Projection Moiré Interferometry

The goal of the LaRC PMI development effort is to mature the PMI technique for the routine acquisition of spatially continuous, full-field model deformation measurements in production wind tunnels. Although other video photogrammetry-based model deformation measurement techniques have been used on a nearly routine basis in NASA facilities, PMI has the unique ability to provide pictures of model deformation, potentially representing the global deformation more accurately. The quantitative deformation measurement accuracy and resolution goals for PMI are 0.25- and 0.1-mm respectively, with ± 0.025 degree localized twist measurement capability. These accuracy and resolution specifications are to be achieved for a 1.2-x 1.2-meter instrument field-of-view, typical of large wind tunnel applications.

PMI is an optically simple, non-contacting measurement technique used since the 1970s for surface topology and shape characterization. The fundamentals of PMI are well known, but only recently have PMI and similar systems been used to quantitatively measure wind tunnel model deformations while under aerodynamic load. The elegance of the technique is its optical simplicity, requiring only two major components: a projector and a video camera. The intricacies of the technique lie within the image processing routines required to obtain quantitative deformation data. PMI as applied to measuring wind tunnel model deformation is shown schematically in figure 1.

Figure 1.