Dynamic shape measurement system for laser materials processing

We describe a laser-based profilometry system that demonstrates high-resolution, dynamic surface height measurements in hostile conditions. The robust system measures transient distortions of continuous surfaces under the conditions of high stray light and mechanical disturbances that are typically found in laser drilling and welding. A coherent fringe pattern is projected onto the object surface using a binary phase grating interferometer. A charge-coupled device (CCD) camera fitted with a narrow-bandpass filter, centered on the optical wavelength of the projected fringes, records images of the illuminated workpiece while rejecting unwanted light from the high-power processing laser. The fringe pattern in each image is analyzed by the Fourier transform method, enabling dynamic shape measurement at the camera frame rate. The fringe order is identified uniquely by the dynamic projection of a laser spot aligned with a single fringe. An in-situ calibration produces height maps that are corrected for perspective and lens distortion errors. The unique measurement capability of the system has enabled the first dynamic shape measurements to be made during laser forming. Results are presented for the laser forming of 80x80 mm titanium alloy coupons, over which area a height accuracy of ±12 µm was achieved.