crown glass vs optical glass

crown glass and optical glass both are nouns.

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

As nouns, optical glass is a hypernym of crown glass; that is, optical glass is a word with a broader meaning than crown glass:

crown glass: optical glass of low dispersion and low refractive index
optical glass: clear homogeneous glass of known refractive index; used to make lenses

crown glass (noun)	optical glass (noun)
optical glass of low dispersion and low refractive index	clear homogeneous glass of known refractive index; used to make lenses
a glass blown into a globe which is later flattened and spun to form a disk