In the real application environment of field engineering, a large variety of metrology tools are required by the technician to inspect part profile features. However, some of these tools are burdensome and only address a sole application or measurement. In other cases, standard tools lack the capability of accessing irregular profile features. Customers of field engineering want the next generation metrology devices to have the ability to replace the many current tools with one single device. This paper will describe a method based on the ring optical gage concept to the measurement of numerous kinds of profile features useful for the field technician. The ring optical system is composed of a collimated laser, a conical mirror and a CCD camera. To be useful for a wide range of applications, the ring optical system requires profile feature extraction algorithms and data manipulation directed toward real world applications in field operation. The paper will discuss such practical applications as measuring the non-ideal round hole with both off-centered and oblique axes. The algorithms needed to analyze other features such as measuring the width of gaps, radius of transition fillets, fall of step surfaces, and surface parallelism will also be discussed in this paper. With the assistance of image processing and geometric algorithms, these features can be extracted with a reasonable performance. Tailoring the feature extraction analysis to this specific gage offers the potential for a wider application base beyond simple inner diameter measurements. The paper will present experimental results that are compared with standard gages to prove the performance and feasibility of the analysis in real world field engineering. Potential accuracy improvement methods, a new dual ring design and future work will be discussed at the end of this paper.
A portable 3-D shape measurement system based on a combined stereovision and phase shifting method which can
realize big scale objects measurement is proposed. This system uses two pre-calibrated cameras and one projector which
do not need to be calibrated. During the whole measurement procedure, the projector is used to project a visibilitymodulated
fringe pattern on the object and is relatively fixed to the object. The two cameras are set up for stereovision
and grab fringe images simultaneously. The cameras can be moved to as many positions as needed to capture single
views and these single views can then be transformed into the same global coordinate system to reconstruct the whole 3-
D model. Since the phase value at each pixel is used to assist stereo matching only, it does not have to be accurate and
the errors caused by inaccurate phase measurement, for example, periodic errors due to the nonlinearity of the
projector's gamma curve are eliminated. These two high frame rate cameras can grab images as fast as 180 fps. Using
the visibility-modulated fringe pattern the phase information in one direction and fringe visibility information in the
other direction can be obtained simultaneously for stereo matching. Therefore only three images are needed for a single
view, which means the image acquisition time for each view is just 13.9ms. Experimental results are presented to show
the feasibility of this method.
KEYWORDS: Cameras, Fringe analysis, Phase shifting, Projection systems, Image acquisition, 3D image processing, 3D image reconstruction, 3D modeling, 3D acquisition, Imaging systems
The use of a color visibility-modulated fringe pattern is proposed to further accelerate image acquisition in the newly
proposed combined stereovision and phase shifting method for 3-D shape measurement. The method uses two cameras
and one projector and can eliminate errors caused by inaccurate phase measurement. In order to eliminate the need for
pattern switching and thus make real-time image acquisition possible, the use of a visibility-modulated fringe pattern
was previously proposed. This modified pattern is sinusoidal in one direction as in a conventional fringe pattern, but is
visibility-modulated in the other direction. Using this modified pattern we can achieve pixel-level phase matching in
both directions without changing the fringe pattern. In this paper, a color visibility-modulated fringe pattern is
introduced to further accelerate image acquisition. To obtain the three fringe images simultaneously, we encode the three
phase-shifted fringe patterns into the R, G, and B channels of a color pattern and project it onto the object via a color
projector. The color fringe image is then taken by two single-CCD color cameras simultaneously and each decoded into
three fringe images. The problems specifically associated with color systems, such as color coupling and color
imbalance, will be shown to have much less effect on the measured results. With this technique, the acquisition speed is
limited only by the frame rate of the camera, which significantly reduces the errors caused by object motions.
Experimental results are presented to support claims of the proposed method.
A new method, which combines the stereovision and phase shifting techniques, is proposed for more accurate 3-D shape
measurement. This method uses two cameras and one projector and can eliminate errors caused by inaccurate phase
measurement, for example, periodic errors due to the nonlinearity of the projector's gamma curve. The two cameras are
set up for stereovision. The projector is used to project phase-shifted fringe patterns onto the object twice with the fringe
patterns rotated by 90 degrees in the second time. Fringe images are taken by the two cameras from different directions
simultaneously. The resulting phase maps are used to assist stereo matching at the pixel level. The coordinates of the
object surface are calculated based on triangulation. Since the phase value at each pixel is used to assist stereo matching
only, it does not have to be accurate. This means that the projector does not need to be calibrated, which simplifies the
system calibration. Errors due to inaccurate phase measurement are significantly reduced because the two cameras
produce phase maps with the same phase errors. This combined method is better than stereovision method alone because
it provides higher resolution and easier stereo matching. It is better than the phase shifting method alone because it
eliminates the need of accurate phase measurement in order to ensure high measurement accuracy. Experimental results
and comparisons with the typical phase shifting method are presented to show the effectiveness and advantages of this
newly proposed method.
We propose to use a visibility-modulated fringe pattern to enable real-time image acquisition in the newly proposed
combined stereovision and phase shifting method for 3-D shape measurement. The combined stereovision and phase
shifting method uses two cameras and one projector and can eliminate errors caused by inaccurate phase measurement,
such as periodic errors due to the nonlinearity of the projector's gamma curve. In order to achieve pixel-to-pixel
matching between the two cameras, we previously used two phase-shifted fringe patterns, one with fringes in the vertical
direction and the other in the horizontal direction. This means that the projected fringe pattern has to be switched during
the image acquisition process, which slows down the process. As a result, measurement of dynamically changing objects
is difficult. In this paper, we propose to use a visibility-modulated fringe pattern to eliminate the need of the second
fringe pattern. This new fringe pattern is sinusoidal in the horizontal direction as in a conventional fringe pattern, but is
visibility-modulated in the vertical direction. With this new pattern, we can obtain the phase information in one direction
and fringe visibility information in the other direction simultaneously for stereo matching. Since no pattern changing is
necessary during the image acquisition process, the image acquisition time can be reduced to less than half of the time
previously required, thus making the measurement of dynamically changing objects possible. Experimental results are
presented to demonstrate the effectiveness of the proposed method.
We propose a nonlinear calibration method for improving the accuracy of structured light systems that use cameras and
projectors. Previously we have developed a systematic method for the calibration of projectors as well as structured light
systems for 3-D shape measurement. However, we used only a linear model and did not consider lens distortions of the
camera and the projector. As a result, measurement accuracy was limited. In this paper, we develop nonlinear models for
both the camera and the projector and apply them to develop a nonlinear algorithm for 3-D shape measurement. The aim is to improve system accuracy by reducing the nonlinear error caused by lens distortion. Experimental results on
nonlinear camera and projector calibration, comparison of 3-D measurement errors with linear and nonlinear camera and
projector models, as well as 3-D shape measurement of some sample objects are presented.
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