sun hat vs sun helmet

sun hat and sun helmet both are nouns.

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

As nouns, sun helmet is a hyponym of sun hat; that is, sun helmet is a word with a more specific, narrower meaning than sun hat:

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

Other hyponyms of sun hat include pith hat, pith helmet, topee, topi, sunbonnet.

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