X-Ray Vision

A power that lets one see through almost anything. It can usually be turned on or off at will, and the objects that you can see through and how far you see through them vary.

Pete: So you're telling me you never once looked in the girl's locker room?

Clark: Well, maybe once.

—Smallville

How the power actually works, especially if it's implied to be actual X-rays, is confusing. Whether your eyes or your glasses are sensitive to X-rays, you would still need a powerful X-ray source on the other side of whatever you were trying to see through, and even then you'd only see things that were X-ray opaque. Presumably you'd "see" things thus revealed in a "colour" for which we normal, X-ray-blind people have no name.

X-ray scopes, goggles, visors and what-have-you are perennially popular items in video games. Sometimes this has unintentionally humorous effects, as when some character's repel-everything super armor turns out to be completely radiotransparent, allowing you to see his/her skeleton beneath.

In the real world, there is a technology called "X-ray backscatter imaging" which allows for a certain amount of selective viewing. However, it requires bulky beamshaping equipment around an X-ray tube, which is of considerable size, not counting the high-voltage power supply and water-cooling system to keep from melting the tube's cathode. And then you have to scan the beam across the target, and use large and heavy detectors to pick up the extremely faint backscattered X-rays. The smallest mobile X-ray backscatter (not counting "portal"-style backscatter machines, which can cheat in a number of ways) machine is built into a 20-foot shipping container. It isn't impossible to miniaturize a backscatter machine, but it would take a whole lot of really surprising breakthroughs in a whole lot of scientific fields.

Also, all those X-rays would kill most people due to all the exposure to that much radiation. Given this, the common usage of X-ray vision for voyeuristic purposes has even worse implications than invasion of privacy—it's downright dangerous to the people who are being ogled.

See also: Super Senses. Similar to Vein-O-Vision and Volcanic Veins.

Examples of X-Ray Vision include:

Anime and Manga

Characters with the Byakugan power in Naruto (Hyuuga family members) have general all-purpose awesome vision, including the ability to see people through objects. Used mostly to see chakra lines, it's also been shown to allow vision of a person's skeletal structure, and fans have speculated that it might have... other advantages (and disadvantages^{[dead link]}).
When Shiki in Tsukihime concentrates in the right way, he can see through things like skin in order to 'kill' tainted blood as it moves through his veins. Doing this too hard leaves him almost entirely blind for a period, during which all he can see are lines and points of death.
Some kinky Fan Art for Dennou Coil tends to reverse this, using the city's augmented reality glasses to cover some streaking girls—that is, the girls are using augmented reality to make them look like they're wearing clothing to anyone wearing the AR glasses (i.e., everyone), when in reality they're, er, not. This means that to see "through" clothing, you have to take the glasses off.
- An example from Siraha and another from Kakkii here (and see-through here with Barbie Doll Anatomy underneath) -- mostly tame pictures, but definitely a NSFW website. This has even gained it's own tag on the Danbooru system -- "Reverse X-Ray".
Kogarashi's "Maid Guy Scan" ability in Kamen no Maid Guy. He does use it to see through girls' clothes, but only with good intentions...such as telling his master, Naeka, that she needs to lose some weight. Nobody really likes it when he does this.
- Particularly because the ONLY people he really uses this is on women.
One chapter of To LOVE-Ru has Rito put on a special pair of glasses that Lala uses to work on her inventions. As this is To Love Ru, the glasses just happen to make him see through women's clothing.
Mii Konori of A Certain Scientific Railgun calls her ability Clairvoyance, but it works just like the X-Ray Vision that everyone else here has.

Comic Books

Comic book Superman looks at a wall and sees Lois, in natural colour, tied up behind it. Or he sees a bomb inside a ship that he's swimming past. Apparently, his eyes emit something that goes right through masonry or steel, but bounces off skin, clothing, dynamite, etc. at wavelengths that look just like ordinary light. However, like real X-rays, it's stopped by sufficiently dense materials (although, Pre-Crisis, only lead stopped it).
- One Hand Wave for this is that it's not X-rays, but actually a combination of his telescopic vision and microscopic vision; he looks between the molecules of whatever he's looking through. To which those with even a passing understanding of what makes an object opaque in the first place will say: No.
  - Unless, of course, he's also a probability warper at a quantum level...
- Another ties into the "psionic Superman" explanation that was quite popular a couple decades back (and which seems to have influenced the John Byrne reboot). This one posits that it's actually a form of clairvoyance, and that its inability to go through lead is merely a psychological limitation.
- Another post-Crisis theory says that he can detect the minor electromagnetic energy emitted when cosmic ray particles pass through matter, and his brain just interprets it as a visual image subconsciously constructed from the energy patterns. These emissions might be partially blocked by lead (or any sufficiently dense material).
- However, in The Golden Age of Comic Books and at least part of The Silver Age of Comic Books, it was definitely X-rays. In fact, this is where his heat vision originally came from; he was able to focus the X-rays enough to cook whatever he was looking at. Or in one Superboy comic, turn gold into lead. One odd side-effect of this was that, until the Post-Crisis reboot, his heat vision couldn't melt lead. (This ignores the fact that an actual beam of highly focused X-rays could in fact melt lead, if it were sufficiently intense.)
Ultra Boy of the Legion of Super-Heroes originally had this as his only power, but was later upgraded to "Any of Superman's powers, but only one at a time".
- Note that originally the wrinkle was that he could melt lead, unlike Superboy (see above), which humorously led some writers to have him do nothing but melt lead just to drive the point home.
British juvenile comic character X Ray Specs sometimes sees other people as skeletons, sometimes not. Sometimes he sees the crucial thing at the right time, sometimes not.
DC Comics character 'Hitman' tried out for the Justice League once and was rejected. He said that he hadn't expected to get in, he just wanted to get a look at Wonder Woman with his X-Ray Vision. He left with a leer.
Empowered's supersuit has "imaging functions" which, among other things, grants her this power. It's detailed enough to detect an aneurysm in the head of the Punch Clock Villain who was guarding her, which saves his life.
One character in Target Comics, the White Streak, had x-ray vision.
An extremely disturbing use in Supreme Power where Mark Milton goes to a strip club, takes a woman home, and has sex with her - while having his x-ray vision on enough to see her bones and muscles beneath the skin. Mark also used to use it to spy on girls in high school.
A pair of x-ray-specs is the first of his parents' gadgets that Chase finds in Runaways. Naturally, he realises what they do by seeing Nico and Karolina in their underwear.

What is it, Chase?
What do those things do?
Nothing. Nothing at all.

The protagonist of Strontium Dog is able to emit α particles from his eyes, which somehow allows him to see through walls and metal.
- α particles ~~are the size of a~~ are helium nuclei and can be stopped by paper.

Film

In xXx, Xander has a pair of X-ray binoculars with a built-in digital camera which can see through anything from clothes to brick walls, depending on what the viewer focuses on.
The movie X! The Man With X-Ray Eyes. The main character takes eyedrops to increase the frequencies of light that he can see; he goes from seeing everyone with no clothes on to seeing organs to seeing skeletons to seeing a Cosmic Horror at the center of the universe. He attempts to stop it by gouging them out but in an alternate ending (which may or may not be an Urban Legend), he can still see.

Literature

The Life and Times of the Thunderbolt Kid by Bill Bryson. Young Bill recounts how Superman's x-ray vision isn't an actual x-ray, more of cross section, and how even that was useless for looking through women's clothing as breasts would be restrained by bras and other articles of clothing. Therefore Bill developed "Thundervision" which was able to loosen restrictive clothing while looking trough it.

Live-Action TV

Smallville manages to use this properly. More or less. Clark's vision is indeed similar to X-rays, barring a few occasions involving girls' locker rooms which are mostly played for laughs. The lack of any kind of contrast invalidates it slightly, but it still makes more sense than the classic comic-book version.
In the 1980s Twilight Zone, some kids catch a leprechuan and force him to grant their wishes in exchange for freedom. One wishes for X-Ray vision. Of course, when he tries to see through girl's clothing, all he sees are their bones and organs, like a real X-Ray machine.
Similarly to above, deconstructed in Dossier on Detective Dubrovsky. A psychic admits that he can see through things but when his female listener is disturbed (or aroused) by this prospective he sorrowfully assures her that instead of all the good parts he can only see bones and organs and the bowels' contents.^{[please verify]}

Toys

Bionicle has the Mask of X-Ray Vision; Kanohi Akaku or just "Akaku" for short.
- The Great Ruru also has this.

Video Games

In World of Warcraft, engineers can make Gnomish X Ray Specs which can see through other players' clothes.
- Armor, but not undergarments. Not that this helps much with the "GAAHH! NAKED DWARF!" reaction.
Metroid: Samus can gain X-ray vision as a powerup to let her see through false floors. In the Prime subseries she can also use this to find enemy weakpoints and track invisible enemies. Prime 3 lets you kill many tough enemies in a single blow if you find their weakpoint!
- Also, one of the few somewhat realistic portrayals. X-ray vision mode is bright and white, similar to what X-ray scans look like, and is anything but regular-colored that someone like Superman sees in. You can also see the internal structure of things; like Samus's hand inside her Arm Cannon. I guess the suit doesn't have very good radiation shielding.
  - Sure doesn't.
  - Except for the fact that it does have good radiation shielding. When something can fend off Phazon without disintegrating, it should at least stand up to X-Rays.
Done a bit better in the video game GoldenEye: Rogue Agent with M.R.I. (Magnetic Resonance Imaging: http://en.wikipedia.org/wiki/MRI ); this vision mode allows you to see the silhouettes of enemy (and allied) soldiers through walls in a vaguely transparent white, but no more detail than that. It still doesn't work like it should, though...
The video game 007 James Bond 007: Nightfire has a blue tinted heat vision vision mode that gives enemy's silhouettes in misty red past obstacles. It works through solid stone walls and functions in ways typical of "x-ray" vision though.
Deus Ex and sequel has a vision augmentation that allows first night vision and then the ability to see people and animals though walls as figures made of white smoke.
Batman: Arkham Asylum has 'detective mode' which allows you to see enemies and NPC's as skeletons some distance away and though walls. It also shows important items and breakable walls.

Web Comics

Tedd's glasses in El Goonish Shive had an "X-Ray" feature, designed specifically to see concealed weaponry. Whether they worked with actual X-rays, we don't know, since he removed the feature—Tedd is a pervert, but he's against "this kind of voyeurism". Later a guard at some MIB facility used much the same to check agent Wolf.
Vriska and Rose both have this in Homestuck, Rose gets hers from being the Seer of Light and Vriska has it from her Vision Eightfold.
In Freefall, Ab²y is specifically said to be backscatter X-Ray equipped. We never see the world through his eyes, but given how hard Freefall usually is, we can assume that it works realistically.
Schlock Mercenary obviously is hi-tech enough for this, and the resident Mad Scientist got such glasses.

Web Original

Mitch Calrus of Fine Structure has this superpower along with intangibility.
- This uses some kind of four-dimensional light, and he can control how far he sees. Great for voyeurism if you happen to be into women's cross-sections. Mitch isn't:

"I can't just look at the skin below the clothes. I can't just peel away layers like that. It's a focus depth thing. And people just look red on the interior. Icky red and other nasty colours. Watching blood circulate isn't fun, it's horrific."

At the Whateley Academy in the webfiction Whateley Universe, there's a boy code-named Peeper. His only power is the ability to stare at people and look through their clothes. Maybe he could do something interesting with locked boxes or what-have-you, but he's not interested. This makes him one of the most annoying pains on campus, as he's constantly annoying the hot girls.
- In his combat final, we learn that he can also see through things like the fake rocks people use to hide door keys, so he can get into a lot of people's houses too.

Western Animation

The M-Ray Contact Lenses gadget on Totally Spies!, so-called because they were supposedly designed for seeing through metal, but could be used for seeing through anything, like brick or concrete, or even like telescopes for reading tiny writing from across the room.
Captain Hero in Drawn Together has this power. In one episode, he used it to spy on Foxxy, which resulted in her getting a brain tumour.
Powerpuff Girls
See-More, a H.I.V.E. Five villain in Teen Titans, has X-ray vision that lets him see through Starfire's clothes, which causes her to "cover" herself in embarrassment.
The title characters of Phineas and Ferb make X-Ray glasses on one occasion. They try to mass-produce them, but their carrot supply winds up mysteriously disappearing.

Real Life

Adverts in comics for X-Ray Specs. Okay, any boy stupid enough to part with money in the hope of seeing through girls' clothes deserves to be ripped off, but shouldn't enough people have "seen through" this con by now and let the sellers go out of business?
- You can buy those cheap in novelty shops. Yes, they still don't actually work, but then it's a novelty item, so you shouldn't expect too much.
- Some apps for mobile phones feature the same thing, saying they'll convert the camera of your cellular in a X-Ray one. Same as per X-Ray goggles mentioned above.
Truth in Television. During the Cold War the KGB invented an X-ray device so powerful it could show the inside of a lock, enabling a safecracker to see the movement of the tumblers. Unfortunately the device emitted powerful bursts of radiation when being used — KGB agents would joke that you could tell a veteran safecracker by his lack of teeth!
Backscatter Technology
You can't simply press a button on Photoshop to do this (yet), but you can do something similar if the subject in the photo is wearing light and thin clothes.