X-ray

X-rays were a type of radiation. They were used in medical imaging. They were also produced naturally by a variety of stellar phenomena.

In 2265, the USS Enterprise (NCC-1701) attempted to save Mestiko from the X-ray radiation emitted by a rogue pulsar, but were only partially successful. 

In 2269, when a mirror image duplicate of Spock was created, James T. Kirk asked Leonard McCoy if X-rays could be used to tell the two apart, but since Vulcans had perfect bilateral symmetry, this was impossible. 

In 2270, the Enterprise encountered X-ray storms while in orbit of a naked singularity. 

In 2348, the USS Stargazer (NCC-2893) disabled several ships belonging to Acamarian Gatherers by luring them towards a red dwarf just before it gave off an x-ray flare. 

In 2365, the USS Enterprise (NCC-1701-D), out of desperation, used X-rays in an attempt to scan a Cognoscente weapon, to no avail.