light beam vs low beam

light beam and low beam both are nouns.

Word	Noun	Adjective	Verb	Adverb
light beam	Yes	No	No	No
low beam	Yes	No	No	No

As nouns, low beam is a hyponym of light beam; that is, low beam is a word with a more specific, narrower meaning than light beam:

light beam: a column of light (as from a beacon)
low beam: the beam of a car's headlights that provides illumination for a short distance

Other hyponyms of light beam include heat ray, high beam, moon-ray, moon ray, moonbeam, sunbeam, sunray, laser beam.

light beam (noun)	low beam (noun)
a column of light (as from a beacon)	the beam of a car's headlights that provides illumination for a short distance