lightness vs sunniness

lightness and sunniness both are nouns.

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

As nouns, sunniness is a hyponym of lightness; that is, sunniness 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
sunniness: lightness created by sunlight

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

lightness (noun)	sunniness (noun)
the trait of being lighthearted and frivolous	lightness created by sunlight
the visual effect of illumination on objects or scenes as created in pictures	the quality of being cheerful and dispelling gloom
the property of being comparatively small in weight
having a light color
the gracefulness of a person or animal that is quick and nimble
a feeling of joy and pride