double refraction vs birefringence

double refraction and birefringence both are nouns.

Word	Noun	Adjective	Verb	Adverb
double refraction	Yes	No	No	No
birefringence	Yes	No	No	No

As nouns, double refraction and birefringence are synonyms defined as:

double refraction and birefringence: splitting a ray into two parallel rays polarized perpendicularly

double refraction (noun)	birefringence (noun)
splitting a ray into two parallel rays polarized perpendicularly	splitting a ray into two parallel rays polarized perpendicularly