GB2444961A - Imager with detector elements sensitive to radiation of different polarisations - Google Patents

Imager with detector elements sensitive to radiation of different polarisations Download PDF

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Publication number
GB2444961A
GB2444961A GB0625803A GB0625803A GB2444961A GB 2444961 A GB2444961 A GB 2444961A GB 0625803 A GB0625803 A GB 0625803A GB 0625803 A GB0625803 A GB 0625803A GB 2444961 A GB2444961 A GB 2444961A
Authority
GB
United Kingdom
Prior art keywords
polarisation
imager
radiation
elements
detector
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
GB0625803A
Other versions
GB0625803D0 (en
Inventor
Andrew Haining
Robert Craig
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Thales Holdings UK PLC
Original Assignee
Thales Holdings UK PLC
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Thales Holdings UK PLC filed Critical Thales Holdings UK PLC
Priority to GB0625803A priority Critical patent/GB2444961A/en
Publication of GB0625803D0 publication Critical patent/GB0625803D0/en
Priority to PCT/EP2007/064057 priority patent/WO2008077839A1/en
Publication of GB2444961A publication Critical patent/GB2444961A/en
Withdrawn legal-status Critical Current

Links

Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J1/00—Photometry, e.g. photographic exposure meter
    • G01J1/42—Photometry, e.g. photographic exposure meter using electric radiation detectors
    • G01J1/4228—Photometry, e.g. photographic exposure meter using electric radiation detectors arrangements with two or more detectors, e.g. for sensitivity compensation
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J1/00—Photometry, e.g. photographic exposure meter
    • G01J1/02—Details
    • G01J1/04—Optical or mechanical part supplementary adjustable parts
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J1/00—Photometry, e.g. photographic exposure meter
    • G01J1/02—Details
    • G01J1/04—Optical or mechanical part supplementary adjustable parts
    • G01J1/0407—Optical elements not provided otherwise, e.g. manifolds, windows, holograms, gratings
    • G01J1/0429—Optical elements not provided otherwise, e.g. manifolds, windows, holograms, gratings using polarisation elements
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J4/00—Measuring polarisation of light
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B23/00—Telescopes, e.g. binoculars; Periscopes; Instruments for viewing the inside of hollow bodies; Viewfinders; Optical aiming or sighting devices
    • G02B23/12—Telescopes, e.g. binoculars; Periscopes; Instruments for viewing the inside of hollow bodies; Viewfinders; Optical aiming or sighting devices with means for image conversion or intensification
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
    • G02B27/28—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00 for polarising
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
    • G02B27/28—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00 for polarising
    • G02B27/283—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00 for polarising used for beam splitting or combining
    • G02B27/285—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00 for polarising used for beam splitting or combining comprising arrays of elements, e.g. microprisms
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
    • H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
    • H01L27/144—Devices controlled by radiation
    • H01L27/146—Imager structures
    • H01L27/14643—Photodiode arrays; MOS imagers
    • H01L27/14658—X-ray, gamma-ray or corpuscular radiation imagers

Abstract

An imager comprising a detector 14 having at least one element sensitive to radiation having a first polarisation and at least one element sensitive to radiation having a second polarization, and a means 12 for simultaneously directing from the same scene or point of a scene radiation of the first polarisation onto the first polarization sensitive element and radiation of the second polarisation onto the second polarization sensitive element. Preferably, the detector 14 consists of a two dimensional array of elements.

Description

Animater The present invention relates to an imager. In particular, the
invention relates to a thermal imager, sometimes referred to as an infrared camera, that is capable of distinguishing objects by the analysis of the polarisation of radiation emitted from these objects.
Background of the Invention
Objects of interest in a sccne, particularly man-made objects, may be identified by the tO polarisation of light or infrared radiation emitted by them. This identification is facilitated if images of the scene for two directions of polarisation can be produced which are accurately registered in position and time.
Polarisation sensitive thermal imagers with registered images are known. One uses separate detectors for each waveband and superposes images of these detectors using a polarisation sensitive bearnsplitter. A problem with this is that using separate detectors makes the imager large and costly. Another known imager uses a non-polarisation sensitive detector and a rotating filter wheel, so that alternate frames from the imager give information about alternate polarisation directions. A problem with this is that since the images are not registered in time, they will not be properly superposed for moving objects. A third known thermal imager uses an array of elements, alternately sensitive to different wavebands. However, these fail to provide accurate registration, because different sensitivity elements are sampling spatially different parts of the scene.
Summary of the Invention
According to the present invention, there is provided an imager, preferably a thermal imager, for imaging a scene, the imager having: a detector having at least one element sensitive to radiation having a first polarisation and at least one element sensitive to radiation having a second polarisation, and directing means for simultaneously directing from the same scene or part of a scene radiation of the first polarisation onto the first polarisation element and radiation of the second polarisation onto the second polarisation element.
By providing means for simultaneously directing different polarisation radiation from the same scene or point of a scene onto different areas of the detector, there is provided a simple and effective mechanism for providing images in two polarisation directions, captured at the same time, and accurately registered. This can be achieved with relatively simple optics and detector arrays and without the need for any moving parts.
Preferably, the detector comprises a two-dimensional array of detector elemcnts. In at least one dimension of the array, alternate elements may be sensitive to different directions of polarisatiori.
The first polarisation elements may be arranged in spaced apart rows, with rows of the second polarisation elements positioned between them, so that alternate rows of first and second polarisation elements are provided.
The first polarisation elements may be arranged in spaced apart columns, with columns of the second polarisation elements positioned between them, so that alternate columns of first and second polarisation elements are provided.
The first and second polarisation elements may be arranged as a checkerboard.
Preferably, the first polarisation elements are sensitive to vertical polarisation and the second polarisation elements are sensitive to horizontal polarisation.
The detector may be a Quantum Well Infrared Photodetector (QWJP). This type of detector is sensitive in a narrow waveband, which means that dispersion, and so blurring of the image, is small.
The directing means may comprise a bi-rethugent device, preferably in the form of a bi.refrrngent plate that has different refractive indexes for the two directions of polarisation. This displaces ray paths so that the image in one polarisation direction is displaced relative to the other, and consequently radiation of the first polarisation is S directed onto one or more of the first polansation elements and radiation of the second polarisation is directed onto one or more of the second polarisation elements. The directing means are configured so that displacement of the rays corresponds to the pitch of the array.
The directing means may comprise a singLe eLement, such as a single bi-refringent element or a plurality of elements.
According to another aspect of the present invention, there is provided a method for imaging a scene comprising: simultaneously directing from the saint scene or part of the scene radiation of a first polarisation onto a first polarisation element and radiation of a second poLarisation onto a second polarisation element and using the simultaneously captured first and second polarisation radiation to form one or more images of the scene.
Brief Description of the Drawiugs
Vanous aspects of the invention will now be described by way of example only, and with reference to the accompanying drawings, of which: Figure 1 is a schematic view of a polarisation sensitive camera; Figure 2 is a front view of a polarisation sensitive detector for use in the camera of Figure 1; Figure 3 is a front view of a polarisation sensitive detector for use in the camera of Figure 1, and Figure 4 is a front view of a polarisation sensitive dctector for use in the camera of' Figure 1.
Specific Description of the Drawings
Figure 1 shows a thermal imager that has an objective lens 10, a tilted bi-refringent plate 12 that is able to displace radiation of different polarisations by different amounts and a single detector 14. Preferably, the detector 14 is a Quantum Well Infrared Photodetector (QWIP) with a narrow response in each waveband. Between the lens 10 and the plate 12 other optics may be provided, depending on the nature of the camera. However, this is not essential.
The detector 14 is a single unit that has a two-dimensional array of detector elements arranged so that the pitch between each element is the same. Figure 2 shows an example of such a detector. In this, elements in the first, third and fifth rows etc are sensitive to radiation that has vertical polarisation, whereas elements in the second and fourth rows etc are sensitive to radiation that has horizontal polarisation. This means that in each column of the array, alternate elements are sensitive to different directions of polarisation.
The lens 10 images radiation from a scene onto the detector 14. This radiation has at least two polarisation directions. The radiation passes through the bi-refringent plate 12, which displaces the components of the image. The displacement is slightly different for the two polarisation directions, so that different areas of the detector 14 are illuminated with radiation having different polarisation directions. The material, thickness, and tilt of the plate 12 are chosen so as to make the displacement difference equal to the pitch of the elements in each column of the detector array 14.
Radiation from one particular scene point, that is the same point, passes through the bi-refringent element 12, which moves the first and second polarisation components of this radiation by exactly the pitch of the detector element array. Hence, in the example of Figure 2, radiation from the same point may be imaged simultaneously onto one of the vertical polarisation sensitive elements of the top row and the corresponding horizontal polarisation sensitive element of the second row. In this way, simultaneous registered images for each polarisation are provided using a single detector element array 14 and a single bi-refringent element.
Radiation incident on the elements is integrated. This integration is simultaneous for every clement of the array. For each detector element, an electrical signal is produced corresponding to the integrated amount of radiation. The information from the complete array is then read out rapidly, with alternate pixels providing separate images for the two directions of polarisation. Processing electronics (not shown) transfer the electrical information from the detector array 14 to video displays either one for each polarisation direction or a combined display. Alternatively, the images can bc used for further vidco processing. In some applications, a single display will be used to display an image synthesised from the two images, as an example the sum of the two signals being displayed as brightness, and the difference as colour.
Techniques for processing images using data captured by radiation sensitive detectors are well known in the art and so will not be described in detail.
hi one embodiment, for the 3 to 5 p.m waveband, the bi-refringent element is made of magnesium fluoride, and is 10.5 mm thick, tilted at an angle of 20 . This produces a displacement difference of 20 p.m. However, other bi-refringent materials and physical arrangements may be used, For example, materials such as Thallium Arsenic Selenide or Cadmium Sulphide could be used for the 8 -12 p.m waveband. The bi-refringent device may be colour corrected, although this is not essential for narrow band detectors such as QWIPs.
A skilled person will appreciate that 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 patterns for the polarisation detector elements may be used, for example vertical stripes or a checkerboard, as shown in Figures 3 and 4. Equally, other combinations of detector pitch and plate may be used, provided the dispersion between the different polarisation images matches their detector element spacing.
Whilst the bi-refringent plate is shown in Figure 1 as being directly in front of the detector, it may be at any of a number of positions in the optical path. Furthermore, bi-refringent devices other than a simple plate can be used, for example a prism in a parallel-light space. Also, although a refractive bi-refringent element is described, a reflective or diffractive device could also be used. Accordingly, the above description of the specific embodiment is made by way of example only and not for the purposes of limitation. It is clear that minor modifications may be made without significant changes to the operation described.

Claims (20)

  1. claims I. An irnager for imaging a scene, the imager having: a detector
    having at least one element sensitive to radiation having a first polarisation and at least one element sensitive to radiation having a second polarisation, and directing means for simultaneously directing from the same scene or part of a scene radiation of the first polarisation onto the first polarisation element and radiation of the second polarisation onto the second polarisation element.
  2. 2. An imager as claimed in claim 1 wherein the detector comprises a plurality of first and second elements,
  3. 3. An imager as claimed in claim 2 wherein the detector comprises a two-dimensional array of detector elements.
  4. 4. An imager as claimed in claim 2 or claim 3 wherein alternate elements are sensitive to different directions of polarisation.
  5. 5. An imager as claimed in claim 4 wherein the first and second polarisation elements are arranged in alternate rows.
  6. 6. An imager as claimed in claim 4 wherein the first and second polarisation elements are arranged in alternate columns.
  7. 7. An imager as claimed in claim 4 wherein the first and second polarisation elements are arranged in a checkerboard layout.
  8. 8. An imager as claimed in any of the preceding claims wherein the detector is a Quantum Well Infrared Photodetector (QWIP).
  9. 9. An imager as claimed in any of the preceding claims wherein the directing means comprise a bi-refringent material.
  10. 10. An imager as claimed in claim 9 wherein the directing mcans comprises a bi-refringent plate.
    S
  11. ii. An imager as claimed in any of the preceding claims, wherein the directing means are configured so that displacement of the raxiiation of different polarisations corresponds to the pitch of the first and second elements.
  12. 12. An inlager as claimed in any of the preceding claims comprising a lens for imaging a scene onto the detector.
  13. 13. An iniager as claimed in claim 12 wherein the directing means are provided between the lens and the detector.
  14. 14. An imager as claimed in any of the preceding claims wherein the directing means consist of a single element.
  15. 15. An imager as claimed in any of claims I to 14 wherein the directing means comprise a plurality of elements.
  16. 16. An imager as claimed in any of the preceding claims wherein the or each first polarisation element is sensitive to vertical polarisation and the or each second polarisation element is sensitive to horizontal polarisation.
  17. 17. An imager as claimed in any of the preceding claims wherein the imager is a thermal imagcr.
  18. IS. A method for imaging a scene comprising: simultaneously directing from the same scene or same part of a scene radiation of a first polarisation onto a first polarisation element and radiation of a second polarisation onto a second polarisation element and using the simultaneously captured first and second polarisation radiation to form one or more images of the scene.
  19. 19. A thermal imager as described hereinbefore with reference to the accompanying drawings.
  20. 20. An imaging method as described hereinbefore with reference to the S accompanying drawings.
GB0625803A 2006-12-22 2006-12-22 Imager with detector elements sensitive to radiation of different polarisations Withdrawn GB2444961A (en)

Priority Applications (2)

Application Number Priority Date Filing Date Title
GB0625803A GB2444961A (en) 2006-12-22 2006-12-22 Imager with detector elements sensitive to radiation of different polarisations
PCT/EP2007/064057 WO2008077839A1 (en) 2006-12-22 2007-12-17 An imager

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
GB0625803A GB2444961A (en) 2006-12-22 2006-12-22 Imager with detector elements sensitive to radiation of different polarisations

Publications (2)

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GB0625803D0 GB0625803D0 (en) 2007-02-07
GB2444961A true GB2444961A (en) 2008-06-25

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WO (1) WO2008077839A1 (en)

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0549482A2 (en) * 1991-12-05 1993-06-30 Eastman Kodak Company Apparatus for focus error detection
US5898167A (en) * 1974-09-26 1999-04-27 Olympus Optical Co., Ltd. Optical pick-up head and integrated type optical unit for use in optical pick-up head
US6310345B1 (en) * 1999-10-12 2001-10-30 The United States Of America As Represented By The Secretary Of The Army Polarization-resolving infrared imager
US6410917B1 (en) * 2000-01-18 2002-06-25 The United States Of America As Represented By The Secretary Of The Army Polarization-sensitive corrugated quantum well infrared photodetector array

Family Cites Families (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5438414A (en) * 1993-01-22 1995-08-01 The Johns Hopkins University Integrated dual imaging detector
FR2812942B1 (en) * 2000-08-08 2002-10-31 Commissariat Energie Atomique POLARIZED LIGHT FLUORESCENCE IMAGING DEVICE
US7511293B2 (en) * 2005-02-25 2009-03-31 Nanometrics Incorporated Scatterometer having a computer system that reads data from selected pixels of the sensor array

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5898167A (en) * 1974-09-26 1999-04-27 Olympus Optical Co., Ltd. Optical pick-up head and integrated type optical unit for use in optical pick-up head
EP0549482A2 (en) * 1991-12-05 1993-06-30 Eastman Kodak Company Apparatus for focus error detection
US6310345B1 (en) * 1999-10-12 2001-10-30 The United States Of America As Represented By The Secretary Of The Army Polarization-resolving infrared imager
US6410917B1 (en) * 2000-01-18 2002-06-25 The United States Of America As Represented By The Secretary Of The Army Polarization-sensitive corrugated quantum well infrared photodetector array

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Publication number Publication date
GB0625803D0 (en) 2007-02-07
WO2008077839A1 (en) 2008-07-03

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