Superman's eyes change, and emit a visible beam in The Mechanical Monsters.
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X-Ray Vision is a term that describes the ability to see through solid objects. Kryptonians, Daxamites and Martians are among those who possess this sense.

The method by which this is possible is unclear. In most cases, the ability can more accurately be described as "see-through vision" since what the viewer sees contains more information than what an X-ray scan can provide.

The TV series Smallville took the term literally, depicting what Clark sees both as an X-ray image and a clear view of normally obstructed objects. When he first develops the power in the season one episode "X-ray" he explains it to his parents, "Sometimes I can see through things, other times it's like an X-ray."

In Kryptonians, the power is attributed to the Photonucleic Effect, meaning that the ability can only be used in the presence of a yellow star. Kryptonians and Daxamites can see through any object with the exception of lead, while Martians can see through any object.