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.
Some villains have tried to hide from Superman's x-ray vision by using lead-lined containers, believing that would make anything set up like that somehow invisible to him. However, the Joker learned the hard way about the reality of that limitation when he claimed to have Superman's friends held hostage and hidden in a number of lead lined coffins throughout Metropolis with a limited air supply. Actually, the Joker had his hostages with him and was fleeing the city with them in an lead-lined truck, expecting Superman to be too busy franticly searching the city. Unfortunately for the supervillain, all Superman had to do was to ascend above the city and do a wide scan of the cityscape with his x-ray vision for the coffins, which were immediately spotted among the few opaque objects in his scan. Thus Superman was able to quickly search those coffins and then track down and capture the Joker before he escaped considering that lead is an unusual metal to use all over a vehicle body, which made it instantly stand out to a similar wide scan.
