Picometer-differential twice-exposed element for three-dimensional measurement with extremely long depth of field

The accuracy of optical three-dimensional (3D) shape measurement is always influenced by the defocusing of a projection or imaging system. In this paper, a novel optical element made by picometer-differential twice-exposed holography, called a picometer comb, is proposed to generate the projection pattern for 3D shape measurement. Two interference fields with picometer-scale period difference are recorded on a substrate to fabricate the picometer comb by twice-exposed laser holography; this element reconstructs the diffraction field, which is essentially the interference between the holograms of two object waves with a slight angle. This picometer comb has the advantage of the generation of a stable light field distribution with extremely long depth of field and small divergence angle. We demonstrate that this diffraction field provides a solution for non-defocusing 3D shape measurement.