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

Projection moire 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.