Imaging Method Improves Scanning Speed for Precise 3D Surface Measurement

A new technique for 3D surface measurement, called fringe photometric stereo (FPS), could improve the speed and accuracy of surface measurements taken for industrial inspection, medical applications, robotic vision, and other purposes.

The FPS method for acquiring and reconstructing 3D surface measurements was developed by a research team at the University of Electronic Science and Technology of China.

FPS speeds up the image scanning process and enables high-quality, unambiguous 3D reconstruction by requiring fewer fringe patterns than traditional fringe projection profilometry (FPP).

Researchers developed a method for acquiring and reconstructing high-quality 3D surface measurements that is faster and more accurate than traditional techniques. The new fringe photometric stereo (FPS) method requires fewer protected fringe patterns than traditional fringe projection profilometry (FPP), which speeds up the scanning process. Courtesy of Ce Zhu, University of Electronic Science and Technology of China.

To measure and reconstruct 3D surfaces using traditional FPP, a series of phase-shifted light patterns are projected onto an object’s surface. The reflected images are then captured and the phase differences analyzed to create a 3D map of the surface.

The long scanning time required for FPP comes primarily from the high number of multiple-frequency fringe images required to analyze phase differences. FPP uses triangulation to convert the phase data into continuous values for an accurate representation of the shape or surface.

FPS bypasses the need for phase unwrapping and reduces the number of fringe images required by using single-frequency fringe patterns. It analytically relates the depth gradient to the phase gradient, which allows it to retrieve the continuous 3D surface directly from the high-frequency wrapped phase. In this way, FPS enables 3D reconstruction for continuous objects without the need for phase unwrapping.

“Traditional 3D imaging works by comparing two viewpoints, similar to how our eyes work together to judge depth,” professor Ce Zhu, who led the research, said. “In contrast, our new approach ‘feels’ the surface by projecting light patterns, almost like running a hand over it to detect changes. This can reduce the number of patterns used by more than two-thirds, which greatly speeds up the scanning process, and surprisingly, is even more accurate than the old technique.”

The traditional phase-shifting profilometry method requires projecting multiple fringe patterns and analyzing numerous images, resulting in long scanning times. In contrast, the new method (bottom) significantly reduces the number of frames needed, making the process faster and more accurate. Courtesy of Ce Zhu, University of Electronic Science and Technology of China.

Only a camera and a projector are required to use FPS. There is no need for dual, multiple, or light field cameras. In experiments, the FPS technique was found to mitigate additive Gaussian noise better than traditional temporal phase unwrapping.

To test the new technique, the researchers set up an experimental system consisting of a 1280 × 960 camera with an 8-mm lens and a projector with a resolution of 912 × 1140. They used the setup to take measurements of single objects and groups of objects with continuous surfaces, including a human hand, a paper mask, a cloth toy, gypsum geometries, and clay handicrafts. They also validated the FPS approach using standard plane and sphere models, demonstrating that FPS effectively suppresses noise compared to traditional FPP.

The rapid scanning speed and high precision demonstrated by FPS make it a promising technique for high-precision industrial inspection. For example, it could be used for surface-mount inspection in printed circuit board manufacturing, defect detection in new energy batteries, corrosion inspection of oil pipelines, or speaker diaphragm deformation measurement.

“Our approach is ideal for applications demanding real-time scanning, including industrial applications like detecting defects in printed circuit boards, batteries, or oil pipelines as well as medical procedures such as diagnostics and implant customization,” Zhu said. “It could also help advance robotics by improving human-robot interaction or offering vision guidance for tasks like folding clothes.”

According to Zhu, FPS could be particularly useful for customizing prosthetics. “It can quickly acquire high-precision surface information from the residual limb, reducing errors associated with manual measurements and improving the fit of the prosthesis,” he said. “This would also eliminate the need to apply plaster or other materials to the skin, making the experience much more comfortable for the patient.”

Although the FPS method has been found to improve scanning speed and accuracy for scenes with continuous surfaces, its ability to reconstruct objects with sudden changes in depth requires further development. The team is addressing this challenge by incorporating established surface reconstruction techniques from photometric stereo into FPS. This enhancement could enable FPS to analyze more complex scenes, further widening its reach as a 3D surface imaging and image reconstruction tool.

The research was published in Optica (www.doi.org/10.1364/OPTICA.531601).