Long term birefringence change in polycarbonate substrate has been studied at high temperature or with stress, and the equation relating the birefringence change rate with temperature or stress was experimentally determined. At high temperature, the change in birefringence was found to be logarithmic in time and can be explained in terms of a stress relaxation. The effect of stress on the change in birefringence was analyzed from a viscoelastic point of view, and it was found that the change in birefringence was proportional to the 0.31 power of time. It is concluded, from the evaluation of the birefringence change under operating environment, that a polycarbonate substrate has high stability in regard to birefringence when used for magneto-optical disks.
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