Improvement of elliptically polarized white light technique for photoviscoelasticity

Viscoelastic materials such as rubbers and plastics are widely used not only in industrial fields but in daily life. The mechanical properties of the materials are remarkably dependent on time and temperature. Photoviscoelastic technique has been used for stress field measurement in the kind of material. However, the conventional techniques are not suitable to measure fringe order and the principal direction of birefringence simultaneously. Yoneyama et al proposed an elliptically polarized white light technique which can measure fringe order and the principal direction of birefringence simultaneously with a single exposure. However, the technique involves still three severe problems. First, the accuracy in determination of the principal direction of birefringence is severely influenced not only by noise but by the accuracy of fringe order determination, because the principal direction of birefringence is determined by utilizing the attenuation of light intensity and the fringe order. Second, it is difficult to determine fringe order automatically and accurately. And the third, this technique needs long time to manipulate images. In this paper, the authors describe how to improve the technique and to solve the problems. In this situation, the polarization angle for the largest attenuation in light intensity is calculated and examined to obtain good results. Also, an algorithm for the determination of fringe order is developed. As the results, the angle for the highest attenuation is obtained as 45 degrees, and the fringe order is successfully determined by applying a window field reference technique.