looker-on vs viewer

looker-on and viewer both are nouns.

Word	Noun	Adjective	Verb	Adverb
looker-on	Yes	No	No	No
viewer	Yes	No	No	No

As nouns, viewer is a hypernym of looker-on; that is, viewer is a word with a broader meaning than looker-on:

looker-on: someone who looks on
viewer: a close observer; someone who looks at something (such as an exhibition of some kind)

Other hypernyms of looker-on include looker, spectator, watcher, witness.

looker-on (noun)	viewer (noun)
someone who looks on	an optical device for viewing photographic transparencies
	a close observer; someone who looks at something (such as an exhibition of some kind)