Fig. 3a, the beam steering element M1 may be moved to a different position in order to change the wavelength of the output beams transmitted through the aperture stop AP2 of the second telecentric system SYS2. For example, the orientation of the beam steering element M1 may be selected such that light of the beams LB5a, LB5b, LB5c having the wavelength λι may be blocked by the aperture stop AP2, and light of the beams LB5a', LB5b', LB5c' having the wavelength λ₂ may pass through the aperture stop AP2. The orientation angle a of the beam steering element M1 may be set to 012 in order to set the central wavelength λο to the value λ₂. Referring to Fig. 3b, the spectral position λο of the passband PB1 may be adjusted by changing the input angle φ_!Ν of light beams LB4a, LB4b, LB4c coupled to the second telecentric system SYS2. The input angle φ_!Ν of a light beam means the angle between the centerline of said light beam and the axis AX2 of the second telecentric system SYS2. The imaging device 500 may comprise a beam steering unit BSU1 . The beam steering unit BSU1 may be arranged to change the spectral position λο of the passband PB1 by changing the input angle φ_!Ν of light beams LB4a, LB4b, LB4c impinging on the converging lens (LNS21 or LNS22) of the second telecentric system SYS2. The light beams LB4a, LB4b, LB4c may have a predetermined wavelength λι . The beam steering unit BSU1 may comprise e.g. the beam steering element M1 and an actuator ACT1 . The actuator ACT1 may be arranged to turn the beam steering element M1 . The beam steering element M1 may be e.g. a mirror. In an embodiment, the beam steering unit BSU1 may be arranged to turn the second telecentric system SYS1 with respect to the first telecentric system SYS1 (see Figs. 10a, 10b). Referring to Fig. 4, the spectral transmittance Τ(λ) of the imaging device 500 may depend on the wavelength λ such that the imaging device 500 has a spectral passband PB1 . The passband PB1 may have an upper limit λυ and a lower limit λι_. The spectral transmittance Τ(λ) of the imaging device 500 may have a maximum value T_MAX- The upper limit λυ and the lower limit λι_ may refer to spectral positions where the spectral transmittance Τ(λ) is equal to 50% of the maximum value T_MAX- The passband PB1 may have a spectral width A R/VHM- FWHM means "full width at half maximum". The spectral width A R/VHM is equal to the difference λυ-λι_. λο denotes the center of the passband PB1 . λο may denote the spectral position of the passband PB1 . The center λο may be equal to the average ((λυ+λι_)/2) of the limits λυ, λι_. The spectral position of the center λο may be changed e.g. by changing the orientation of the beam steering element M1 . The spectral width A R/VHM may depend on the width w_Api of the aperture stop AP1 and/or on the width w_Ap2 of the aperture stop AP2. The spectral width A R/VHM may be changed by changing the width w_Api of the aperture stop AP1 and/or on the width w_Ap2 of the aperture stop AP2. The spectral width A R/VHM may remain substantially constant during scanning the spectral position of the passband PB1 . The spectral width A R/VHM may be e.g. smaller than 50 nm, smaller than 40 nm, smaller than 30 nm, smaller than 20 nm, or even smaller than 10 nm.

Depending on the widths w_Api , w_Ap2, the shape of the passband PB1 may be e.g. substantially triangular, substantially trapezoidal, or substantially rectangular.

The maximum spectral transmittance T_MAX of the device 500 may be e.g. higher than or equal to 10%, or even higher than or equal to 50%. λ_ΜΐΝ and MAX may denote the limits of the spectral tuning range RNG1 of the device 500. The spectral position λο of the passband PB1 may be adjusted within the tuning range from λ_ΜΐΝ to MAX- The spectral position λο of the passband PB1 may be adjusted within the tuning range from λ_ΜΐΝ to λ_ΜΑχ so that the limits λυ, λι_ remain within said tuning range. The device 500 may optionally comprise one or more optical cut-off filters to define the limit λ_ΜΐΝ and/or the limit λ_ΜΑχ- A cut-off filter may comprise e.g. a colored glass filter and/or an interference filter. The components and the materials of the device 500 may be selected such that the tuning range RNG1 of the device 500 may overlap e.g. the ultraviolet region, the visible region and/or the infrared region. The limit λ_ΜΐΝ may be e.g. longer than or equal to 250 nm. The limit λ_ΜΑχ may be e.g. shorter than or equal to 30 μηη. In particular, using a reflective diffraction grating G1 may provide substantial freedom to implement a desired tuning range RNG1 .

The tuning range RNG1 of the device 500 may cover e.g. the range from 400 nm to 700 nm. The lower limit λ_ΜΐΝ of the tuning range may be e.g. substantially equal to 400 nm, and the upper limit λ_ΜΑχ of the tuning range may be substantially equal to 700 nm.

For example, the following spectral widths A R/VHM may be provided by tuning the same imaging device 500 and by using same image sensor SEN1 :

A FWHM = 21 nm when λο = 650 nm (red color),

A FWHM = 20.5 nm when λο = 550 nm (green color),

A FWHM = 20 nm when λο = 550 nm (blue color),

Referring to the second curve from the top of Fig. 4, an input beam (e.g. the beam LB1 a) received from the object OBJ1 may have a spectral intensity distribution Ιι(λ). For example, the spectral intensity distribution Ιι(λ) ("spectrum") may have a value Ιι(λι) at the wavelength λι , and spectral intensity distribution Ιι(λ) ("spectrum") may have a value Ιι(λ₂) at the wavelength λ₂. The spectrum Ιι(λ) may represent the spectral intensity of light LB1 a received from a certain point Pa of the object OBJ1 .

The imaging device 500 may operate as a tunable filter, which allows transmission of spectral components of the spectrum Ι ι(λ) in the selected spectral range, and which prevents transmission of spectral components of the spectrum Ιι(λ) which are outside the selected spectral range.

Referring to the lowermost curve of Fig. 4, only the selected spectral band SB1 of the light received from the object OBJ1 may be transmitted to the image sensor SEN1 . The selected spectral band SB1 may be obtained by multiplying the spectral intensity distribution Ιι(λ) with the spectral transmittance Τ(λ) of the imaging device 500. For example, the center of the passband PB1 may be tuned to the spectral position λι to measure the value Ιι(λι) at the wavelength λι . For example, the center of the passband PB1 may be tuned to the spectral position λ₂ to measure the value Ιι(λ₂) at the wavelength λ₂. The image sensor SEN 1 may provide a signal value S₂, which is proportional to the integral over the selected spectral band SB1 . The image sensor SEN1 may comprise a plurality of detector pixels DET1 . A detector pixel DET1 may provide a signal value S₂( i ,u,v) which is proportional to the integral over the selected spectral band SB1 of light (e.g. the beam LB6a) impinging on said detector pixel DET1 . The signal value S₂( i ,u,v) of each detector pixel DET1 may be optionally converted into a calibrated intensity value X( i ,u,v) by using one or more calibration parameters stored in a memory. The imaging device 500 may be arranged to measure one or more intensity values Χ(λι ,υ,ν).

Referring to Fig. 5, the image sensor SEN1 may comprise a plurality of detector pixels DET1 arranged in a two-dimensional array. The active area of the image sensor SEN 1 may be in a plane defined by directions SU and SV. A position on the image sensor SEN 1 may be specified by coordinates u,v. The image sensor SEN 1 may comprise a detector pixel DET1 , which is located at a position u,v.

All detector pixels DET1 of the image sensor SEN1 may be exposed substantially simultaneously. The signals S₂ obtained from the detector pixels DET1 may represent substantially the same exposure period, and the signals S2 may also be formed according to the same passband PB1 .

The optical image IMG2 may simultaneously cover a plurality of detector pixels of the sensor SEN1 in order to analyze spatial variations of optical spectrum at different points of the two-dimensional image IMG2.

The signal values S2 obtained from the detector pixels of the image sensor SEN1 may together form a digital image, which represents the selected spectral band PB1 , SB1 . The intensity values X determined from the signal values S2 may together from a digital image, which represents the selected spectral band PB1 , SB1 .

The exposure time of the image sensor SEN1 when capturing a single image IMG2 may be e.g. in the range of 10^"6 s to 10^"3 s. The exposure time may be e.g. in the range of 10^"3 s to 1 s. The exposure time may be e.g. in the range of 10^"6 s to 1 s. The optical image IMG2 may be captured by the pixels DET1 such that the pixels DET1 are exposed substantially simultaneously to the light of the image IMG2. The digital image (i.e. the image frame) captured by the image sensor SEN1 may be an instantaneous image. The digital image provided by the image sensor SEN1 may represent the selected spectral band PB1 of the optical image IMG2, which was formed on the image sensor SEN1 between the start of the exposure time and the end of the exposure time. The exposure of substantially all pixels of the image sensor SEN1 may start substantially simultaneously and the exposure of substantially all pixels may stop substantially simultaneously. Simultaneous exposure of the pixels may be used e.g. when capturing an image in a transient or rapidly changing situation. The number of detector pixels DET1 of the image sensor SEN1 may be e.g. higher than or equal to 10³, higher than or equal to 10⁴, higher than or equal to 10⁵, higher than or equal to 10⁶, or even higher than or equal to 10⁷. The image sensor SEN1 may be e.g. a CCD sensor or a CMOS sensor. CCD means charge coupled device. CMOS means Complementary Metal Oxide Semiconductor. The image sensor SEN1 may be selected such that it may detect light over the desired tuning range RNG1 . The image sensor SEN1 may be selected such that it can detect e.g. UV-light, visible light and/or IR-light.

The image sensor SEN1 may be optionally equipped with an auxiliary filter, e.g. with an RGB Bayer filter. However, using the image sensor SEN1 without the Bayer filter may provide improved spatial resolution.

In an embodiment, the exposure of the detector pixels may also be controlled by using a rolling shutter. The pixels of the image sensor SEN1 may be exposed by using the rolling shutter such that the exposure of a second row or column of pixels may start later than the exposure of a first row or column of pixels, and the exposure of the second row or column of pixels may stop later than the exposure of the first row or column of pixels. Even when using the rolling shutter, the image may be captured without moving the beam steering element M1 . The beam steering element M1 does not need to be adjusted during the time period between the start of the exposure of first detector pixel and the end of exposure of the last detector pixel.

Fig. 6 shows several dimensions related to the operation of the imaging device 500. L0 may denote the distance between the object and the input aperture stop AP1 . L1 may denote the distance between the aperture stop AP1 and the input lens LNS1 1 . L2 may denote the distance between the input lens LNS1 1 and the auxiliary image IMG1 . L5 may denote the distance between the lens LNS22 and the aperture stop AP2. L6 may denote a distance between the focusing lens LNS31 and the image sensor SEN1 .

The object OBJ1 may have a width w₀. The aperture of the aperture stop AP1 may have a width w_Api . The aperture of the input lens LNS1 1 may have a width wi_i i . The auxiliary image IMG1 may have a width wi . The dispersive element G1 may have a width w_Gi . The aperture of the converging lens LNS22 may have a width w_L22. The aperture of the aperture stop AP2 may have a width w_AP2. The second image IMG2 formed on the image sensor SEN1 may have a width w₂. The active area of the image sensor SEN1 may have a width WSENI -

The distance L1 may be substantially equal to the focal length fi of the first telecentric system SYS1 . The distance L2 may be substantially equal to the focal length fi of the first telecentric system SYS1 . The distance L5 may be e.g. in the range of 40% to 60% of the focal length f₂ of the second telecentric system SYS2. in particular, the distance L5 may be substantially equal to 50% of the focal length f₂ of the second telecentric system SYS2.

The distance L0 between the object and the input aperture stop AP1 may be e.g. longer than 10 times the distance L1 . In an embodiment, the distance L0 between the object and the input aperture stop AP1 may be e.g. longer than 100 times the distance L1 . The object OBJ1 may be at infinity.

The imaging device 500 may provide a horizontal image magnification, which is equal to the ratio w₂/w₀.

The ratio w_Ln/w_Api may be e.g. greater than 10, in order to provide a relatively narrow passband PB1 and/or a sharp image IMG2. The ratio WLI I/WAPI may be even greater than 20. The ratio w_L22/w_Ap₂ may be e.g. greater than 10, in order to provide a relatively narrow passband PB1 and/or a sharp image IMG2. The ratio w_L22/w_Ap₂ may be greater than 20, or even greater than 40.

The spatial resolution of the image IMG2 formed in the image sensor SEN1 may be substantially diffraction-limited. In particular, the spatial resolution of the image IMG2 may be determined by the width w_Api of the aperture of the first aperture stop AP1 . The width w_APi may be greater than the width w_AP2 in order to provide high spatial resolution and narrow passband PB1 . The width w_Api of the aperture of the aperture stop AP1 may be e.g. in the range of 0.2 mm to 2 mm. The width w_Api of the aperture of the aperture stop AP1 may be e.g. in the range of 0.5 mm to 1 .5 mm. The width w_Api of the aperture of the aperture stop AP1 may be e.g. substantially equal to 1 mm.

The width w_Ap2 of the aperture of the aperture stop AP2 may be e.g. in the range of 0.1 mm to 2 mm. The width w_APi of the aperture of the aperture stop AP1 may be e.g. in the range of 0.2 mm to 1 .0 mm. The width w_APi of the aperture of the aperture stop AP1 may be e.g. substantially equal to 0.5 mm.

The distance L1 may be e.g. in the range of 50 mm to 200 mm.

The focal length fi of the first telecentric system SYS1 may be e.g. in the range of 50 mm to 200 mm. The focal length fi of the first telecentric system SYS1 may be e.g. substantially equal to 100 mm.

The width w₂ of the image IMG2 formed on the image sensor SEN1 may depend on the focal length of the focusing optics LNS31 . The focal length of the focusing optics LNS31 may be e.g. in the range of 2 mm to 10 mm. The focal length of the focusing optics LNS31 may be e.g. substantially equal to 3.5 mm. The width w₂ of the image IMG2 formed on the image sensor SEN1 may be e.g. substantially equal to 1 .5 mm.

The width w_Ln of the lens LNS1 1 may be e.g. in the range of 10 mm to 50 mm. The width w_Ln of the lens LNS1 1 may be e.g. substantially equal to 25 mm.

The width w_L22 of the converging lens LNS22 may be e.g. in the range of 10 mm to 50 mm. The width w_L22 of the converging lens LNS22 may be e.g. substantially equal to 25 mm.

The focal length f₂ of the second telecentric system SYS2 may be e.g. in the range of 25 mm to 100 mm. The focal length f₂ of the first telecentric system SYS1 may be e.g. substantially equal to 50 mm. The grating constant of the diffraction grating G1 may be e.g. in the range of 600 to 1200 line pairs per mm. The grating constant of the diffraction grating G1 may be e.g. 830 line pairs per mm. The optical axis AX1 of the first telecentric system SYS1 may be defined by the center of the aperture of the aperture stop AP1 and the center CP1 1 of the input lens LNS1 1 . The optical axis AX2 of the second telecentric system SYS2 may be defined by the center of the aperture of the aperture stop AP2 and the center CP22 of the converging lens LNS22.

The width w_Api may be e.g. substantially equal to 1 .0 mm, the width w_Ap2 may be e.g. substantially equal to 1 .5 mm, the height h_Api may be e.g. substantially equal to 2 mm, and the height h_Ap2 may be e.g. in the range of 0.5 mm.

Referring to Fig. 7, the imaging device 500 may comprise an actuator unit ACT1 to change the spectral position λο of the passband PB1 of the imaging device 500. The imaging device 500 may comprise a control unit CNT1 . The control unit CNT1 may be arranged to provide a control signal SMI for controlling the actuator unit ACT1 . The relationship between a selected spectral position and a control signal value SMI may be determined by using one or more spectral calibration parameters. The control unit CNT1 may be configured to provide the control signal SMI to the actuator unit ACT1 based by using the one or more spectral calibration parameters. The imaging device 500 may optionally comprise a memory MEM2 for storing one or more spectral calibration parameters. The actuator unit ACT1 may comprise e.g. a stepper motor. The actuator unit ACT1 may comprise e.g. a stepper motor driver configured to drive the stepper motor according to the control signal SMI - The stepper motor driver may also be integrated in the control unit CNT1 .

The imaging device 500 may comprise a communication unit RXTX1 to receive commands and/or to signal values. COM1 denotes a communication signal. The communication unit RXTX1 may be arranged to communicate e.g. via an electric cable, via an optical cable, and/or in a wireless manner.

The image sensor SEN1 may provide a sensor signal S2. The imaging device 500 may be arranged to obtain the sensor signal S2 form the image sensor SEN1 , and to provide calibrated intensity values Χ(λ) from the sensor signal S2 by using one or more intensity calibration parameters stored in a memory.

The imaging device 500 may optionally comprise one or more data processors for determining calibrated intensity values values Χ(λ) from the sensor signals S2 by using one or more intensity calibration parameters stored in a memory.

The imaging device 500 may optionally comprise a memory MEM1 for storing sensor signal values S2 and/or calibrated intensity values Χ(λ).

The imaging device 500 may optionally comprise a user interface UIF1 to receive user input from a user. For example, the user may select the spectral position λο of the passband PB1 by using the user interface UIF1 . The user interface UIF1 may comprise e.g. a touch screen. The user interface UIF1 may comprise e.g. one or more keys.

The imaging device 500 may comprise a memory MEM3 for storing computer program code PROG1 . The computer program PROG1 may comprise computer program code configured to, when executed on at least one processor, cause an apparatus or the device 500 to tune the spectral position λο specified by user input received via the user input UIFI or specified by a command received via the communication unit RXTX1 . The processing of the measured sensor signals S2 may also take place outside the imaging device 500, e.g. in a portable computer of a user.

The spectral scale of the imaging device 500, i.e. the relationship between the control signal SMI and the spectral position λο may be calibrated e.g. by using light obtained from a laser or from a spectral calibration lamp. The spectral scale of the device 500 may be determined by calibration measurements, e.g. by using the excitation spectrum of a gas discharge lamp. The gas discharge lamp may contain e.g. argon, neon, xenon, krypton, hydrogen, or mercury.

Referring to Fig. 8, the imaging device 500 may comprise a first dispersive element G1 and a second dispersive element G2. The imaging device 500 may comprise e.g. a first grating G1 and a second grating G2. The first dispersive element G1 may cause an anamorphic effect, i.e. that the image magnification of the device in the horizontal direction may be different from the image magnification of the device in the vertical direction. The device 500 may further comprise a second (auxiliary) dispersing element G2 to improve image quality. The anamorphic effect may be at least partly compensated e.g. by using a second dispersive element G2. The second dispersive element G2 may be arranged to at least partly compensate the anamorphic effect. The second dispersive element G2 may be arranged to at least partly reduce the difference between vertical image magnification and horizontal image magnification of the device 500. For example, the second dispersive element G2 be arranged to operate at the diffraction order -1 so that the second telecentric system and an optional focusing unit may form a compensated image to the image sensor SEN1 .

When using the first dispersive element G1 and the second dispersive element G2, the auxiliary image IMG1 may be formed to a location, which is optically between the dispersive elements G1 and G2. In particular, the first telecentric system SYS1 may be arranged to form the auxiliary image IMG1 to a position, which is substantially at the mid-way between the dispersive elements G1 and G2.

The beam steering element M1 may provide a bundle of steered beams LB4a, LB4b, LB4c by reflecting light of the deflected beams provided by the first dispersive element G1 . The second dispersive element G2 may be arranged to provide a second bundle of deflected beams by deflecting light of the steered beams LB4a, LB4b, LB4c such that said second bundle of deflected beams is coupled to the second telecentric system SYS2.

In general, the compensation of the different image magnifications may be performed e.g. by using one or more optical elements which treat the horizontal direction in a different manner than the vertical direction. The compensation of the different image magnifications may be performed e.g. by using one or more cylindrical lenses. The gratings G1 , G2 may be blazed grating to increase spectral transmittance at the selected spectral band and/or in order to provide more effective blocking of spectral components which are outside the selected spectral band. The dispersive element G1 , G2 may be a prism. The dispersive element G1 , G2 may comprise a prism. The dispersive element G1 , G2 may comprise a prism-grating-prism unit.

Referring to Fig. 9, the first dispersive element G1 may be implemented by using a prism grating prism unit. The first dispersive element G1 may comprise prisms PR1 1 , PR12 and a diffraction grating GR1 . The second dispersive element G2 may be implemented by using a prism grating prism unit. The second dispersive element G2 may comprise prisms PR21 , PR22 and a diffraction grating GR2.

Referring to Fig. 10a and 10b, the central wavelength λο of the spectral band of input light transmitted to the image sensor SEN1 may be selected by changing the orientation of the second telecentric system SYS2 with respect to the first telecentric system SYS1 . The second telecentric system SYS2 may be supported such that the orientation of the second telecentric system SYS2 may be changed with respect to the first telecentric system SYS1 . The device 500 may comprise a beam steering unit BSU1 to change the input angle φ_!Ν of the light beams coupled to the second telecentric system SYS2 according to the desired central wavelength _c. The beam steering unit BSU1 may change the input angle φ_!Ν by changing the orientation of the axis AX2 of the second telecentric system SYS2 with respect to the direction of the deflected light beams LB3a, LB3b, LB3c. The beam steering unit BSU1 may change the input angle φ_!Ν by changing the orientation of the axis AX2 of the second telecentric system SYS2 with respect to the dispersive element G1 . The beam steering unit BSU1 may comprise a pivot mechanism. The beam steering unit BSU1 may comprise an actuator ACT1 to turn the second telecentric system SYS2. The beam steering unit BSU1 may comprise a frame FRAME2 to support the second telecentric system SYS2. The actuator ACT1 may be arranged to turn the frame FRAME2 with respect to the first telecentric system SYS1 .

Fig. 10a shows the imaging device 500 when the axis of the second telecentric system SYS2 has a first orientation angle βι with respect to the axis of the first telecentric system SYS1 . The input angle φ_!Ν of the light beams LB3a, LB3b, LB3c having the wavelength λι may be substantially equal to zero, and light of the beams LB3a, LB3b, LB3c may subsequently pass through the aperture stop AP2. The first spectral component having a first wavelength λι may pass through the aperture stop AP2 to the image sensor SEN1 , wherein the aperture stop AP2 may block the second spectral component having a second wavelength λ₂.

Fig. 10b shows the imaging device 500 of Fig. 10a when the axis of the second telecentric system SYS2 has a second different orientation angle β₂ with respect to the axis of the first telecentric system SYS1 . In that case the input angle φ_!Ν of the light beams LB3a, LB3b, LB3c having the wavelength λι may be substantially deviate from zero. The first spectral component having a first wavelength λι may be blocked by the aperture stop AP2, wherein the second spectral component having the second wavelength λ₂ may pass through the aperture stop AP2 to the image sensor SEN1 . The orientation angle β of the second telecentric system SYS2 may be set to βι in order to set the central wavelength λο to a value λι . The orientation angle β of the second telecentric system SYS2 may be set to β₂ in order to set the central wavelength λο to a value λ₂. In an embodiment, the lens LNS31 and the image sensor SEN1 may also be replaced with an eye of the user. The image IMG2 may be formed on the retina of the user. Referring to Fig. 1 1 , the grating G1 of the imaging device 500 may also be a reflective diffraction grating. The grating G1 may be a blazed reflective diffraction grating in order to maximize spectral transmittance at the passband. The central wavelength λο of the spectral band of input light transmitted to the image sensor SEN1 may be selected by changing the orientation of the beam steering element M1 . The beam steering element M1 may be e.g. a mirror. The grating G1 may provide a bundle of deflected light beams. The beam steering element M1 may provide a bundle of steered beams by reflecting light of the deflected light beams. The position a of the beam steering element M1 may be adjustable in order to select the direction of the steered beams with respect to the axis of the second telecentric system SYS2.

Using the reflective diffraction grating G1 may provide considerable freedom to select the tuning range RNG1 . Using the reflective diffraction grating may enable operation in the UV region, in the visible region, and/or in the IR region. The tuning range may be a portion of the range 250 nm to 30 μηη.

The grating constant of the grating G1 may be e.g. 600 grooves/mm, and the tuning range may be e.g. from 900 nm to 1600 nm.

Coupling light from the grating G1 to the beam steering element M1 may allow using a simple and/or stable mechanical construction, as the grating G1 does not need to be moved with respect to the first telecentric system SYS1 when the position of the beam steering element M1 is changed. However, the grating G1 and the beam steering element M1 may also be arranged in a different order. The beam steering element M1 may provide a bundle of steered beams by reflecting light received from the first telecentric system SYS1 , the grating G1 may provide a bundle of deflected light beams by deflecting light of the steered beams, and the second telecentric system SYS2 may receive the deflected light beams. In this case, adjusting the beam steering element M1 may comprise adjusting the position of the grating G1 with respect to the first telecentric system SYS1 , in order to keep the image IMG1 sharply focused on the grating G1 . The beam steering element M1 may be a mirror M1 arranged to provide a bundle of steered beams, and the first dispersive element G1 may be arranged to provide the deflected light beams by diffracting light of the steered beams, wherein the angular position a of the mirror M1 may be adjustable in order to select the direction of the deflected beams with respect to the axis AX2 of the second telecentric system SYS2.

Referring to Fig. 12, the imaging device 500 does not need to comprise the focusing lens LNS31 and/or the image sensor SEN1 . The focusing lens LNS31 and/or the image sensor SEN1 may be parts of an external camera CAM1 . The imaging device 500 may be used as an attachment or add-on imaging spectrograph, which may positioned in front of the camera CAM1 or in front of an eye of a user. The imaging device 500 may be attached to a camera CAM1 , wherein the image IMG2 may be captured by the camera CAM1 .

Output light from the imaging device may be coupled to the objective LNS31 of the camera CAM1 so that the objective LNS31 of the camera CAM1 may form the optical image IMG2 on the image sensor SEN1 of the camera CAM1 .

For example, a mobile phone may comprise the camera CAM1 , wherein the output light provided by the imaging device 500 may be coupled to the camera CAM1 of the mobile phone. The output light from the imaging device 500 may also be coupled to the eye of a user such that the optical image is formed on the retina of the user. The imaging device 500 may be arranged to operate as virtual display, which forms an optical image on the on the retina of the user. The second image IMG2 may be formed on the image sensor SEN1 or on the retina of an eye of a user. The lens LNS31 and the image sensor SEN1 may be replaced with the eye of the user.

The method may comprise:

- forming the first image IMG1 of the object OBJ1 by using the first telecentric system SYS1 to form a first image IMG1 of the object OBJ1 ,

- forming a bundle of output beams LB5a, LB5b from light received from the first image IMG1 by using a second telecentric system SYS2, and

- forming the second image IMG2 from light of the output beams LB5a, LB5b, wherein the first dispersive element G1 is positioned between the first telecentric system SYS1 and the second telecentric system SYS2 to provide a bundle of deflected light beams LB3a, LB3b such that the direction of the deflected light beams LB3a,LB3b depends on the wavelength λ, and wherein the second telecentric system SYS2 provides said output beams LB5a,LB5b from light of the deflected light beams LB3a,LB3b.

The image sensor SEN1 may provide a digital image may capturing the second optical image IMG2. An anamorphic effect caused by the first dispersive element G1 may also be compensated by numerical image processing of the captured digital image. A data processor may be configured to provide a compensated digital image from a digital image obtained from the image sensor SEN1 .

The aperture stops AP1 , AP2 may also be called e.g. as slits.

The first dispersive element (G1 ) may be a transmissive diffraction grating.

The first dispersive element (G1 ) may be a reflective diffraction grating (see Fig. 1 1 ).

The first dispersive element (G1 ) may comprise a prism.

The object OBJ1 may be a real or virtual object. For example, the object OBJ1 may be a tangible piece of material. The object OBJ1 may be a real object. The object OBJ1 may be e.g. in solid, liquid, or gaseous form. The object OBJ1 may comprise a sample. The object OBJ1 may a combination of a cuvette and a chemical substance contained in the cuvette. The object OBJ1 may be e.g. a plant (e.g. tree or a flower), a combustion flame, or an oil spill floating on water. The object may be e.g. the sun or a star observed through a layer of absorbing gas. The object OBJ1 may be a display screen, which emits or reflects light of an image. The object OBJ1 may be an optical image formed by another optical device. The object OBJ1 may also be called as a target. The light LB1 may be provided e.g. directly from a light source, by reflecting light obtained from a light source, by transmitting light obtained from a light source. The light source may comprise e.g. an incandescent lamp, a blackbody radiator, an infrared light emitting glow-bar, a tungsten halogen lamp, a fluorescent lamp, or a light emitting diode.

The object OBJ1 does not need to be a part of the imaging device 500. However, the imaging device 500 may be arranged to operate as a part of a measuring apparatus. The measuring apparatus may comprise e.g. cuvette. The cuvette of the measuring apparatus may be used as the object for the imaging device 500. The device 500 may be used e.g. for remote sensing applications where it may have high transmittance at the wavelength band selected for imaging. The spectrometer 500 may be used e.g. for monitoring spatial variations of color of an object. The spectrometer 500 may be used e.g. for absorption measurement, where the passband PB1 is matched with an absorption band of light obtained from the object. The spectrometer 500 may be used e.g. for a fluorescence measurement, where the passband PB1 is matched with fluorescent light.

The image sensor SEN1 may be sensitive e.g. in the ultraviolet, visible and/or infrared region. The materials of the components of the device 500 may be selected such that the tuning range RNG1 may overlap the ultraviolet, visible and/or infrared region. The combination of the device 500 and the image sensor SEN 1 may be arranged to measure spectral intensities e.g. in the ultraviolet, visible and/or infrared region. The term "light" may refer to electromagnetic radiation in the ultraviolet, visible and/or infrared regime.

SX, SY, and SZ denote orthogonal directions.

The distance L5 between the between the lens LNS22 and the aperture stop AP2 may also be substantially equal the focal length f₂ of the second telecentric system SYS2. For the person skilled in the art, it will be clear that modifications and variations of the devices and the 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.

Claims

1 . An imaging device (500), comprising:

- a first telecentric system (SYS1 ) to form a first image (IMG1 ) of an object (OBJ1 ),

- a first dispersive element (G1 ) positioned between the first telecentric system (SYS1 ) and the second telecentric system (SYS2) to provide a bundle of deflected light beams (LB3a,LB3b) such that the direction of the deflected light beams (LB3a,LB3b) depends on the wavelength (λ), wherein the second telecentric system (SYS2) is arranged to form the output beams (LB5a,LB5b) from light of the deflected light beams (LB3a,LB3b), and the second telecentric system (SYS2) comprises an aperture stop (AP2) to reject light which is outside a selected spectral range (PB1 ).

2. The device (500) of claim 1 wherein the first dispersive element (G1 ) is a diffraction grating.

3. The device (500) of claim 1 or 2 wherein the first telecentric system (SYS1 ) is arranged to form the first image (IMG1 ) on the first dispersive element (G1 ).

4. The device (500) according to any of the claims 1 to 3, wherein the device (500) comprises a beam steering unit (BSU1 ) arranged to adjust the spectral position (λο) of the spectral range (PB1 ) by adjusting the input angle (ΦΙΝ) of light beams (LB4a, LB4b) coupled to the second telecentric system (SYS2).

5. The device (500) according to any of the claims 1 to 4 comprising a beam steering element (M1 ) positioned between the first telecentric system (SYS1 ) and the second telecentric system (SYS2) to adjust the input angle (φ_!Ν) of light beams (LB4a, LB4b) coupled to the second telecentric system (SYS2).

6. The device (500) of claim 5 wherein the beam steering element (M1 ) is a mirror arranged to provide a bundle of reflected beams (LB4a, LB4b) by reflecting light of the deflected light beams (LB3a, LB3b) such that the reflected beams (LB4a, LB4b, LB4c) are coupled into the second telecentric system (SYS2), the second telecentric system (SYS2) is arranged to provide the output beams (LB5a, LB5b, LB5c) from the light of the reflected beams (LB4a, LB4b, LB4c), and the position (a) of the mirror (M1 ) is adjustable in order to select the direction of the reflected beams (LB4a, LB4b) with respect to the axis (AX2) of the second telecentric system (SYS2).

7. The device (500) according to any of the claims 1 to 6, wherein the device (500) comprises an actuator unit (ACT1 ) to change the direction of the axis (AX2) of the second telecentric system (SYS2) with respect to the first dispersive element (G1 ).

8. The device (500) according to any of the claims 1 to 7, wherein the first telecentric system comprises an aperture stop (AP1 ) and a focusing lens (LNS1 1 ), and wherein the distance (L1 ) between the aperture stop (AP1 ) and the focusing lens (LNS1 1 ) is substantially equal to the focal length (fi ) of the first telecentric system (SYS1 ).

9. The device (500) according to any of the claims 1 to 8, wherein the second telecentric system (SYS2) comprises a converging lens (LNS22) and an aperture stop (AP2), the aperture of the converging lens (LNS22) has a first width (wi_22), the aperture stop (AP2) has a second width (w_Ap2), and the ratio (wi_22/w_Ap2) of the first width (w_L22) to the second width (w_Ap2) is greater than 10.

10. The device (500) according to any of the claims 1 to 9, comprising a focusing unit (LNS31 ) to focus light of the output beams (LB5a) to an image sensor (SEN1 )

1 1 . The device (500) according to any of the claims 1 to 10, comprising an image sensor (SEN1 ) to capture the second image (IMG2).

12. The device (500) according to any of the claims 1 to 1 1 , wherein the device comprises a second dispersive element (G2), and the first telecentric system (SYS1 ) is arranged to form the first image (IMG1 ) such that the position of the first image (IMG1 ) is between the first dispersive element (G1 ) and the second dispersive element (G2).

13. The device (500) according to any of the claims 1 to 12, wherein the output beams (LB5a, LB5b) pass through the aperture stop (AP2).

14. The device (500) according to any of the claims 1 to 13, wherein the device (500) comprises an image sensor (SEN1 ) to capture the second image (IMG2), the image sensor (SEN1 ) comprises a plurality of detector pixels (DET1 ) arranged in a two-dimensional array, and wherein the device (500) is arranged to operate such that substantially the whole active area of the image sensor (SEN1 ) is simultaneously exposed to light transmitted through the aperture stop (AP2).

15. The device (500) of claim 14, wherein the device (500) is arranged to operate such that substantially the whole active area of the image sensor (SEN1 ) is simultaneously exposed to light, which represents the same selected passband (PB1 ).

16. The device (500) according to any of the claims 1 to 13, wherein the device (500) comprises an image sensor (SEN1 ) to capture the second image (IMG2), the image sensor (SEN1 ) comprises a plurality of detector pixels (DET1 ) arranged in a two-dimensional array, and the device (500) is arranged to operate such that substantially all detector pixels (DET1 ) of the image sensor (SEN1 ) are exposed simultaneously to light transmitted through the aperture stop (AP2).

17. The device (500) according to any of the claims 1 to 13, wherein the device (500) comprises an image sensor (SEN1 ) to capture the second image (IMG2), the image sensor (SEN1 ) comprises a plurality of detector pixels (DET1 ) arranged in a two-dimensional array, and the device (500) is arranged to operate such that substantially all detector pixels (DET1 ) of the image sensor (SEN1 ) are exposed substantially simultaneously at the selected spectral passband (PB1 ).

18. A method for capturing an image (IMG2), the method comprising:

wherein a first dispersive element (G1 ) positioned between the first telecentric system (SYS1 ) and the second telecentric system (SYS2) provides a bundle of deflected light beams (LB3a,LB5b) such that the direction of the deflected light beams (LB3a) depends on the wavelength (λ), and wherein the second telecentric system (SYS2) provides said output beams (LB5a,LB5b) from light of the deflected light beams (LB3a,LB3b).

19. The method of claim 18, wherein the output beams (LB5a, LB5b) pass through an aperture stop (AP2), and wherein the aperture stop (AP2) rejects light which is outside a selected spectral passband (PB1 ).

20. The method of claim 18 or 19 wherein the image sensor (SEN1 ) comprises a plurality of detector pixels (DET1 ) arranged in a two-dimensional array, and substantially all detector pixels (DET1 ) of the image sensor (SEN1 ) are exposed simultaneously to light transmitted through the aperture stop (AP2).