Measurement of Mechanical Strain Using Chromatic Monitoring of Photoelasticity

The present contribution describes a chromatic processing approach for quantifying the two dimensional, polychromatic interference patterns produced by a strained photo-elastic element and recorded with a CCD camera. The outputs from the three R, G, B channels of the camera covering a selected area of the interference pattern are processed to yield three chromatic parameters which are H (dominant signal wavelength), L (nominal signal strength), S (effective wavelength spread of signal). It is shown that the value of each of the three parameters varies with strain in a quasi cyclical manner, all being out of phase with each other. Consequently the strain measurement range and sensitivity can both be optimized by the use of the appropriate chromatic parameter within different strain ranges.