Are his eyes emitting dangerous levels of radiation that penetrate all matter(except lead, of course)? Is it a form of clairvoyance(far seeing)? Something else?
If he could just see X-Rays, that would be pretty remarkable in itself (though only useful if the target was standing in front of an X-Ray source) but numerous accounts suggest his eyes produce X-Rays (allowing him to destroy inconvenient pieces of film and such) which invites the question of how these rays bounce and return to him, passing through the target (twice?).
Of course the unstated issue is why he doesn’t used such an ability all the time, including casually scanning any opponent before facing him in battle to see if he’s carrying any small lead boxes that might contain, I dunno, kryptonite or something. For a guy with X-Ray vision and faster-than-bullet sped, Superman sure gets taken by surprise a lot.
As with everything in comic books, it depends on the writer.
For a couple of decades, it might as well have been magic. No one gave it much thought, so it just worked, no explanation needed.
Then there was a period of time when his powers (yes, all of them) were retconed into being an extension of a “telekinetic field.” Basically, it wasn’t that Superman was super-strong or invulnerable, it’s just that he was a really powerful telekinetic, and that was just how his powers appeared to outside observers. X-ray vision wasn’t really made of x-rays, but was a kind of telekinetic “sense” that his brain interpreted as being able to see through things.
Just before the New 52, it was explained that Superman was able to see the entire electromagnetic spectrum: radio waves, X-rays, gamma, the whole shebang.
After the New 52, the new Superman was able to emit light from his eyes - basically, his X-ray vision was an extension of heat vision, but he was able to select what kind of light, even ultra-low frequency or microwave. As expected, this was super overpowered, and it wasn’t too long before he lost his powers completely.
Now, the Superman from the nineties has made his way to the New 52 universe and taken over from the New 52 Superman, so I guess the telekinetic explanation is back for a little while, or at least until the writers decide to shake things up again.
Well, it couldn’t be traditional x-rays because those need the emitter to be on one side of the object/person and the sensor to be on the other because they recorded the relative amount of x-rays that passed through the target. Superman must use backscatter x-ray. (Or he could use millimeter wave radio.) Either way, he’d have to shoot the beams from his eyes.
Also, this.
Even back in the Gold and Silver Ages, this was handled inconsistently. In general, Superman had the ability to see inside things when he wanted to. No explanation given (or necessary). Some took this to mean that he could see further into the spectrum than ordinary mortals*. But sometimes it was implied that his x-ray vision was someone related to his heat vision, which implied that he was emitting radiation and receiving back reflections, or something. Kinds like x-ray band radar.
however it worked , it apparently wasn’t “on” all the time – Supes seemed to perceive the world as everyone else did, and not as the bizarre spectacle of people walking around as skeletons, or something.
*A professor at one of my schools once gave a lecture on X-ray astronomy b y entitling it something like “seeing the Universe as Superman Does”
I have a vague recollection that at one point his X-ray vision was explained as part of his microscopic vision. He could see through things by looking between the atoms. This would probably have been in the late 50s or early 60s.
That was John Byrne’s explanation in the post-Crisis reboot, late 1980’s.
If this is any guide, the biomechanical process (at 5:22) involves pupil-switching.
So when, in the movie, Lois Lane asked him to announce the color of her panties…
I don’t know if The Adventures of Superman is canon, but in the October 8th episode back in 1955 The Seven Souvenirs a crook runs a scheme to trick Superman into using his X-Ray vision on some daggers in an attempt to transmute them into radium. If Superman’s X-Ray vision can do that, it has to be based on emitting some kind of radiation, and is not a passive receptor.
He definitely emitted X-rays several times in the Superboy comics. Using it similarly as heat vision to melt glass or other stuff
It works very well, thank you.
I do remember a line in one of the books immediately after Crisis, where Kal was trying to use his x-ray vision on a building that was infested with alien gunk. He clearly stated that it helps if he knows what he is supposed to be looking at, making x-ray vision (for that writer at that time) a type of clairvoyance.
I’m more curious about the genesis of the heat vision. I read DC as a kid in the 1960s and often bought the “80 Page Giant” volumes of reprints from the '50s, and a lot of them had Supes saying “I will use the HEAT OF MY X-RAY VISION to melt this…thing.”
When did the heat vision stop being an adjunct and become a real super power?
I mean, it’s one hell of an addition. That was Sun Boy’s whole schtick, and it got him into the Legion of Super Heroes.
The Superman Analog “The Plutonian” in the classic Irredeemable Series had an interesting explanation for his powers that was the most plausible (in fiction) I’m ever seen for Superman type powers.
11ith panel down
What was REALLY odd about that phase of the character was that because his X-Ray vision was established as ineffective against lead, I think there were a couple of instances where the comics said the heating function didn’t work against lead either - which is just bizarre.
I understand how an alien from a higher gravity planet could leap tall buildings here on Earth.
But how does Superman fly?! 
Sky hooks.
One instance was in Superboy #98 (July 1962), the first appearance of the character Ultraboy. Ultraboy was undercover in Smallville, trying to discover Superboy’s secret identity, aided by his “Penetra-vision” which could see through anything, including lead. Further, Ultraboy’s equivalent of heat-vision could melt lead, a feat that Superboy admitted he was incapable of, though this make no sense.