WO2008129284A1

WO2008129284A1 - Polarimetric imaging apparatus

Info

Publication number: WO2008129284A1
Application number: PCT/GB2008/001392
Authority: WO
Inventors: Daniel Robert Hayter; Greg Innes; David Leslie Jordan
Original assignee: Qinetiq Limited
Priority date: 2007-04-19
Filing date: 2008-04-21
Publication date: 2008-10-30
Also published as: GB0707584D0; GB0916669D0; GB2460567A

Abstract

A polarimetric imaging apparatus 2 and method for distinguishing objects of interest within a scene, said apparatus comprising means for resolving electromagnetic radiation received from the scene having a first substantially circular polarisation state into a first image and for resolving electromagnetic radiation received from the scene having a substantially circular polarisation state of opposite handedness to that of the first circular polarisation state into a second image, means for comparing the first and second images so as to identify differences there-between, and means for providing an output indicative of the difference between the first and second images.

Description

POLARIMETRIC IMAGING APPARATUS

The present invention relates to a polarimetric imaging apparatus and in particular to a polarimetric imaging apparatus sensitive to circularly polarised electromagnetic radiation. The present invention also relates to a method of distinguishing objects of interest within a scene using substantially circularly polarised light emanating therefrom.

Polarimetric imagers are typically used to determine information about objects within a scene by making polarisation of light within the scene more readily perceptible to a human viewer, either in real time, or in viewing recorded images. Example applications of polarimetric imagers include inspection of crops in order to locate disease, inspection of soil surfaces to locate areas of disturbance corresponding with buried objects, and discrimination between man-made objects and natural objects in cluttered environments. In the latter case, man-made objects within a naturally illuminated scene will tend to reflect light in a particular polarisation state, whereas natural objects tend to reflect light with a random polarisation state and hence will appear unpolarised.

By way of background to the present invention, known polarimetric imagers use a variety of configurations and techniques in order to determine information about the states and relative degrees of polarisation of light emanating from objects within a scene. For example, US 3,992,571, US 5,264,916, US 5,345,308 and US 5,598,298 all describe polarimetric imagers having mechanically rotatable linear polarisers.

US 3,992,571 , US 5,264,916, GB 2,268,022A, US 5,543,917 and US RE37.752E (reissue of US 5,557,324) describe polarimetric imagers employing multiple fixed linear polarisers. A third configuration comprising a liquid crystal rotator in combination with a fixed linear polariser is described in US 5,404,225, US 5,726,755, and US RE37J52E.

Although the abovementioned polarimetric imagers are sensitive to linear and elliptical polarisation states, sensitivity decreases as the polarisation state approaches circular. Indeed, the abovementioned imagers are incapable of detecting all polarisation states; specifically they will not detect circular polarisation states. This inability to detect circularly polarised electromagnetic radiation has not hitherto been considered to be a shortcoming, especially for passive polarimetric imagers, since the amount of circularly polarised ambient light emanating from objects within a naturally illuminated scene has traditionally been thought to be negligible.

By way of example, US 5,726,755 referenced above discusses a conventional polarimetric imaging technique and an imager in terms of manipulating and detecting a polarisation plane. Accordingly, it is inherent from the nomenclature used in US 5,726,755 that the technique and imager described therein operate exclusively using linearly polarised light.

Furthermore, US RE 37,752 specifically teaches that unpolarised light reflected from objects is almost always partially linearly polarised, thereby teaching away from using circularly polarised light for imaging objects within a scene.

In the interests of clarity, references in the prior art to elliptically polarised light merely indicate that the light has some substantially polarised component rather than being unpolarised. Such references to elliptically polarised light do not imply that the light is specifically circularly polarised.

Notwithstanding the foregoing, new research undertaken by the applicant has revealed that there is a significant amount of circularly polarised light generated from man-made objects within a naturally illuminated scene.

Indeed, this research has unexpectedly shown that imaging a naturally illuminated scene passively using a polarimetric imager sensitive to circularly polarised light has advantages over conventional methods which employ linear or elliptical polarisation states.

By way of example, the present polarimetric imaging apparatus is an improvement over conventional polarimetric imagers which modulate between linear polarisation states as the latter require a constant relative orientation between the scene, the source of illumination, and the imager for reliable and repeatable operation. In contrast, the present polarimetric imaging apparatus operates repeatably irrespective of the relative orientation between the scene, the source of illumination, and the imaging apparatus because the resolving means is not affected by the angle of orientation thereof about the optical axis. The abovementioned advantage facilitates reliable imaging from moveable platforms, for example from moving vehicles. Furthermore, the present polarimetric imaging apparatus provides more consistent performance in situations where electromagnetic radiation illuminating a scene may have a changeable angle of incidence over time. A practical example of this is in the natural environment where ambient illumination varies as a function of the position of the sun angle throughout the day. Accordingly, the present imaging apparatus is less adversely affected by the angle of the sun or the time of day.

Hence, contrary to conventionally accepted wisdom, a polarimetric imager sensitive to circularly polarised light has been shown to provide a practical means for determining information about objects within a scene, and specifically for discriminating between man-made objects and natural objects within cluttered environments. Hence, in this respect the present invention overcomes a technical prejudice in the prior art.

Accordingly, it is an object of the invention to provide a polarimetric imaging apparatus which mitigates at least some of the disadvantages of the conventional devices described above. It is a further object of the invention to provide an improved imaging apparatus optimally sensitive to substantially circularly polarised electromagnetic radiation and to a method of imaging a scene using substantially circularly polarised electromagnetic radiation emanating there-from.

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According to a first aspect of the present invention, there is now proposed a polarimetric imaging apparatus for distinguishing objects of interest within a scene, said apparatus comprising means for comparing electromagnetic radiation received from the scene having a first substantially circular polarisation state and electromagnetic radiation received from the scene having a substantially circular polarisation state of opposite handedness to that of the first circular polarisation state so as to identify differences there-between, and means for providing an output indicative of the difference there-between. The electromagnetic radiation having the first substantially circular polarisation state may comprise left circular polarised electromagnetic radiation, and the electromagnetic radiation of opposite handedness may comprise right circular polarised electromagnetic radiation.

Preferably, the comparing means comprises means for resolving electromagnetic radiation received from the scene having the first substantially circular polarisation state into a first image and for resolving electromagnetic radiation received from the scene having the substantially circular polarisation state of opposite handedness to that of the first circular polarisation state into a second image.

The present polarimetric imaging apparatus is an improvement over conventional polarimetric imagers which modulate between linear polarisation states as the latter require a constant relative orientation between the scene, the source of illumination, and the imager for reliable and repeatable operation. In contrast, the present polarimetric imaging apparatus operates repeatably irrespective of the rotational orientation between the scene and the imaging apparatus because the resolving means is not affected by the angle of orientation thereof about the optical axis. In addition, the present polarimetric imaging apparatus operates more reliably irrespective of the relative geometry of the scene, source of illumination and imaging apparatus.

The abovementioned advantage facilitates reliable imaging from moveable platforms, for example from moving vehicles. Furthermore, the present polarimetric imaging apparatus provides more consistent performance in situations where electromagnetic radiation illuminating a scene may have a changeable angle of incidence over time. A practical example of this is in the natural environment where ambient illumination varies as a function of the position of the sun angle throughout the day. Hence, the present imaging apparatus is less adversely affected by the angle of the sun or the time of day. Preferably, the polarimetric imaging apparatus is adapted in use to passively distinguish objects of interest within the scene using ambient electromagnetic radiation.

Advantageously, the resolving means is arranged to resolve electromagnetic radiation into said first and second images concurrently.

Conveniently, the resolving means comprises a first circular analyser arranged to resolve the received electromagnetic radiation into said first image and a second circular analyser arranged to resolve the received electromagnetic radiation into said second image.

Advantageously, the first circular analyser comprises a first linear polariser arranged in optical communication with a first retarder configured to receive electromagnetic radiation from the scene, and wherein the second circular analyser comprises a second linear polariser arranged in optical communication with a second retarder configured to receive electromagnetic radiation from the scene.

In a preferred embodiment, the first retarder is arranged to convert electromagnetic radiation having the first substantially circular polarisation state to the first image consisting of electromagnetic radiation having a first linear polarisation state.

Preferably, the second retarder is arranged to convert electromagnetic radiation having the second substantially circular polarisation state to the second image consisting of electromagnetic radiation having a second linear polarisation state substantially orthogonal to the first linear polarisation state.

Within the first circular analyser, the first retarder is arranged to convert circular polarised electromagnetic radiation received from the scene of a first handedness to electromagnetic radiation of a first linear polarisation state (the first image). Similarly, within the second circular analyser, the second retarder is arranged to convert circular polarised electromagnetic radiation received from the scene having opposite handedness into electromagnetic radiation of a second linear polarisation state (the second image), the first and second linear polarisation states being substantially orthogonal. Advantageously, the first and second retarders have a retardance of 90 degrees. At least one of the first and second retarders may comprise a quarter-wave plate.

In a preferred embodiment, the first linear polariser has a transmission axis arranged to substantially transmit the first image there-through and the second linear polariser has a transmission axis arranged to substantially transmit the second image there-through.

In the interests of clarity, each linear polariser transmits electromagnetic radiation unhindered in one orientation only. As will be readily apparent to the skilled person, the intensity of electromagnetic radiation emerging from the polariser is reduced as a function of the cosine of the angle between the azimuth of the electromagnetic radiation and the orientation of the transmission axis of the polariser (Malus' Law wherein I₀₁₁₁ = I_1n cos² θ). In other words, only electromagnetic radiation having an optical field parallel to the transmission axis of the linear polariser will pass there- through essentially unaffected. Electromagnetic radiation having an optical field orthogonal to the transmission axis of the linear polariser will be completely blocked.

Within the present polarimetric imaging apparatus, the first and second linear polarisers have their respective transmission axes arranged substantially orthogonally with respect to each other.

Advantageously, the polarimetric imaging apparatus further comprises a first sensor arranged to sense the first image transmitted through the first circular analyser and a second sensor arranged to sense the second image transmitted through the second circular analyser.

In a preferred embodiment the comparing means comprises a processor adapted in use to determine the difference between an output from the first sensor and an output from the second sensor and to provide said output indicative of the difference between the first and second images.

Preferably, the outputs from the first and second sensors and the output indicative of the difference between the first and second images comprise spatial images of the scene. Advantageously, the processor is adapted in use to determine differences in image intensity between the outputs of the first and second sensors and to denote objects of interest within the scene corresponding with said intensity differences. For example, the differences in image intensities may be determined by performing a pixel-wise comparison of identical spatial regions of the first and second images.

In a preferred embodiment, the output means comprises a display adapted to display the output indicative of the difference between the first and second images.

Where the output indicative of the difference between the first and second images comprises a spatial image of the scene, objects of interest are preferably distinguished in use within the image of the scene by varying at least one of hue, saturation and intensity of spatial regions within said image corresponding with said objects of interest.

Advantageously, the polarimetric imaging apparatus further comprises means for focusing electromagnetic radiation received from the scene onto the focal plane array, first passing through the first and second circular analysers. For example, the focusing means may comprise a lens, a zoom lens, a telescope, etc.

Conveniently, the polarimetric imaging apparatus further comprises a beam splitter arranged in optical communication with the first and second circular analysers. In this embodiment, the beam splitter may be configured to receive electromagnetic radiation from the scene and to separate said received electromagnetic radiation along two optical paths, the first optical path leading to the first circular analyser and the second optical path leading to the second optical analyser.

Preferably, the first and second retarders comprise a common retarder configured to receive electromagnetic radiation from the scene arranged in optical communication with the first and second linear polarisers via the beam splitter.

In a preferred embodiment, the beam splitter comprises a polarising beam splitter and the first and second linear polarisers are comprised of said polarising beam splitter. Optionally, the polarimetric imaging apparatus further comprises a source of electromagnetic radiation for actively illuminating the scene. The source may be an unpolarised source emitting randomly polarised electromagnetic radiation.

The polarimetric imaging apparatus may comprise a source of substantially polarised electromagnetic radiation for actively illuminating the scene with substantially polarised electromagnetic radiation. The source of substantially polarised electromagnetic radiation may comprise a source of substantially linearly polarised electromagnetic radiation for actively illuminating the scene with substantially linearly polarised electromagnetic radiation. Alternatively, or in addition, the source of substantially polarised electromagnetic radiation may comprise a source of substantially circularly polarised electromagnetic radiation for actively illuminating the scene with substantially circularly polarised electromagnetic radiation. Illuminating the scene with substantially circularly polarised electromagnetic radiation enables objects of interest to be distinguished within the scene with maximum contrast to their natural surroundings.

According to a second aspect of the present invention, there is now proposed a method of distinguishing objects of interest within a scene comprising the steps of:

(i) receiving electromagnetic radiation from the scene,

(ii) resolving electromagnetic radiation received from the scene having a first substantially circular polarisation state into a first image,

(iii) resolving electromagnetic radiation received from the scene having a substantially circular polarisation state of opposite handedness to that of the first circular polarisation state into a second image,

(iv) comparing the first and second images so as to identify differences there-between,

(v) providing an output indicative of the difference between the first and second images.

The method preferably comprises passively distinguishing objects of interest within the scene using ambient electromagnetic radiation received there-from. For example, objects of interest within the scene may be distinguished passively using ambient electromagnetic radiation reflected from objects within the scene.

Advantageously, the steps of resolving the electromagnetic radiation received from the scene into said first and second images are performed concurrently.

Preferably, the step of resolving the electromagnetic radiation received from the scene having a first substantially circular polarisation state comprises converting said first substantially circular polarisation state into the first image consisting of electromagnetic radiation having a first linear polarisation state.

Advantageously, the step of resolving the electromagnetic radiation received from the scene having a second substantially circular polarisation state comprises converting said second substantially circular polarisation state into the second image consisting of electromagnetic radiation having a second linear polarisation state substantially orthogonal to the first linear polarisation state.

Conveniently, the first and second images and the output indicative of differences there-between comprise spatial images of the scene and the step of comparing the first and second images comprises determining differences in image intensity therebetween and denoting objects of interest within the scene corresponding with said intensity differences.

In a preferred embodiment, the method comprises the step of distinguishing objects of interest within the scene by varying at least one of hue, saturation and intensity of spatial regions within said output corresponding with said objects of interest.

Optionally, the method comprises the additional step of actively illuminating the scene with electromagnetic radiation. The electromagnetic radiation may be substantially randomly polarised electromagnetic radiation.

The step of actively illuminating the scene may comprise illuminating the scene with substantially polarised electromagnetic radiation. The step of actively illuminating the scene may comprise illuminating the scene with substantially linearly polarised electromagnetic radiation. Alternatively, or in addition, the step of actively illuminating the scene may comprise illuminating the scene with substantially circularly polarised electromagnetic radiation. Illuminating the scene with substantially circularly polarised electromagnetic radiation enables objects of interest to be distinguished within the scene with maximum contrast to their natural surroundings.

The invention will now be described, by example only, with reference to the accompanying drawings in which;

Figure 1 shows a schematic view of a polarimetric imaging apparatus according to one embodiment of the present invention.

Figure 2 shows a schematic view of a polarimetric imaging apparatus according to a second embodiment of the present invention.

Figure 3 shows a schematic view of a polarimetric imaging apparatus according to a third embodiment of the present invention.

Figure 4 shows a schematic view of a polarimetric imaging apparatus according to another embodiment of the present invention.

Figures 5a - 5d illustrate simulated images from the polarimetric imaging apparatus of figures 1 - 4. Specifically, figures 5a - 5c show sub-images from the polarimetric imaging apparatus indicative of the processing steps by which an object within a scene reflecting light having a substantially circular polarisation state is distinguished. Figure 5d shows a simulated image of the scene in which said identified object is highlighted.

The present polarimetric imaging apparatus operates by forming images of a scene using left and right circularly polarised light emanating there-from. Manmade objects of interest within the scene can thus be readily distinguished since they give rise to electromagnetic radiation having a high circular polarised component (either strongly left or right circularly polarised), whereas natural objects give rise to substantially randomly polarised electromagnetic radiation. Without limitation, spatial images comprising predominantly left or right circularly polarised components are analysed within the present imaging apparatus to identify regions there-within having minimum or maximum image intensity denoting objects of interest. Without limitation, spatial images of the scene corresponding with the left circularly polarised component are compared with spatial images of the scene corresponding with the right circularly polarised component to provide optimum image contrast, thereby facilitating detection of objects of interest within the scene.

The present polarimetric imaging apparatus detects left and right circularly polarised components by converting left and right circular polarised electromagnetic radiation into linearly polarised electromagnetic radiation having mutually orthogonal linear polarisation states. The two orthogonal linear polarisation states are subsequently separated by selectively filtering the linearly polarised electromagnetic radiation using linear polarisers. The linear polarisers are orientated such that their respective transmission axes are mutually substantially orthogonal. Hence, electromagnetic radiation having a first linear polarisation state corresponding with a first circularly polarised component will be substantially transmitted by one of the linear polarisers and substantially blocked by the other linear polariser. Similarly, electromagnetic radiation having a second linear polarisation state corresponding with a circularly polarised component of opposite handedness will be substantially blocked by one of the linear polarisers but substantially transmitted by the other linear polariser.

Referring now to the drawings wherein like reference numerals identify corresponding or similar elements throughout the several views, figure 1 shows a schematic view of a polarimetric imaging apparatus according to a first embodiment of the present invention. In the first embodiment of the invention, the polarimetric imaging apparatus 2 comprises separate optical analysers for respectively resolving left and right circular polarised electromagnetic radiation received from a scene.

The first optical analyser comprises a first linear polariser 4 arranged in optical communication with a first optical retarder 6 configured to receive electromagnetic radiation from the scene via a first lens 8. The first optical analyser is arranged to detect left circular polarised electromagnetic radiation emanating from objects of interest within the scene. The first optical analyser detects said left circular polarised electromagnetic radiation by firstly converting said left circular polarised electromagnetic radiation into electromagnetic radiation having a first linear polarisation state. To this end, the first optical retarder 6 has a retardance of 90 degrees. Without limitation, the first optical retarder 6 comprises a quarter-wave plate.

Electromagnetic radiation having the first linear polarisation state is detected by selectively filtering the output from the first optical retarder 6 using the first linear polariser 4. This is achieved by orientating the transmission axis of the first linear polariser 4 with respect to the first optical retarder 6 such that electromagnetic radiation having the first linear polarisation state passes unhindered through the first linear polariser 4.

The intensity of electromagnetic radiation emerging from the polariser 4 is reduced as a function of the cosine of the angle between the azimuth of the electromagnetic radiation and the orientation of the transmission axis of the polariser 4 (according to Malus' Law; where l_out = Im cos² θ). Accordingly, the first linear polariser 4 progressively attenuates linearly polarised electromagnetic radiation emanating from the first optical retarder 6 as the angle between the azimuth of the electromagnetic radiation and the orientation of the transmission axis of the polariser 4 increases.

Hence, the first linear polariser 4 transmits a proportion of any randomly polarised and circularly polarised electromagnetic radiation emanating from the first optical retarder 6 but will substantially block electromagnetic radiation having a linear polarisation state orthogonal to the first linear polarisation state. Electromagnetic radiation passing through the first linear polariser 4 is detected by a sensor 10 which provides an output 40 corresponding thereto to the processor 20 for further analysis. Without limitation, the sensor 10 comprises a camera capable of recording a spatial image of the scene. The sensor 10 may comprise any device capable of detecting radiation from within the electromagnetic spectrum, for example a focal plane array, a CCD array, a CMOS array, an infrared imager, and a thermal imager.

The configuration of the second optical analyser is similar to that of the first optical analyser.

The second optical analyser comprises a second linear polariser 12 arranged in optical communication with a second optical retarder 14 configured to receive electromagnetic radiation from the scene via a second lens 16.

The first and second lens 8, 16 are arranged so as to provide substantially identical images of the scene to the first and second optical retarders 6, 14.

The second optical analyser is arranged to detect right circular polarised electromagnetic radiation emanating from objects of interest within the scene. The second optical analyser detects said right circular polarised electromagnetic radiation by firstly converting said right circular polarised electromagnetic radiation into electromagnetic radiation having a second linear polarisation state orthogonal to the first linear polarisation state. Accordingly, the second optical retarder 14 has a retardance of 90 degrees. Without limitation, the second optical retarder 14 comprises a quarter-wave plate.

Electromagnetic radiation having the second linear polarisation state is detected by selectively filtering the output from the second optical retarder 14 using the second linear polariser 12. This is achieved by orientating the transmission axis of the second linear polariser 12 with respect to the second optical retarder 14 such that electromagnetic radiation having the second linear polarisation state passes unhindered through the second linear polariser 12. The intensity of electromagnetic radiation emerging from the polariser 12 is reduced as a function of the cosine of the angle between the azimuth of the electromagnetic radiation and the orientation of the transmission axis of the polariser 12 (according to Malus' Law; where l_out = I_1n cos² θ). Accordingly, the second linear polariser 12 progressively attenuates linearly polarised electromagnetic radiation emanating from the second optical retarder 14 as the angle between the azimuth of the electromagnetic radiation and the orientation of the transmission axis of the polariser 12 increases.

Hence, the second linear polariser 12 transmits a proportion of any randomly polarised and circularly polarised electromagnetic radiation emanating from the second optical retarder 14 but will substantially block electromagnetic radiation having the first linear polarisation state.

Electromagnetic radiation passing through the second linear polariser 12 is detected by a sensor 18 which provides an output 44 corresponding thereto to the processor 20 for further analysis. Without limitation, the sensor 18 comprises a camera capable of recording a spatial image of the scene. The sensor 18 may comprise any device capable of detecting radiation from the electromagnetic spectrum, for example a focal plane array, a CCD array, a CMOS array, an infrared imager, and a thermal imager.

In use, the outputs 40, 44 from the first and second sensors 10, 18 are compared by the processor 20 to identify differences there-between indicative of objects of interest with in the scene.

Specifically, where the outputs 40, 44 from the first and second sensors 10, 18 comprise spatial images of the scene, said spatial images 40, 44 are compared by determining differences in image intensity between the two images 40, 44 to identify spatial regions there-within corresponding with objects of interest in the scene. For example, each spatial image 40, 44 typically comprises a plurality of picture elements or "pixels" and the differences in image intensity is determined by performing a pixel- wise comparison of identical spatial regions of the two images 40, 44.

By way of further explanation, natural objects 24 will appear in the same spatial regions of both images 40, 44 and with substantially the same image intensity. In contrast, manmade objects of interest within the scene will appear in the same spatial regions of both images 40, 44 but with different image intensities due to the predominant left or right circular polarised electromagnetic radiation emanating there-from.

For example, referring to 5a, a manmade object of interest within the scene giving rise to electromagnetic radiation having a predominant left circular polarised component will appear in the first image 40 as a spatial region 42 of high image intensity, i.e. such object will appear bright within the image 40. Referring now to figure 5b by way of comparison, the same object will appear in the second image 44 as a spatial region 46 of low image intensity, i.e. such object will appear dark within the image 44. As mentioned above, natural objects 24 will appear in the same spatial regions of both images 40, 44 and with substantially the same image intensity.

Similarly, a manmade object of interest within the scene giving rise to electromagnetic radiation having a predominant right circular polarised component will appear in the first image 40 as a spatial region of low image intensity, i.e. such object will appear dark within the image 40. Whereas, the same object will appear in the second image 44 as a spatial region of high image intensity, i.e. such object will appear bright within the image 44.

Referring to figure 5c, the comparison of the two images 40, 44 is performed by differencing said images, for example by subtracting absolute values of pixels in one image 40 from those of related pixels in the other image 44. Additional image processing is optionally performed after the differencing process to correct for any minor discrepancies in pixel intensity levels between the two images 40, 44, for example a thresholding procedure is optionally performed. An intermediate image 48 is thus created comprising a spatial region 50 corresponding with regions of images 40, 44 having substantially different image intensity. Said spatial region 50 is indicative of the presence and location of an object of interest in the scene giving rise to circularly polarised electromagnetic radiation.

Referring now to figure 5d, objects of interest within the scene are distinguishing by varying at least one of the hue, saturation and intensity of spatial region 50 and superimposing the spatial region 50 highlighted in this manner onto an output image 52 of the scene, for example onto the first image 40, the second image 44 or a composite image comprising the sum of the first and second images 40, 44. The output image 52 is viewed directly by an observer or is optionally recorded for subsequent analysis.

In a second embodiment of the present polarimetric imaging apparatus, the first and second optical analysers share common elements in order to eliminate duplication of elements. Referring to figure 2, according to the second embodiment of the present polarimetric imaging apparatus, the first and second optical analysers share a common input lens 32. In this embodiment, the first and second optical retarders 6, 14 are configured to receive electromagnetic radiation from the scene via a beam splitter 30 and a common lens 32. The remainder of the polarimetric imaging apparatus is the same as described above with reference to figures 1 and 5.

The common lens 32 and the beam splitter cooperate to provide substantially identical images of the scene to the first and second optical retarders 6, 14, thereby reducing misalignment errors between the first and second optical analysers over the first embodiment.

Further elements within the first and second optical analysers may be shared therebetween to reduce duplication of elements within the imaging apparatus. Referring to figure 3, according to a third embodiment of the present polarimetric imaging apparatus, the first and second optical analysers share a common optical retarder 34. In this embodiment the first and second optical retarders 6, 14 of the foregoing embodiments are replaced by the common optical retarder 34 arranged between the common input lens 32 and the beam splitter 30. Hence, in use, circularly polarised electromagnetic radiation received from the scene is converted by the common optical retarder 34 into electromagnetic radiation having substantially mutually orthogonal first and second linear polarisation states. The linearly polarised electromagnetic radiation comprising both first and second linear polarisation states is divided substantially equally by the beam splitter 30 between the first and second linear polarisers 4, 12.

The remainder of the polarimetric imaging apparatus is the same as described above with reference to figures 1 , 2 and 5.

A final element which may be shared by the first and second optical analysers is the linear polariser, further reducing duplication of elements within the imaging apparatus 2. Referring to figure 4, according to a fourth embodiment of the present polarimetric imaging apparatus, the first and second optical analysers share a common polarising beam splitter 36. In this embodiment the first and second linear polarisers 4, 12 of the foregoing embodiments are replaced by the common polarising beam splitter 36 arranged between the common optical retarder 34 and the first and second sensors 10, 18.

In common with the embodiment of figure 3, circularly polarised electromagnetic radiation received from the scene is converted by the common optical retarder 34 into electromagnetic radiation having substantially mutually orthogonal first and second linear polarisation states. However, in the embodiment of figure 4 the common polarising beam splitter 36 analyses said linearly polarised electromagnetic radiation and simultaneously separates the same into electromagnetic radiation having first and second polarisation states respectively.

The common polarising beam splitter 36 is configured to transmit electromagnetic radiation having the first linear polarisation state unhindered to the first sensor 10 whilst substantially blocking electromagnetic radiation having the second linear polarisation state from said first sensor 10. Similarly, electromagnetic radiation having the second linear polarisation state is transmitted unhindered to the second sensor 18 whilst electromagnetic radiation having the first linear polarisation state is substantially blocked from said second sensor 18.

The remainder of the polarimetric imaging apparatus is the same as described above with reference to figures 1 , 2, 3 and 5.

The present polarimetric imaging apparatus is capable of imaging using electromagnetic radiation in a variety of wavebands. For example, without limitation the imaging apparatus is operable in the ultraviolet (UV), visible, near infrared (NIR), short- wave infrared (SWIR), medium-wave infrared (MWIR), and long-wave infrared wavebands.

The present polarimetric imaging apparatus is primarily a passive imager adapted to image a scene using naturally occurring ambient electromagnetic radiation reflected from objects within the scene. Optionally, the polarimetric imager comprises a source of electromagnetic radiation arranged to actively illuminate the scene with electromagnetic radiation. Active illumination of the scene is used to supplement the ambient electromagnetic radiation occurring naturally within the environment, or alternatively instead of ambient electromagnetic radiation when there is no ambient electromagnetic radiation occurring naturally within the environment.

Embodiments of the present polarimetric apparatus have been described hereinbefore in terms of an imaging apparatus that produces a spatial polarimetric image of the scene via the use of a pair of two-dimensional focal plane detector arrays. However, the first and second sensors need not comprise two-dimensional focal plane arrays, but may comprise alternative sensors.

For example, in applications where there is relative movement between the polarimetric imaging apparatus and the scene / objects in the scene, two one-dimensional focal plane arrays can be used to form a two-dimensional image. A specific example of this is when the sensors are mounted on a moving vehicle; in this case the forward motion of the vehicle provides the second, along track, dimension. Another specific example is the use of a static polarimetric imaging apparatus for imaging objects which move within the scene; in this case the movement of the object across the field of view of the imager provides the second dimension.

In a further alternatively arrangement, the two focal plane arrays can be replaced by two simple point detectors, such as two photodiodes, to provide a non-imaging system where only the mean polarimetric signal in the region being viewed is provided.

In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

The scope of the present disclosure includes any novel feature or combination of features disclosed therein either explicitly or implicitly or any generalisation thereof irrespective of whether or not it relates to the claimed invention or mitigates any or all of the problems addressed by the present invention. The applicant hereby gives notice that new claims may be formulated to such features during the prosecution of this application or of any such further application derived there-from. In particular, with reference to the appended claims, features from dependent claims may be combined with those of the independent claims and features from respective independent claims may be combined in any appropriate manner and not merely in the specific combinations enumerated in the claims.

Claims

1. A polarimetric imaging apparatus for passively distinguishing objects of interest within a scene using ambient electromagnetic radiation received therefrom, said apparatus comprising means for comparing electromagnetic radiation received from the scene having a first substantially circular polarisation state and electromagnetic radiation received from the scene having a substantially circular polarisation state of opposite handedness to that of the first circular polarisation state so as to identify differences there-between, and means for providing an output indicative of the difference there-between.

2. A polarimetric imaging apparatus according to claim 1 wherein the comparing means comprises means for resolving electromagnetic radiation received from the scene having the first substantially circular polarisation state into a first image and for resolving electromagnetic radiation received from the scene having the substantially circular polarisation state of opposite handedness to that of the first circular polarisation state into a second image.

3. A polarimetric imaging apparatus according to claim 2 wherein the resolving means is arranged to resolve electromagnetic radiation into said first and second images concurrently.

4. A polarimetric imaging apparatus according to claim 2 or 3 wherein the resolving means comprises a first circular analyser arranged to resolve the received electromagnetic radiation into said first image and a second circular analyser arranged to resolve the received electromagnetic radiation into said second image.

5. A polarimetric imaging apparatus according to claim 4 wherein the first circular analyser comprises a first linear polariser arranged in optical communication with a first retarder configured to receive electromagnetic radiation from the scene, and wherein the second circular analyser comprises a second linear polariser arranged in optical communication with a second retarder configured to receive electromagnetic radiation from the scene.

6. A polarimetric imaging apparatus according to claim 5 wherein the first retarder is arranged to convert electromagnetic radiation having the first substantially circular polarisation state to the first image consisting of electromagnetic radiation having a first linear polarisation state.

7. A polarimetric imaging apparatus according to claim 6 wherein the second retarder is arranged to convert electromagnetic radiation having the second substantially circular polarisation state to the second image consisting of electromagnetic radiation having a second linear polarisation state substantially orthogonal to the first linear polarisation state.

8. A polarimetric imaging apparatus according to any of claims 5 - 7 wherein at least one of the first and second retarders comprises a quarter-wave plate.

9. A polarimetric imaging apparatus according to any of claims 5 - 8 wherein the first linear polariser has a transmission axis arranged to substantially transmit the first image there-through.

10. A polarimetric imaging apparatus according to any of claims 5 - 9 wherein the second linear polariser has a transmission axis arranged to substantially transmit the second image there-through.

11. A polarimetric imaging apparatus according to any of claims 5 - 10 further comprising a first sensor arranged to sense the first image transmitted through the first circular analyser and a second sensor arranged to sense the second image transmitted through the second circular analyser.

12. A polarimetric imaging apparatus according to claim 11 wherein the comparing means comprises a processor adapted in use to determine the difference between an output from the first sensor and an output from the second sensor and to provide said output indicative of the difference between the first and second images.

13. A polarimetric imaging apparatus according to claim 12 wherein the outputs from the first and second sensors and the output indicative of the difference between the first and second images comprise spatial images of the scene.

14. A polarimetric imaging apparatus according to claim 13 wherein the processor is adapted in use to determine differences in image intensity between the outputs of the first and second sensors and to denote objects of interest within the scene corresponding with said intensity differences.

15. A polarimetric imaging apparatus according to claim 13 or 14 wherein the output means comprises a display adapted to display the output indicative of the difference between the first and second images.

16. A polarimetric imaging apparatus according to claim 15 wherein in use objects of interest are distinguished within the image of the scene by varying at least one of hue, saturation and intensity of spatial regions within said image corresponding with said objects of interest.

17. A polarimetric imaging apparatus according to any of the claims 4 - 16 further comprising means for focusing electromagnetic radiation received from the scene through the first and second circular analysers.

18. A polarimetric imaging apparatus according to any of claims 4 - 17 further comprising a beam splitter arranged in optical communication with the first and second circular analysers.

19. A polarimetric imaging apparatus according to claim 18 wherein the first and second retarders comprise a common retarder configured to receive electromagnetic radiation from the scene arranged in optical communication with the first and second linear polarisers via the beam splitter.

20. A polarimetric imaging apparatus according to claim 19, the beam splitter comprising a polarising beam splitter and wherein the first and second linear polarisers are comprised of said polarising beam splitter.

21. The use of a polarimetric imaging apparatus according to any of the preceding claims for detecting naturally occurring circularly polarised electromagnetic radiation emanating from a scene.

22. The use of a polarimetric imaging apparatus according to any of claims 1 - 20 for passively distinguishing objects of interest within a scene using naturally occurring circularly polarised electromagnetic radiation emanating from the scene

23. The use of a polarimetric imaging apparatus according to any of claims 1 - 20 for comparing the intensity of naturally occurring electromagnetic radiation having a first circular polarisation state emanating from an object within a scene with that of naturally occurring electromagnetic radiation having a second circular polarisation state of opposite handedness to that of the first circular polarisation state emanating from said object.

24. A method of passively distinguishing objects of interest within a scene using ambient electromagnetic radiation received therefrom comprising the steps of:

(i) receiving electromagnetic radiation from the scene,

(iv) comparing the first and second images so as to identify differences therebetween,

25. A method according to claim 24 comprising passively distinguishing objects of interest within the scene using ambient electromagnetic radiation received therefrom.

26. A method according to claim 25 wherein the steps of resolving the electromagnetic radiation received from the scene into said first and second images are performed concurrently.

27. A method according to any of claims 24 - 26 wherein the step of resolving the electromagnetic radiation received from the scene having a first substantially circular polarisation state comprises converting said first substantially circular polarisation state into the first image consisting of electromagnetic radiation having a first linear polarisation state.

28. A method according to claim 27 wherein the step of resolving the electromagnetic radiation received from the scene having a second substantially circular polarisation state comprises converting said second substantially circular polarisation state into the second image consisting of electromagnetic radiation having a second linear polarisation state substantially orthogonal to the first linear polarisation state.

29. A method according to any of claims 24 - 28 wherein the first and second images and the output indicative of differences there-between comprise spatial images of the scene and the step of comparing the first and second images comprises determining differences in image intensity there-between and denoting objects of interest within the scene corresponding with said intensity differences.

30. A method according to claim 29 comprising the step of distinguishing objects of interest within the scene by varying at least one of hue, saturation and intensity of spatial regions within said output corresponding with said objects of interest.