How to Detect a Two-way Mirror

Camera filming through a two-way-mirror
Camera filming through a two-way-mirror.

 Chris Weeks/Hulton Archive/Getty Images

The viral message below, that is circulating online, purports to give practical tips on how to tell a two-way mirror from an ordinary one. This viral message has been circulating since May 1999 and is considered partly true.

The following example of the forwarded email was contributed in the same year and follows cases of people who have installed these two-way mirrors in female changing rooms and more. Read the message, "How to Detect a Two-way Mirror" below, consider Peter Kohler's analysis that follows, and learn more about how two-way mirrors actually function in real life.

Example of the Forwarded Email

When we visit bathrooms, hotel rooms, changing rooms, etc., how many of you know for sure that the seemingly ordinary mirror hanging on the wall is a real mirror, or actually a 2-way mirror (i.e. they can see you, but you can't see them)?
There have been many cases of people installing 2-way mirrors in female changing rooms. It is very difficult to positively identify the surface by just looking at it. It's time to get paranoid. So, how do we determine with any amount of certainty? Just conduct this simple test:
Place the tip of your fingernail against the reflective surface and if there is a GAP between your fingernail and the image of the nail, then it is a GENUINE mirror. However, if your fingernail DIRECTLY TOUCHES the image of your nail, then BEWARE, for it is a 2-way mirror!
So if not at home and changing before a mirror, do the "fingernail test". It doesn't cost you anything. It is simple to do, and it might save you from getting "visually raped"!
Share this with your girlfriends.

Analysis by Peter Kohler

Despite the overemphatic tone used in the above text, which is of course what keeps it in circulation, the fingernail test does work as described in most situations. Below, a few fuzzy points involved will be cleared, as well as a suggestion of several other possible ways to identify a two-way mirror.

For the Sticklers Among Us

Some companies in the window glass and mirror trade call them "two-way mirrors" and some call them "one-way mirrors" though there seems to be no distinction between the two names. Both names refer to a product known as Mirropane. Promotional literature from the LOF Architectural Specialty Glass company states that the product registered under the name "Mirropane E.P. Transparent Mirror" is "formed using LOF's patented chemical vapor deposition process on 1/4 Grey tint glass."

As to exactly how that works or what reflective metal is involved, it seems to be a trade secret, although the good folks at Morehouse Glass in Portland, Oregon suggest that tin or nickel are the likeliest choices. It's probably not silver, as suggested in the missive under scrutiny.

The product can be heat-treated for maximum strength and can also be laminated to make it scratch resistant. For example, if someone decided to use the product for a mirror in a changing room, it would not be easily scratched by a belt buckle or other light brushings-up-against. The product can also be made considerably bullet-proof.

More Information About How Two-way Mirrors Work

Mirropane is treated on the subject or first surface of the glass, and the recommended lighting ratio for surveillance purposes is 10:1, with the subject side being ten times brighter than the observer side. The fingernail test described above works for the very reason stated, namely that there is no glass between an object and the reflective surface if the mirror is touched.

There are other first-surface mirrors as well that are not two-way, but these are used primarily in precision optical instruments or in scientific experiments using lasers, where the refraction from the glass would be an interference. Mirropane is commonly used in prisons and police stations, in psychological observation rooms, and in security situations which can include many types of businesses where viewing customers or employees is deemed necessary or desirable.

Additional Ways to Identify Mirropane

Below are some other ways to identify Mirropane from an ordinary, second-surface mirror.

William Beaty, an electrical engineer in Seattle, says to

"Simply turn the lights off in [the] room, then place a bright flashlight against the mirror surface. If there is a hidden chamber behind the mirror, the flashlight will illuminate it, and since you're in a darkened room, you'll see the hidden chamber."

A deputy from the Washington County Police Department in Oregon concurs and suggests that even a penlight will work for this test, though not nearly as well. He further suggests that, if you are in a room, such as a changing room, where you cannot turn off the lights on your side, hold your eyes near to the glass surface and cup your hands around them on either side to eliminate most of the light from your visual field. Then, you ought to be able to see through the treated glass, as Mirropane will permit about a 12 percent passage of light from the lit side to the hidden chamber, if there is one.

Douglas Brown, a part-time field researcher, and writer who works for Powell's Books, Inc. in Portland, Oregon, has some clever advice to share. He makes the point that there is a clear auditory difference between Mirropane and regular mirrors, because of how they are installed. Scratch on the surface with your knuckle or fingernail, he says. In most situations, you'll be able to hear the difference in the sound produced. Ordinary mirrors have backing material which will dull the sound, while windows have open air behind them and will reverberate more.

Employees of Morehouse Glass make the point that any mirror hung in front of a wall will be a mirror, plain and simple. This is because Mirropane will be a pane of glass installed into the wall, like any other window, and will have window framing evident, not mirror glass molding around it.