irradiation vs heat ray

irradiation and heat ray both are nouns.

Word	Noun	Adjective	Verb	Adverb
irradiation	Yes	No	No	No
heat ray	Yes	No	No	No

As nouns, heat ray is a hyponym of irradiation; that is, heat ray is a word with a more specific, narrower meaning than irradiation:

Other hyponyms of irradiation include high beam, moon-ray, moon ray, moonbeam, sunbeam, sunray, laser beam, low beam.

irradiation (noun)	heat ray (noun)
(medicine) the treatment of disease (especially cancer) by exposure to a radioactive substance	a ray that produces a thermal effect
the apparent enlargement of a bright object when viewed against a dark background
(Pavolvian conditioning) the elicitation of a conditioned response by stimulation similar but not identical to the original stimulus
(physiology) the spread of sensory neural impulses in the cortex
a column of light (as from a beacon)
the condition of being exposed to radiation