Photoelasticity as an experimental method for analyzing stress fields in
mechanics was developed in the early thirties by the pioneering works of
Mesnager in France and Coker and Filon in England. Almost concurrently,
Föppl, Mesmer, and Oppel in Germany contributed significantly to what
turned out to be an amazing development. Indeed, in the fifties and
sixties a tremendous number of scientific papers and monographs
appeared, all over the world, dealing with various aspects of the method
and its applications in experimental stress analysis. All of these
contributions were based on the so-called Neumann-Maxwell
stress-opticallaw; they were developed by means of the classical methods
of vector analysis and analytic geometry, using the
conventionallight-vector concept. This way of treating problems of
mechanics by photoelasticity indicated many shortcomings and drawbacks
of this classical method, especially when three-dimensional problems of
elasticity had to be treated and when complicated load and geometry
situations existed. Meanwhile, the idea of using the Poincare sphere for
representing any polarization profile in photoelastic applications was
introduced by Robert in France and Aben in the USSR, in order to deal
with problems of polarization oflight passing through aseries of optical
elements (retarders andjor rotators). Although the Poincare-sphere
presentation of any polarization profile con- stitutes a powerful and
elegant method, it exhibits the difficulty of requiring manipulations in
three-dimensional space, on the surface of the unit sphere. However,
other graphical methods have been developed to bypass this difficulty.