Objective  Due to its high speed and high precision, fringe projection profilometry has been widely used in many fields, such as automatic online inspection of mechanical parts, automobile manufacturing, cultural heritage protection. However, traditional fringe projection uses a single exposure time or a single projection intensity to measure objects with high dynamic range (HDR). Overexposure will occur in areas with large reflectance, which exceeds the maximum brightness range of the camera sensor, resulting in the failure to obtain true intensity and accurate three-dimensional (3D) data. To solve this issue, this paper proposes a HDR object surface 3D measurement method utilizing the different color channel responses of a color camera based on monochrome fringe projection.

  Methods  In this paper, a 3D measurement method of HDR object surface based on monochrome fringe projection is proposed. In this method, the blue fringe patterns are projected onto the surface of the measured object, and the color camera captures the color image from another perspective. The two fringe patterns corresponding to the blue and green channels from the captured fringe images are separated. Mask image of blue and green channels are generated by selecting a group of pixels with unsaturated and maximum modulation from the blue-green channel fringe patterns. The HDR image is synthesized by the mask images of blue and green channels and fringe patterns of blue and green channels (Fig.3). Then, phase calculation methods and system calibration are applied to achieve 3D measurement of objects with high dynamic range.

  Results and Discussions   To demonstrate the effectiveness of the proposed method, a metal flat plates and a metal spherical part with HDR surface were tested. Comparative experiments were conducted between the separated blue channel fringe patterns and the synthesized HDR images to verify the effectiveness of the proposed HDR method (Fig.8, Fig.10). The proposed method can provide accurate 3D measurement results without measurement errors caused by pixel saturation. To quantitatively evaluate the accuracy of the method proposed, an artificial standard step surface were measured by Zhang's method and the proposed HDR images (Fig.12). 3D data of the step surface measured by the coordinate measuring machine (CMM) can be used as the ground truth. The difference between the measured data and the ground truth are shown (Tab.1). It can be seen that the accuracy of the proposed method is slightly higher than Zhang's method. Compared with the existing HDR methods, the proposed method has the advantage of fewer images and without additional hardware facilities.

  Conclusions  The 3D measurement technique for HDR object surface based on monochrome fringe projection is proposed by utilizing the different color channel responses of a color camera. The blue fringe patterns are projected onto the surface of the tested object, and captured by the color CCD camera. Fringe patterns corresponding to the blue and green channels are separated from the captured color fringe patterns. A group of pixels with unsaturated and maximum modulation from the blue and green channel fringe patterns are selected to generate mask images of the blue and green channels. The HDR image is synthesize by the mask images of the blue and green channels and fringe patterns of blue and green channels. Then the absolute phase is obtained by the phase calculation method, and the HDR object is measured by the calibrated system. The three-step phase-shifting method and an optimal three-fringe selection method are applied to obtain the wrapped phase and unwrapped phase, respectively. A total of 9 color images are required to reconstruct the 3D shape of HDR objects. Compared to traditional methods, the proposed method has the advantages of reducing the number of projected images and improving measurement efficiency.