Phase Shift Profilometry Method Research Articles

3D measurement using fringe projection profilometry

This work is devoted to measuring the depth of the 3D object using the structured light method, in particular, phase shift profilometry. Theoretical studies on the methods of three-dimensional measurement systems and fringe projection profilometry are presented. The phase shift profilometry method with an improved calculation of the frequency of sinusoidal patterns is applied. In practical implementation in the environment (20 cm × 30 cm), the algorithm is tested on a stepped object consisting of eight steps with a difference of 150 mm between two successive steps. In this case, the achievable error for measuring such an object is 20 mm. Our method has great potential in industrial applications where the measurement of the smoothing of the surface of the object is needed to find the defect in the surface with high accuracy without contacting the object.

Adaptive Phase Error Suppression Concerning 3D surface Deformation Measurement on Color Digital Fringe Projection Profilometry

3D surface measurement based on phase-shifting profilometry (PSP) has been actively developed in recent years. Three color channels of RGB that are modulated to generate a one-shot PSP method is a concept of color digital fringe pattern profilometry (CDFPP). The CDFPP is a promising technique for the 3D imaging profile of dynamic surface deformation if several phase errors in the one-shot PSP method can be suppressed. This work proposes a processing scheme for phase error suppression schemes (PESS) based on retrieving the modulated sinusoidal fringe and color fringe normalization in PSP using RGB color channel. The processing of PESS consists of tunable bandpass filtering (BPF) followed by fringe normalization. The initial BPF function is defined based on a smoothing spline data set of frequency and power spectrum from the baseline color fringe image. The predefine BPF function could be tunable during the imaging process by considering each frame's condition and RGB channel spectrum mapping. The corrected fringe images are then normalized from the color imbalance, and the phase shift is calculated using the conventional three-step PSP. For evaluation, PESS is performed to reconstruct simulator membrane deformation from four different static profiles and tested to observe the 3D surface of continuous membrane deformation. The PESS could suppress the phase errors of less than 30% less absolute errors than the conventional method and successfully reconstruct the 3D surface for low-frequency continuous membrane deformation with minimizing phase errors.

Two-wavelength phase-shifting method with four patterns for three-dimensional shape measurement

Fringe projection technique has been widely used for three-dimensional (3D) shape measurement. However, it remains challenging to achieve high-speed measurement. A two-wavelength phase-shifting profilometry method with only four patterns is presented. Specifically, all these four patterns contain two wavelength components. The short wavelength component was used to compute the wrapped phase map, while the long one was used to unwrap the wrapped phase map. The performance of the proposed method was validated by both simulation study and experimental results.

Motion-artifact-free dynamic 3D shape measurement with hybrid Fourier-transform phase-shifting profilometry.

Fourier-transform profilometry (FTP) and phase-shifting profilometry (PSP) are two mainstream fringe projection techniques widely used for three-dimensional (3D) shape measurement. The former is well known for its single-shot nature and the latter for its higher measurement resolution and precision. However, when it comes to measuring the dynamic objects, neither approach is able to produce high-resolution, high-accuracy measurement results that are free from any depth ambiguities and motion-related artifacts. Furthermore, for scenes consisting of both static and dynamic objects, a trade-off between measurement precision and efficiency has to be made, suggesting that using a single approach can yield only suboptimal results. To this end, we propose a novel hybrid Fourier-transform phase-shifting profilometry method to integrate the advantages of both approaches. The motion vulnerability of multi-shot PSP can be overcome, or at least significantly alleviated, through the combination of single-shot FTP, while the high accuracy of PSP can also be preserved when the object is motionless. We design a phase-based, pixel-wise motion detection strategy that can accurately outline the moving object regions from their motionless counterparts. The final measurement result is obtained by fusing the determined regions where the PSP or FTP is applied correspondingly. To validate the proposed hybrid approach, we develop a real-time 3D shape measurement system for measuring multiple isolated moving objects. Experimental results demonstrate that our method achieves significantly higher precision and better robustness compared with conventional approaches where PSP or FTP is applied separately.

Three-dimensional measurement with reflection suppression based on high-dynamic range images

To deal with the three-dimensional (3-D) point cloud loss caused by object reflection in the active fringe projection 3-D measurement, an active reflection suppression method for 3-D measurement is proposed. The method employs high-dynamic range images obtained by multiple exposure image fusion and a three-wavelength phase-shift profilometry method to achieve high-precision 3-D measurement of reflective objects. Experimental results show that, compared to traditional 3-D measurement methods, the proposed one can more effectively handle reflections thereby avoiding 3-D point cloud loss in the measurement of reflective objects.

Motion-induced error reduction by combining Fourier transform profilometry with phase-shifting profilometry.

We propose a hybrid computational framework to reduce motion-induced measurement error by combining the Fourier transform profilometry (FTP) and phase-shifting profilometry (PSP). The proposed method is composed of three major steps: Step 1 is to extract continuous relative phase maps for each isolated object with single-shot FTP method and spatial phase unwrapping; Step 2 is to obtain an absolute phase map of the entire scene using PSP method, albeit motion-induced errors exist on the extracted absolute phase map; and Step 3 is to shift the continuous relative phase maps from Step 1 to generate final absolute phase maps for each isolated object by referring to the absolute phase map with error from Step 2. Experiments demonstrate the success of the proposed computational framework for measuring multiple isolated rapidly moving objects.

A new phase unwrapping algorithm based on Three Wavelength Phase Shift Profilometry method

Phase Shift Profilometry (PSP) method is widely used in 3D shape measurement. The advantage of the PSP method is that it is less sensitive to the surface reflectivity variations and the object can be measured point by point. But if only a single wavelength λ1 is used, the step height cannot be larger than λ1/4. In order to measure step height object, Gray Code (GC) and Multi-Wavelength Phase Shift Profilometry (MWPSP) method are used. But the GC method cannot solve the problem of the objects with a variety of colors under different illumination. The shortcoming of MWPSP method is that its speed is too slow because a lot of images with multi-wavelengths should be captured by the system. In order to solve the problem of measurement speed and measurement range, a new phase unwrapping algorithm based on Three Wavelength Phase Shift Profilometry (TWPSP) is proposed in this paper. A series of fringe images only with three wavelengths λ1, λ2 and λ3, are captured by the 3D measurement system. Three equivalent wavelengths λ12, λ23and λ123 based on the original three wavelengths are calculated to perform the phase unwrapping algorithm. Compared with the GC method, the proposed method can measure the objects with different colors under different illuminations. Compared with the MWPSP method, the proposed method is faster and high precision. Experiments demonstrate that the proposed method is robust, high precision, low noise and suit to measure the object with variety colors under different illumination. It can be used to measure arbitrary step height and multiple objects together.

3-D Measurement of Solder Paste Using Two-Step Phase Shift Profilometry

A two-step phase shift profilometry method (2-step PSP) with prefiltering and postfiltering stages is proposed to reconstruct the 3-D profile of solder paste. Two sinusoidal patterns which are pi-out-of-phase are used in the 3-D reconstruction. The new method uses only two fringe patterns rather than four as the four-step phase shift profilometry (4-step PSP). In Fourier transform profilometry (FTP), a bandpass filter is required to extract the fundamental spectrum from the background and higher order harmonics due to camera noise and imperfectness of the pattern projector. By using two pi-out-of-phase sinusoidal fringe patterns, the background term can be eliminated directly by taking the average of the two fringe patterns. The fringe pattern which is close to its ideal form can also be recovered from the averaging process. Prefiltering is utilized in filtering raw images to remove noise causing higher order harmonics. Hilbert transform is then used to obtain the in-quadrature component of the processed fringe pattern. Postfiltering is applied for reconstructing an appropriate 3-D profile.