New scattered light method in three-dimensional photoelasticity

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Abstract
The principle of polarization of scattered light is used to determine the principal stresses within a specimen. The effect of a series of birefringent media is shown to be equivalent to that of a single birefringent medium coupled with a rotation of the principal axes. This requires a knowledge of the state of polarization of the emergent beam, i.e. the ellipticity of the light ellipse, and Poincaré's method is developed to show that the energy of the emergent beam may be expressed in terms of ellipticity as the sum of two components, one independent of the orientation a of the analyser and one dependent on a; a photomultiplier is used to convert these into a continuous alternating voltage which may be measured by d.c. and a.c. voltmeters in parallel. Similarly, the phase difference may be determined in terms of voltage ratios. The accuracy of measurements of orientation is to within 0.2° and of phase difference to within 1°. With this method of measurement it is possible to use a convergent beam and this increases the voltages obtainable.

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