WO1999021123A1

WO1999021123A1 - Optical filtering device

Info

Publication number: WO1999021123A1
Application number: PCT/GB1998/003116
Authority: WO
Inventors: Meirion Francis Lewis
Original assignee: The Secretary Of State For Defence
Priority date: 1997-10-21
Filing date: 1998-10-19
Publication date: 1999-04-29
Also published as: GB9722142D0

Abstract

An optical filtering device for selectively transmitting radiation from a scene having wavelengths within a selected wavelength interval comprising dispersing means arranged to provide spatial dispersion as a function of wavelength without substantial angular dispersion and modulating means arranged to selectively transmit radiation within at least one wavelength interval from at least one direction of radiation incident on the device. The device may also include means for forming an image of the scene. The device may comprise first and second spectrally dispersing elements, such as first and second diffraction gratings or first and second prisms, wherein the latter dispersing element counteracts the angular dispersion of the former. The device may also include a spatial light modulator, such as a ferroelectric liquid crystal spatial light modulator, for selectively transmitting radiation within a selected wavelength interval. This enables rapid switching between the selectively transmitted wavelength intervals. The device may therefore be used in a pattern recognition system, for forming a colour image of the scene to aid identification, or may be used for improved resolution colour photography or active imaging.

Description

OPTICAL FILTERING DEVICE

This invention relates to an optical filtering device. In particular, although not exclusively . it relates to a device suitable for transmitting radiation ot selected wavelengths m an optical system The device may have particular application in a pattern recognition system, for example tor production line inspection The device may also have application in colour photography

Pattern recognition is a useful technique tor recognising faces, signatures, fingerprints or bariknotes for security purposes Also, it mav be used for production line inspection to identify or inspect objects on the production line Although such recognition tasks can be easy to achieve

humans, they aie difficult to accomplish with computers The problem is computationalh intensive and a viable solution has been sought for many years Most attempts to produce autonomous pattern recognition systems have employed the intensity pattern That is, a pattern essentially equivalent to the scene photographed by a black and white camera or film Furthermore, colour information may also be helpful for identification purposes

er. if colour content is required, the computational effort required for analysis b\ computer is significantly greater For example, for computations on the green, led and blue content in a scene, the computational effort required is three times more

The following citations provide general background to the present invention; EP 0 548 830 A. US 5 285 254. US 5 455 674 and US 5 457 530. EP 0 548 830 A describes a spectrometer comprising a spatial light modulator (SLM) in which parallel rays of light from a point source are dispersed by a prism and the resulting spectrum is input to an SLM. Portions of the spectrum are selected by selectively activating or deactivating portions of the SLM. US 5 457 530 also describes a spectrometer which makes use of a diffracting element and an optical shutter to achieve a similar effect. US 528 254 relates to a monochromator. comprising diffractive elements and a rotating mechanical shutter. The existing systems, however, do have several limitations in some applications. For example, the monochromator in US 528 254 produces a light beam which varies repetitively in wavelength. It has the further limitation that the spectral output has a temporal and spatial dependence. Furthermore, the above systems are concerned only with the dispersion of light from a radiative point source, not an image scene.

It is an object of the invention to provide an optical filtering which overcomes these limitations. The invention has application in colour photography where electronic cameras are now replacing conventional film. In such electronic cameras, the scene is captured on a CCD camera and read out electrically and stored. However, compared to a black and white camera, the use of a colour camera is more costly and also of lower resolution. The invention provides improved resolution in colour photography and at less expense. The device may also be used, for example, in a pattern recognition system or in colour photography to overcome the problems encountered using conventional svstems. According to one aspect of the invention, an optical filtering device for selectively transmitting radiation from a scene having wavelengths within a selected wavelength interval comprises;

dispersing means arranged to provide spatial dispersion as a function of wavelength and

modulating means arranged to selectively transmit radiation within at least one wavelength interval from at least one direction of radiation incident on the device.

The optical filtering device preferably comprises a first spectrally dispersing element and a second spectrally dispersing element, wherein the second spectrally dispersing element is arranged to counteract angular dispersion introduced by the first spectrally dispersing element and wherein the modulating means is an electronically controlled spatial light modulator.

The device has the advantage that a particular selected wavelength interval or intervals from the scene may be selectively transmitted while radiation outside of the selected interval or intervals may be selectively blocked or reduced in intensity. This may be achieved by suitably addressing the modulating means to selectively transmit radiation within the required wavelength interval or intervals.

The device may also comprise means for forming an image of the scene from radiation which has undergone spatial dispersion and selective wavelength transmission. The dispersing means may typically comprise a first spectrally dispersing element and a second spectrally dispersing element, wherein the second spectrally dispersing element is arranged to counteract angular dispersion introduced by the first spectrally dispersing element. For example, the dispersing elements may be diffraction gratings, prisms or dichroic filters. The device may also include dispersing elements having other optical functions.

The device may also include reflecting means arranged to define a folded light path between the first spectrally dispersing element and the second spectrally dispersing element.

The device may also include means for producing an intermediate focus in an optical path between the first and second spectrally dispersing elements.

The modulating means may be an electronically controlled spatial light modulator, for example a ferroelectric liquid crystal spatial light modulator. A ferroelectric liquid crystal spatial light modulator has the advantage of providing rapid switching between transmitting and blocking states. Alternatively, a nematic liquid crystal spatial light modulator may be used. This provide continuous control of the relative intensities of different wavelengths transmitted through the device. A one or two dimensional spatial light modulator may be used. The device may be used in a pattern recognition system. In particular, the device may be used to aid identification of objects or people or for inspection purposes. For example, such a system may be used to confirm the colour content of banknotes, postage stamps, security documents or passports. Alternatively, the device may be used in an imaging system for generating images of the scene having modified colour content.

According to another aspect of the invention, a method for selectively transmitting radiation from a scene having wavelengths within a selected wavelength interval comprising the steps of;

spatially dispersing radiation from the scene as a function of wavelength and

selectively transmitting radiation within at least one wavelength interval from at least one direction of radiation incident on the device.

Preferably, the method comprises the step of spatially dispersing radiation from a scene as a function of wavelength through a first spectrally dispersing element and a second spectrally dispersing element, wherein the second spectrally dispersing element is arranged to counteract angular dispersion introduced by the first spectrally dispersing element.

In a further preferred embodiment, the method comprises the step of modulating radiation transmitted through the second spectrally dispersion element using an electronically controlled spatial light modulator. The method may comprise the further step of forming an image of the scene from radiation which has undergone spatial dispersion and selective wavelength transmission.

The method may comprise the further steps of;

(i) illuminating a sample with selectively transmitted radiation within one or more wavelength interval.

(ii) measuring the emission spectrum from the sample corresponding to the one or more wavelength interval and

(iii) comparing the one or more measured emission spectrum with emission spectra of known samples to enable identification of the sample.

According to another aspect of the invention, a hyperspectral imaging method comprises the steps of;

A hyperspectral imaging method comprising the steps of:

(ii) spatially dispersing radiation from an object or scene as a function of wavelength,

(iii) selectively transmitting radiation within at least one wavelength interval from at least one direction of radiation emitted from the object or scene.

(iii) forming an image of the object or scene from radiation which has undergone spatial dispersion and selective wavelength transmission and

(iv) comparing the emission spectrum of the imaged objects or scenes with emission spectra of known objects or scenes to enable identification of the object or scene.

Preferably, the method comprises the step of spatially dispersing radiation from the object or scene as a function of wavelength through a first spectrally dispersing element and a second spectrally dispersing element, wherein the second spectrally dispersing element is arranged to counteract angular dispersion introduced by the first spectrally dispersing element.

The method may comprise the step of modulating radiation transmitted through the second spectrally dispersion element using an electronically controlled spatial light modulator so as to selectively transmit radiation within at least one wavelength interval from at least one direction of radiation. The invention will now be described, by example only, with reference to the following figures in which;

Figure 1 is a schematic drawing of an example of an optical filtering device of the invention.

Figure 2 schematically illustrates transmissive diffraction gratings which may be used in the device of the invention.

Figure 3 schematically illustrates the angular dispersion of radiation when incident on a diffraction grating,

Figure 4 illustrates dispersion of white light rays in two diffraction gratings arranged such that angular dispersion in one grating is counteracted in the other.

Figure 5 shows virtual images of a white light source produced by two diffraction gratings.

Figure 6 shows the Figure 4 arrangement with the addition of an image forming lens.

Figure 7 shows a schematic diagram of an embodiment of the invention, in which a spatial light modulator is located at an intermediate focus between dispersing elements, and

Figure 8 illustrates a device of the invention comprising two prisms as dispersive elements. Referring to Figure 1. there is shown a schematic drawing of an example of an optical filtering device of the invention, indicated generally by 1. The device 1 comprises first and second diffraction gratings 2.3. an input lens 4, and output lens 5 and a mirror 6. The device also comprises a spatial light modulator 7. In this example, the device 1 is used to view a scene 8.

In operation, white light radiation 9 from the scene to be viewed 8 is input through the input lens 4. In practice, the device 1 is most likely to be used to view a distant scene, but in Figure 1 this is simulated by the scene 8. white light 9. the diffuser 10 and the lens 4. The subsequent operation of the device will be described initially with reference to a beam of radiation 1 1 from a particular point in the scene 8.

The diffraction gratings 2.3 are of like dispersion and are disposed so that the angular dispersion introduced by the second grating 3 substantially counteracts the dispersion introduced by the first grating 2. White light radiation 1 1 is incident on the first diffraction grating 2 where it is angularly dispersed. The dispersed radiation then passes to the mirror 6 where it is reflected onto the second diffraction grating 3 to counteract the dispersion introduced by the first grating 2. The first and second gratings 2.3. therefore produce a spatial, wavelength dependent dispersion of light from the scene 8. This scene may be viewed by a camera 12 sensitive to the particular wavelength range of interest. Alternatively, the scene may be viewed directly by the human eye in which case it may be necessary to view the scene through a diffuser situated at the image plane. P. to ensure that light of all wavelengths enters the pupil of the eye.

The diffraction gratings 2.3 in Figure 1 are reflective gratings and therefore the light follows a folded path through the apparatus. Of course, transmissive grating elements mav also be used. The modulator 7 may be a liquid crystal spatial light modulator having a transparent pixel, exemplified by the shaded pixel 14b. and an opaque pixel, exemplified by the pixel 14a (cross-hatched pixel). The pixels in the larger, clear area of the modulator 7 may be either opaque or transparent depending on the requirements of the user. On application of an external addressing voltage (not shown), the modulator 7 may switch any of its pixels so addressed to an opaque state so that only certain pixels transmit radiation incident on the modulator 7 at a particular spatial position. Only radiation within a particular wavelength interval or intervals, angularly dispersed to these positions, will therefore be transmitted through the modulator 7. Alternatively, certain pixels may be switched to a partially opaque state so as to selectively reduce the intensity of radiation within a particular wavelength interval or intervals.

The modulator 7 may have any number of pixels but. in practice, for applications in the visible wavelength region only three pixels may be required to enable selective transmission of red, blue and green radiation. For modulators having a larger number of pixels, the required regions of opacity may be obtained by forming a cluster of opaque pixel clusters. For a liquid crystal spatial light modulator, the positions of opaque pixels such as 14b are controlled by applying a suitable voltage across selected pixels by suitable supply means (not shown). Radiation dispersed along a particular path, within a selected wavelength interval, can therefore be selectively transmitted as required by blocking unwanted radiation wavelengths by use of opaque pixels or pixel clusters. If it is desired to modify the colour content of an image, this may be achieved by reducing the intensity of radiation passing through certain pixels of the modulator, rather than blocking them altogether. Examples of modulators which may be included in the device 1 may be found in "SLM Technology: Materials. Devices and Applications", Marcel-Dekker Inc., Ed. Uzi Efron (1994). Analogue devices, e.g. nematic liquid crystal devices, have a response of the order of kHz and binary devices, e.g. ferroelectric liquid crystal devices, have a response of the order of MHz. In some applications, binary devices can also be used to achieve analogue response by time dithering the pixels. The fast response times of the SLM therefore enable the optical filter device to be operated at very high speeds, and spectral selection can be achieved rapidly. A further advantage of the device is that a spectral output can be produced which is both spectrally and temporally arbitrary.

The optical filtering device 1 enables the spectral content of a scene 8 to be electrically controlled, the modulator 7 being electrically controlled, so that only particular wavelengths in the spectrum will be recorded at the imaging camera (or observed by the eye) at any one instant. Furthermore, liquid crystal spatial light modulators are capable of being switched at very high speed, at least up to 10 kHz. This therefore enables rapid switching between red. blue and green light transmission to the image plane. This has useful applications in pattern recognition where colour information from a scene needs to be processed rapidly. It may also have applications in. for example, colour photography, allowing the three primary colours to be recorded separately in sequence.

In one embodiment of the device, the intensity of radiation at a particular wavelength may be controlled by the use of detectors arranged to detect radiation incident on the spatial light modulator pixels. Intensity information obtained from the detectors may be utilised in a feedback logic circuit to actively control the degree of opacity of certain pixels and hence the spectral content of the image. This would enable the amount of. for example, red or blue light in the imaged scene to be controlled automatically. This may be used to correct for imperfect (i.e. coloured) lighting conditions which occur, for example, at sunset or in artificial lighting. The function of the mirror 6 shown in Figure 1 is to simply fold radiation 1 1 passing through the device so that the device may be more compact. The mirror 6 is not therefore essential to the device, but may in practice be used for convenience to reduce the overall size of the device. If the mirror is not used, and the second grating 3 may take a position such as that of the mirror shown in Figure 1. with radiation output from the device through a lens 5 which would be positioned to the left hand side of the figure.

Preferably, the diffraction gratings 2.3 shown in Figure 1 are blazed to enhance their efficiency in the wavelength band of interest and reduce spurious diffraction orders. Alternatively, the diffraction gratings may be replaced with any dispersive elements which can be arranged such that the second dispersive element substantially counteracts the angular dispersion of light of the first. For example, two prisms may be used, of like shape, composition and dispersion and are disposed such that their angular dispersions are in substantially the same plane but opposite. Other optical arrangements may also be used to achieve the desired effect. Additional mirror components may also be included in a prism device to fold the optical path.

Figure 2 illustrates the angular dispersion of an input beam of radiation 1 1. The incoming radiation 1 1 is diffracted and thereby angularly dispersed by a diffraction grating, D. in accordance with the following well known grating equation;

sin α -i- sin β = nλ/d ( 1 )

where α and β are respectively the angles of incidence and diffraction at the first grating, λ is the wavelength of incident light, d . is the grating line spacing and n is any integer, positive or negative, or zero. Figure 3 shows an alternative arrangement comprising first and second transmissive diffraction gratings 20.21. fixed optical stops. 22 and 23. and a selective modulator 24, and illustrates the effect produced by such an arrangement for incident white light (the arrangement does not make use of a folded light path as no mirror is used, as in Figure 1). The arrangement shown in Figure 3 receives white light indicated by a ray 1 1a from a point in a remote scene (not shown).

The first grating 20 is blazed in first order i.e. n in Equation (1) is +1. The first fixed stop 22 defines an entrance aperture. The beam of radiation 26 emergent from the first diffraction grating 20 is dispersed, with the outer limits of this divergent beam indicated by a blue ray 26B and a red ray 26R. The divergent beam 26 is incident on the second grating 21. which is blazed in first order and accurately counteracts the dispersion introduced by the first grating 20. In consequence, the second grating 21 in first order transmits a beam 27 in which angular dispersion is substantially zero. The transmitted beam 27 has parallel red and blue rays 27R and 27B with rays of intervening colours therebetween.

The device shown in Figure 3 may also include a second fixed optical stop 23 to block all transmitted light other than that in grating first order. The stop 23 has an aperture 28 to transmit such light, as is required. However, the second optical stop 23 is not essential and may not be required in many applications.

As shown, the selective spatial light modulator 24 has an opaque pixel 24b which blocks transmission of green light (i.e. intervening rays between red and blue light). Light of other wavelengths is transmitted by the selective modulator 24 and may be transmitted to a camera (not shown). Alternatively, for example, if the opaque pixel 24b is transparent, and all others are opaque, only green light will be transmitted to the camera. The field of view of the device is described with reference to Figure 4 which shows first and second diffraction gratings 30,31 and two white light rays 32.33, from different parts of a remote scene, which are inclined to an optical axis 34. The white light rays 32.33. incident on the device give rise to respective parallel beams 37,38 which retain the angular separation between them on transmission through the device. A lens (not shown) is required for image formation, such as the objective lens of a camera to which radiation 37.38 is transmitted.

Typically, the field of view of the device will be limited to a few degrees in the plane of the figure. However, in the orthogonal plane the field of view is much greater. For example, a device has been made which has a +/- 20° field of view in the orthogonal plane. However, in certain applications the limited field of view in one direction does not introduce a problem and is quite acceptable. For example, for production line inspection, where objects to be inspected or identified may be scrolled in front of an imaging system on a moving conveyor belt, it may be necessary to image objects in a line. This only requires a substantial field of view in one dimension and the limited field of view in an orthogonal direction does not matter. If required, magnifying optics within the device may be used to reduce the angular separation of beams 37.38.

Clearly the dispersion of the rays in exaggerated in Figure 4 and. in practice, blue rays from one point will overlap with red rays from another. However, in some applications it is useful to block, say. red rays from one particular point in a scene, even though red rays from other parts of the scene will not be affected in the same way. Another practical consideration in the device of the invention is the depth of field i.e. the range of distant objects in focus for a given configuration. As discussed below, this aspect of the device is acceptable in practice. The depth of field of the device is illustrated in Figure 5. where imaging properties of the gratings 30 and 31 are illustrated schematically. A white light source 43. situated close to the device, produces a divergent white light beam 40 incident on the first grating 30 (the distance between the white light source and the device is exaggerated for clarity). Angular dispersion in the first grating 30 and counteraction thereof in the second grating 31 produces divergent red and blue beams 41 and 42 which form virtual images of the point source 43. Beams of intervening wavelengths are not shown. A selective modulator 44 is arranged for selectively transmitting the required wavelengths, as described previously.

The red and blue beams diverge from virtual images 46 and 47 as indicated by dotted lines 48 and 49. These images may be viewed by using a lens to focus the beams 41 and 42. Figure 5 demonstrates that a white light source gives rise to dispersed coloured images in a system incorporating mutually counteracting wavelength dispersive elements in the form of gratings 30 and 31. One or more of these coloured images can be controlled by transmitting only certain wavelengths, or transmitting certain wavelengths by a reduced amount, with the modulator 44.

Figure 5 illustrates the lateral separation of virtual images of a light source very close to a device of the invention as an example of wavelength smearing. The spatial dispersion introduced by the gratings 30 and 31 causes aberration which is negligible for remote scenes, although it becomes progressively more significant as the object distance reduces, as in the example shown in Figure 5. The spatial dispersion is similar in magnitude to the separation between the gratings 30 and 31 i.e. the order of a few centimetres. For objects tens of metres away, the lateral smearing of a few centimetres according to wavelength is not apparent, since it is less than the spatial resolution of the system. In one system which has been built, the depth of field extended from at least 50 m to infinity. Furthermore, for observation of close objects, suitable optics can be used to make it appear as if the objects are at infinity, (as shown in Figure 1).

Referring to Figure 6. the grating arrangement and ray diagram of Figure 4 is shown once more with the addition of a focusing lens 60. Elements previously described are like referenced. The lens 60 brings each of the beams 37 and 38 to a respective focus 61 or 62 in a focal plane 63. The lens 60 may be the objective lens of a camera and the focal plane 63 may be occupied by an array of detecting elements. The location of each of the foci 61 and 62 is dependent on the angle of incidence of the respective rays 32 and 33. This demonstrates that a pair of mutually counteracting dispersive elements may be incorporated in an imaging system in accordance with the invention. It should be noted that the angular separation of rays 32 and 33 is greatly exaggerated for clarity of illustration. Referring now to Figure 7. an alternative configuration of the device is shown, comprising two diffraction gratings 90.91. Parallel light 80 transmitted by the first grating 90 is focused by a first lens 81 at an intermediate focal plane 82. Light 83 diverging from the focal plane 82 is rendered parallel by a second lens 84, the lenses 81 and 84 being separated by the sum of their focal lengths f, and f₂ respectively. Parallel light from the second lens 84 passes to the second grating 91 for diffraction. The spatial light modulator 7 is located at the intermediate focal plane 82. If the SLM modulator is analogue, the SLM 7 can be programmed to transmit or absorb any particular wavelength i.e. it can be programmed to transmit a particular spectrum. As described previously, an analogue response can be obtained using binary devices, such as ferroelectric devices, by time-dithering the pixels, providing a fast SLM response is available. Alternatively, silicon micromachined SLMs or other types of SLM may be employed.

In the figure, light from a broadband source 1 10 is transmitted through the input lens 112. to form beam 80. prior to transmission through the first diffraction grating 90. Using this arrangement, a white light source can be filtered to provide an arbitrary spectrum. As shown, the arrangement shown in Figure 7 allows a white light source 100 to produce an output beam 81 of arbitrary spectrum. This may be of use for generating special effects in the entertainment industry or for colour matching e.g. paints, dyes, clothes, carpets. In this application, the user may compare a white object, illuminated with coloured light, with a coloured object illuminated with white light.

An additional lens (not shown) may also be included in the arrangement, beyond the grating 91. to enable spectral components to be reformed for subsequent detection at a pixel. It should also be noted that the lens 1 12 is not necessary for all applications. This embodiment of the invention has application in the identification of samples, such as biological samples, through their absorption spectra. The sample may be placed in the path of the coloured light beam 81 before being focussed on to a detector. The absoφtion spectrum can be measured by passage of one narrow wavelength band at a time, by varying transmission of the pixels of the SLM. followed by computer analysis to determine the required information. For example, this may be used to measure the concentration of a particular species within a biological sample. The process can be speeded up by measuring a few spectra and matching them to the absoφtion spectrum of known species. Effectively, this is comparison between the measured absoφtion spectrum and a finite series of known templates.

A related application is the identification of. say, a distant object from its emission spectrum. This is known as hyperspectral imaging. Using the arrangement shown in Figure 7. incoming light from a distant point (corresponding to light beam 80 with lens 112 omitted) is passed through a modulator 7 (at the intermediate focal plane 82) and is finally imaged onto a small output detector (not shown). This isolates the point of interest from the background scenery, although alternatively a finite object could be imaged onto a detector array. As described above, the spatial light modulator 7 can be controlled to effectively provide a large number of narrowband filters implemented sequentially, followed by computer processing. If it is only required to rapidly distinguish between a few possible emitters, a relatively small number of carefully selected SLM filters may be used to distinguish between these emitters. The arrangements shown and described with reference to Figures 1-7 could equally be implemented using prisms instead of diffraction gratings. Referring to Figure 8, there is shown a device comprising two prisms 101.102. the latter being arranged to counteract the dispersion introduced by the former.

A white light ray 103 entering the first prism 101 is dispersed and a beam 104. having dispersed red and blue rays. 104R and 104B respectively, emerges from the prism 101. The beam 104 is then counter-dispersed by the second prism 102, as described previously for a two-grating device. A modulator 105 may then be located after the second prism 102 to selectively transmit certain wavelengths. In addition, mirrors may also be included in the device to provide for reflection of light from the first prism 101 to the second 102.

The examples described which employ gratings may use prisms instead and vice versa. However, the invention is not restricted to use of a pair of prisms or a pair of diffraction gratings and other dispersive elements may also be used, such as a pair of dichroic filters. It may also be convenient and desirable to obtain the dispersive functions of these pairs of devices by other optical means or by a combination of means. The important feature is that the second dispersive element substantially counteracts the angular dispersion of the first. This leaves spatial dispersion without a degree of angular dispersion sufficient to degrade system optical properties unacceptably. When employed in an imaging system, angular dispersion should be counteracted to a degree at which optical aberrations become of acceptable proportions. It may also be desirable to use a combination of dispersive and other functions such as a diffraction grating on a mirror, as used in a conventional spectrometer. With reference to Figure 1, for example, the diffraction grating 3 may be a curved, mirrored diffraction grating, having a focusing and a dispersive function, which may remove the need for the lens 5.

The foregoing description has discussed selective transmission of visible wavelengths. However, the optical filtering device is not restricted to the visible region of the spectrum, and operation in the infra-red or ultraviolet is also possible. For example, the device may have useful application in infrared imaging systems.

In particular, the device may be used in a pattern recognition system to give colour information as well as spatial information relating to an object to be viewed. Pattern recognition systems have been limited in the past to systems which make use of intensity patterns for comparative puφoses. which is essentially equivalent to the scene photographed by a black and white camera. Using the optical filtering device of the invention, a pattern recognition system could make use of the colour content of objects as red, green and blue images may be generated in rapid succession, as described previously. The device is particularly well suited to production line inspection applications and to horizontal recognition applications, where information in a wide field of view is only required in one dimension. For example, the device may be used in the observation of scrolling objects on a conveyor belt. Despite the limited field of view in one dimension, the device may also have application in colour photography applications. In imaging systems for use in the visible part of the spectrum, it is often desirable to generate the red, blue and green data from an image rapidly. Increasingly in photography, electronic means, such as CCD cameras, are being used to replace photographic film to record images. Although conventionally colour photography can be achieved with colour cameras, there is an inherent loss of resolution with such systems, due to the need for green, red and blue filtering for each pixel. The present device may be used to enable the red, blue and green content of a scene to be recorded in rapid succession, as the spatial light modulator (e.g. modulator 7 in Figure 1) is switched, at full resolution.

Embodiments described have employed one dimensional spectral dispersion and spatial light modulation. However, it may also be possible to use a two-dimensional spatial light modulator. Such a modulator may also be located at an intermediate focal plane as shown at 82 in Figure 7. It is also possible to employ optical light path directions and modulator locations other than those described. In Figure 3, for example, the light propagation direction may be reversed so that input radiation becomes represented by 27B and 27R. and output radiation by 1 1a. By this means, coloured images may be generated from separate inputs of. say, red. green and blue light.

Claims

1. An optical filtering device for selectively transmitting radiation from a scene having wavelengths within a selected wavelength interval comprising;

2. The optical filtering device of claim 1. wherein the dispersing means comprise a first spectrally dispersing element and a second spectrally dispersing element, wherein the second spectrally dispersing element is arranged to counteract angular dispersion introduced by the first spectrally dispersing element and wherein the modulating means is an electronically controlled spatial light modulator.

The optical filtering device of claim 1 or 2, and further comprising

means for forming an image of the scene from radiation which has undergone spatial dispersion and selective wavelength transmission.

4. The optical filtering device of claim 1 or 2 wherein the dispersing means comprise a first spectrally dispersing element and a second spectrally dispersing element, wherein the second spectrally dispersing element is arranged to counteract angular dispersion introduced by the first spectrally dispersing element.

5. The optical filtering device of claim 4. including reflecting means arranged to define a folded light path between the first spectrally dispersing element and the second spectrally dispersing element.

6. The device of claim 4 wherein the first and second spectrally dispersing elements are diffraction gratings.

7. The device of claim 4 wherein the first and second spectrally dispersing elements are prisms.

8. The device of claim 4 wherein the first and second spectrally dispersing elements are dichoric filters.

9. The device of claim 4 including means for producing an intermediate focus in an optical path between the first and second spectrally dispersing elements.

10. The device of claim 9. wherein the spatial light modulator is located substantially at the intermediate focus.

11. The device of claim 1 wherein the modulating means is an electronically controlled spatial light modulator.

12. The device of any of claims 1-11 wherein the spatial light modulator is a ferroelectric liquid crystal spatial light modulator.

13. The device of any of claims 1-11 wherein the spatial light modulator is nematic liquid crystal spatial light modulator.

14. The device of claim 12 or 13 wherein the spatial light modulator provides spatial modulation in two dimensions.

15. An imaging system for generating images of the scene having colour content comprising the optical filtering device of claim 1 or 2.

16. A method for selectively transmitting radiation from a scene having wavelengths within a selected wavelength interval comprising the steps of;

spatially dispersing radiation from the scene as a function of wavelength and

17. The method of claim 16. comprising the step of spatially dispersing radiation from a scene as a function of wavelength through a first spectrally dispersing element and a second spectrally dispersing element, wherein the second spectrally dispersing element is arranged to counteract angular dispersion introduced by the first spectrally dispersing element.

18. The method of claim 17. comprising the further step of modulating radiation transmitted through the second spectrally dispersion element using an electronically controlled spatial light modulator.

19. The method of any of claims 16-18. comprising the further step of forming an image of the scene from radiation which has undergone spatial dispersion and selective wavelength transmission.

20. The method of any of claims 16-19 and further comprising the steps of ;

21. A hyperspectral imaging method comprising the steps of;

(iii) selectively transmitting radiation within at least one wavelength interval from at least one direction of radiation emitted from the object or scene,

22. The method of claim 21, comprising the step of spatially dispersing radiation from the object or scene as a function of wavelength through a first spectrally dispersing element and a second spectrally dispersing element, wherein the second spectrally dispersing element is arranged to counteract angular dispersion introduced by the first spectrally dispersing element.

23. The method of claim 22, comprising the step of modulating radiation transmitted through the second spectrally dispersion element using an electronically controlled spatial light modulator so as to selectively transmit radiation within at least one wavelength interval from at least one direction of radiation