Double light refraction
Birefringence occurs when there are impurities in the polycarbonate of a disc or when the polycarbonate is stressed when molded. The light is then refracted in more than one direction; signal response to the optical pickup is degraded.
Certain types of material have this double refraction. The substrate used for CD-Rs has this property, and it is caused by the way the molecules are organized and by the internal tensions created when the disc is molded. Excessive birefringence can cause interference and read errors.