Scene based nonuniformity compensation for starting focal plane arrays

• US 4,975,864 A
• Filed: 01/26/1989
• Issued: 12/04/1990
• Est. Priority Date: 01/26/1989
• Status: Expired due to Term

• Current Assignee: Raytheon Company (Rtx Corporation)
• Original Assignee: Hughes Aircraft Company (Rtx Corporation)
• Inventors: Knight, Donald K., McKeefery, Raymond L., Dan, Chuong D., Ramirez, Marvin J., Sendall, Robert
• Primary Examiner(s): Lall, Parshotam S.
• Assistant Examiner(s): ZANELLI, MICHAEL J
• Application Number:
  US07/302,534
• Time in Patent Office: 677 Days
• Field of Search: 364/570, 364/571.01, 364/571.04, 364/571.05, 364/571.07, 364/571.08, 364/572, 364/575, 364/582, 250/332, 382/50, 382/52, 382/54, 358/113
• US Class Current: 382/275
• CPC Class Codes: H04N 25/674 based on the scene itself, e.g. defocusing
• CPC Class Codes: H04N 5/33 Transforming infrared radiation cameras or camera modules for generating image signals from infrared radiation H04N23/20; circuitry of SSIS for transforming infrared radiation into image signals H04N25/20

Abstract

Apparatus and methods for providing nonuniformity compensation of staring infrared focal plane array imaging systems, or other video imaging ssytem, or the like. The invention comprises a processor which implements nonuniformity compensation of the detectors comprising the array. The processor generates compensation terms that are stored in an offset term memory and which are subsequently combined with the output signals from the array. The processing accomplished by the present invention normalizes all detector elements in the array such that they all appear to respond to infrared energy in an identical manner. The processor comprises a median filter which selectively implements cross (X) shaped and plus (+) shaped filters. An antimedian calculator computes the antimedian of the output of the median filter. This value comprises the difference between the central pixel of a respective filter and the median value of all pixels in the cross (X) or plus (+) shaped filter. A third filter samples each of the signals from the detector array and compares them to a preset value indicative of an anticipated scene intensity level determined by the operator to provide an output signal indicative of the difference. Control circuity selects which output signal of the filter circuits is to be used to compensate the detector signals during a particlar video field. The ouptut signals of the antimedian calculator and average filter comprise sign information which is indicative whether the central pixel value is less than, equal to or greater than the median, or whether the central pixel is less than, equal to or greater than the preset value, respectively. The control circuitry increments or decrements the value of the offset terms stored in the offset term memory in resonse to the signal provided by the selected antimedian calculator or third filter, and convergence rate information supplied by the control circuitry which controls the rate of convergence of the offset terms toward the scene average.

Claims

1. A method of processing signals derived from an array of detector elements to compensate for nonuniformities between detector elements such that each detector element appears to respond to received energy in an identical manner, said method comprising the steps of:

selectively processing output signals derived from the detector elements by means of an antimedian cross (X) shaped filter to determine the difference between the value of the central signal in the cross (X) shaped filter and the median value of all signals comprising the filter and for providing a first output signal indicative of the difference;

selectively processing output signals derived from the detector elements by means of an antimedian plus (+) shaped filter to determine the difference between the value of the central signal in the plus (+) shaped filter and the median value of all signals comprising the filter and for providing a second output signal indicative of the difference;

selectively processing output signals derived from the detector elements and comparing each of them to a preset intensity value to determine the difference therebetween and for providing a third output signal indicative of the difference; and

adjusting the value of the compensation term associated with the detector element corresponding to the central signal by a predetermined amount in response to a selected one of the output signals to adjust the value of the compensation term to bias the detector output toward a uniform response.

2. A method of processing signals derived from an staring array of detector elements to compensate for nonuniformities between detector elements such that each detector element appears to respond to received energy in an identical manner, said method comprising the steps of:

selectively processing output signals derived from the detector elements of the staring array by means of an antimedian cross (X) shaped filter to determine the difference between the value of the central signal in the cross (X) shaped filter and the median value of all signals comprising the filter and for providing a first output signal indicative of the difference;

selectively processing output signals derived from the detector elements of the staring array by means of an antimedian plus (+) shaped filter to determine the difference between the value of the central signal in the plus (+) shaped filter and the median value of all signals comprising the filter and for providing a second output signal indicative of the difference;

selectively processing the signals derived from each of the detector elements of the staring array and comparing them to a preset intensity value to determine the difference therebetween and providing a third output signal indicative of the difference; and

adjusting the value of the compensation term associated with the detector element corresponding to the central signal by a predetermined amount in response to a selected one of the output signals to adjust the value of the compensation term to bias the detector output toward a uniform response.

3. The method of claim 2 wherein the step of selectively processing output signals from the detector elements by means of an antimedian cross (X) shaped filter comprises the steps of:

determining the median of detector signals within a cross (X) shaped window;

comparing the median to the value of the central element in the window;

determining the antimedian of the central element in the window if the value of the central element is unequal to the median; and

comparing the antimedian to zero, and incrementing the central value by a predetermined amount if the antimedian value is greater than zero and decrementing the central value by a predetermined amount if the antimedian value is less than zero.

4. The method of claim 2 wherein the step of selectively processing output signals from the detector elements by means of an antimedian plus (+) shaped filter comprises the steps of:

determining the median of detector signals within a plus (+) shaped window;

comparing the median to the value of the central element in the window;

determining the antimedian of the central element in the window if the value of the central element is unequal to the median; and

comparing the antimedian to zero, and incrementing the central value by a predetermined amount if the antimedian value is greater than zero and decrementing the central value by a predetermined amount if the antimedian value is less than zero.

5. A signal processor for use with an imaging system comprising an array of detector elements, which implements nonuniformity compensation of the detector elements, said processor comprising:

filter means for sampling a subset of signals derived from the array, which subset corresponds to a plurality of detector elements in the array including a selected detector element which is to be compensated, and for selectively processing the subset of signals by means of an antimedian cross (X) shaped filter and an antimedian plus (+) shaped filter, and for providing an output signal from the selected filter which is indicative of the difference between the signal derived from the selected detector element and the median value of the signals processed by the selected filter, and for providing an output signal from the selected filter which is indicative of the sign of the filter computation indicating whether the selected signal is equal to, greater than or less than the median;

comparator means for processing signals from the array and for comparing each of the signal levels to a preset signal level indicative of an anticipated scene intensity level, and for providing an output signal indicative of the sign of the comparison indicating whether the anticipated scene level is equal to, greater than or less than the preset signal level;

selection means coupled to the filter means and the comparator means for selectively choosing an output signal therefrom and for providing an output signal corresponding to the selected signal;

offset memory means coupled to said selection means for storing and subsequently combining offset compensation terms with output signals from the array;

controller means coupled to the comparator means for providing the preset signal level thereto, and coupled to the filter means and comparator means for selecting an output signal from the antimedian cross (X) shaped filter, the antimedian plus (+) shaped filter, or the comparator means, and coupled to the selection means for controlling the selection means to adjust the compensation terms stored in the offset memory means as a function of a predetermined rate of convergence in response to the selected output signal;

whereby nonuniformities in the detector elements of the array are compensated for by adjustment of the compensation terms such that each detector element appears to respond to infrared energy in an identical manner.

6. A signal processor for use with a staring focal plane array infrared imaging system which implements scene based nonuniformity compensation of the detectors comprising the array, said processor comprising:

first filter means for processing signals from the focal plane array by means of an antimedian cross (X) shaped filter and for providing an output signal therefrom which is indicative of the difference between the median of the signals processed thereby and the central signal within the cross (X) shaped filter;

second filter means for processing signals by means of an antimedian plus (+) shaped filter and for providing an output signal therefrom which is indicative of the difference between the median of the signals processed thereby and the central signal within the plus (+) shaped filter;

average filter means for comparing the signal level of each of the signals derived from detectors in the array to a preset signal level, and for providing a third output signal which is indicative of the difference therebetween;

offset memory means for storing and subsequently combining offset compensation terms with processing signals from the focal plane array;

controller means coupled to the first filter means, the second filter means and the average filter means for controlling the selection of the filter means utilized to adjust the offset compensation terms and for processing the output signals provided by the selected filter to adjust the offset compensation terms as a function of the output signal of a selected filter means;

whereby nonuniformities in the detector elements of the array are compensated for such that each detector element appears to respond to infrared energy in an identical manner.

7. A signal processor for use with a focal plane array infrared imaging system which implements scene based nonuniformity compensation of the detectors comprising the array, by determining and adjusting nonuniformity compensation terms stored in an offset term memory and which are combined with output signals from the array, said processor comprising:

register means for sampling a matrix of output signals from the array;

first selection means for selectively sampling a plurality of signals from the matrix in the form of a cross (X) shaped set of signals and a plus (+) shaped set of signals;

comparison means coupled to the first selection means for determining the respective median value of signals comprising the cross (X) shaped set of signals and the plus (+) shaped set of signals, and for providing an output signal which is indicative of the median of the compared signals;

second selection means coupled to the first selection means and the comparison means for responding to the output signal from the comparison means and for selecting the median value of the respective cross (X) shaped set of signals and plus (+) shaped set of signals;

antimedian calculation means coupled to the register means and the second selection means for determining the difference between the median value provided by the second selection means and the central signal of the matrix of signals;

comparator means for comparing the intensity level of each of the signals derived from the detector array to a preset signal value, and for providing an output signal indicative of the sign of the comparison indicating whether the output signal value is equal to, greater than or less than the preset signal level;

controller means coupled to the comparator means for providing the preset signal value thereto, and coupled to the first selection means and comparator means for selecting an output signal from the cross (X) shaped set of signals, the plus (+) shaped set of signals, or the comparator means, which selected signal is employed to adjust the compensation terms, and for controlling adjustment of the compensation terms in response to the selected signal.