UV vs sun-ray

UV and sun-ray both are nouns.

Word	Noun	Adjective	Verb	Adverb
UV	Yes	No	No	No
sun-ray	Yes	No	No	No

As nouns, sun-ray is a hyponym of UV; that is, sun-ray is a word with a more specific, narrower meaning than UV:

UV: radiation lying in the ultraviolet range; wave lengths shorter than light but longer than X rays
sun-ray: a ray of artificial ultraviolet light from a sunray lamp

Other hyponyms of UV include sunray.

UV (noun)	sun-ray (noun)
radiation lying in the ultraviolet range; wave lengths shorter than light but longer than X rays	a ray of artificial ultraviolet light from a sunray lamp