Image analysis – Image enhancement or restoration – Image filter
Reexamination Certificate
1998-10-19
2001-06-05
Lee, Thomas D. (Department: 2724)
Image analysis
Image enhancement or restoration
Image filter
C382S103000, C382S255000, C382S218000, C250S347000
Reexamination Certificate
active
06243498
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to image and data processing systems and techniques. More specifically, the present invention relates to systems and techniques for adaptive non-uniformity compensation for focal plane arrays of infrared detectors and the like.
2. Description of the Related Art
Focal plane arrays are used in military, astronomical and other applications. For example, in military applications, focal plane arrays are often used for target acquisition and tracking. The seekers of missiles often use arrays of image detectors sensitive to energy in the visible or (more typically) infrared portion of the electromagnetic spectrum. Unfortunately, these arrays, referred to as focal plane arrays are subject to anomalies such as detector to detector nonuniformity in sensitivity, gain/offset and fixed pattern noise. While numerous techniques are known in the art for addressing detector to detector nonuniformity in sensitivity, gain/offset, fixed pattern noise continues to be problematic.
Fixed pattern noise (FPNs) are sensor fixed artifacts induced by non-uniform response of the focal plane array (FPA). The non-uniform response causes the FPA output to be spatially varying even when illuminated by a uniform source. Techniques such as stored non-uniformity correction can correct for the non-uniform array response under static conditions. But dynamic inputs, such as changing photon flux induced by dome heating, requires dynamic or adaptive non-uniformity compensation.
Accordingly, nonuniformity compensation (NUC) systems have been developed to address detector to detector nonuniformities. In particular, adaptive nonuniformity compensation systems (ADNUC) have been developed to address fixed pattern noise in focal plane arrays of image detectors. Traditional ADNUC systems use an additive feedback algorithm wherein a correction-offset term is accumulated from an error-term which is generated from the filtered output image. The correction term is then subtracted from the next input image. Therefore, depending on the feedback coefficients (the nonlinear transfer functions), it takes about 10-30 image frames for the ADNUC systems to reduce the FPN from an initial high value to a low equilibrium value. While nominally effective, this system limits the response time of the system.
In addition, conventional ADNUC systems are not designed to remove temporally correlated temporal noise. Further, because FPNs are fixed on the focal plane and hence are temporally correlated, they present a more difficult false alarm problem than that presented by temporally uncorrelated temporal noise (TN). In traditional ADNUC systems, the accumulated correction-offset terms will cause fixed-pattern artifacts, which may lead to a high number of false alarms for target detection by the missile tracking system (i.e., the “tracker”).
Further, traditional systems do not remove hot-dome shading effects. Hot dome shading is a heating of the missile dome due to aerodynamic friction effects. The heat on the dome creates a thermal background image which causes a filter mismatch in the tracker and thereby limits the performance of the system.
Hence, a need remains in the art for a system and technique for addressing fixed pattern noise in focal plane arrays. Specifically, there is a need for a system and technique for rapidly addressing fixed pattern noise, including temporal noise and dome shading, in focal plane arrays of infrared image detectors.
SUMMARY OF THE INVENTION
The need in the art is addressed by the adaptive nonuniformity compensation system and method of the present invention. The inventive system is adapted for use with a focal plane array of electromagnetic energy detectors and is adapted to receive first and second interleaved frames of image data from electromagnetic energy received from at least a portion of a scene within a field of view of the detectors. The first frame is a focused frame and the second frame is a blurred frame. In a feed-forward path the inventive system compares the first frame to the second frame and provides an error signal in response thereto. In a main path, the system multiplies (or divides) at least a portion of the second frame of image data with the error signal to provide an noise error corrected output signal.
In the preferred embodiment, the error signal is scaled prior to being multiplied or divided by the second frame. An anti-mean (high pass) filter is provided to remove dome shading effects from the frames of image data. In the best mode, the anti-mean filter is disposed in the main path and blurred and focused outputs therefrom are weighted, averaged and stored. The weighted, averaged and stored focused frames are compared to the weighted, averaged and stored blurred frames to provide a fixed pattern noise error signal. A temporal noise error signal is identified from the weighted, averaged and stored focused frames. The fixed pattern and temporal noise error signals are fed forward and shunted from a current frame using multiplication or division. Thereafter, a constant mean value may be added to provide the output signal. Pixel replacement can be consolidated into a single circuit and positioned prior to the anti-mean filter.
The invention provides a feedforward shunting system and technique which reduces fixed pattern noise on a frame by frame basis without accumulation of error terms from prior frames. The system uses the focused and blurred frame pairs and anti-mean filters to find spatial nonuniform regions in the image. For those pixels identified to be fixed pattern noise, the pixel value derived from the focal plane array is shunted (divided by a large number, typically 8 to 64). For non-fixed pattern noise pixels, the focal plane array output is unmodified. This shunting process leads to faster adaptation and results in reduced residual FPN and artifact generation.
Hence, the invention provides a system and technique for rapidly addressing fixed pattern noise, including temporal noise and dome shading, in focal plane arrays of infrared image detectors.
REFERENCES:
patent: 4975864 (1990-12-01), Sendall et al.
patent: 5400161 (1995-03-01), Lambert, Jr.
patent: 5721427 (1998-02-01), King et al.
patent: 5903659 (1999-05-01), Kilgore
Braunreiter Dennis C.
Chen Hai-Wen
Schmitt Harry A.
Collins David W.
Lee Thomas D.
Lenzen, Jr. Glenn H.
Raytheon Company
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