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Birefringence is the decomposition of a ray of light into two rays when it passes through certain material. An extraordinary ray is produced along with the ordinary ray. Of course, when thinking of a laser reading from an optical disc, the importance of this issue with regard to DVD discs is apparent. Due to stresses in the plastic of a DVD disc, this distortion of light can be produced and cause issues. For this reason, birefringence must be kept within specification and so is measured by disc manufacturers.