Abstract
We propose a liquid crystal (LC)-based 3D optical surface profilometer that can utilize multiple fringe patterns to extract an enhanced 3D surface depth profile. To avoid the optical phase ambiguity and enhance the 3D depth extraction, 16 interference patterns were generated by the LC-based dynamic fringe pattern generator (DFPG) using four-step phase shifting and four-step spatial frequency varying schemes. The DFPG had one common slit with an electrically controllable birefringence (ECB) LC mode and four switching slits with a twisted nematic LC mode. The spatial frequency of the projected fringe pattern could be controlled by selecting one of the switching slits. In addition, moving fringe patterns were obtainable by applying voltages to the ECB LC layer, which varied the phase difference between the common and the selected switching slits. Notably, the DFPG switching time required to project 16 fringe patterns was minimized by utilizing the dual-frequency modulation of the driving waveform to switch the LC layers. We calculated the phase modulation of the DFPG and reconstructed the depth profile of 3D objects using a discrete Fourier transform method and geometric optical parameters.
Highlights
The demand for acquiring 3D information has increased, accompanied by improvements in several 3D applications like 3D displays, 3D printing technologies, 3D medical or dental imaging systems, and 3D vision modules for robot or vehicle applications [1,2,3,4,5,6]
We propose an liquid crystal (LC)-based dynamic fringe pattern generator (DFPG) for a more compact 3D optical surface profilometer system, which can generate multiple fringe patterns to enhance the 3D depth extraction and avoid the optical phase ambiguity in analyzing the 3D depth profile from distorted fringe patterns induced by the surface morphologies
The multi-spatial frequency properties of the interference fringe patterns were achieved by applying a turn-on voltage on one of the switching slits prepared with the twisted nematic (TN) LC mode
Summary
The demand for acquiring 3D information has increased, accompanied by improvements in several 3D applications like 3D displays, 3D printing technologies, 3D medical or dental imaging systems, and 3D vision modules for robot or vehicle applications [1,2,3,4,5,6]. To avoid the optical phase ambiguity from the fringe patterns distorted by the surface morphology, multiple sets of interference patterns were needed, and phase unwrapping or geometrical parameter methods were applied to reconstruct surface depth profiles. The depth extraction method based on the geometrical parameters could provide the depth and position value informations in the absolute coordinate system without the phase unwrapping process [10] These optical techniques do not damage the surface morphology of an object even for a soft surface. The electrical switching properties of the phase modulation and the slit spacing controls for the interference patterns of the DFPG are characterized and the 3D depth profile reconstruction from the distorted fringe patterns using the discrete Fourier transform (DFT) method and geometric optical parameters is presented
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