It is demonstrated that far-infrared interferometry can be used successfully to measure birefringence of crystalline polymer samples thick enough to be opaque at visible wavelengths. The use of a strong, optically-pumped laser as a source of radiation allows absorbing plastics to be measured. An injection molded, high-density polyethylene specimen showed a birefringence of about 2%, whereas a compression-molded sample of the same material showed no measurable birefringence.
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Jacobsson et al. (1982) studied this question.
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