GB2457306A - An imaging apparatus and method - Google Patents

Abstract

An image capture for imaging a scene comprises an array 10 of differently sensitive detector elements (fig. 2; 20, 22, 24 and 26), each detector element having a respective one of a plurality of different sensitivities, means 2, 4 and 6, which may be lenses, for directing radiation from the scene to form an image at the array; and processing means 12 for reading and processing response signals from the detector elements. The processing means is configured to operate in a plurality of modes, comprising a first processing mode in which it produces signals dependent upon the sensitivity of the detector elements to obtain sensitivity dependent image data, and a second processing mode in which it produces sensitivity independent image data. The processes may produce images at different resolutions, and the images may be displayed on displays 14 and 16. The sensor elements on the array may be sensitive to light of different polarization or wavelength and may be quantum well infra-red photodetectors. A microscanner 8 or the array itself may be moved so that incident radiation is directed onto a different sensor elements in turn (fig. 5).

Description

1 2457306 An Imaging Anparatus The present invention relates to imaging, and in particular to infra-red imaging capable of distinguishing radiation having different polarisations or being in different wavelength bands.

Objects of interest in a scene (particularly man-made objects) may be detected or identified by the polarisation of visible light or infra-red radiation that they emit, or by the ratio of the radiation, for instance visible or infra-red radiation, that they emit at different wavelength bands. Various polarisation-sensitive or waveband-sensitive imagers are known, which may be used to identif' the polarisation or wavebands of emitted radiation.

Objects of interest may also be detected or identified using a high resolution imager, which does not discriminate between different wavebands or polarisations but instead provides images with the maximum possible spatial resolution.

In field situations it can be important to detect and discriminate objects or targets, particularly man-made objects. However, such objects are often at a distance or partially concealed and may be difficult to see, particularly at night. It has been found, pursuant to the present invention, that in such circumstances one of the polarisation-sensitive, waveband-sensitive or high resolution techniques may be most suitable for detection of the presence of an object, but another of the techniques may be most suitable for subsequent identification or discrimination of the object.

Various different polarisation-sensitive imagers are known. A known imager comprises an alternating array of detector elements having different polarisation sensitivities, and in operation an image is scanned across the array in small steps to provide registerd polarisation-sensitive images. In another known imager, a polarisation-sensitive beamsplitter is used to direct radiation having different polarisations to different detector arrays. In a further known imager, a birefringent optical element is used to displace incoming radiation having mutually perpendicular polarisations by different amounts and to direct the radiation to an array of alternating, polarisation sensitive detectors. In another known imager, a rotating polarising filter wheel is arranged relative to a non-polarisation sensitive detector array and operated so that alternate image frames generated by the detector array are representative of radiation having different polarisations.

Such known polarisation-sensitive imagers can be bulky, requiring the use of filter wheels or more than one detector array in some cases. Furthermore, if high resolution images in addition to polarisation-sensitive images are required a separate high-resolution imager is used. In addition, the time needed to obtain and process polarisation sensitive images can be relatively long, leading to a potential loss of useful image data whilst waiting for the polarisation sensitive images to be obtained and analysed.

Turning to known waveband-sensitive imagers, visible light colour cameras that are sensitive to radiation of different wavelengths are of course well known. Such colour cameras include, for example, an array of alternating or stacked sensor elements each sensitive to one of red, green or blue light, or separate arrays of sensor elements each array sensitive to one of red, green or blue light. However, such alternating or stacked sensor element arrangements, or separate arrays, are either not suitable for detection of infra-red radiation or would be bulky and expensive if used for infra-red radiation.

Hyperspectral cameras for infra-red radiation are also known. Such hyperspectral cameras analyse radiation from a target or subject at each of a large number, usually hundreds, of different wavelengths. However, the signal processing required for hundreds of wavebands is necessarily complex, and is difficult to achieve in real time.

Again, if high-resolution images are desired in addition to hyperspectral images, a separate high-resolution imager may be required.

It is an aim of the present invention to provide an improved, or at least alternative imaging apparatus.

In a first, independent aspect there is provided an imaging apparatus for imaging a scene comprising:-an array of differently-sensitive detector elements, each detector element having a respective one of a plurality of different sensitivities; means for directing radiation from the scene to form an image at the array; and processing means for reading and processing response signals from the detector elements, wherein the processing means is configured to operate in a plurality of modes, comprising a first processing mode in which it processes response signals in dependence upon the sensitivity of the detector elements from which the response signals were obtained to obtain first, sensitivity-dependent image data, and a second processing mode in which it processes response signals to obtain second, sensitivity-independent image data.

Thus, a particularly versatile imaging apparatus may be provided, which may operate in either or both a higher spatial-resolution mode and a lower spatial-resolution, sensitivity-dependent mode. The apparatus may be particularly useful for the detection and identification of objects or targets, particularly in low light conditions and/or in the field. It has been found that one of the imaging modes can be most effective for detection of the presence or absence of an object or target, and the other of the imaging modes can be most effective for identification of the object or target. The apparatus may provide a thermal imager or infra-red camera capable of operating in two modes: a polarimetnc mode for distinguishing objects by their polarisation characteristics; and a high resolution mode for distinguishing objects by their shape.

The processing means is preferably configured to operate in the first processing mode and the second processing mode in succession. The processing means is preferably connected to at least one display, and is configured to pass the first image data and/or second image data to the at least one display for generation and display of images corresponding to the first and/or second image data.

Each detector element may comprise a photo-sensitive detection area, and electrical connections to the detection area which may be used to read response signals from the detection area.

Preferably the first image data and the second image data are each representative of the image of the scene, and the second image data represents the image with higher spatial resolution than the first image data.

Preferably the apparatus further comprises means for causing relative movement of the image and the array, configured to operate according to one of a plurality of scan modes, in each scan mode the image and the array of differently-sensitive detector elements being placed in a different series of relative positions.

By providing the means for causing relative movement, it can be ensured that the same portion or portions of the image can be directed to detector elements having different sensitivities. The apparatus may further comprise switching means for switching between scan modes.

Those features are particularly important and so in another independent aspect there is provided an imaging apparatus for imaging a scene comprising:-an array of differently-sensitive detector elements, each detector element having a respective one of a plurality of different sensitivities; means for directing radiation from the scene to form an image at the array; means for causing relative movement of the image and the array, configured to operate according to one of a plurality of scan modes, in each scan mode the image and the array of differently-sensitive detector elements being placed in a different series of relative positions; switching means for switching between scan modes; and processing means for reading and processing response signals from the detector elements.

Preferably the processing means is configured to select a processing mode in dependence upon the scan mode according to which the detector signals were obtained.

As the same array of detector elements is used in both scan modes, a particularly rapid and reliable switching may be obtained.

The plurality of scan modes may comprise a first scan mode and a second scan mode, and in operation the image and the array are placed in a first series of relative positions according to the first scan mode, or a second, different series of relative positions according to the second scan mode, and preferably the separation of relative positions in the second series is smaller than the separation of relative positions in the first series Thus, the second scan mode can be used to provide image data representing an image with higher spatial resolution than the first scan mode. The number of different relative positions in the second series may be greater than in the first series.

The processing means may be configured to process response signals arising from the first series of relative positions according to the first processing mode, and to process response signals arising from the second series of relative positions according to the second processing mode.

By using the second processing mode to process response signals from the second series of relative positions, it can be ensured that the second image represents an image with even higher spatial resolution.

The apparatus may be configured to switch to the second scan mode whilst the processing means is processing response signals read from the detector elements during the first scan mode or whilst the processing means is processing the first image data arising from those response signals.

The processing means may be configured to read response signals from the detector elements during the second series of relative positions whilst processing response signals that were read from the detector elements during the first series of relative positions or whilst processing the first image data arising from those response signals.

Thus, opportunities to observe the scene using the apparatus, and to obtain response signals are not lost, and rapid switching between the first and second scan modes can be provided.

The first series of relative positions may be such that distances between relative positions in the first series are substantially equal to separations of detector elements in the array.

Preferably, for at least one relative position, and preferably each relative position, in the first series, the distance between that relative position and the next relative position in the first series is substantially equal to a distance between neighbouring detector elements.

The first series of relative positions may be such that there is at least one portion of the image that is directed to a respective detector element at each of the relative positions of the first series.

Thus, image data for different sensitivities may be obtained for the same image portions, and it can be ensured that the image data for different sensitivities represent images that are in spatial registration with each other, and spatially registered sensitivity-dependent information may be obtained.

The first series of relative positions may be such that, in operation, for at least one portion of the image, a respective response signal is obtained in response to that portion of the image from a detector element of each sensitivity.

Thus, image data for all of the different sensitivities can be obtained for the same image portions, and it can be ensured that the image data for all of the different sensitivities represent images that are in spatial registration with each other The second series of relative positions may be such that there is at least one relative position for which the distance between that relative position and at least one other relative position in the second series is substantially equal to a fraction of the distance between neighbouring detector elements. Preferably the fraction of the distance is 1/2, 1/4, or 1/8.

The second series of relative positions may be such that at least one portion of the image is directed to a respective detector element at each of some of the second series of relative positions and is directed to a respective position between detector elements at each of some other of the second series of relative positions.

The first image data and the second image data preferably each comprises a plurality of pixel signals, each pixel signal corresponding to a respective portion of the image. The processing means may be configured to process each pixel signal of the second image signal independently of the sensitivity of the detector elements from which the pixel signal arose, to cause the display of a corresponding pixel.

The processing of response signals in the first processing mode may comprise processing response signals generated in response to the same image portion by detector elements of different sensitivities in dependence on the sensitivity of the detector elements, to provide at least one sensitivity-dependent pixel signal representative of that image portion.

The at least one sensitivity-dependent pixel signal may comprise a plurality of pixel signals, each pixel signal being for a respective sensitivity. Alternatively, the at least one sensitivity-dependent pixel signal may be a single pixel signal that includes a plurality of features each corresponding to a different sensitivity, or at least one feature representative of differences between response signals from detector elements of different sensitivities.

The processing of response signals in the second processing mode may comprise generating a respective pixel signal from at least one response signal generated at each detector element and combining the pixel signals independently of the sensitivity of the detector elements to which they correspond, to provide the second image data.

Preferably the second processing mode comprises generating, for each relative position, a set of pixel signals, each pixel signal corresponding to a respective detector element, and combining the sets of pixel signals to produce an interlaced second image signal.

At least some, and preferably all, of the detector elements may be for detection of infra-red radiation.

The plurality of different sensitivities may be two different sensitivities or four different sensitivities.

The different sensitivities may be different polarisation sensitivities, and each detector element may have a respective one of a plurality of different polarisation sensitivities, or the different sensitivities may be different wavelength sensitivities and each detector element may be sensitive to a respective one of a plurality of different wavelength bands.

A detector element having a particular polarisation sensitivity may be considered to be a detector element have a maximum sensitivity to radiation having a particular polarisation direction. The detector elements of the array may be substantially identical in structure, and may be positioned on the array with different orientations or may have at least one component having a different orientation, with the polarisation sensitivity of each detector element being dependent on the relative orientation of the detector element or component. The directions of polarisations may be defined relative to each other andlor relative to an x or y axis of the array of detector elements.

In the case where each detector element has a respective one of a plurality of different polarisation sensitivities, it is preferable that each detector element has substantially the same sensitivity to unpolarised radiation as each other detector element. Thus the detector elements may be used easily in either a polarisation-sensitive mode or a polarisation insensitive mode.

The different polarisation sensitivities may be a sensitivity to radiation having a polarisation substantially equal to 00, a sensitivity to radiation having a polarisation substantially equal to 900, a sensitivity to radiation having a polarisation substantially equal to 45°, and a sensitivity to radiation having a polarisation substantially equal to 135°.

In the case where each detector element is sensitive to a respective one of a plurality of different wavelength bands, at least one, and preferably each of the wavelength bands may have an upper and/or lower limit that is in the infra-red range of wavelengths.

Alternatively, one or more of the detector elements may have an upper andlor lower limit that is in the visible range of wavelengths.

A detector element being sensitive to a particular wavelength band may be considered to be a detector element having a maximum sensitivity to radiation having a wavelength within that wavelength band. The upper and lower limits of the wavelength band may be defined as being those wavelengths at which the sensitivity is equal to 12 of the maximum sensitivity in question. The term waveband is used interchangeably with the term wavelength band herein.

Preferably none of the wavelength bands overlaps with any other of the wavelength bands.

The width of each wavelength band may be less than or equal to I gm, or less than or equal to 0.5gm, and is preferably less than or equal to 0.3gm.

The centre of at least one, and preferably each, of the wavebands may be between 0.7gm and 1000gm, or between 1.4gm and 100gm, and preferably is between 3gm and 15gm.

At least one and preferably all of the wavebands may be in the mid-wavelength infra-red region. At least one of the wavebands may be in the visible region and/or at least one of the wavebands may be in the long-wavelength infra-red region.

At least one of the wavelength bands may be an infra-red wavelength band and at least one of the wavelength bands may be a visible wavelength band, to provide mixed infra- red and visible light imaging. Preferably, two of the wavebands are in the mid-wavelength infra-red region, one of the wavebands is in the long-wavelength infra-red region and one of the wavebands is in the visible region, which has been found to provide a particularly effective mixed infra-red and visible light imaging.

The mid-wavelength infra-red region may be considered to be between 3pm and 8tm, and the long-wavelength infra-red region may be considered to be between 8pm and l5pm.

Each detector element may comprise a quantum well device, preferably a quantum well infra-red photo-detector (QWIP), as the polarisation sensitivity of such QWIPs is relatively easy to select, and as they are able to provide substantially the same sensitivity to unpolarised radiation. Some, and preferably all, of the quantum well infra-red photo-detectors may include a linear diffraction grating, and in that case the polarisation sensitivity of a detector element may be selected by selecting the angle of its diffraction grating.

The array of detector elements may comprise a plurality of non-overlapping sub-arrays, each sub-array comprising a single detector element of each different sensitivity.

Each sub-array may be a square or rectangular sub-array, and preferably each sub-array has a substantially identical arrangement of detector elements to each other sub-array.

In a further, independent aspect of the invention there is provided a method of imaging a scene comprising providing an array of differently-sensitive detector elements, each detector element having a respective one of a plurality of different sensitivities, directing radiation from the scene to fonn an image at the array, reading and processing response signals from the detector elements, and selecting a processing mode from a plurality of processing modes, and processing the response signals in accordance with the selected processing mode, wherein the processing modes comprise a first processing mode according to which response signals are processed in dependence upon the sensitivity of the detector elements from which the response signals were obtained to obtain first, sensitivity-dependent image data, and a second processing mode according to which response signals are processed to obtain second, sensitivity-independent image data.

In a further independent aspect, there is provided apparatus substantially as described herein, with reference to one or more of the accompanying drawings.

In another independent aspect, there is provided a method substantially as described herein, with reference to one or more of the accompanying drawings.

Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, apparatus features may be applied to method features and vice versa.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is an illustration of an imaging apparatus according to the preferred embodiment; Figure 2 is an illustration of a polarisation sensitive detector array included in the apparatus of Figure 1; Figure 3 is a flowchart illustrating in overview operation of the preferred embodiment; Figure 4 is a flowchart illustrating in overview the first and second scan modes used in by the preferred embodiment; Figure 5 is an illustration of the movement of a single image portion, or pixel, on the detector array of the apparatus of Figures 1 and 2 in one cycle, according to a first scan mode; Figure 6 is an illustration of the movement of a single image portion, or pixel, on the detector array of the apparatus of Figures 1 and 2 in one cycle, according to a second scan mode; and Figure 7 is a graph showing the response characteristics of differently wavelength-sensitive detector elements included in a detector array used in variants of the described embodiments.

Figure 1 shows a side-view of an imaging apparatus according to the preferred embodiment, in the form of a thermal camera. The apparatus includes an optical system comprising an arrangement of lenses 2 4 6 aligned with a microscanner 8, and a detector array 10 comprising a two-dimensional array of polarisation sensitive detector elements.

The microscanner 8 is linked to a control processor, or processing electronics, 12 that controls movement of the microscanner in operation. The polarisation sensitive array is connected to the control processor or processing electronics 12, which in operation stores and processes response signals from the detector elements. The control processor or processing electronics is connected to displays 14 16.

The arrangement of lenses 2 4 6 is aligned with a scene to be imaged, along the line of sight of the apparatus and, in operation, infra-red radiation from the scene passes through the arrangement of lenses 2 4 6 and the microscanner 8 and forms an image on the detector array 10.

Part of the detector array 10 is shown in plan view in Figure 2, and comprises a two-dimensional array of polarisation sensitive detector elements 20 22 24 26, with different elements being sensitive to different directions of polarisation arranged in a repeating pattern of four in a checkerboard pattern. Each repeating pattern of four detector elements 20 22 24 26 can be considered to be a sub-array. The elements are sensitive to one or other of four different directions of polarisation (0°, 45°, 90° or 135°) in the preferred embodiment. Each of the detector elements are separated from their nearest neighbours in the horizontal and vertical directions by a characteristic pitch step of 2Ojim, measured from the centre of each detector element to the centres of the nearest neighbours. In the preferred embodiment, the detector array 10 is an array of 640 by 512 detector elements.

Each detector element comprises a quantum well infra-red photodetector (QWIP).

QWIP detectors require a diffraction grating of some form for normal operation. Each of the QWIPs included in the preferred embodiment includes a linear diffraction grating positioned in front of a detection area, and the polarisation sensitivity of each QWIP is determined by the angle of its linear diffraction grating. The QWIPs are essentially identical in structure to each other, apart from the angle of the diffraction grating, and have essentially the same response characteristics to unpolarised radiation as each other, and may have a similar sensitivity to unpolarised radiation as unmodified QWIP detectors, which are uniformly sensitive to all directions of polarisation.

The microscaimer 8 comprises a lens and associated piezo-electric actuators that are operable to drive the lens by selected distances in a series of steps. Each movement of the lens causes a movement of the image relative to the array. The apparatus has different modes of operation, and the distances by which the lens is driven, and the corresponding displacement of the image on the array, is different for each mode.

Operation of the apparatus is illustrated in overview in Figure 3. The apparatus operates firstly according to a first scan mode to obtain a first set of response signals, and then switches to a second scan mode to obtain a second set of response signals, as illustrated in the flow chart of Figure 4.

In the first scan mode, the microscanner 8 drives the lens such as to cause movements of the image relative to the array. The microscanner 8 moves the image in a cyclic series of four positions, separated by steps of 2Otm in the x and/or y directions, corresponding to the pitch step of the detector array The four positions of the series are chosen so that each image portion, made up of radiation from a corresponding scene point or region, is held stationary over a detector element of each of the four different types of polarisation sensitivity in turn. As the array and image are moved so that the same image portion is directed to a detector element of each different polarisation sensitivity type, it is ensured that images produced using the detector elements of each different type are in registration with each other.

The image is held stationary at each of the four positions relative to the array during a detection interval, or integration time, between movements. The integration time is 4ms in the preferred embodiment. Each detector element of the array detects and integrates received radiation during the integration time. The integration is simultaneous for every detector element of the array, and each detector element produces an electrical signal corresponding to the integrated amount of radiation for that detector element.

The electrical signals of the detector elements are read out by the control processor at the end of the integration time, to produce a field of data, comprising a response signal in respect of each detector element. The combined time to integrate and read out the electrical signals is 10 ms. The time taken to displace the image between measurements is negligible compared to the integration and read out times and so 100 fields of data can be produced per second.

An example of the movement of one particular image portion, or pixel, corresponding to one point or region of the scene is shown in Figure 5. The displacement cycle begins with the image portion or pixel 30 being aligned with one of the detector elements 22 having a polarisation sensitivity direction of 0° and held for the integration and read out time. The image portion is then moved, and held for the integration and read out time, over each of the detector elements 24 20 and 22 in turn, before returning to the detector element 26 to complete a single displacement cycle. The displacement cycle is then be repeated a desired number of times.

The control processor processes the four successive fields of data generated during each displacement cycle in dependence on the polarisation sensitivity of the detector elements for each of the four fields, according to a first processing mode, to produce one frame of image data. As the array has 640 by 512 elements, each frame of data has 320 by 256 image points or pixels, with four numbers (one for each polarisation direction) associated with each pixel. Each set of four numbers makes up a sensitivity dependent pixel signal.

Each frame is then processed by the control processor 12, again according to the first processing mode, to provide a displayed image or images. In the preferred embodiment, a single display 14 is used to display an image synthesised from the four fields making up each frame. In that example, the sum of the four signals for each pixel is used to determine the brightness of that pixel, and differences between the four signals are used to determine the colour of the pixel. Other parameters, such as polarisation direction or intensity, or a measure of anomaly of each portion of the image from the rest of the image, may also be calculated and displayed.

The frame of image data is also optionally subject to further processing by the control processor 12, according to the first processing mode, for instance by application of a pattern recognition process in order to detect the presence or absence of one or more objects of possible interest in the image.

The displacement cycle of the first scan mode is repeated a desired number of times, and a frame is generated and processed for each displacement cycle.

As soon as the displacement cycle of the first scan mode has been completed the desired number of times, the apparatus switches to the second, high resolution scan mode. The switching between the two modes is carried out by the control processor 12 changing the microscanner drive voltage waveforms. The change is made in 250ms or less, and appears instantaneous to the user.

The control processor 12 can continue to process data arising from the first scan mode, according to the first processing mode, after the apparatus has switched to the second scan mode and in parallel with the reading of response signals from the array during the second scan mode. That is particularly useful when the control processor 12 applies a further processing, such as a pattern recognition process to the image data, as it means that any delays in such further processing do not cause any loss of image data, and that opportunities to observe the scene are not wasted.

In the second scan mode, the microscanner is again operated to cause a repeating cycle of relative movement of the image and the array. The microscanner 8 again displaces the image in a cyclic pattern of four relative positions, and again holds the image stationary for the integration and read out time between displacements. The integration and readout of electrical signals from the detector elements is carried out in the same way as for the first scan mode. However, in contrast to the first scan mode, the displacements of the image according to the second scan mode are by a half pitch step of lOjim, rather than by a full pitch step of 2Otm.

An example of the movement of one particular image portion, or pixel, corresponding to one point or region of the scene, according to the second scan mode is shown in Figure 6. The displacement cycle begins with the image portion or pixel 50 being aligned with one of the detector elements 26 having a polarisation sensitivity direction of 450 and held for the integration and read out time. The image portion is then moved, and held for the integration and read out time, over positions midway between three pairs of detector elements 26 20, 26 22, and 26 24 before returning to the detector element 26 to complete a single displacement cycle. The displacement cycle is then repeated a desired number of times.

Four successive fields of data are generated from each displacement cycle, and are processed by the control processor 12 according to a second processing mode to make up one frame of data. As the relative positions according to the second scan mode are separated only by half a pitch step rather than a whole pitch step, the spatial resolution of the frame of image data in each of the x and y directions is double that obtained in the first scan mode.

According to the second processing mode, the control processor 12 does not process the four fields of data for each frame in dependence upon the polarisation sensitivity of the detection elements from which they were generated. Instead the four fields of data are combined independently of the polarisation sensitivity to form an image frame representing a single, high resolution image, which is displayed on display 16.

Thus, according to the preferred embodiment, the polarisation-sensitive image is displayed on display 14 and the high resolution image of the same scene is displayed on another display 16, enabling comparison of corresponding features in the two images.

The control processor 12 can, optionally, be used to process corresponding polarisation-sensitive and high resolution image frames arising from the first and second modes of operation together in order to provide improved object detection and identification.

In certain alternative embodiments, the image is kept stationary relative to the array of detector elements, rather than being moved in a displacement cycle. The response signals from the detection elements are processed either according to the first processing mode to provide polarisation-dependent image data, or according to the second processing mode to provide polarisation-independent image data. As the image is kept stationary, the polarisation-dependent image data is not registered, and pixel signals from detector elements of different polarisation sensitivities are representative of adjacent, rather than identical, image portions.

In other alternative embodiments, there is a single, repetitive displacement cycle, instead of the two different displacement cycles of the first and second modes of operation. In one such alternative embodiment, there is a single scan mode in which the displacement cycle is made up of eight relative positions, that includes each of the four relative positions of the first mode, together with four further relative positions midway between those four relative positions.

The control processor 12 in that one such alternative embodiment operates in one of two different processing modes. In one processing mode the control processor processes response signals independently of the polarisation sensitivity. Response signals from different detector elements (including detector elements of different polarisation sensitivities) arising from the same portions of the image for each cycle are averaged or summed, and the fields of data for each cycle are interlaced to produce a frame providing a high resolution, non-polarimetric image. Alternatively or additionally, response signals from different detector elements arising from the same portions of the image in each cycle are used in a uniformity correction procedure, to provide for uniform response between different detector elements.

In the other processing mode of that one such alternative embodiment the control processor processes response signals in dependence on the polarisation sensitivity of the detector elements from which they arose. The control processor interlaces two fields of data arising from each cycle, four numbers representing the four polarisation directions being associated with each pixel signal of each of the two fields of data. Thus, interlaced, registered polarisation-dependent images may be produced.

In variants of all of the embodiments described above, the array of polarisation sensitive detector elements may be replaced by an array of differently wavelength-sensitive detector elements. The variation of sensitivity with wavelength of received radiation for wavelength-sensitive detector elements used in some examples of such variants is shown in Figure 7. Such variants operate in the same way as the described embodiments, but the processing and display of data according to such variants is carried out in dependence upon waveband-sensitivity rather than polarisation sensitivity. For instance, the four numbers making up each pixel signal for each frame of data in the first mode represent a portion of the image for each of the four wavebands rather than for each of the four polarisation sensitivities.

Other microscanner mechanisms may be used in place of the piezoelectric actuator and moving lens arrangement described above. For instance, a tilting plate or tilting mirror may be used instead of a moving lens. Alternatively, the detector array itself, rather than the radiation forming the image, may be displaced.

It will be understood that further variations of the disclosed arrangements are possible without departing from the invention. For example, whilst the invention is described primarily with reference to a thermal imager, it could equally be applied to other cameras, such as visible light cameras. In addition, alternative arrangements of the detector elements in the array and alternative microscanner displacement patterns may be used in alternative embodiments. For instance, in one alternative embodiment the use of detector elements having two rather than four different waveband or polarisation sensitivities, and the use of a modified microscanner displacement pattern, provides for an increased frame rate or image resolution whilst providing determination of two rather than four polarisation directions or imaging or two rather than four wavebands. The relative positions included in the series of relative positions, and the order of the series, may also be varied.

It will be understood that the present invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention.

Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination.

Claims (4)

1. CLAIMS: 1. An imaging apparatus for imaging a scene comprising:-an array of differently-sensitive detector elements, each detector element having a respective one of a plurality of different sensitivities; means for directing radiation from the scene to form an image at the array; and processing means for reading and processing response signals from the detector elements, wherein the processing means is configured to operate in a plurality of modes, comprising a first processing mode in which it processes response signals in dependence upon the sensitivity of the detector elements from which the response signals were obtained to obtain first, sensitivity-dependent image data, and a second processing mode in which it processes response signals to obtain second, sensitivity-independent image data.
2. 2. Apparatus according to Claim 1, wherein the first image data and the second image data are each representative of the image of the scene, and the second image data represents the image with higher spatial resolution than the first image data.
3. 3. Apparatus according to Claim 1 or 2, wherein the apparatus further comprises means for causing relative movement of the image and the array, configured to operate according to one of a plurality of scan modes, in each scan mode the image and the array of differently-sensitive detector elements being placed in a different series of relative positions.
4. 4. Apparatus according to Claim 3, further comprising switching means for switching between scan modes, and preferably the processing means is configured to select a processing mode in dependence upon the scan mode according to which the detector signals were obtained.S. Apparatus according to Claim 3 or 4, wherein the plurality of scan modes comprises a first scan mode and a second scan mode, and in operation the image and the array are placed in a first series of relative positions according to the first scan mode, or a second, different series of relative positions according to the second scan mode, and preferably the separation of relative positions in the second series is smaller than the separation of relative positions in the first series 6. Apparatus according to Claim 5, wherein the processing means is configured to process response signals arising from the first series of relative positions according to the first processing mode, and to process response signals arising from the second series of relative positions according to the second processing mode.7. Apparatus according to any of Claims 3 to 6, wherein the apparatus is configured to switch to the second scan mode whilst the processing means is processing response signals read from the detector elements during the first scan mode or whilst the processing means is processing the first image data arising from those response signals.8. Apparatus according to any of Claims 3 to 7, wherein the first series of relative positions is such that distances between relative positions in the first series are substantially equal to separations of detector elements in the array.9. Apparatus according to any of Claims 3 to 8, wherein the first series of relative positions is such that, in operation, for at least one portion of the image, a respective response signal is obtained in response to that portion of the image from a detector element of each polarisation sensitivity.10. Apparatus according to any of Claims 3 to 9, wherein the second series of relative positions is such that there is at least one relative position for which the distance between that relative position and at least one other relative position in the second series is substantially equal to a fraction of the distance between neighbouring detector elements.11. Apparatus according to any preceding claim, wherein the processing of response signals in the first processing mode comprises processing response signals generated in response to the same image portion by detector elements of different sensitivities in dependence on the sensitivity of the detector elements, to provide at least one sensitivity-dependent pixel signal representative of that image portion.12. Apparatus according to any preceding claim, wherein the processing of response signals in the second processing mode comprises generating a respective pixel signal from at least one response signal generated at each detector element and combining the pixel signals independently of the sensitivity of the detector elements to which they correspond, to provide the second image data.13. Apparatus according to any preceding claim, wherein the different sensitivities are different polarisation sensitivities, and each detector element has a respective one of a plurality of different polarisation sensitivities, or the different sensitivities are different wavelength sensitivities and each detector element is sensitive to a respective one of a plurality of different wavelength bands.14. Apparatus according to any preceding claim, wherein the array of detector elements comprises a plurality of non-overlapping sub-arrays, each sub-array comprising a single detector element of each different sensitivity.15. Apparatus according to Claim 14, wherein each sub-array is a square or rectangular sub-array, and preferably each sub-array has a substantially identical arrangement of detector elements to each other sub-array.16. Apparatus according to any preceding claim, wherein at least some of the detector elements are for detection of infra-red radiation.17. Apparatus according to any preceding claim, wherein each detector element comprises a quantum well device, preferably a quantum well infra-red photodetector (QWIP).18. A method of imaging a scene comprising providing an array of differently-sensitive detector elements, each detector element having a respective one of a plurality of different sensitivities, directing radiation from the scene to form an image at the array, and reading and processing response signals from the detector elements, wherein the processing comprises switching between a first processing mode according to which response signals are processed in dependence upon the sensitivity of the detector elements from which the response signals were obtained to obtain first, sensitivity-dependent image data, to a second processing mode according to which response signals are processed to obtain second, sensitivity-independent image data.19. An apparatus substantially as described herein, with reference to one or more of the accompanying drawings.20. A method substantially as described herein, with reference to one or more of the accompanying drawings.