Optics: measuring and testing – By polarized light examination – With light attenuation
Reexamination Certificate
1999-10-25
2001-07-03
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By polarized light examination
With light attenuation
Reexamination Certificate
active
06256099
ABSTRACT:
FIELD OF THE INVENTION
The present invention refers, in general, to methods and systems for videogrammetry and, in particular, to methods and a system for measuring three-dimensional spatial coordinates and for external camera calibration required for that measurement.
BACKGROUND OF THE INVENTION
There is a known method in photogrammetry for analyzing two-dimensional images, captured on photographic film, to produce three-dimensional coordinate measurements. At least two images of an overlapping surface portion are required. Modern photogrammetry has incorporated computers to carry out many of its functions, but the process has not yet been entirely automated. Videogrammetry has added two major improvements to the art of photogrammetry. Firstly, rather than recording images on film it records images directly on opto-electronic imaging devices, such as photosensitive diodes or other imaging surfaces. And secondly, since the images are immediately digitized and stored in a computer, they are readily accessible to manipulation and analysis by computational methods.
Attempts have been made to develop better systems and methods for measuring three-dimensional spatial coordinates. For example, U.S. Pat. No. 5,589,942 dated Dec. 31, 1996, granted to Gordon, for a “Real time three dimensional sensing system” discloses a system which utilizes two flexibly located cameras for receiving and recording visual information with respect to a sensed object illuminated by a series of light planes. Each pixel of each image is converted to a digital word and the words are grouped into stripes, each stripe comprising contiguous pixels. One pixel of each stripe in one image is selected and an epi-polar line of that point is drawn in the other image. The three dimensional coordinate of each selected point is determined by establishing the point on said epi-polar line which also lies on a stripe in the second image and which is closest to a known light plane. This system uses a complex pattern of projected stripes, which creates difficulties in finding homologous points in the pair of images, and leads to false positives and false negatives in homologous point identification. The system requires the light plane to be known.
U.S. Pat. No. 4,627,734, dated Dec. 9, 1986, granted to Rioux, for a “Three dimensional imaging device and method” discloses a three-dimensional imaging system operating in accordance with the known active triangulation method, and employing a laser beam that is projected onto an area of the surface to be examined. The solution is characterized by synchronized scanning of projected and deflected beams. The main disadvantage of the system consists in the fact that the position and orientation of the projected light beam must be known in order to calculate three dimensional coordinates.
U.S. Pat. No. 5,440,392 dated Aug. 8, 1995, granted to Pettersen et al. for a “Method and system for point by point measurement of spatial coordinates” describes a technical solution for point by point measurement of spatial coordinates. A touch probe, including three point-sized light sources at known local coordinates, touches the point to be measured. Using a camera to acquire an image of the three light sources, the system calculates the three dimensional coordinates of the point touched. It is obvious that this solution requires physically touching the object to be measured and is time-consuming.
SUMMARY OF THE INVENTION
There is accordingly a need for a method and system which overcome, or at least alleviate, the disadvantages of the prior art. It is therefore desirable to offer a technical solution which is faster, non-contact, and calibrated quickly and easily, without the use of special equipment.
Broadly, the method of measuring three-dimensional spatial coordinates starts by synchronizing at least two cameras so that they can capture simultaneous images. Then the cameras are set up with the object to be measured in their fields of view. A simple pattern is then projected onto the surface of the object, and simultaneous images which include the projected pattern are captured. The images are processed by a computer, which uses conventional software algorithms to find the pattern images, calculate centroids from the pattern images, group the centroids into homologous pairs, and calculate three-dimensional spatial coordinates.
In one aspect of the above method, wherein the projected simple pattern is a spot, images are discarded and recaptured, if necessary, until there is just one centroid in each camera image. Therefore, grouping these centroids into a homologous pair is trivial.
In another aspect of the above method, wherein the projected simple pattern is a stripe, it is necessary to segment the stripe images into sets of continuous stripe images. In one camera image, each continous stripe image is sampled at a known interval, and a centroid is calculated across the stripe for each sample, yielding a series of two dimensional points in the camera image. For each point in the series, the known internal and external calibrations of both cameras are used to determine a corresponding two-dimensional point on a stripe within the other camera image. Each such pair of corresponding points are grouped as a homologous pair.
Generally, the method of external camera calibration starts by synchronizing at least two cameras, so that they capture simultaneous images. Then, the cameras are placed so that their fields of view include a surface that will be used for calibration, and a simple pattern is projected onto that surface. A pair of simultaneous images of the simple pattern are captured. The images are processed by a computer, which uses conventional software algorithms to find the pattern images, to discard any images in which the pattern is not found, to calculate centroids of the pattern images, and to group the centroids into homologous pairs. Then, the pattern is projected onto a different portion of the surface, and new images are captured and processed, until the required number of homologous pairs have been found. The computer uses software for the methods of resection and ray bundle adjustment, known in photogrammetry, to compute an external calibration, with relative scale, of the positions and orientations of the cameras. Then, the system is used to acquire three-dimensional spatial coordinates from the surface of a target object of known dimensions. From this, a ratio is calculated and applied to the external calibration, in order to transform the relative scale into a usable absolute scale.
Broadly, the system used to carry out the above methods comprises a computer provided with software to execute the methods, at least two cameras containing image planes divided into a multiplicity of photosensitive sites, and a light projector capable of projecting a simple pattern onto a surface. Simultaneously exposed images of the simple pattern are captured and stored in computer memory.
Alternatively, the system can use charge couple devices, charge injection devices, or complementary metal-oxide semiconductors for image planes.
Alternatively, the system can use a light projector that is hand-held or coupled to a computer-controlled electro-mechanical beam positioner.
Alternatively, the system can use a light projector that is a laser device, a light-emitting diode (LED) device, or a light projector using focussing optics to project an image onto an external surface.
Alternatively, the system can use a light projector that uses collimating or focussing optics.
REFERENCES:
patent: 5193120 (1993-03-01), Gamache et al.
patent: 5589942 (1996-12-01), Gordon
patent: 5852672 (1998-10-01), Lu
patent: 5886767 (1999-03-01), Snook
Chow Jeffrey Brian
Cornwall Gordon Emery
James Robert Newton
Kaufman Frederick
Lang Curtis Earle
Cornwall Gordon Emery
Font Frank G.
Kaufman Frederick
Nguyen Tu T.
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