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How to Detect a Two-Way Mirror

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Forwarded email purports to give practical advice on how to tell the difference between a two-way mirror and an ordinary mirror.

Description: Email rumor
Circulating since: May 1999
Status: Partly true

Email text contributed by Tracy T., May 12, 1999:


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: I'll tell you straightaway that, 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. But allow me to clear up a few fuzzy points involved, as well as suggest 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" — don't ask me why; there seems to be no distinction between the two names. They both 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 even 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 nickle 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 (so if, say, some nut decided to use this 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, in case you are thinking about buying some for the watchtower in the maximum security prison you've just had built.

For those who "just want to know":

Sure enough, 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.

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