Optics: measuring and testing – Shape or surface configuration – Triangulation
Reexamination Certificate
1999-05-14
2003-04-15
Smith, Zandra (Department: 2877)
Optics: measuring and testing
Shape or surface configuration
Triangulation
C382S154000
Reexamination Certificate
active
06549288
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to a three-dimensional (“3D”) measurement/digitization system and method, and in particular to a portable 3D digitization system and method which facilitate acquisition of data relating to 3D profiles of objects for subsequent computer-aided processing and reproduction of the 3D profiles of objects by shape digitizing.
BACKGROUND OF THE INVENTION
Speed, accuracy, and portability have been recurrent and difficult to achieve goals for devices that scan, measure or otherwise collect data about 3D objects for purposes such as reproduction. With the advent of computers, such devices have useful application in many fields, such as digital imaging, computer animation, topography, reconstructive and plastic surgery, dentistry, internal medicine, rapid prototyping, and other fields. These computer-aided systems obtain information about an object and then transform the shape, contour, color, and other information to a useful, digitized form.
The technology currently available for shape digitizing falls into two different but related groups: mechanical systems and optical systems. All systems within those two general categories struggle with the basic criteria of speed, accuracy, portability and ability to digitize the color texture image of an object.
A mechanical system acquires data about an object through the use of a probe that has a sensitive tip. The mechanical system scans an object by moving its probe tip across the object's surface and taking readings. Generally, the probe connects to a mechanical arm, and the system tracks the probe's position in space using angle measuring devices as the arm moves. The system calculates the position of the probe with coordinates known from the angle measuring devices.
Although mechanical systems scan with generally high accuracy, the rate at which a mechanical system acquires the data is relatively slow and can take several hours for scanning. A typical mechanical system measures only one point at a time, and no information is obtained about the material properties of the object such as its color.
As an alternative to mechanical systems, there are several types of optical object shape digitizers which fall into two basic categories: systems based on triangulation and alternative systems. A triangulation system projects beams of light on an object and then determines three-dimensional spatial locations for points where the light reflects from the object. Ordinarily, the light source is located at a certain distance from the light detector, and relative positions of the components and the direction of the light beam need to be known. A single dot system projects a single beam of light which, when reflected, produces a single dot of reflection. A scan line system sends a plane of light against the object which projects on the object on a line and reflects as a curvilinear-shaped set of points describing one contour line of the object. The location of each point in that curvilinear set of points can be determined by trigonometric triangulation.
Some single dot optical scanning systems use a linear reflective light position detector to read information about the object. In such systems a laser projects a dot of light upon the object. The linear reflected light position detector occupies a position relative to the laser which allows the determination of a 3D location for the point of reflection. A single dot optical scanner with a linear reflected light position detector can digitize only a single point at a time. Thus, a single dot optical scanning system, like mechanical system described above, is relatively slow in collecting a full set of points to describe an object. Single dot optical scanners are typically used for applications such as industrial engineering. The digitizing speed is usually limited by the mechanics of the scanning system, i.e., the moving and positioning of the laser beam. A scanning head can be mounted on a high-precision, but costly, positioning system to take a digitized image of the object's shape with generally good accuracy. However, because of the high cost, slow speed and difficulty of obtaining material properties such as colored texture, single dot optical scanners find generally only limited applications.
Scan line systems offer one solution to the speed bottleneck of single point triangulation system. Those systems typically employ a 2D imager, such as a charge coupled device (CCD) camera, for signal detection. The system projects a light plane (i.e., a laser stripe) instead of just one dot and read the reflection of multiple points depicting the contour of an object at a location that is at a distance from the CCD camera and from which the position can be triangulated. Some embodiments of the scan line-type system attach the CCD camera to a rotating arm or a moving platform. During scanning, either the object moves on a known path relative to the camera and laser, or the camera and laser, together, move around the object. In any case, such systems usually depend on this type of fixed rotational movement and typically use a bulky, high-precision mechanical system for positioning. Because of the use of mechanical positioning devices, resealing flexibility can be very limited, e.g., a scanner designed for objects the size of a basketball may not be useful for scanning apple-sized objects.
Some laser stripe triangulation systems currently available are further limited because the laser stripe stays at a fixed angle relative to the camera, and the system makes its calculations based on the cylindrical coordinates of its rotating platform. The mathematical simplicity in such a projection system complicates the hardware portion of these devices as they typically depend on the rotational platform mentioned. Also, the simplified geometry does not generally allow for extremely refined reproduction of topologically nontrivial objects, such as objects with holes in them (e.g., a tea pot with a handle). Full realization of triangulation scanning with a non-restrictive geometry has not been achieved in the available devices.
Apart from optical triangulation systems (single dot or structured line systems), there are alternative optical scanning systems which present a scanning solution different from those employing triangulation techniques. Range meters, depth-from-focus and multi-camera systems ate among those categorized as “alternative” systems. Range meter systems typically use a pulsed laser and mechanical scanning techniques to project a dot laser across then measure the time or phase delay of the reflected signal. As range meter systems typically incorporate a single dot method of data collection, they are intrinsic to single-point scanners, and they typically do not acquire material properties of the object.
Another type of alternative scanning system is a stereoscopic system which uses several CCD cameras located at known distances from each other. The captured images are processed with a pattern recognition system which finds matching points in different images of the object, thereby obtaining the shape/contour information. One advanced stereoscopic system uses 6 high-resolution CCD cameras. Since matching points can not be identified on flat and texture-less parts of the object a special grid needs to be projected on the object to facilitate geometry reconstruction. In spite of that, data omissions frequently occur, and thus the method is not very reliable since the quality depends on the material reflective properties.
In the depth-from-focus method two images of the object are acquired with cameras focused to focal planes located closer and further away than the object. By comparing the defocused images the depth information can be obtained. To facilitate the depth reconstruction a special checkerboard grid is typically projected on the object. The method suffers from the problems reverse to the problems of stereoscopic imaging: the objects with rich texture can not be reliably processed. Also, the technique, similar to the stereoscopy, resul
Abadjev Vadim
Afanassenkov Andrei
Bernstein Vladimir
Lebedev Alexei
Migdal Alexander
Kenyon & Kenyon
Smith Zandra
Viewpoint Corp.
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