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OK, so the creepy hotel attendant has just shown you to your room, and all you want to do is relax, enjoy your privacy and do some naked jumping jacks. But! That mirror... could he possibly be... watching you?? Is he perhaps... eating stale doughnuts while... breathing hard and sweating and... sitting up very straight and... staring with eyes locked wide open and... filming... and breathing... MY GOD IS HE WATCHING ME RIGHT NOW???

Two-way mirrors, folks, you never know when they're going to be your one-way ticket onto somebody's freaky website. At least, you never know until you read this node. There is a very simple test to determine whether you're just looking into your reflection or into a federal agent's Patriot Acting eyes.

Just extend your index finger. If you don't have an index finger, borrow somebody else's. Put the tip of your finger on the glass of the mirror. On a normal, God-fearing, red-blooded one-way mirror, there will appear to be a small amount of space between your finger and your finger's reflection. IF! on the other hand, the mirror is a Satanic, communistic, terrorist-sympathizing two-way mirror, your finger will appear to touch your reflection. That's it folks. And remember, if by this or any other means you discover that you're under investigation as a possible threat to national security, you're not allowed to inform anyone that you are being investigated. Thanks Bush, we didn't much care for the whole freedom thing anyway.

If you want to see if a mirror is two-way or not, put your cupped hands to it and look through them. If you block out the ambient light, you'll be able to see anyone on the other side, as long as they're not in pitch darkness (which they can't be, actually; even if there are no lights on the dark side of the mirror, it will at least be illuminated by the photons coming through it from the light side).

If the dark side is very very dark, then you can illuminate it with a flashlight through the mirror. You may need a friend to hold the flashlight, or an improvised shade you can use one-handed, to do this.

A mirror with no glass in front of its reflective layer is not necessarily a two-way mirror. It's a front surface mirror, all right, but that front reflective surface may be backed by an opaque layer, just like the reflective surfaces in regular back surface mirrors, which have glass in front of the reflective layer.

Most mirrors, including two-way ones, are of the back surface type, because the glass in front stops the reflective layer from getting scratched, or corroding. Front surface mirrors are delicate; delicate things do not last long in hotel rooms. Or in police interrogation rooms, for that matter.

What makes a mirror one- or two-way is its transparency. Without a backing, the thin reflective layer of a modern mirror is likely to be almost as see-through as mirrored sunglasses. If a mirror is reasonably transparent, it can work as a two-way mirror if there's someone in a dimly lit area on the other side of it. The reflection of the person on the lighter side of the mirror is so much brighter than the image of the person on the darker side that it drowns it out. It doesn't matter what side of the glass the reflective layer is on.

Ordinary glass windows work quite well as two-way mirrors, at night. If the lights are on inside and there's no illumination outside, the people inside will see their own reflection in the glass, but people outside will be able to see in. In this case, the glass is working as a front and back surface mirror, since there's no special reflective coating at all.

If you want to conduct some two-way mirror optics experiments of your own, try using compact discs that aren't printed all over their label side. CDs are highly reflective, but also moderately transparent - actually, some of them are very transparent. You can see your face in them; you can also see a lamp through them. And it doesn't matter which way round you hold the CD.

Reference regarding the front-side-equals-two-way myth:
http://www.snopes.com/spoons/noose/mirror.htm

My, oh my. Two writeups in this node about how to discover the presence of two-way mirrors, but none about how they work or how to install them! Disappointing, E2, Disappointing!!

How a two-way mirror works

Two-way, or transparent mirrors work like this: A regular pane of glass has a 50 to 70 % reflective coating put on it, resulting in a pane of glass that reflects an amount of the light back. This coating is usually a metallic coating of some sort, but the exact metal depends on how and where the mirror is produced.

You have probably noticed that when it is dark outside, and you look at a window, that it works as a mirror, and that you cannot see anything on the outside of your house, yet people on the outside can see you easily.

The rest of the reflective qualities are achieved using this phenomenon - if 50% is reflected (giving the window the initial appearance of a mirror already), and darkness on the other side of the mirror is added, you get a virtually 100 % reflective surface, but one that can be seen through just fine from the other side.

Because of the reliance of darkness, the mirror has a weakness: Light. ANY light - a lit cigarette, the blinking indication light of a mobile phone, a computer screen - will be visible from the mirror-side of the window, jeopardising the stealth of the two-way window.

Why install a two-way mirror?

  • Install one in your bathroom to spy on your house guests.
  • Install one in your kids' bedroom to make sure they don't do things they are not supposed to.
  • Install one in your bosses' office, so you can spy on the future plans of the business

Okay, all of the above are probably illegal and definitely immoral. So why would anybody want a two-way window?

They are often used as security devices. You have probably seen mirrored walls in liquor stores, banks, customs offices, and other places. The point is that if a potential criminal is not sure if they are being watched, chances of them committing a crime are lessened. it is easier to avoid a camera mounted on the roof of a store (it only points one way, or swivels in a predictable pattern) than someone sitting behind a one-way mirror (you don't know where they are looking, if they have a video camera, or perhaps even a gun).

Anyway - there are plenty of legitimate reasons for wanting to install two-way mirrors. So here is the recipe for how to do it:

Preparing the spy room

To install a two-way mirror, ideally you should have a small room adjacent to the room you want to look into. In general, it is a good idea to visualise a window covering the whole area of where your mirror is going to be.

Behind your mirror, you will want the room to be completely darkened. If stealth is one of the important factors, you may consider soundproofing the room (if not soundproofing, at least put carpet on all hard objects, such as the floor, tables, and walls where you are likely to touch the walls), remember to put felt or other soft material between the door and the frame, consider using extra quiet door handles, and keep them hinges oiled! If the entrance to the room is from a room that is lit (i.e from a hallway or something), you might want to consider putting two doors in - similar to the light sluice in a darkroom.

If you don't have the real estate available to take an entire room for your spying, it is also possible to do it differently: In the room you want to spy on, add an extra wall, approximately 2 feet from one of the side walls. Behind the wall, put up one or more remote controlled cameras - now, you have a room that can be completely darkened at all times, and you have the comfort of using the cameras whenever you need to see into the room.

A last tip is this: Get a large piece of cardboard or thin styrofoam that fits exactly over the size of the mirror pane, on the backside of the mirror. paint it black (yes, like the song). When you are not using your mirror installation, make sure you put this firmly into place. That way, even using the techniques described above, it becomes very difficult for the untrained eye to discover that they are, in fact, looking at a two-way mirror.

Installing the mirror

This is something you cannot usually do yourself - The glass is more expensive than normal glass (I have found it for $7 per square feet on www.reflectionproducts.com, but any security products or glass products dealer should be able to help you out, and good quality two-way mirror glass is somewhat more expensive), and if you want stealth, it can be a little tricky to get it to set in a frame correctly.

if you feel brave, however, there is really no limit to what you can do: Get some kind of idea on what kind of mirror would fit into your kind of room. Just make sure that it is a type that LOOKS as thought it cannot be taken down. If it looks as if it can be taken down, people are likely to try and take it down - even for innocent reasons, such as moving it to the bathroom as a shaving mirror. If they do that, and suddenly stare into cameras or your embarassed face, that isn't too good.

The installation is pretty much the same as installing a regular mirror - just take care that the reflective surface is not damaged - something that is easily done, because the reflective surface is often steamed onto the glass panes. it looks great, but can be somewhat fragile.

Well, it looks as if you have managed to install everything correctly.

Happy spying!

-30-

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