X-rays were a high energy type of electromagnetic radiation. X-ray photons were used for medical and dental imaging, as well as weapons systems. They are also produced naturally by a variety of stellar phenomena, including pulsars and quasars.
Gosis' species used x-rays to get a look inside a lifeform. In 2152, Temec created a radiograph of Captain Jonathan Archer by using x-rays. (ENT: "The Communicator")
In 2364, Wesley Crusher reported that the deflectors of the USS Enterprise-D were being hit by X-rays. Worf adjusted the settings to compensate for the rays' effect. (TNG: "Symbiosis")
Talarian warships used lasers that emitted x radiation. (TNG: "Suddenly Human")
While working on a way to separate Tuvok and Neelix following their transporter accident, The Doctor referred to the use of barium as a contrast agent for the "primitive imaging technology called X-rays." (VOY: "Tuvix")
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