lightness vs brightness

lightness and brightness both are nouns.

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

As nouns, brightness is a hyponym of lightness; that is, brightness is a word with a more specific, narrower meaning than lightness:

lightness: the visual effect of illumination on objects or scenes as created in pictures
brightness: the location of a visual perception along a continuum from black to white

Other hyponyms of lightness include aura, aureole, gloriole, glory, halo, nimbus, sunniness, highlight, highlighting.

lightness (noun)	brightness (noun)
the trait of being lighthearted and frivolous	the location of a visual perception along a continuum from black to white
the visual effect of illumination on objects or scenes as created in pictures	the quality of being luminous; emitting or reflecting light
the property of being comparatively small in weight	intelligence as manifested in being quick and witty
having a light color
the gracefulness of a person or animal that is quick and nimble
a feeling of joy and pride