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Basic Experimental Mechanics


Projection Moiré


Structural Materials & Mechanics Research Group




Basic principles

The fringe projection technique allows measuring the 3D shape of objects using very simple experimental apparatus. A grating is projected onto the object of interest and an image is collected by a digital camera that views the object surface from a different direction. The separation of the projected fringes from a straight lines in the object image is related to the surface shape. The following expression describes the intensity distribution of the fringe pattern: I(x,y)=A(x,y)+B(x,y)cos(w(x,y)x+f) Where the modulated phase w(x,y) is an unknown, and is proportional to the surface height. A novel computational algorithm has been developed in the University of Sheffield to efficiently calculate the phase map from a single object image and a reference image, that can either be artificially generated (a virtual grating) or an image recorded of a flat plane without the object of interest. The phase obtained using this method is wrapped in the interval (-2p,2p), and must be unwrapped and calibrated to yield a continuous map with the information about the out-of-plane dimensions of the object. This technique has applications in non-destructive testing, quality control and machine vision.



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