Phase errors due to speckles in laser fringe projection

When measuring a three-dimensional shape with triangulation and projected interference fringes it is of interest to reduce speckle contrast without destroying the coherence of the projected light. A moving aperture is used to suppress the speckles and thereby reduce the phase error in the fringe image. It is shown that the phase error depends linearly on the ratio between the speckle contrast and the modulation of the fringes. In this investigation the spatial carrier method was used to extract the phase, where the phase error also depends on filtering the Fourier spectrum. An analytical expression for the phase error is derived. Both the speckle reduction and the theoretical expressions for the phase error are verified by simulations and experiments. It was concluded that a movement of the aperture by three aperture diameters during exposure of the image reduces the speckle contrast and hence the phase error by 60%. In the experiments, a phase error of 0.2 rad was obtained. (C) 2010 Optical Society of America