radiancy vs brightness

radiancy and brightness both are nouns.

Word	Noun	Adjective	Verb	Adverb
radiancy	Yes	No	No	No
brightness	Yes	No	No	No

As nouns, brightness is a hypernym of radiancy; that is, brightness is a word with a broader meaning than radiancy:

radiancy: the quality of being bright and sending out rays of light
brightness: the location of a visual perception along a continuum from black to white

radiancy (noun)	brightness (noun)
the quality of being bright and sending out rays of light	the location of a visual perception along a continuum from black to white
	the quality of being luminous; emitting or reflecting light
	intelligence as manifested in being quick and witty