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, 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!