sun helmet vs sun hat

sun helmet and sun hat both are nouns.

Word	Noun	Adjective	Verb	Adverb
sun helmet	Yes	No	No	No
sun hat	Yes	No	No	No

As nouns, sun hat is a hypernym of sun helmet; that is, sun hat is a word with a broader meaning than sun helmet:

sun helmet: a lightweight hat worn in tropical countries for protection from the sun
sun hat: a hat with a broad brim that protects the face from direct exposure to the sun

Other hypernyms of sun helmet include sunhat.

sun helmet (noun)	sun hat (noun)
a lightweight hat worn in tropical countries for protection from the sun	a hat with a broad brim that protects the face from direct exposure to the sun