Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1998-12-10
2003-05-13
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C382S255000
Reexamination Certificate
active
06563100
ABSTRACT:
STATEMENT OF GOVERNMENT INTEREST
The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
FIELD OF THE INVENTION
The present invention relates generally to detectors, and in particular to the enhancement of a display by refining an approximation of the actual energy received at a detector.
BACKGROUND
The measurement of radiance underlies many operations in remote sensing such as determining spectral signature (e.g., via measuring spectral directional reflectance factor, R) and sensing spatial imagery. Since optical detectors measure irradiance, the radiance is inferred and not directly measured. As a result there is a problem in the processing of irradiance measurement data in order to determine radiance that does not appear to be directly addressed in the literature. These errors intrinsic to the scene being viewed are produced by the unpredictable non-uniform illumination of the instantaneous-field-of-view (IFOV) of the individual optical detectors in an array of such detectors comprising a sensor.
These considerations are applicable to both passive and active sensors and to sensing via reflected and/or emitted radiance. They are also applicable to almost all detectors in the visible, infrared and ultraviolet bands that are designed to respond to the total incident optical power in whatever wavelength or bandwidths they are sensitive. Further background is provided in the following references:
Dereniak, E. L. and G. D. Boreman, “Infrared Detectors and Systems,” John Wiley, NY, 1996, pp. 152-3.
Hoist, G. C., “Electro-Optical Imaging System Performance,” SPIE Opt. Eng. Press, 2000, pp. 6,201, and 203.
Kaufman, Y. J., “The atmospheric effect on remote sensing and its correction,” Chap. 9 of “Theory and Applications of Optical Remote Sensing,” G. Asrar, Ed., John Wiley, NY, 1989, pp. 336-7.
McKenna, Charles M., Personal communication, May 1997.
Miller, J. L. and E. Friedman, “Photonics Rules of Thumb: Optics, Electro-Optics, Fiber Optics, and Lasers,” McGraw-Hill, NY, 1996, p. 269.
Nicodemus, F. E., J. C. Richmond, et al., “Geometrical considerations and nomenclature in reflectance,” National Bureau of Standards, U.S. Dept. Commerce, 1977, p. 37.
Nussbaum, A. and R. A. Phillips, “Contemporary Optics for Scientists and Engineers,” Prentice-Hall, Englewood Cliffs, N.J., Chap. 10, 1976, p. 273.
Schulz, M. and L. Caldwell, “Nonuniformity correction and correctability of infrared focal plane arrays,” Infrared Phys. & Tech., 36, 1995, pp. 763-777.
Stover, J. C., “Optical Scattering: Measurement and Analysis,” 2nd ed., SPIE Opt. Eng. Press, 1995, pp. 12-19.
During operation, individual detectors record the total irradiance E in some spectral band illuminating the non-infinitesimal solid angle subtended by each of the detectors. This is normally used to calculate an average radiance (L
av
) based on the presumption of a uniform illumination of that solid angle. Usually the pattern of illumination is in fact non-uniform. This non-uniformity is not, and probably cannot, in principle, be calibrated out by any in-lab, pre- or post-operational usage procedure. The reasons for this are that the non-uniformities: vary unpredictably with the specific portion of the particular scene being viewed, vary in an unpredictable way from detector to detector in the array since they are usually arranged to view different portions of the scene, and vary unpredictably from instant to instant for time-varying scenes and for arrays in relative motion with respect to the scenes being viewed.
The non-uniformity in illumination of the Instantaneous Field of View (IFOV) of a single detector can be due to relatively fine surface features, relatively small objects, changes in reflectance or emittance due to relatively abrupt changes in topography or ground or object composition, and relatively abrupt changes in surface altitude within the IFOV of a single detector. The IFOV is a function of individual detector geometry and other characteristics. This value is related to the projected solid angle of a detector indicative of the portion of a scene that the detector is capable of viewing at any one time. Both the IFOV and the projected solid angle are essentially dictated by the design of the sensor and need no further elaboration for their respective derivations.
Despite the variation in illumination within any one solid angle subtended by an optical detector, the single measured irradiance, E, from one detector produces a single output, i.e., a pixel in an image (for an imaging sensor) that has a single magnitude of brightness corresponding to L
av
in the detector's spectral band. This pixel covers a small, but non-infinitesimal, area of the image.
An image typically consists of an orderly array of a very large number of pixels that, when viewed as a whole, display the image. Irradiance (E) and radiance (L) are fundamentally related through the mathematical derivative relation
L
=
ⅆ
E
ⅆ
Σ
(
1
)
(where &Sgr;=projected solid angle) and its inverse (integral) relation
E=∫Ld&Sgr;
(2)
This effectively leads to characterizing the illuminated detectors as averaging the unpredictable non-uniform radiance received over the various lines-of-sight within their IFOV. Only when the illumination of the detector's IFOV is uniform or nearly so, is there little or no error from using a radiance/irradiance geometric relation specific to the detector geometry, to determine the radiance from the detector response.
In terms of average radiance incident on the i
th
detector, L
avi
, (averaged over the projected solid-angle &Sgr;
i
of the IFOV of the i
th
detector)
L
av
i
=
∫
θ
L
i
(
θ
,
Ω
)
ⅆ
Σ
i
∫
ⅆ
Σ
i
(
3
)
(where &thgr;=zenith angle and &OHgr;=azimuth angle and d&Sgr;
i
=cos &thgr;
i
d&thgr;
i
d&OHgr;
i
) the irradiance illuminating the i
th
detector can be expressed as
E
i
=&Sgr;
i
(
L
av
i
). (4)
Clearly, when L
i
(&thgr;,&OHgr;)=constant, then L
i
can be inferred from
L
i
=
E
i
Σ
i
(
5
)
However, when the unpredictable illumination of the IFOV is strongly non-uniform and the detector response still is related to the average radiance value, the user must consider what relationship that average radiance value has to his desired measure. Such a desired measure might be the peak radiance within the IFOV or the radiance along the line-of-sight through the geometric center of the IFOV or some other measure. The choice of measure will generally depend to what purpose the data will be applied. The difference between the desired and inferred measures is termed the error. Such errors contribute to image distortion.
Presently known procedures typically correct for extrinsic sources of non-uniform illumination fields for which exist some physical models to guide the correction. These extrinsic sources usually are contributed from one or more of the following mechanisms: platform motion; media lying between surface viewed (or ground) and platform (e.g., turbulence); and sensor optics and/or detectors.
In dealing with such causes of non-uniform illumination of individual detectors in an array, the physics of the phenomenon is invoked in a model to reduce the effect of non-uniform illumination. Assume that the non-uniformity of illumination is intrinsic to the scene viewed. Thus, a physical model of an intrinsic process cannot be invoked. A reference that provides a broad catalogue of error sources and effects is Holst, ibid.
A non-uniformity intrinsic to the viewed scene is similar to the spatial photoresponse non-uniformity of detector arrays in which the photoresponsivity of the individual detectors in the array have manufacturing differences to include unequal “aging” rates of component materials. That is referred to as pattern noise, contributed by variations among detectors. The error source addressed b
Berger Henry
Bosch Edward Howard
Baugher Jr. Earl H.
Le Que T.
Luu Thanh X.
The United States of America as represented by the Secretary of
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