Radiant energy – Calibration or standardization methods
Reexamination Certificate
1999-07-02
2001-11-20
Hannaher, Constantine (Department: 2878)
Radiant energy
Calibration or standardization methods
C250S208100
Reexamination Certificate
active
06320186
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to infrared detectors, and more particularly to a method of compensating for non uniformities among detector elements of an infrared detector array.
BACKGROUND OF THE INVENTION
Infrared detectors provide thermal images for temperature measurement and heat detection. They are used for various applications, such as for military, industrial, and medical applications. In its simplest form, an infrared detector is a device, such as a photosensitive diode, that generates an electric current when exposed to infrared radiation. This current is dependent on the intensity and wavelength of the radiation and can be used in many different ways to produce an infrared picture.
Infrared detectors may be configured as a single element (detector), a small array of elements, a long linear array, or a full two-dimensional array. When the detector is a full two-dimensional array, the entire image is recorded at once, and the array is referred to as a “staring” array. However, with smaller arrays, the image is scanned over the array. The small array requires a serial scan to sweep the image in two-dimensions, whereas the linear array requires a “pushbroom” scan to sweep the image across the array in one dimension.
The current produced by an infrared detector is amplified and processed to provide a more useful detector output. The processing reduces interference due to external and internal causes, such as electrical noise.
The ideal response of an infrared detector array is that each detector element exhibit the same linear voltage response for given temperature changes in the irradiation of the array. However, one type interference with a good detector signal is electrical noise due to detector non-uniformity among detector elements. The uniformity differences have both spatially and temporally dependent causes.
A number of methods have been tried for compensating non uniformity of infrared detector arrays. Generally, all involve some sort of data processing. Some methods use a uniform calibration source, typically using a chopper and controlled temperature. Other methods are scene-based, which means that they use an image comprised of one or more objects or patterns. The scene-based methods may be further categorized into mechanical and non-mechanical methods.
The “dithered scan” method of non uniformity compensation is a scene-based mechanical method. The detector array views a scene through suitable optics. During a given time frame, the incident flux is sensed by each detector element. At the end of the time frame, the array data is delivered for processing and the array is displaced (“dithered”) a fixed distance, typically a distance equal to the width or height of one detector element, in either the horizontal or vertical direction. The scene flux is assumed to be stable throughout the dither cycle. Thus, during the next time frame, each detector element is exposed to the flux seen by one of its neighbors during the prior time frame. These detector pairs can be “linked” analytically, such as by averaging their outputs. By a suitable choice of a dither pattern, each detector can be linked with several of its neighbors, to adjust gain and offset differences. A dithered scan approach is described in an article by William F. O'Neil, “Dithered Scan Detector Compensation”,
Proc. IRIS Passive Detectors
, 1992, Vol. 1, pp. 123-134.
SUMMARY OF THE INVENTION
One aspect of the invention is a “one-step” method of calculating offset correction values for detector elements of an infrared detector array. The array is dithered with respect to the scene that is to be imaged. The dither motion is such that pairs of neighboring detector elements are exposed to the same scene flux. The method assumes the scene flux is stable over the dither cycle. With this assumption, neighborhood averaging is used to adjust offset differences, based on their responses to the same scene irradiance. The neighborhood averaging uses the sum of “shifted image differences”, where shifted images (matrices of detector output values) from a first field and a second field, respectively, are differenced. This sum is divided by four. A scene term may be added to compensate for the bias.
The “one-step” method is the mathematical equivalent of “two-step” methods. Specifically, for offset correction calculations, a detector element and its neighbors are exposed to the same scene flux. This permits calculation of a local response average, which is subtracted from the detector element's output to determine the output correction for that detector element.
Generally, for a given image, a complete non-uniformity correction process involves calculations for each detector element. A multiplicative gain correction factor is calculated and applied to that detector element's output. The method is iterative, such that new gain correction updates become constant (equal to unity), reducing gain errors to smaller and smaller errors. Then an offset correction is interactively calculated and subtracted, such that the new offset correction updates become constant (equal to zero). The image can be displayed during the iterations with successively smoothed gain and offset errors, or the display can be delayed until the correction values become stable to some predetermined level.
The method can be mathematically described with matrix a notation, which shows that the method is equivalent to a low pass filtering process. For each detector element, the calculations use local neighbor averages, which include values representing the detector element whose error is being calculated as well as values representing its neighbors. As a result, the method is a true low pass filter, as compared to previous techniques. It better smooths the high frequency components of the gain and offset matrices.
REFERENCES:
patent: 5144132 (1992-09-01), Kitakado
patent: 5276319 (1994-01-01), Hepfer et al.
patent: 5514865 (1996-05-01), O'Neil
patent: 5838813 (1998-11-01), Kancler et al.
patent: 5925875 (1999-07-01), Frey
patent: 6184527 (2001-02-01), Young
patent: WO 98/26582 (1998-06-01), None
Woolfson M.G., “Electonric LOS Jitter Compensation for Staring Sensor”, Westinghouse Electric Corporation, SPIE, vol. 1762, pp. 317-326, Jul. 19, 1992.
McCormack Kent
Turner Larry A.
Young Ching-ju Jennifer
Baker & Botts L.L.P.
Gabor Otilia
Hannaher Constantine
Raytheon Company
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