Photography – Having means to reflect image to film
Reexamination Certificate
2000-09-29
2003-09-09
Mahoney, Christopher (Department: 2851)
Photography
Having means to reflect image to film
C396S089000, C396S544000
Reexamination Certificate
active
06616347
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to apparatus and methods for optical image acquisition and analysis. In particular, it relates to passive techniques for measuring the ranges of objects and their locations in three dimensions.
In many surveying and related applications, it is desirable to rapidly measure the locations of all of the visible objects in a scene in three dimensions. Conventional passive image acquisition and processing techniques are effective for determining the bearings of objects, but do not adequately provide range information.
Various active techniques are used for determining the range of objects, including direct measurement, radar, sonar, scanned laser and structured light methods. Direct measurement involves holding rulers and the like up to the objects of interest or reference locations and reading the scale. This usually requires two people, and can be error-prone, time consuming and sometimes dangerous. Measurements in three dimensions can be difficult. Other active techniques involve transmitting energy to the object and monitoring the reflection of that energy. These methods have several shortcomings. They often fail when the object does not reflect the transmitted energy well or when the ambient energies are too high. Production of the transmitted energy requires special hardware that consumes power and is often expensive and failure prone. When several systems are operating in close proximity, the possibility of mutual interference exists. Scanned laser systems can be slow. Sonar is prone to errors caused by wind. Most of these active systems do not produce enough information to identify objects.
Range information can be obtained using a conventional camera, if the object or the camera is moving a known way. The motion of the image in the field of view is compared with motion expected for various ranges in order to infer the range. However, the method is useful only in limited circumstances.
Another approach is known as photogrammetry. Distinctive markers are placed within the scene, and photographs are taken from a number of angles. The locations of the markers in the resulting images are measured and triangulation is performed to infer the camera locations and, eventually, locations of objects of interest. This method is very time-consuming and tedious.
Other approaches make use of passive optical techniques. These generally break down into stereo and focus methods. Stereo methods mimic human stereoscopic vision, using images from two cameras to estimate range. Stereo methods can be very effective, but they suffer from a problem in aligning parts of images from the two cameras. In cluttered or repetitive scenes, such as those containing soil or vegetation, the problem of determining which parts of the images from the two cameras to align with each other can be intractable. Image features such as edges that are coplanar with the line segment connecting the two lenses cannot be used for stereo ranging.
Focus techniques can be divided into autofocus systems and range mapping systems. Autofocus systems are used to focus cameras at one or a few points in the field of view. They measure the degree of blur at these points and drive the lens focus mechanism until the blur is minimized. While these can be quite sophisticated, they do not produce point-by-point range mapping information that is needed in some applications.
In focus-based range mapping systems, multiple cameras or multiple settings of a single camera are used to make several images of the same scene with differing focus qualities. Sharpness is measured across the images and point-by-point comparison of the sharpness between the images is made in a way that effects of the scene contrast cancel out. The remaining differences in sharpness indicate the distance of the objects at the various points in the images.
Systems of this last type are described by Pentland, who used two or more cameras with differing apertures to obtain simultaneous images. A bulky beamsplitter/mirror apparatus is placed in front of the cameras to ensure that they have the same view of the scene. This multiple camera system is too costly, heavy, and limited in power to find widespread use. A variation is described in U.S. Pat. No. 5,365,597, where a camera system includes dual camera optics in which a beamsplitter is used within the lens system to simplify the optical design. Another improvement of Pentland's multiple camera method is described by Nourbakhsh et al. (U.S. Pat. No. 5,793,900). Nourbakhsh et al. describe a system using three cameras with different focus distance settings, rather than different apertures as in Pentland's presentation. This system allows for rapid calculation of ranges, but sacrifices range resolution in order to do so. The use of multiple sets of optics tends to make the camera system heavy and expensive. It is also difficult to synchronize the optics if overall focus, zoom, or iris need to be changed. The beamsplitters themselves must be large since they have to be sized to full aperture and field of view of the system. Moreover, the images formed in this way will not be truly identical due to manufacturing variations between the sets of optics.
An alternative method that uses only a single camera is described by Nakagawa et al. in U.S. Pat. No. 5,151,609. This approach is intended for use with a microscope. In this method, the object under consideration rests on a platform that is moved in steps toward or away from the camera. A large number of images can be obtained in this way, which increases the range-finding power relative to Pentland's method. In a related variation, the camera and the object are kept fixed and the focus setting of the lens is changed step-wise. Even in a static situation, such as a surveying application, the time to complete the measurement can be excessive. Even if the scene and the camera location and orientation are static, the acquisition of multiple images by changing the camera settings is time consuming and introduces problems of control, measurement, and recording of the camera parameters to associate with the images. Also, changing the focus setting of a lens may cause the image to shift laterally if the lens rotates during the focus change and the optical axis and the rotation axis are not in perfect alignment.
Thus, it would be desirable to provide a simplified method by which ranges and locations of objects can be determined easily and accurately. In particular, it would be desirable to provide a method by which range-mapping for substantially all objects in the field of view of a camera can be provided easily and accurately. It is further desirable to perform these measurements using relatively simple, portable equipment. It would also be desirable to produce a detailed photographic record of a scene, from which location measurements can be made as their need becomes apparent.
SUMMARY OF THE INVENTION
In one aspect, this invention is a camera system comprising
a) a camera having a camera line of sight, an image sensor and a means for capturing images formed on the image sensor;
b) a focussing system for focussing light to form an image on the image sensor;
c) an optical displacement unit (ODU) that receives light from an offset line of sight that is different from the camera line of sight and directs the light into the camera along or parallel to the camera line of sight, the ODU being rotatable about a rotation axis that is the same as or parallel to the camera line of sight but different than the offset line of sight,
wherein all elements of the focussing system, except for an optional elements for increasing field of view, are located between the ODU and the image sensor.
In a second aspect, this invention is a camera system comprising
a) a camera having a camera line of sight, an image sensor and a means for capturing images formed on the image sensor;
b) a focussing system for focussing light to form an image on the image sensor; and
c) an optical displacement unit (ODU) that receives light from an offset li
Gary C. Cohn PLLC
Mahoney Christopher
LandOfFree
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