We attempted to apply the caustic method to the frozen-stress model to analyze a concentrated load P located in a straight line. The caustic patterns formed by a concentrated load for the frozen-stress model are theoretically calculated by the optical constants that were previously evaluated under the frozen-stress condition. Comparison between optical constants in the frozen-stress state and those at room temperature showed the relationship of plus-minus signs in them to be unstable and values varied considerably under some conditions, thus making it complicated to measure them. In this study, we present some practical methods for determining the value of P from the caustic patterns. In addition, we also propose a simple technique for measuring the optical constants in the frozen-stress state. The caustic patterns obtained in our experiment are consistent with theoretically calculated ones. Furthermore, whether evaluated by the proposed technique or determined by the method using the deviation of light due to stress gradient, these optical constants are the same.

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