Determination of the relationship between object distance and optimal defocus level for fringe projection systems

In fringe projection measurement, the suitable defocus of the projector can improve the measurement accuracy. This paper presents a method to determine the optimal defocus level in different object distances. In each object distance, this method adjusts the focal length of the projector through some special patterns, and the plane plate is measured by binocular fringe projection measurement system in different defocus levels. Then, the fitting plane error in different defocus levels is polynomial fitted, and the optimal defocus degrees are calculated on the fitting curve. Finally, the corresponding relationship between the object distance and the optimal defocus level is obtained by spline interpolation fitting. Experimental results show that this method can be successfully applied to fringe projection measurement with any object distance, and the measurement accuracy is improved by more than 10%.