Patent number 8605161 is assigned to
The following quote was obtained by the news editors from the background information supplied by the inventors: "This invention relates to imaging systems, and more particularly to relaxing limitations on scene motion across the imager to address issues such as imaging system pointing and stabilization limitations, non-uniformity compensation (NUC), dead pixel clumps, burn-in mitigation, field-of-view (FOV) extension, super-sampling, pixel-phase induced noise in frame summing, and staying within the limits of optical stabilization actuators.
"Imaging systems image a field-of-view (FOV) of a scene along a line-of-sight (LOS) onto an image detector that captures images a certain frame rate. The 'scene' is what the imaging system is looking at such as an object of interest in a background. It is well known and widely accepted that any motion of the scene across the image detector produces smearing of the scene in the image during the frame. Such motion and smearing are tightly coupled. The higher the rate of motion the greater the amount of smearing. The tradeoff of rate of motion vs. smearing constitutes a system level trade space.
"Motion of the scene across the image detector has two components. A first motion component represents motion that the system would like to remove during frame integration. This component may include intentional motion of the imaging system across the scene (e.g. LOS scan to cover a larger area across the frame), unintentional motion of the image detector across the scene (e.g. platform jitter) and scene motion (e.g. the object of interest is moving relative to the background). A second motion component represents intentional scene motion across the imaging detector (as opposed to across the scene) that must not be removed. This component may be induced for such purposes as enabling pixel non-uniformity compensation, mitigating the effects of dead pixel clumps in the imager or the effects of burn-in such as vidicon ghosting, or for enabling effective FOV enlargement across a sequence of frames (as in step-stare, TDI, or non-TDI scanners).
"The imaging system may implement optical-stabilization to remove the first motion component. The system measures the unintentional and intentional motion of the detector across the scene and subtracts the motion from the estimated scene motion to produce an actuation signal. The actuation signal drives an actuator to control LOS to cancel the first motion component. The actuator may reposition the image detector, one or more optical components of the system's optical focusing system or a gimbal on which the imaging system is mounted. Because the actuator has a limited dynamic range, it may need to be continuously re-centered to stay within actuation limits. Re-centering produces intentional motion of the scene across the detector.
"Ideally, optical stabilization removes all unintentional sources of scene motion across the image detector. But it does not, in fact must not, remove the second motion component representing intentional scene motion across the imager from frame-to-frame. Therefore, in existing practice, the system designer must always balance the inherent trade-off of smearing vs. rate of intentional motion across the image detector.
"In certain imaging systems the frame rate is increased to relax the motion vs. smear tradeoff, and the consequent signal-to-noise (SNR) loss is mitigated by registering and summing multiple frames, producing a sum-image having a higher SNR ratio than the individual frames. For example, when the frame rate is increased by 5.times. and sum-frames are produced at the previous frame rate, the single-frame smear is reduced by about a factor of five. Frame summing is sometimes done to improve SNR induced by other system limitations, such as single-frame integration time, but it is still limited by registration and smear.
"Frame summing also produces another system sensitivity. The same motion that produces smear causes shifts in the sub-pixel phase of the image from frame-to-frame. Since the individual images are inherently pixelized, this produces an effective misregistration of the images to be summed, which causes the summing to increase the effective smearing. It also increases effective noise based on the non-repeatability of the smearing function (of the sub pixel phase histories) from sum frame to sum frame. In some cases, this blurring offsets much of the SNR benefit of summing multiple images together.
"The 'register and sum' function may be performed off of the image detector, in a computer or in dedicated logic, or may be performed on the image detector (e.g. orthogonal transform charge coupled device (CCD) or time domain integration (TDI) CCD). The advantage of performing the register and sum in the detector is a much higher frame rate (per summed frame), limiting the time over which smear occurs, and consequently the smear. But, this cannot fully eliminate the smear from intentional and necessary motion.
"Conventional optical stabilization cannot fix remaining smear either, because as stated previously this would require canceling the intentional motion. For example, scanning TDI uses registered 'frames' of a multi-column detector to improve performance over a single-column detector, but relies on image motion across the detector to scan the scene. Complete stabilization would cancel this motion. Thus, the designer is always left with at least one frame of smear embedded the summed frame."
In addition to the background information obtained for this patent, VerticalNews journalists also obtained the inventor's summary information for this patent: "The following is a summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not intended to identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description and the defining claims that are presented later.
"The present invention provides a system and method for decoupling within-frame motion from any intentional scene motion across the image detector from frame-to-frame, and more particularly the rate of intentional scene motion from smearing of the scene in the detected image with each frame. The invention may also control the sub-pixel phase of the image from frame-to-frame. In register/sum applications, controlling the sub-pixel phase from frame-to-frame either substantially eliminates misregistration of the images at full-pixel resolution or allows for super-sampling of the images onto a higher resolution grid. The ability to decouple intentional scene motion from smearing and to control the sub-pixel phase from frame-to-frame defines a new trade space that relaxes the limitations on intentional scene motion across the image detector to address issues such as imaging system pointing and stabilization limitation, non-uniformity compensation (NUC), dead pixel clumps, burn-in mitigation, FOV extension, super-sampling, pixel-phase induced noise in frame summing, and staying within the limits of optical stabilization actuators.
"In an embodiment in which an imaging system includes an image detector configured to capture images of a scene along a LOS in a sequence of frames, an actuator is driven during frames to control the LOS to provide optical stabilization to remove detector motion relative to the scene and is driven between frames to provide intentional scene motion in a discrete step across the detector from frame-to-frame.
"In an embodiment, the actuator is driven continuously during the frame to cancel scene motion across the image detector and is driven in a single discrete step between frames to provide intentional scene motion in a discrete step across the image detector from one frame to the next. Driving the actuator in multiple discrete steps between frames will still produce intentional scene motion in a single discrete step across the detector from one frame to the next.
"In an embodiment, the intentional scene motion across the detector is quantized to an integer multiple of a predetermined step size (e.g. a full-pixel). Driving the actuator in quantized step sizes provides control over the sub-pixel phase of the image between frames.
"In an embodiment, a sequence of images are registered based on the quantized motion of the actuator and summed to form a sum image. In one case, the intentional scene motion is quantized and the images are summed at the center sub-pixel phase to provide an image at full-pixel resolution. In another case, the intentional motion is a sub-pixel dither signal; the images are mapped according to their sub-pixel phase to a higher resolution grid to form a higher resolution image. The sub-pixel dither signal could be quantized as well provided the quantization step-size is sub-pixel.
"These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which:
BRIEF DESCRIPTION OF THE DRAWINGS
"FIG. 1 is a block diagram of an optically-stabilized imaging system;
"FIG. 2 is a flow diagram illustrating the steps and scene motion in detector space of intra-frame optical stabilization of a scene during frames and inter-frame intentional scene motion between frames;
"FIG. 3 is a diagram illustrating an embodiment of an actuation signal in scene space for LOS control for continuous optical stabilization during frames and discrete intentional scene motion between frames;
"FIG. 4 is a diagram illustrating the overlay of two unregistered digital images and the sum of the two registered digital frames comparing for different techniques of incorporating intentional scene motion across the imager with optical stabilization;
"FIG. 5 is a flow diagram of an embodiment for computing an actuation signal to control LOS to implement intra-frame optical stabilization and inter-frame intentional scene motion;
"FIG. 6 is a diagram of an actuation signal for control LOS for continuous optical stabilization during frames and discrete intentional scene motion to re-center the actuator between frames without smearing;
"FIGS. 7a and 7b are diagrams illustrating the problem posed by dead pixel clumps in the image detector and mitigation of the problem using inter-frame intentional scene motion without smearing;
"FIGS. 8a and 8b are diagrams illustrating the problem posed by burn-in in the image detector due to motion of a bright source across the scene and mitigation of the problem using inter-frame intentional scene motion without smearing;
"FIGS. 9a through 9d are diagrams illustrating FOV extension using inter-frame intentional scene motion; and
"FIG. 10 illustrates super-sampling of the image using inter-frame intentional scene motion."
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