WO2008124446A1

WO2008124446A1 - Dynamic spectral imaging device with spectral zooming capability

Info

Publication number: WO2008124446A1
Application number: PCT/US2008/059154
Authority: WO
Inventors: Michael R. Wang
Original assignee: University Of Miami
Priority date: 2007-04-06
Filing date: 2008-04-02
Publication date: 2008-10-16

Abstract

The present invention provides a dynamic spectral imaging system with spectral band zooming and selection capability that can adapt to different application requirements and significantly reduce the size of the captured spectral image data cube. The imaging system may employ a diffraction grating to disperse the spectral information of the captured image and may further include a dynamic spatial filter at the Fourier plane to select the spectral channel and spectral band width for each spectral image. With a limited fixed spectral channel number, the system can provide both coarse and fine spectral image viewing and capturing.

Description

DYNAMIC SPECTRAL IMAGING DEVICE WITH SPECTRAL ZOOMING CAPABILITY

FIELD OF THE INVENTION

The present invention relates to spectral imaging, and in particular, to optical signal processing in a dynamic spectral imaging system. BACKGROUND OF THE INVENTION

Spectral imaging is a technique that acquires and analyzes 3-D image data consisting of 2-D spatial image and 1-D spectral information. A spectral image can be acquired by a spectral scanning (λ-scan) of a conventional 2-D spatial image using a filter-wheel or by pushbroom scanning in one spatial axis (y-axis) of a line spectral image (x-λ image). The scanning approach typically involves relatively long acquisition times, which delays the time needed to build the 3-D image data cube. The delays may result in spatial or spectral-signature artifacts appearing in a motion scene, and as such, scanning-based approaches are more suitable for imaging relatively stationary objects. For a fast moving scene, a computed-tomography imaging spectrometer capable of capturing of 3-D spectral image information in a single image snapshot has been reported. However, the main drawback to this approach is the time consuming computing recovery of the image data cube. The above spectral imaging techniques typically use a fixed number of spectral channels to establish a fixed image data cube. To satisfy a variety of application requirements, the spectral band generally must be large while the spectral resolution must be fine. Such parameters result in a large image data cube for capturing, saving, transferring, and processing. However, for a specific application, only a portion of the image data cube may be useful while a majority of the image data may be discarded, especially in the spectral domain. As such, an ideal spectral imaging system should thus offer spectral band selection features that can adjust to a large spectral band for spectral coarse viewing and to a narrow spectral band for spectral fine viewing while capturing a limited number of spectral channels to speed up the spectral imaging system for real-time imaging applications. For example, in a particular application, only spectral image channels of interest may be captured and processed without wasting significant resources on the collection and processing of unnecessary spectral channels. The imaging Fourier Transform ("FT") spectrometer and the dynamic tunable filter spectrometers, such as liquid crystal tunable filter ("LCTF") and acousto-optics tunable filter ("AOTF"), could possibly realize such spectral band zooming and selection functions. However, the FT approach often requires time-consuming angular scanning to acquire data. Moreover, an LCTF approach needs multiple LC stages to achieve a wide spectral band selection range that results in low light efficiency. The drawbacks of the AOTF approach may include a limited optical aperture of the crystal and the resulting small spectral band selection range.

Accordingly, it is desirable to provide a dynamic spectral imaging system having spectral band zooming selection capabilities that can adapt to different application requirements and significantly reduce the size of the captured spectral image data cube.

SUMMARY OF THE INVENTION

The present invention provides a dynamic spectral imaging system with spectral band zooming and selection capabilities that can adapt to different application requirements and significantly reduce the size of the captured spectral image data cube. In particular, the present invention provides a dual-channel spectral imaging system having agile spectral band access and spectral bandwidth tuning capabilities. The dynamic spectral imaging system may employ a diffraction grating to disperse the spectral information of the captured image, and may further include a dynamic spatial filter at the Fourier plane to select the spectral channel and spectral band width for each spectral image. With a limited fixed spectral channel number, the dynamic spectral imaging system can provide both coarse and fine spectral image viewing and capturing. In particular, the present invention provides a spectral imaging system, including a spatial filter operable to selectively adjust the spectral band width for a captured spectral image. A diffraction grating operable to disperse spectral information of the captured spectral image prior to reaching the spatial filter may also be included, where the spatial filter is positioned at a Fourier plane of the system, and includes a spatial light modulator, a scanning slit, and/or an acoustic optic tunable filter. The present invention further provides a spectral imaging system, including a first imaging channel, the first imaging channel including a first spatial filter operable to selectively adjust the spectral band width of a captured spectral image for projection onto a first image plane; and a second imaging channel, the second imaging channel including a second spatial filter operable to selectively adjust the spectral band width of a captured spectral image for projection onto a second image plane. At least one of the first and second spatial filters may include an acoustic optic tunable filter, and/or a spatial light modulator. A diffraction grating operable to disperse spectral information of the captured spectral image to the first and second imaging channels may also be included, and the diffraction grating may include a sinusoidal holographic phase diffraction grating. The first imaging channel may include a first lens and a second lens, the first spatial filter being disposed therebetween, while the second imaging channel may include a third lens and a fourth lens, the second spatial filter being disposed therebetween. The present invention also provides a method for spectral imaging, including capturing an image having spectral information; dispersing the spectral information to a first imaging channel and a second imaging channel; filtering the spectral information along the first imaging channel to a first spectral bandwidth; and filtering the spectral information along the second imaging channel to a second spectral bandwidth different from the first spectral bandwidth. BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein like designations refer to like elements, and wherein:

FIG. 1 is a schematic of an embodiment of a dynamic spectral imaging system constructed in accordance with the principles of the present invention;

FIG. 2 is a schematic of an embodiment of a spectral filtering subsystem constructed in accordance with the principles of the present invention; FIG. 3 is a graphical illustration of spectral filtering examples having centers located at 475nm and 472nm with a different spectral selectivity bandwidth of 9nm and 50nm; FIG. 4 is a graphical illustration of the experimental and theoretical divergent angle dependent spectral resolution at 550nm with a 0.4mm slit width;

FIG. 5 is a graphical illustration of the experimental and theoretical slit width dependent spectral bandwidth at 500nm center wavelength and a 0.57° incident beam divergence angle;

FIG. 6 is a graphical illustration of a fine and coarse spectral selection bandwidth for exemplary images;

FIG. 7 is a graphical illustration of a fine and coarse spectral selection bandwidth for exemplary images at center wavelength of 482 nm at a fixed slit width of 1.4 mm;

FIG. 8 is a graphical illustration of a slit width dependent spectral selection bandwidth; and

FIG. 9 is a graphical illustration of a slit width dependent transmission curve at a center wavelength of 550 nm for the coarse spectral channel. DETAILED DESCRIPTION OF THE INVENTION

The present invention advantageously provides a dual-channel spectral imaging system with a spectral zooming and selection capability that can adapt to different application requirements and significantly reduce the size of the captured spectral image data cube. The spectral imaging system may include a diffraction grating to disperse the spectral information of the captured original image, and may further include a dynamic spatial filter at the Fourier plane to select the spectral channel and spectral band width for each spectral image captured. The diffraction grating may include, for example, a holographic sinusoidal phase grating, blazed diffraction grating, non-holographic surface etched or ruled diffraction grating, and/or a dispersion prism, among others. With a limited fixed spectral channel number, the spectral imaging system can provide coarse spectral image viewing and capturing at a large spectral band setting for rough identification of objects of potential interest. By switching to a narrow spectral band of interest, the spectral imaging system of the present invention may provide desired fine spectral image viewing and capturing for further spectral image evaluation at higher resolutions.

An embodiment of the spectral imaging system of the present invention is illustrated in Figures 1 and 2. For example, a remote scene is imaged on a sinusoidal grating located at (u,v) plane through lens L₀. The grating disperses the image light beam into -1, 0, and +1 diffraction orders where both the -1 and +1 diffraction orders carry rainbow color information that forms a spectral strap on the Fourier plane (ξ ,η) of the lens L₁. The zero order is simply a portion of the image beam without dispersion while both the -1 and +1 diffraction orders are spectrally dispersed. In the present example, a +1 diffraction order was used, but of course, other orders may be used for a particular application and the presently described configuration is intended for illustrative purposes. A dynamic spatial filter such as a spatial light modulator (SLM) can selectively open a narrow slit to allow a particular spectral wavelength λ with bandwidth Δλ to pass through while blocking all other spectral contents of the +1 diffraction order. The passing light forms a sharp image after lens L₂ on a imaging detector array at (x',y') plane. By selectively opening N different slits sequentially on the dynamic spatial filter, N spectral images can be formed on the imaging detector array and acquired to form the desired 3-D spectral image data cube for the current scene.

To describe the operation principle of the spectral imaging system, let the image of the scene captured by imaging lens Lo on the grating be represented by G(u,v). The transmittance function of the sinusoidal holographic phase grating for the +1 diffraction order can be described as (Eq. 1):

T(^vW₁ (^) - expC/ - 2π ^■ /, - «)

Here, /_A is the frequency of the holographic grating and ΠI_A is the peak to peak excursion of the phase delay. At the Fourier plane of the lens L₁, the spectral amplitude is described as (Eq. 2):

E(ξ,n) - mf_ξ - f_Aj_η)

where, H(fξ , f_η) = F{ G(u,v) } indicates the Fourier transform of the image

G(M, v).

From Eq. 2, it can be deduced that a diffracted monochromatic wave by the grating is propagating in a common diffraction direction provided that the incident image beam is collimated. Such a diffracted monochromatic wave of wavelength λ has a common focusing spot at the point (ξ / (λ */) =/_A =/_ξθn the Fourier plane. Different diffracted monochromatic waves form different spectral spots on the Fourier plane. For white light incidence, the dispersion results in a colorful spectral strap on the Fourier plane with different spectral contents spatially separated. By using a dynamic spatial filter such as a spatial light modulator (SLM) or a narrow slit to selectively pass specified wavebands, a desired spectral image can be formed on the imaging detector array CCDl.

The transmission function of the spatial filter at the Fourier plane can be written as (Eq. 3):

S(ξ,_η) = rect(^{ξ ~ λ}°J- ^f* )

It is a rectangular function of width 2D and with its center located at ξ = λo * /

Through the 4-/lens system, the field distribution on the imaging detector array CCDl is given by (Eq. 4):

where C" is a complex amplitude constant.

Despite the spectral selection by the spatial filter at the Fourier plane ("FP"), clear images of different wavebands of the original image at the grating can be formed on the imaging detector array CCDl provided that the spectral selection slit width is not too small to result in diffraction blurring of the image in the direction perpendicular to the slit.

In this architecture, the long focal length lens was used as the system fore capture lens in order to decrease the input light angle. For a collimated light, a very clear spectrum distribution exists at the Fourier plane. Moreover, with a narrower slit, a higher spectral resolution may be achieved. However, when the images are formed by a converging imaging light on the grating rather than a collimated light, there may be some spectral overlap/crosstalk at the Fourier plane depending on the converging angle of the image light. This accordingly results in a limitation of the spectral resolution. In other words, the minimum selected spectral bandwidth cannot be further reduced when the slit width reaches a reasonably small value.

In a particular application, a minimum slit width of approximately 0.4 mm or larger may be employed as (a) reduction of the slit width further may not help the spectral image resolution but may decrease imaging light power capturing at the CCDl and (b) reducing the slit width may cause potential diffraction blurring to the image at the CCDl.

From this analysis, it is apparent that the 4-f based spectral selection is dynamic and suitable for spectral zooming imaging when the imaging beam angle is small. As the imaging beam angle is increased for wider field of view, this spectral selection technique can be used for coarse spectral selection and zooming. In order to achieve fine spectral resolution, an AOTF may be adopted in the fine spectral imaging channel. Of course, other filtering mechanisms may also be used, including, for example, LCTF, tunable Fabry-Perot structures, and/or photonic bandgap tunable filters, just to name a few.

In principle, an acoustic optical tunable filter is a device utilizing acoustic wave generating volume phase grating inside the crystal to diffract the incoming optical wave and thus realizing the acousto-optic spectral filtering function based on Bragg diffraction. By tuning the acoustic wave frequency, the generated grating can selectively diffract a particular spectral band of the incoming image beam to a diffracted beam channel while leaving the remaining image beam propagating un- diffracted. The diffracted image beam and the un-diffracted image beam are separated by a certain angle. An optical lens may then be used to project the diffracted image beam with desired spectral band to the imaging detector array CCD2.

Because of the grating Bragg diffraction effect, the AOTF device will only allow certain wavelength or waveband that meets the Bragg condition be diffracted to the diffracted beam channel for imaging. A theoretical spectral resolution of AOTF has been approximated as Eq. (5):

AZ = ¹ b ^■ L ^■ sin θ, where b is the scattering constant, L is the interaction length between acoustic and optical waves, and O₁ is the incident optical beam propagating angle relative to the optical axis.

The tuning acoustic frequency is given by Eq. (6): n: - K

1 - ( 1 - 112

Λ n] - sinC ø, - θ_a ) where n_e( O₁ ) and n_o are the extraordinary and ordinary refractive indices of the AOTF crystal with each corresponding to the incident and diffracted optical wave, respectively, /is the acoustic frequency, Vis acoustic velocity, and θ_a is the angle of propagation direction of the acoustic wave. The AOTF offers fast tuning, narrow passband, and large acceptance angle.

For example, tuning speeds of 250 μsec may be achieved, the spectral bandwidth can be as small as 0.5 nm with spectral operation range of 400 nm to 650 nm, and incident beam angle can be as large as 4°. The small spectral selection bandwidth makes it suitable for hyperspectral imaging applications As an example, a spectral imaging system prototype was constructed from the above -described architecture. The scene object was a projection slide attached on a ground glass and illuminated using a white light source, where the object distance was approximately 3 meters. The image lens L₀ having a focal length of 300 mm and the sinusoidal holographic phase grating having a grating period of 1000 lp/mm were used. The lenses Li and L₂ included identical focal length of 50 mm, and the spatial filter included a width adjustable slit or a transmission SLM.

To demonstrate the spectral imaging with spectral zooming, the entire visible spectral band from 400 nm to 700 nm was initially covered with 6 spectral images using slit width of 3.1 mm to achieve 50-nm spectral bandwidth for each image. A set of six spectral images each with 50 nm bandwidth was captured (not shown). After zooming into a spectral width of 9 nm using slit width of 0.4 mm, six spectral images were acquired in the reduced spectral band width from 454 nm to 499 nm. Spectral imaging with spectral zooming to different spectral bands and different spectral resolutions was successfully demonstrated. For example, Figure 3 shows two spectral filtering examples with centers located at 475 nm and at 472 nm of the captured images.

To facilitate real time spectral zooming and selection, a spatial light modulator with fast update rate may be used as a dynamic spatial filter. The number N of images in each spectral image set can be small for fast spectral image acquisition but sufficient for desired spectral image evaluation. The dynamic adjusting characteristics of the SLM make it possible to dynamically random select the desired spectral band and bandwidths within the whole spectral operation range. There are various commercially available SLMs including liquid crystal and digital micro-mirror devices. The digital micro-mirror devices can respond up to a few microseconds for real time imaging applications, such as for atmosphere investigations. A transmissive SLM of 60 Hz speed was used to realize the adjustable slit for spectral selection experiment. The spectral selection is similar to Fig. 3 with spectral bandwidths of 9 nm, 15.8 nm and 23 nm achieved at the SLM slit widths of 0.4 mm, 0.8 mm and 1.2 mm, respectively.

The spectral resolution of this system may depend on the opening width of the spatial filter at the Fourier plane. The smaller the slit width, the better the spectral resolution will be. However, the spectral resolution is compromised by two factors. First, the incident image beam at the grating is a focused beam rather than a collimated beam since the grating is located on the original image plane. For a fixed wavelength, different angle of the focused beam will results in different spectral location at the Fourier plane. This results in certain spectral cross talks at the opening slit. From the grating equation, an estimate of the spectral resolution changes with the beam input angle alteration may be calculated. Figure 4 shows both the simulated and experimental divergent angle dependent spectral resolution at a fixed 0.4 mm slit width at 550 nm center wavelength. The spectral resolution changes from 3 nm to 11 nm when divergent angle varies from 0.2° to 0.9° showing certain field of view limitation to the optical Fourier transform based structure.

The slit width may also influence the image spectral bandwidth and can be theoretically determined. Figure 5 shows both experimental and theoretical relationships between the spectral resolution and the slit width at 500 nm center wavelength with a fixed 0.57° divergence angle. When the slit width is reduced to some extent, further reduction of the slit width does not improve spectral resolution due to diffraction grating defects and spectral overlapping resulting from incident beam divergence. Experimentation has achieved minimum spectral resolution of about 9 nm at 500 nm center wavelength with a slit width of 0.4 mm and an incident image beam divergent angle of 0.57°.

In Figures 4 and 5, the deviations of experimental results from theoretical values for larger divergent angle or wider slit width are due to the optical system aberrations that were not accounted for in the theoretical simulation. Higher spectral resolution may be achieved by using larger optical aperture and longer focal length at the expense of increased system environmental sensitivity. Similar to other imaging spectrometer systems, such as spectral scanning, LCTF, and AOTF, the narrow spectral band imaging has limited photon stream received by the imaging detector array. High-sensitivity, low-noise imaging detector arrays may thus be used. For applications of strong illumination, the quality of the spectral image data acquired may depend on the resolution of the electronic system analog-to-digital converter, but not necessarily by the photon statistics.

In an additional example, a dual-channel spectral imaging prototype system was constructed based on the architecture shown in Figures 1 and 2. The scene object included a projection slide attached on a ground glass and illuminated using a white light source. The object distance was about 1.5 meters. The image lens L₀ included a focal length of 300 mm, and a sinusoidal holographic phase diffraction grating with period of 1000 lp/mm was located at the image plane to disperse the image spectral contents. The grating diffraction results in +1, 0, and -1 diffraction channels, and the +1 and 0 diffraction channels were employed as the coarse and fine spectral imaging channels, respectively.

In the coarse spectral imaging channel, the lenses Li and L₂ included focal length of 50 mm and 35mm, respectively. A motorized variable slit was mounted on a motorized linear stage for scanning. The slit width was adjustable through a DC motor, while the scanning motion speed was approximately 6 mm/sec. The coarse spectral imaging channel was been calibrated and synchronized through software control of the slit width, slit translation scanning at the Fourier plane, and the spectrally selected image acquisition at CCDl. In the fine spectral image channel, a TeO₂ based non-collinear AOTF was used having a spectral operation range from 450 nm to 800 nm, and including a spectral resolution of 2nm and 12nm at 450 nm and 700 nm, respectively. Acceptance image angle was 2.8° solid angle and the spectrally selected image diffraction angle was 1.35°. The AOTF aperture size was about 5x5mm². The focal length of the lenses placed before and behind the AOTF were 100mm and 150mm, respectively. The selection of relatively long focal length lenses allows improved separation of the diffracted image beam from the un-diffracted beam. The two lenses will reform the original image on the grating surface to AOTF and CCD2 imaging sensor.

To demonstrate the effective operation of the coarse spectral imaging, a set of spectral images were captured at CCDl with lens Lo aperture size of 5.4 mm and slit width of 1.4 mm. The image object included a colorful slide, and the spectral images covered the range from 470 nm to 630 nm. At this slit width, image spectral bandwidth at 568 nm center wavelength is 22 nm as shown in Figures 6 and 7, discussed below.

For fine spectral imaging, a set of six spectral images were captured having center wavelengths of 557 nm, 562 nm, 565 nm, 568 nm, 573 nm, and 579 nm, respectively. The spectral selectivity bandwidths for these fine spectral images are close to 9nm. The corresponding spectral widths for these two channels setting are shown in Figures 6 and 7. The spectral selectivity bandwidth is smaller at shorter wavelengths. The increase of the spectral bandwidth at longer center wavelengths is provided by the operating nature of the AOTF.

The capturing of the spectral images demonstrates real-time operation of the system. The spectral imaging with spectral zooming uses less spectral image frames for real-time operation while still covering a wide spectral range at spectral coarse imaging and a narrow spectral range at spectral fine imaging. The system provides flexibility for the dynamic selection of spectral band and spectral resolution, which may be application dependent. The scanning slit can be replaced by a spatial light modulator (SLM) to eliminate a motion device element and to improve the spectral selection speed in the coarse spectral image channel. The spectral resolution of the fine spectral images may be center wavelength dependent but not tunable. It is typically a set parameter based on the AOTF design and fabrication. On the other hand, the coarse channel spectral resolution can be easily adjusted by slit width or SLM pixels. Figure 6 shows the spectral bandwidth comparison for captured images at the common center wavelength of 568 nm. Figure 7 shows spectral bandwidth comparison at center wavelength of 482 nm, where 6nm spectral resolution is reach by AOTF. In this particular example, the AOTF demonstrated better spectral resolution than the slit selection at the slit width of 1.4 mm. Although the fine spectral resolution of about 6 nm is not significantly smaller than the 9 nm described above, it is however important to note that the present example has a larger incident image beam angle of 1.2° as compared to the previous 0.57°.

Figure 8 shows the measured and fitted slit width dependent spectral resolution at 550 nm center wavelength for the present coarse image channel. Figure 9 shows a set of spectra at a fixed location on the image plane with different slit widths at 550 nm. Reducing slit width further does not necessarily reduce the spectral bandwidth or improve the spectral resolution, but may reduce the image light power captured by the CCDl camera that in turn decreases the image signal-to-noise ratio. At the present image beam angle of 1.2°, the minimum spectral bandwidth achievable by the slit selection is about 12 nm with a 0.6 mm width slit, which is much larger than that of the AOTF. While this demonstrated the applicability of using AOTF for fine spectral image selection, on the other hand, the slit spectral selection has demonstrated its dynamic spectral bandwidth selection capability which may not be achievable by use of an AOTF. The two coarse and fine spectral imaging channels complement to each other and together demonstrate the needed wide spectral range and fine spectral resolution in the spectral imaging system.

In a practical application, the spectral imaging system can operate first in the coarse spectral imaging mode covering a wide spectral range for identifying the existence of potential targets of interest in the scene. Once identified, the system can switch to the fine spectral imaging mode for further characterization and target verification. With few spectral frames captured in each set, the system has a fast update rate depending on the CCD camera used and should be suitable for real-time operation.

In conclusion, a dynamic spectral imaging system with spectral zooming capability is provided. In particular, the system may include a dual-channel spectral imaging system having agile spectral band access and spectral bandwidth tuning capability. A diffractive grating and an acousto-optic tunable filter may be respectively used as spectral dispersion and spectral filtering elements for the two channels. A 4-f spectral filtering channel using an adjustable slit may be set up at the first diffraction order of the grating to realize coarse spectral band selection. The AOTF selectively filters the spectrum of the non-dispersed zero order to realize fine spectral imaging. The spectral zooming function is realized without increasing spectral frame number facilitating real-time spectral imaging operation.

The system may provide the ability to adjust between a wide spectral band for spectral coarse viewing and a narrow spectral band for spectral fine viewing by capturing a few spectral images as a set. The dynamic spectral zooming may be achieved quickly, thereby allowing for real time spectral imaging of a spectral scene of interest without wasting significant resources on unnecessary spectral image channels. The application of high speed AOTF, dynamic SLM filter and high sensitive CCD sensor significantly improves response speed, thus enabling spectral imaging for a variety of real-time applications. For example, the dynamic spectral imaging system may be adapted for infrared spectral imaging applications including target/decoy discrimination, medical procedures, and remote identification of pollutants.

It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described herein above. In addition, unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. A variety of modifications and variations are possible in light of the above teachings without departing from the scope and spirit of the invention, which is limited only by the following claims.

Claims

What is claimed is:

1. A spectral imaging system, comprising: a spatial filter operable to selectively adjust the spectral band width for a captured spectral image.

2. The spectral imaging system according to Claim 1, further comprising a diffraction grating operable to disperse spectral information of the captured spectral image prior to reaching the spatial filter.

3. The spectral imaging system according to Claim 1, wherein the spatial filter is positioned at a Fourier plane of the system.

4. The spectral imaging system according to Claim 1, wherein the spatial filter includes a spatial light modulator.

5. The spectral imaging system according to Claim 1, wherein the spatial filter includes a scanning slit.

6. The spectral imaging system according to Claim 1, wherein the spatial filter includes an acoustic optic tunable filter.

7. The spectral imaging system according to Claim 1, further comprising a 4-f lens system having the spatial filter positioned therein.

8. A spectral imaging system, comprising: a first imaging channel, the first imaging channel including a first spatial filter operable to selectively adjust the spectral band width of a captured spectral image for projection onto a first image plane; and a second imaging channel, the second imaging channel including a second spatial filter operable to selectively adjust the spectral band width of a captured spectral image for projection onto a second image plane.

9. The spectral imaging system according to Claim 8, wherein at least one of the first and second spatial filters includes an acoustic optic tunable filter.

10. The spectral imaging system according to Claim 8, wherein at least one of the first and second spatial filters includes a spatial light modulator.

11. The spectral imaging system according to Claim 8, further comprising a diffraction grating operable to disperse spectral information of the captured spectral image to the first and second imaging channels.

12. The spectral imaging system according to Claim 11, wherein the diffraction grating includes a sinusoidal holographic phase diffraction grating.

13. The spectral imaging system according to Claim 8, wherein the first imaging channel includes a first lens and a second lens, the first spatial filter being disposed therebetween.

14. The spectral imaging system according to Claim 13, wherein the second imaging channel includes a third lens and a fourth lens, the second spatial filter being disposed therebetween.

15. A method for spectral imaging, comprising: capturing an image having spectral information; dispersing the spectral information to a first imaging channel and a second imaging channel ; filtering the spectral information along the first imaging channel to a first spectral bandwidth; and filtering the spectral information along the second imaging channel to a second spectral bandwidth different from the first spectral bandwidth.

16. The method according to Claim 15, wherein at least one of filtering the spectral information along the first imaging channel and filtering the spectral information along the second imaging channel is performed by an acoustic optic tunable filter.

17. The method according to Claim 15, wherein at least one of filtering the spectral information along the first imaging channel and filtering the spectral information along the second imaging channel is performed by a spatial light modulator.