Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
1998-05-26
2001-08-14
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C250S332000, C250S252100
Reexamination Certificate
active
06274869
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to focal plane array signal processing. and, more particularly, to a digital offset correcting signal processor for a microbolometer focal plane array.
2. Discussion of the Related Art
A focal plane array that uses an array of radiation sensors typically suffers from variations in sensor element output from sensor to sensor. This variation comprises two basic components, an offset uniformity error and a gain uniformity error. Offset uniformity error may be measured by blocking external radiation from the radiation sensing elements of the array and measuring the variations in output from each sensor element. Gain uniformity error may be measured by presenting a uniform radiant field to the focal plane array and noting the variations in sensor element output.
Until the present invention, focal plane array offset correction has been limited to analog circuits that attempt to subtract a real time or stored analog signal from a detected signal. Such approaches provide poor offset compensation in relation to the accuracy of the apparatus and method of the present invention for providing offset correction.
One such type of radiation sensor is a bolometer. A bolometer absorbs infrared radiation. A bolometer has a large temperature coefficient of resistance and, as a result, absorbed radiation changes the temperature of the bolometer and, therefore, the bolometer's resistance. Bolometer imaging systems are sensitive to long wave infrared, typically in a wavelength range of 8-12 micrometers. Such infrared systems form images from variations in the amount of radiation emitted from objects within a field of view.
SUMMARY OF THE INVENTION
Variations in the outputs of bolometer detector elements may be translated to a nonuniformity in linear slope response from sensor element to sensor element. A gain factor may then be calculated for each element from the linear slope response. The output of each element may first be offset, and then gain corrected to produce a uniform response,
The invention provides a method and apparatus for setting the offset of output signals for an array of bolometers. A pixel-by-pixel offset compensator of the invention is disclosed for a microbolometer focal plane array including a plurality of detector elements. Each one of the plurality of detector elements has a detector output. The apparatus of the invention includes apparatus for providing a separate offset for each detector in the array where the separate offset may be derived from information from an external source. The apparatus further comprises a means for time sharing connected to the providing means, where the means for time sharing cooperates with the providing means to provide different offsets for different elements in the array. In another aspect of the invention, the means for providing a separate offset further comprises a means for adjusting a current to the detector.
In yet another aspect of the invention, the means for providing a separate offset further comprises a means for adjusting a voltage at the detector.
In yet another aspect of the invention, the means for providing a separate offset further comprises a means for connecting a resistance to the detector.
In another aspect of the invention, a pixel-by-pixel offset compensator for a microbolometer focal plane array comprises an array of detector elements. Each one of the array of detector elements has a detector output. Means for providing a time-varying compensation bias for each detector in the array is connected to the array of elements to compensate for effects of change in detector resistance during a bias control signal.
In another aspect of the invention, the means for providing a time-varying compensation bias further comprises a means for compensating that changes the offset current for each element in the array.
In another aspect of the invention, the compensating means further comprises a means for generating a time-varying compensation waveform so as to force the average current to be constant during the bias control signal.
In another aspect of the invention, the means for providing a time-varying compensation bias further comprises a means for compensating by changing the detector bias voltage for each element in the array.
In another aspect of the invention, the compensating means further comprises a means for generating a time-varying compensation waveform so as to force the average current to be constant during the bias control signal.
In another aspect of the invention, a pixel-by-pixel offset compensator for a microbolometer focal plane array comprises a plurality of detector elements including at least one output carrying a signal. Means for generating a dynamic residue by dynamically offsetting the at least one output with an offset is connected at the output. Circuitry is coupled to the output for measuring the dynamic residue for each detector where the dynamic residue comprises the signal at the at least one output minus the offset.
In another aspect of the invention, the circuitry for measuring the dynamic residue further comprises means for integrating a current onto a capacitor, where the integrating means is connected to the at least one output, means for determining when the capacitor reaches a threshold value, means for resetting the capacitor when it reaches the threshold value, and means, connected to the capacitor, for measuring the number of times the capacitor voltage reaches the threshold value so as to measure the offset.
In another aspect of the invention, means for providing a separate preset for each of the plurality of detectors is connected to the capacitor wherein the separate preset is supplied from an external source.
In another aspect of the invention, the offset correction apparatus and the plurality of detector elements are constructed on an integrated circuit.
Other objects, features and advantages of the present invention will become apparent to those skilled in the art through the description of the preferred embodiment, claims and drawings herein wherein like numerals refer to like elements.
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Gagliardi Albert
Hannaher Constantine
Lockheed-Martin IR Imaging Systems, Inc.
Wolf Greenfield & Sacks P.C.
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