DFGM: combining steady fringe patterns to obtain high definition point clouds

The objective of this research project was the reduction of the mesurement time, and the possibility of using a simple slide projector instead of those based on LCD or DLP matrices.

Fig. 1 shows the concept: the projecion of a number of fringe patterns, typical of the GCM, GCPS and PSM techniques is replaced by the projection of a single pattern, as for the SGM approach.

Fig. 1 - Example of a number of fringe pattern project onto surfaces and the novel single pattern projection proposed.

As depicted in Fig. 2, the optical head is simplified, and, in principle, it is possible to design the instrument in such a way that a very compact, low cost optical head is obtained.

The slide projector typically projects steady patterns as the one shown in Fig. 3.

The elaboration follows the DFGM approach and results into two phase maps whose sensitivity to height variations is proportional to the period of the two components. The pattern at higher spatial period is used to save the range of the measurement, and the pattern at the lower period is used to increase the resolution (Fig. 4).

Fig. 4 - Scheme of the combination of the two patterns with high and low period.

The information from the two phase maps is used by the tringulation formula to compute the height of the object (the relative approach is used). The triangulation is performed on a relative basis, i.e., the geometrical parameters of the optical head must be accurately determined and given as input to the system. The shape map is relative with respect to a reference surface (a plane).

Fig. 5 shows an example of the quality of the obtained point clouds. Typical values of the measurement errors are in the range of 0.2 mm – 0.3 mm over an illumination area of 300mm x 230mm, and a measurement interval up to 100mm.

Fig. 5 - Point Cloud example obtained by the system.