High speed fringe projection for dynamic shape measurement using binary phase mask. Part 2: manufacture and test

Dynamic measurement of object shape is now achievable at projection rates of up to 30,000 binary fringe patterns per second from a digital micromirror device (DMD) based projector. This two-part paper describes the design, manufacture and test of a binary phase mask in the projection system that produces high-quality greyscale fringes from the binary patterns, with much reduced phase noise compared to the projector defocus approach that has been used until now. Part 2 describes the experimental verification and characterization of the phase masks, manufactured using photolithography and dry etching from the design methods described in Part 1. The height distribution of two masks with point spread function (PSF) dimensions of 1 × 256 and 4 × 64 pixels were assessed using coherence scanning interferometry and contact surface profilometry. Phase noise was determined using standard temporal unwrapping fringe sequences and found to be 20–50% higher than a traditional 8-bit greyscale approach. The projector lens distortion has been observed, and the corresponding compensation method shown to significantly improve the quality of the projected fringes. Increasing the number of projector frames per camera frame reduced phase noise, to less than one-half the 8-bit values for a 10-slice sequence. Fringe image acquisition rates for the 1-slice and 10-slice sequences are up to 255 × and 25 × faster than for the 8-bit greyscale sequence, respectively. Finally, examples are presented of fringe patterns and wrapped phase maps from a high-speed system used to measure moving objects, with projection and acquisition rates of up to 9515 frames s⁻¹.