为减少多频条纹相移法进行三维形貌测量时的投影条纹数量, 提高测量速度, 提出了采用双频四步相移彩色条纹投影技术来准确测量具有台阶状不连续或孤立物体表面三维形貌的方法。利用计算机将高低两种不同频率的正弦条纹分别输入彩色图像的红色和蓝色通道合成为彩色条纹, 由数字视频投影仪将四步相移彩色条纹投影到被测物体, 然后利用彩色CCD或CMOS相机采集4幅彩色条纹图并存储于计算机中。基于色彩分离技术得到8幅两种不同频率的四步相移条纹图并由图像灰度算法获得1幅反映背景的灰度图像; 由低频条纹确定台阶状的物体边缘, 高频条纹计算物体的形貌, 并通过背景图像的二值化定位阴影和不连续区域。提出的方法仅用4幅彩色图像完成了台阶状不连续或孤立物体三维形貌的准确测量。与现有的多频4步相移条纹投影形貌测量技术需要8~12幅图像相比, 该方法有效地减少了条纹投影与图像采集的数量。

To reduce the projection fringes in three dimensional profile measurement by traditional phase shift method and to improve the measuring speed, a dual-frequency and 4-step phase-shifting color fringe profilometry is proposed to measure the 3-D shapes of objects with step-jumps and discontinuous shapes. Dual-frequency color fringe patterns with 4-steps phase-shifting are generated by computer encoding with four low-frequency and high-frequency sinusoidal fringe patterns. These color fringe patterns are projected onto the tested objects by a digital video projector, and the deformed fringe patterns are captured by a color CCD/CMOS camera and stored into a computer. Then, the recorded fringe patterns are separated into their RGB components and become 8 frames of fringe patterns. One image related to the background is also obtained by the sum of four dual-frequency color fringe patterns. The shapes of tested objects are acquired from high-frequency fringe patterns, and the ambiguous phase jumps caused by steps are eliminated by using low-frequency fringe patterns. To determine the shadow and dark background, the image related to the background is binarized and used for correcting the phase extraction. The experimental results verify that the proposed method can measure exactly the step-jumps and discontinuous shapes with only four images. Compared with existing methods, the presented method can effectively reduce the quantity of information of fringe projection and image acquisition.