Coded aperture imager comprising a coded diffractive mask

• US 7,923,677 B2
• Filed: 02/06/2007
• Issued: 04/12/2011
• Est. Priority Date: 02/06/2006
• Status: Expired due to Fees

• Current Assignee: Qinetiq Limited (QinetiQ Group PLC)
• Original Assignee: Qinetiq Limited (QinetiQ Group PLC)
• Inventors: Slinger, Christopher William
• Primary Examiner(s): Luu, Thanh X
• Application Number:
  US12/278,521
• Publication Number:
  US 20090020714A1
• Time in Patent Office: 1,526 Days
• Field of Search: 250/208.1, 250/237.G, 250/216, 250/363.06
• US Class Current: 250/216
• CPC Class Codes: G01T 1/295 using coded aperture devices, e.g. Fresnel zone plates handling of radiation of particles, e.g. using diaphragms, collimators, diffraction G21K1/00
• CPC Class Codes: G02B 2207/129 Coded aperture imaging
• CPC Class Codes: G02B 27/4205 having a diffractive optical element [DOE] contributing to image formation, e.g. whereby modulation transfer function MTF or optical aberrations are relevant
• CPC Class Codes: G02B 27/4294 in multispectral systems, e.g. UV and visible
• CPC Class Codes: H04N 25/00 Circuitry of solid-state image sensors [SSIS]; Control thereof

Abstract

The present invention relates to an imaging system which employs the same principles as coded aperture imaging. High angular resolution coded aperture imagers require a small aperture size and relatively large spacing between the coded aperture array and the detector. At such high resolutions diffraction effects can start to dominate and can degrade image quality. The present invention provides a detector array which receives radiation from a scene via a coded diffractive mask. The coded diffractive mask is designed such that its diffraction pattern at the waveband of interest is a well conditioned coded intensity pattern having a strong autocorrelation function with low sidelobes. Thus radiation reaching the detector array is diffracted by the diffractive mask but in a defined way and it is the diffraction pattern of the mask which provides the coding. The scene image can then be reconstructed using the same techniques as for conventional coded aperture imaging but using the diffraction pattern of the mask as the aperture function. The coded diffractive mask may be a binary or greyscale mask, may operate in reflection or transmission and may be an amplitude or phase modulating mask.

Claims

1. A coded aperture imager for forming an image from radiation from a field of view, the radiation being within a waveband of operation of the imager, and the imager comprising:

a coded diffractive mask for receiving the radiation and for forming a coded diffraction pattern thereof upon a detector array to provide output data, the mask having a property that for a single point in the field of view and at the imager'"'"'s waveband of operation the said coded diffraction pattern associated therewith is well conditioned with a sharp autocorrelation function having small sidelobes; and

a processor configured to reconstruct an image of the field of view on the basis of an equation representing the detector array output data,

wherein the equation contains a diffraction expression.

2. A coded aperture imager according to claim 1 wherein the coded diffractive mask is one of a binary amplitude mask, an analogue amplitude mask, a binary phase modulating mask and an analogue phase modulating mask.

3. A coded aperture imager according to claim 1 wherein the coded diffractive mask is provided by a reconfigurable coded diffractive mask means.

4. A coded aperture imager according to claim 1 wherein at least one optical element is located in an optical path between the coded diffractive mask and the detector array.

5. A coded aperture imager according to claim 1 including multiple coded diffractive masks for simultaneous use.

6. A coded aperture imager according to claim 1 including at least one filtering means with a passband which is changeable at least one of periodically, aperiodically and in response to a control signal.

7. A coded aperture imager according to claim 6 including different coded diffractive masks for different passbands of the filtering means.

8. A method of forming an image from radiation from a field of view by coded aperture imaging in a waveband of operation, wherein the method incorporates:

arranging a coded diffractive mask to receive the radiation and to form a coded diffraction pattern thereof upon a detector array to provide output data, the mask having a property that for a single point in the scene and at the waveband of operation the said coded diffraction pattern associated therewith is well conditioned with a sharp autocorrelation function having small sidelobes;

representing the detector array output data in terms of an equation which contains a diffraction expression; and

reconstructing an image of the field of view on the basis of the equation.

9. A method according to claim 8 including the step of periodically reconfiguring the coded diffractive mask so as to provide at least one of a different field of view and a different resolution.

10. A method according to claim 8 further comprising the step of reconfiguring the coded diffractive mask and changing the waveband of operation.