Objective  The optical 3D (three-dimensional) topography measurement technology based on phase calculation mainly projects the coded fringe pattern to the surface of the object through the projector. The camera collects the deformed fringe of the object surface from another perspective, and then obtains the absolute phase information of the surface. The 3D topography of the object surface is obtained by calibrating system parameters. The technique requires uniform reflection on the object surface being measured. However, due to the different color and exposure degree on the surface of the highly reflective object, the reflection on the object surface is not uniform, Different modulation degree and different exposure degree appear on the stripes projected on the surface. The traditional fringe projection profilometry cannot carry out effective 3D measurement. In order to effectively use fringe projection technology to measure the 3D morphology of color highly reflective objects, a multi-channel preprocessing binary exposure time selection method has been proposed to measure the 3D morphology of color highly reflective objects.

  Methods  This paper presents a new method to measure 3D morphology of color highly reflective objects. Firstly, histogram is used to quantify the light intensity information of pixel points, and the exposure time is preprocessed to determine a selection area of exposure time. In this exposure time domain, two dichotomies can be used to select the exposure time, and four groups of deformation fringe patterns with different exposure times are collected. The four groups of fringe images are collected for optimal pixel selection, and then the selected pixel points are integrated. The processing of overexposed pixel points can be realized through phase solution (Fig.2). Then the processed pixel information is selected in the three-color channel for optimal light intensity and color to generate the color fringe map. The optimal phase under each color channel is calculated, and the optimal phase within each color channel is fused, so as to obtain the absolute phase of the color highly reflective object surface. Finally, the calibration of the whole measuring system is completed, and the relationship between phase and depth can be determined by the calibration parameters to obtain 3D data of the object surface.

  Results and Discussions  The 3D morphologies of highly reflective color plate (Fig.5) and retro color ceramic cup (Fig.6) were measured by using the proposed measurement method. According to the 3D topography of the reconstructed object surface, it can be clearly seen that the proposed multi-channel preprocessing binary exposure time method can effectively measure the 3D topography of the color highly reflective object surface. Among them, the preprocessing method of binary selection of exposure time for image acquisition realizes that only four exposures can be used to solve the problem of high reflection on the surface of color highly reflective objects, and greatly reduces the number of groups of fringe images collected at different exposure times in the three-color channel. Then the fringe image is selected for the optimal light intensity and color in the three-color channel to solve the color problem of the highly reflective object surface. Finally, the dual problem that high reflection and color surface cannot be measured in 3D at the same time is solved. The experimental results show that the proposed method can solve the dual problems of different surface light absorption degree and local overexposure of high reflection color objects at the same time, which is a technical innovation in the field of fringe projection technology. The preprocessing method of binary exposure time selection reduces the number of fringe images, avoids the selection of exposure time for repeated effects, saves the measurement time, and effectively solves the problem of high reflection. The proposed method is applied to the three-color channel to solve the problem of 3D surface measurement of highly reflective objects.

  Conclusions  In this paper, the problems related to the 3D topography measurement of color highly reflective objects are studied. By analyzing the problems of local overexposure and the different fringe-modulation of different color stripes in different color regions, a new method of multi-channel preprocessing and dichotomized exposure time has been proposed to measure the 3D morphology of color highly reflective objects. In addition, experiments on the 3D topography measurement of color highly reflective objects have been completed on a calibrated experimental system. The experimental results show that the proposed method can effectively measure the 3D topography of color highly reflective objects. This method fills the problem that the structured light fringe projection technique cannot measure color highly reflective objects effectively and expands the application range of structured light fringe projection technique.