Rapid Three-Dimensional Measurement Using Three-Channel Binary Stripe Defocused Projection

Literature Overview

The paper by Zhang Xin, Gai Shaoyan, and Da Feipeng, published in Laser & Optoelectronics Progress (2020, Vol. 57, No. 23), presents a novel three-dimensional surface measurement technique based on three-channel binary stripe defocused projection. The core innovation lies in encoding phase-shifted stripe patterns into three separate color channels (red, green, blue) and projecting defocused binary stripes sequentially within a single camera frame time. This approach achieves simultaneous speed improvement and reconstruction accuracy enhancement compared to traditional color projection methods, with the added capability of measuring colored objects and dynamic objects.

Core Technical Approach

The methodology follows a well-defined sequence: three-channel separated binary digital images serve as projector inputs, and defocused stripes from all three channels are projected sequentially within one camera exposure window. A color camera captures the fused tri-color stripe pattern on the object surface. A critical preprocessing step involves decoupling the three-channel stripes using calibration data obtained from a monochrome camera, which characterizes the color channel aliasing effect inherent in the measurement system. The decoupled stripe information is then used for phase calculation and subsequent three-dimensional reconstruction.

Key Technical Parameters and Process Windows

Interpretation of Technical Points

The defocused projection concept deserves particular attention. In traditional structured light measurement, over-focus or under-focus of projected fringes degrades fringe contrast and, consequently, phase extraction accuracy. However, in this method, the defocused state is deliberately controlled and compensated through the calibration process. The use of binary stripes rather than continuous-tone grayscale patterns further simplifies the projector hardware requirements, as binary patterns do not demand high gray-level resolution. The sequential projection of three channels within a single frame time represents a clever temporal multiplexing strategy that eliminates the need for hardware switching between separate projectors.

The channel aliasing compensation step is particularly important from a metrology standpoint. Color cameras inherently exhibit spectral cross-talk between channels due to imperfect color filter arrays. Without proper decoupling, the extracted phase from each channel would contain systematic errors that propagate into the final 3D reconstruction. The use of a monochrome camera for this calibration ensures a ground-truth reference free from color channel interference.

Integration with Engineering Practice

From the perspective of steel pipe and fitting manufacturing, this measurement technology holds significant potential for several applications. First, in pipe fitting forming inspection, the ability to rapidly capture 3D surface geometry of elbows, tees, and reducers could enable real-time monitoring of forming processes, detecting deviations from nominal geometry before they become critical. Second, the capability to measure colored or dynamically moving objects opens possibilities for in-line inspection during high-speed production where pipes are continuously moving along the production line. Third, the speed advantage over traditional methods could reduce inspection cycle times, improving throughput in manufacturing environments.

However, several practical challenges must be addressed before deployment in industrial pipe inspection settings. The dynamic range and contrast requirements for measuring metallic pipe surfaces, which are inherently reflective, may exceed the capabilities of the described system. Additionally, the calibration procedure using a monochrome camera adds complexity to the setup, and the system would need to be robust against industrial environmental factors such as vibration, dust, and temperature fluctuations.

Key Questions and Reflections

A critical question arises regarding the scalability of this technique to large-diameter pipe surfaces. The field of view and working distance of the structured light system must accommodate the geometry of pipes ranging from small-bore instrumentation tubing to large-diameter line pipes. Another consideration is the integration with existing quality control systems in pipe manufacturing plants, where coordinate measuring machines and laser scanners are already deployed. The value proposition of this method lies in its speed advantage, but engineers must evaluate whether the accuracy achieved is sufficient for the dimensional tolerances specified in standards such as ASME B16.9 for butt-weld fittings or ASME B16.5 for flanges.

Study Insights and Implications

The fundamental insight from this paper is that temporal multiplexing of structured light patterns through color channel separation, combined with deliberate defocus and calibration-based compensation, can achieve a favorable trade-off between measurement speed and accuracy. For pipe and fitting manufacturers, this represents a pathway toward faster, more flexible 3D metrology that can be integrated into production workflows. The method's applicability to colored and dynamic objects is particularly noteworthy, as it suggests potential for non-contact inspection of pipes in transit or during forming operations where surfaces may not be stationary. Future work should focus on adapting this technique to the specific challenges of metallic surface measurement, including specular reflection handling and scale adaptation to large industrial components.