Combined stereovision and phase shifting method: use of a color-coded visibility-modulated fringe pattern

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. This combined method uses two cameras and one projector and can significantly reduce errors caused by inaccurate phase measurement. 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. With this modified fringe pattern, pixel-level stereo matching can be achieved in both directions without having to change 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 to form a color visibility-modulated fringe pattern and project it onto the object via a color projector. The color fringe image is then captured by two single-CCD color cameras simultaneously, each subsequently split into three grayscale fringe images. With this technique, only a single color fringe image is needed for 3-D reconstruction and the image acquisition speed is limited only by the frame rate of the camera, which makes it possible to measure rapidly changing objects. 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, which contributes to the improvement of the system accuracy. Experimental results are presented to demonstrate the feasibility and performance of the proposed method.