Interferometric moiré pattern encoded security holograms with concealed phase object

This paper describes an interferometry-based scheme for concealing phase objects in a security hologram. The decoding process is based on geometrical moiré technique and requires a key for hologram authenticity verification. Decoding process is straightforward and could be carried out in white light, making it suitable for validation of security holograms. The method is described and experimental results are presented.     

Determination of residual stress by use of phase shifting moiré interferometry and hole-drilling method

A phase-shifting moiré interferometry and hole-drilling combined system was developed to determine residual stresses. The relationship between the 2D displacement data of three points around the drilled hole and the residual stresses relieved by hole-drilling was established. The experimental setup consisted of a four-beam moiré interferometer and a computer-controlled hole-drilling system. Two phase shifters controlled by computer were fixed in two of the four optic paths to directly get the displacement data. With special residual stresses calculation software, the phase distributions of the u and v field obtained by moiré interferometry were quickly converted into values of residual stresses. To analyze the accuracy of this experimental system, an aluminum specimen with a blind hole in the center was real-time tensioned in this system. The displacement field obtained by phase shifting moiré interferometry was compared with the finite element method solution. Good agreement was found with respect to each other. As an application, the in-depth residual stresses of a shot-peened aluminum plate were measured by this method, and possible error sources were discussed.     

A novel technique to determine difference contours between digital and physical objects for projection moiré interferometry

Projection moiré interferometry (PMI) is an out-of-plane displacement measurement technique, and consists of capturing reference and deformed images of a grid pattern projected on the test object. By differencing the reference and deformed images of the projected grid pattern, a fringe pattern is generated from which the displacement field can be extracted. Due to the projection-oriented nature of this technique, measuring displacements in applications with non-viewable, hidden, or inaccessible reference surfaces excludes the use of PMI. This paper presents a technique for computing the difference contours between a digital and physical object. A CAD model of the inaccessible surface is converted to a point cloud and a surface interpolation function is implemented to generate a digital displacement field, which can be correlated and differenced from the displacement field of the physical object determined through traditional PMI methods. These techniques are validated by comparing results from an airfoil with other measurement methods.