Photography – Aerial camera
Reexamination Certificate
2000-08-31
2003-12-02
Adams, Russell (Department: 2851)
Photography
Aerial camera
C396S008000, C396S012000, C396S013000, C348S144000, C348S145000, C348S146000, C348S147000
Reexamination Certificate
active
06658207
ABSTRACT:
BACKGROUND OF THE INVENTION
A. Field of the Invention
This invention relates generally to the field of aerial reconnaissance, remote sensing, mapping and surveillance camera systems. Generally speaking, aerial reconnaissance cameras are available in framing and pan scanning configurations, in both film and electro-optical implementations. The present invention relates to both types of camera configurations, in that a roll framing camera such as described herein generates individual frames of imagery, while the smooth rolling operation provides similar scene coverage and inertial load reductions found in pan scanning cameras.
B. Description of Related Art
In prior art framing cameras, an exposure is taken over a large scene of fixed format. The field of view of the camera is stepped across a large area using mechanically driven stepping hardware while using image motion compensation techniques to compensate for forward motion of the aircraft. The field of view of the camera is a function of lens focal length and the geometrical format size of the image recording media. The exposure time is generally controlled by a shutter and is a function of 1) the sensitivity of the photosensitive media, 2) lens transmittance and relative aperture, and 3) available scene brightness. The photosensitive material can be film, an area array Charge Coupled Device (CCD), or any other media which records an image for later retrieval.
Forward Motion Compensation (FMC) is a technique used in framing cameras to correct for the image motion on the recording media caused by forward motion of the aircraft during the exposure interval. This correction is generally introduced by moving the film or the lens to keep the image stationary in the fore/aft direction while the exposure is taking place. In a framing camera, the correction is usually accomplished by moving the film to match the rate of image motion. U.S. Pat. No. 5,668,593 to Lareau et al., assigned to the assignee of the present invention, the contents of which hare incorporated by reference herein, describes a electro-optical step frame camera system in which the forward motion compensation is achieved electronically in the focal plane of the electro-optical detector.
One limitation of a conventional film or CCD framing camera is that only a single FMC rate can be applied to any given frame regardless of the field of view. Consequently, the motion can exactly be corrected for only a portion of the image. When exposure times are short and the field angles small, this is acceptable. However, for larger fields of view and where longer exposure times are required (as at dusk or under other low light level conditions), the differential rate of motion between the film and the image increases with the field angle and can be large enough result in image blur at the edges of the field. A major advance in forward motion compensation in electro-optical framing cameras is disclosed in the Lareau et al. patent, U.S. Pat. No. 5,155,597, assigned to the assignee of the present invention. The Lareau et al. ′597 patent, which is incorporated by reference herein, describes an electro-optical imaging array that accomplishes FMC electronically and without moving parts by dividing the columns of the array into multiple column groups, and by transferring pixel information in the column groups at a rate that substantially matches the rates of image motion in the column groups.
Another operational function of a framing camera is the generation of an overlap between successive frames of imagery. The overlap is used to ensure complete coverage of all areas of the scene, and/or to provide a view of the scene from two different angular perspectives yielding stereo imagery. In a conventional framing camera, the amount of overlap is selectable and nearly always takes place in the direction of flight.
In step frame cameras, the overlap L(OL) of the two frames of imagery is typically of 10% or 12%, or as much as 56%. An overlap of at least 50% allows all imagery in the adjacent frames to be exposed from two different angular perspectives. These images can be recombined by means of a stereo viewing system to achieve depth perception. Such stereo images are often used by a photointerpreter to gather additional information about the scene.
The operation of a film-type framing camera in a stepping mode is known in the art. For example, the article entitled “The KS-146A LOROP Camera System”, Thomas C. Augustyn, SPIE Proceedings Vol.9, Aug. 27-28 1981, paper 309-11 p.76, describes an automatic stepping mode in which the camera cycle rate is proportional to aircraft velocity, altitude and selected depression angle, to achieve 56% overlap for stereo viewing or 12% overlap for maximum flight line coverage. With the camera line of sight normal to the flight path, the scan head provides either 1, 2, 4, or 6 lateral-step cycles. A similar stepping operation for a frame camera is described in the article entitled “KS-127A Long Range Oblique Reconnaissance Camera for RF-4 Aircraft”, Richard C. Ruck and Oliver J. Smith, SPIE Proceedings Vol. 242, Jul. 29-30, 1980 San Diego Paper 242-02, p.22.
Panoramic (pan) camera technology is another well-established means of imaging. In a panoramic scanning camera, the scene is exposed continuously by rotating a scanning mechanism (such as a double dove prism) so as to scan the image across the photosensitive medium. The photosensitive medium is moved in synchronism with the image. In the case of a film camera, this may be accomplished by moving the film at a constant rate past an exposure slit which is located on the lens optical axis. A scan prism located in front of the lens is rotated in synchronism with the film rate such that the image of the scene remains stationary on the film during the exposure period. The slit opening is adjusted to a predetermined width to control exposure time.
One major advantage of a pan camera is its ability to image a large area in the direction across the line of flight. Scan angles across the line of flight on the order of 120 to over 180 degrees are typical. The lens field of view in a pan camera is generally only required to be large enough to cover the width of the film. Overlapping of images and stereo imagery may also be obtained with pan cameras. Image overlap in a conventional fixed mounted pan camera is obtained as in the case of a framing camera, that is, in the common area between successive scans.
FMC for both the film and electro-optical versions of the pan camera is usually accomplished by a conventional electromechanical means. Translating the lens during the scan is a popular means to achieve graded FMC as a function of instantaneous slant range to the scene. As noted above, the FMC can be done electronically as taught in the Lareau et al. U.S. Pat. No. 5,668,593.
Prior art mechanically stepped framing panoramic cameras, such as described in the ′593 patent and in the KS-146A camera are limited in size and the stepping rate by the mass and commensurate inertial loading created by trying to step that mass across the area of interest. Since the size and mass of the camera increases with operation in multiple spectral bands (i.e., with two or more detectors incorporated into the camera), the capability of mechanically stepped cameras is limited to smaller and more limited camera configurations.
Thus, there exists a need in the art for an electro-optical camera which obtains broad area coverage in the manner of a panning or step framing camera without the above limitations. The present invention meets that need by providing a novel roll framing technique for generating broad area coverage with an area array image recording medium, described in more detail herein. The image motion due to camera roll is compensated for electronically in the detector array. The invention is also particularly suitable for larger, more massive, and more complex cameras, including a camera which carries two or more imaging detectors in order to generate frames of imagery in two or more different bands of the electromag
Beran Stephen R.
Partynski Andrew J.
Adams Russell
Koval Melissa J.
McDonnell & Boehnen Hulbert & Berghoff
Recon /Optical, Inc.
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