Infrastructure of time-averaged projection moire fringes in vibration analysis

Time-averaged moire fringes may be produced by projecting a grid onto a vibrating test item surface using a conventional (2 in. x 2 in.) slide-projector and recording with a 35-mm SLR camera. Quantitative analysis of the resultant fringes is generally performed with the help of equations developed by time-averaging the optical intensity of the projected grid throughout the vibration period of the test item surface. In the present paper the authors present a physical description for the production of time-averaged moire fringes involving the splitting of grid lines resulting from their behavior as simple harmonic oscillators. Superimposition of vibrating grid lines produces blurring of the photographic image; regions of zero-amplitude vibration (nodes) may be identified by the clarity of their grid lines, enabling the analysis of mode shapes from the photographs.