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Considering the inherent defects of the traditional triangulation method, always a shadow area is produced in the image collected by the camera. In this work, based on the traditional monocular vision measurement system, an auxiliary light source was added to alleviate the shadow problem. Two symmetrical digital light processing (DLP) projectors were also employed to project the modulated monochrome red fringe patterns and the monochrome blue fringe patterns onto the object, whereas the distorted fringe patterns were captured by the colour camera. The image collected by the colour camera was separated, whereas the deformation fringe information of the measured object was obtained in the red channel and blue channel, respectively. Fusion of red and blue channels information to eliminate measurement shadows. From the experimental results, the root mean square error (RMSE) of the system was about 6µm, and this method can effectively solve the shadow problem in small object measurement. [ABSTRACT FROM AUTHOR]

• Proposed a one-six network for phase calculation. • This article proposes for the first time a one-six network combined with binocular fringe projection for 3D reconstruction. • The method proposed in this article can effectively reduce the drawback of requiring a large number of fringe patterns in binocular fringe projection, achieving high-speed and high-precision measurement. • The network proposed in this article can fuse more feature information and achieve better and faster fitting by setting appropriate drop values. • This article uses composite color fringe patterns, which have more feature information and can provide a better learning for the network. At present, deep learning plays a crucial role in structured light 3D reconstruction. Further, in the field of fringe projection profilometry, learning 3D features from fringes and performing 3D reconstruction are being studied by many researchers. This paper combines deep learning with binocular fringe projection, uses three channels to form a single-composite-color fringe pattern for three different frequencies as the input, and predicts the numerator and denominator required to solve the wrapped phase of the object. The wrapped phase is calculated using an arctangent function. The absolute phase is obtained by unwrapping the multi-frequency heterodyne method, and the absolute phase of the left and right cameras is matched to obtain a disparity map. The parameters obtained through camera calibration can restore the 3D shape of the object, which greatly reduces the number of fringes required; accuracy close to the phase of the training set is achieved. Finally, the experimental results demonstrate the feasibility of this approach. [ABSTRACT FROM AUTHOR]

• This paper for the first time, proposes phase-shifting segmented phase coding and combines bidirectional coding and complementary phase coding. • The application of the phase-shifting segmented phase coding method is evaluated using simulations and experiments. • This paper establishes theoretical model to demonstrate the feasibility of applying phase-shifting segmented phase coding method. • The phase-shifting segmented phase coding method can effectively eliminate fringe order errors by bidirectional coding complementary phases. • Novel phase-shifting segmented phase coding technology can increase the codewords to realize the high-resolution. The phase-coding method is widely used during 3D measurement for attaining absolute phase retrieval. However, traditional the phase-coding method still suffers from low accuracy and insufficient reliability. To address these issues, a phase-shifting segmented phase coding method was proposed in this work for absolute phase retrieval in complex object measurements, which specifically improved upon the segmented method of phase coding. Coding patterns with different phase-shifting were obtained by embedding the codeword into the phase-shifting phase. These patterns were also combined and arranged to create novel segmented coding patterns. In addition, this segmented encoding can be self-connected based on its different phase-shifting information to obtain the complete fringe order, achieving thus absolute phase retrieval of over 36 codewords. To solve the problem of the periodic misalignment of the wrapped phase and fringe order, the complementary phase coding method was encoded by the bidirectional coding method, and the separation method of the bidirectional coding was improved. From experimental results, it was demonstrated that the proposed method has high measurement accuracy and reliability. [ABSTRACT FROM AUTHOR]

Binary fringe defocused projection is a promising method for high-speed, high-accuracy three-dimensional (3D) shape measurement because of the advantages of superfast projection and capability to overcome the nonlinear gamma of the projector. However, the application of this approach for complex surface measurement remains a challenge. Thus, a method that combines binary fringe and phase coding fringe based on defocusing is proposed to achieve fast-speed, high-accuracy 3D shape measurement of complex surfaces. However, in the case of defocusing, as the number of phase coding fringe cycles increases, the fringe orderk(x,y) becomes difficult to judge, errors occur in the jump area, and the periodic dislocation problem becomes worse, ultimately leading to phase unwrapping error. A phase-shifting coding method is thus presented to resolve these issues. This method not only alleviates the difficulty in judging the fringe order as a result of defocusing but also resolves the correction problem, and in turn, the periodic dislocation problem. Thus, measurement accuracy is improved. Experimental results demonstrate the feasibility and superiority of the proposed method. [ABSTRACT FROM AUTHOR]

Binary fringe defocused projection can resolve the problem caused by the nonlinear gamma of the projector. Owing to the intersecting axis measurement system, the broadening of the fringe period on the reference plane can cause measurement errors. Non-uniform periodical binary fringe defocused projection is utilized to overcome this problem. After appropriate defocused projection of non-uniform periodical binary fringe, uniform periodical sinusoidal fringe can be obtained on the reference plane. This method can prevent the nonlinear gamma effect and broadening of the fringe period, and filter high harmonics and high-frequency noise. Three-dimensional (3-D) shape measurement experiments of standard flat are performed with four-step phase-shift method. Experimental results demonstrate that the proposed method exhibits high measurement precision. Highly accurate 3-D measurements of large objects can also be performed with the proposed method. [ABSTRACT FROM AUTHOR]

Abstract: Fringe projection profilometry is widely used for three-dimensional shape measurement. In an oblique-angle projection, the fringe cycle is broadened on the reference plane. Phase errors are mainly caused by the nonlinear gamma of the projector and fringe cycle broadening. This study describes a phase error compensation method to eliminate these phase errors. A look-up table that stores phase errors is constructed for phase error compensation. Based on it, a new height equation is proposed. The experimental results show that the proposed method can compensate for the phase errors of the fringe projection profilometry, thereby improving the measurement accuracy significantly. [Copyright &y& Elsevier]

Abstract: Fringe projection profilometry (FPP) is a widely used three-dimensional profile measurement technique. One vital step in this technique is calibration, which determines the system accuracy. The least-squares method, because of its flexibility and simplicity, is commonly used in system calibration for FPP. However, calibration results are affected by the nonlinear gamma of the projector and projection fringe cycle broadening. This paper proposes a new look-up table (LUT) generation method by analyzing the differences between the real and ideal unwrapped phases. The aforementioned problems could then be solved after the phase error is compensated by the LUT. Finally, the validity of the proposed method is demonstrated through experiments, and the accuracy reaches 0.02mm. [Copyright &y& Elsevier]

Abstract: Dual-frequency fringe Fourier transform profilometry (FTP) is used to measure steep objects. The dual-frequency fringe can easily be obtained through software programming. However, due to the nonlinear gamma of the projector, the second harmonic generated by the nonlinear response of the camera and the presence of noise, digital filtering becomes difficult and measurement errors are introduced, thus reducing measurement accuracy. Therefore a novel method to generate dual-frequency fringe is presented in this study to solve the aforementioned problems. A binary square wave is projected onto the measured objects. By properly defocusing, the fundamental frequency and the third harmonic components can be preserved. The generated dual-frequency fringe is used to measure the 3-D profile of a computer mouse. The experimental results verify the feasibility of this method. The proposed method has higher measurement accuracy compared with the traditional dual-frequency fringe FTP. [Copyright &y& Elsevier]

• A new approach named S-shaped segmental phase encoding is proposed. • The method can expand the range of code words by a segmentation mechanism. • The fringe order judgment accuracy can be guaranteed by Gray code. • Experimental results demonstrated the high accuracy of our method. Phase-based encoding has good immunity and robustness due to its insensitivity to ambient light, camera noise, and surface reflectance of measured objects. However, the number of code words is limited using traditional phase encoding method. In order to solve this problem, this paper proposes a new approach named S-shaped segmental phase encoding for 3D measurement. Specifically, by using a segmentation mechanism, the proposed method can, firstly, expand the range of code words whose number is limited using traditional phase encoding method. Secondly, the measurement accuracy can be improved. In this paper, Gray code is used to encode the phase encoding segment number, such that the fringe order judgment accuracy can be guaranteed. The experimental results demonstrated more accurate and efficient 3D shape measurement than the traditional methods, especially regarding the measurement of complex and isolated objects. [ABSTRACT FROM AUTHOR]