Enhanced three-dimensional reconstruction of surfaces using multicolor gratings

Most optical topolography systems use a single-wavelength laser for projection, using a swept spot, a moving line, or a projected grating. In a typical projected grating system, the gratings are shifted and a series of images is used to recover the 3-D shape of the target. When the series of images is analyzed in the normal phase shift manner, the resulting 2-D phase map typically has phase unwrapping problems due to noise and Nyquist limits. Surfaces with large vertical discontinuities present a problem for phase unwrapping 3-D shape recovery. We look at simultaneously projecting multiple wavelengths onto a surface to help avoid problems in unwrapping the 2-D phase map. Multicolor projection and shape recovery are demonstrated with a white-light Michelson interferometer and with a two-color Mach-Zehnder interferometer. Using multiple wavelengths, it is unnecessary to rotate the interferometer mirror to change the grating pitch and some operations can be done in parallel, which reduces scanning time. Limitations and improvements in the current system are discussed. (C) 2000 Society of Photo-Optical instrumentation Engineers. [S0091-3286(00)00601-2].