crown glass vs optical crown

crown glass and optical crown both are nouns.

Word	Noun	Adjective	Verb	Adverb
crown glass	Yes	No	No	No
optical crown	Yes	No	No	No

As nouns, crown glass and optical crown are synonyms defined as:

crown glass and optical crown: optical glass of low dispersion and low refractive index

Other synonyms of crown glass include optical crown glass.

crown glass (noun)	optical crown (noun)
optical glass of low dispersion and low refractive index	optical glass of low dispersion and low refractive index
a glass blown into a globe which is later flattened and spun to form a disk