illuminance vs luminousness

illuminance and luminousness both are nouns.

Word	Noun	Adjective	Verb	Adverb
illuminance	Yes	No	No	No
luminousness	Yes	No	No	No

As nouns, luminousness is a hypernym of illuminance; that is, luminousness is a word with a broader meaning than illuminance:

Other hypernyms of illuminance include brightness, brightness level, light, luminance, luminosity.

illuminance (noun)	luminousness (noun)
the luminous flux incident on a unit area	the quality of being luminous; emitting or reflecting light