Television – Camera – system and detail – With single image scanning device supplying plural color...
Reexamination Certificate
2002-04-08
2004-12-14
Ho, Tuan (Department: 2612)
Television
Camera, system and detail
With single image scanning device supplying plural color...
C348S294000, C348S340000, C348S342000, C356S419000
Reexamination Certificate
active
06831688
ABSTRACT:
BACKGROUND OF THE INVENTION
A. Field of the Invention
This invention relates generally to hyperspectral and mutispectral imaging systems for aerial reconnaissance and surveillance applications using a wafer-scale Focal Plane Array (FPA) of an area array format. In this art, “hyperspectral” imaging systems refer to those in which radiation at thirty or more discrete wavelengths are imaged. Imagers that image a lesser but plural number of spectral bands are referred to as “multispectral” imagers. Such systems are used in various applications, including Department of Defense satellite systems and commercial land resource management imaging systems.
The invention further relates to a scanning method for directing a spectrally separated image of the object under surveillance over the FPA in a predetermined direction and rate. In another aspect, the invention further relates to a time delay and integrate (TDI) clocking technique in which the FPA is separated into discrete sub-arrays, in which the length of TDI clocking within each sub-array is predetermined according to the spectral responsiveness of the detector material to each spectral band detected by each sub-array. The present invention provides improvement over the prior art by significantly increasing the area of coverage scanned by a multispectral or hyperspectral imaging system.
B. Description of Related Art
Hyperspectral imaging is a well established art based on the science of spectroradiometers and spectral imaging. In hyperspectral imaging, image radiation is separated into its spectral components using optical elements such as filters, prisms or diffraction gratings, and the separated radiation is imaged by a detector. This section will briefly review the state of the art and cite to several references and patents discussing background art. The entire content of each of the references and patents cited in this document is incorporated by reference herein.
Discussion of the basic hyperspectral imaging technology can be found in such references as
The Infra
-
Red
&
Electro
-
Optical Handbook
, Vol. 1, George J. Zissis, Editor, Sources of Radiation, pp. 334-347. Spectroradiometers applied to reconnaissance applications are called imaging spectrometers, wherein each image is collected in a number of spectral wavelengths.
The fundamental design of imaging spectrometers can be traced to Breckinridge, et. al., U.S. Pat. No. 4,497,540. In this patent, a scene is imaged on slit and dispersed using a diffraction grating or prism to illuminate a detector array. All spectral channels are acquired simultaneously and the field of view in each channel is precisely the same. This can be contrasted to “spectral imaging”, in which different spectral channels are acquired sequentially in time, i.e. through filters, and are not inherently spatially registered to one another.
The common elements of a hyperspectral imaging spectrometer system are shown in FIG.
1
. Target imagery is projected through a fore-optic
10
into the aperture slit
12
of a spectrometer system
14
. The spectrometer
14
includes an optical grating
16
, which serves to diffract the image into its spectral elements. The diffracted image is imaged by camera optics
18
onto a focal plane array detector
20
. Each image
22
generated by detector extends in one spatial direction, typically across the line of flight. A slice of the image in that spatial direction is imaged in a large number (thirty or more) different bands of the spectrum. As the platform flies pasts the terrain of interest (or as different terrain is scanned onto the imager with a scanning device) successive images
22
are generated, each in one spatial direction. When the series of images are combined, they yield an “image cube”
24
, covering the terrain of interest in two orthogonal directions in a multitude of spectra.
The width of the slit
12
is directly proportional to the amount of radiation, and therefore the signal-to-noise-ratio, of the system. And, it is inversely proportional to the spatial resolution. That is, the wider the slit, the worse the spatial resolving power of the system. On the other hand, the spectral resolution of the system is dependent on the optical parameters of the diffraction grating and the number of pixel elements of the detector. The higher the quality (blaze) of the diffraction grating
16
and the greater the number of pixels on the focal plane
20
, the higher the spectral resolution.
In a typical operation, the hyperspectral system is moved over the scene of interest either by moving the entire system (i.e. “pushbroom”) or by movement of a scan mirror (not shown). As objects enter the field of view of the aperture slit
12
, they are diffracted into their spectral colors and projected through the camera lens
18
to the detector array
20
. Thus, each line of the target area is projected as a two dimensional image onto the detector, with one dimension representing a spatial axis of the image in-line-of-flight (ILOF) or cross-line-of-flight (XLOF), depending on camera orientation and scanning mode, and the other dimension representing the spectrum of the image. The combination of the scanning action of the camera, either by pushbroom or scan mirror, with the frame rate of the focal plane array, determines the number of “frames” of the resulting image.
A number of frames of the image will provide spatial resolution in the scan direction. Typically, a design will call for roughly equal resolution in both directions. In this fashion, an image “cube”
24
is thus created wherein the hyperspectral imagery is subsequently processed to detect object features of interest to the image analyst. There exists a tremendous body of knowledge regarding the processing of hyperspectral imagery and how it applies to target “phenomenology”, which is outside the scope of this disclosure. For up to date background information on the processing of hyperspectral imagery see, for example, SPIE Proceedings, Vol. 3372
, Algorithms for Multispectral and Hyperspectral Imagery IV,
Apr. 13-14, 1998.
A common example of a hyperspectral imaging spectrometer is the NASA HIRIS instrument, which is designed to image the earth's surface from an 800-km altitude. The earth's surface is focused on a narrow slit and then dispersed and refocused onto a two dimensional focal plane array. Each time the satellite progresses around the earth by the equivalent of one pixel, the focal plane is read out and one frame of the image “cube” is created. It is commonly recognized that systems such as HIRIS have the limitations of limited resolution, limited exposure time and spatial smearing. There exists a large open literature which discusses technological issues pertaining to hyperspectral imaging systems, major development programs and existing national and commercially available systems. See, for example, Kristin Lewotsky, “Hyperspectral imaging: evolution of imaging spectrometry”,
OE Reports,
November 1994. For a comprehensive reference list of imaging spectrometers, see Herbert J. Kramer,
Observation of the Earth and Its Environment—Survey of Missions and Sensors
, Second Edition, Springer-Verlag, 1994.
Spectroscopy using a large format, two-dimensional CCD in a scanning configuration is disclosed in Bilhorn, U.S. Pat. No. 5,173,748 entitled
Scanning Multi
-
Channel Spectrometry using a Charge
-
Coupled Device
(
CCD
)
in Time
-
Delay Integration
(
TDI
)
Mode
. Bilhorn discloses a technique in which the performance of a spectrometer is improved by scanning the dispersed spectrum across a Time Delay and Integrate (TDI) CCD array synchronously with the clocking of the charge within the array. This technique in Bilhorn affords an accumulation of signal charge from each wavelength of the spectrum, which eliminates the need to compensate for throughput variations commonly found when using other detector approaches such as photodiode arrays, and increases the overall response of the spectrometer. This invention was designed for the use of determining the spectrum of a polychromatic light
Beran Stephen R.
Karins James P.
Lareau Andre G.
Lintell Robert J.
Pfister William R.
Ho Tuan
McDonnell Boehnen & Hulbert & Berghoff LLP
Recon /Optical, Inc.
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