Differential confocal over-range determination method based on an information theory

The existing differential confocal axial three-dimensional (3D) measurement method cannot determine whether the surface height of the sample in the field of view is within its effective measurement range. Therefore, in this paper, we propose a differential confocal over-range determination method (IT-ORDM) based on an information theory to determine whether the surface height information of the sample to be examined is within the effective measurement range of the differential confocal axial measurement. First, the IT-ORDM finds the boundary position of the axial effective measurement range by the differential confocal axial light intensity response curve. Then the effective intensity measurement ranges of the pre-focus axial response curve (ARC) and the post-focus ARC are determined by the correspondence between the boundary position and the ARC. Finally, the intersection operation of the pre-focus image of effective measurement and the post-focus image of effective measurement is used to realize the extraction of the effective measurement area of the differential confocal image. The experimental results show that the IT-ORDM can effectively determine and restore the 3D shape of the measured sample surface at the reference plane position in the multi-stage sample experiments.