Claim: Looking for a gap between an object and its reflection is a way to distinguish two-way mirrors from ordinary mirrors. With the recent advancements in miniaturized electronics, it’s hard for any of us to know we’re not being spied upon these days; cameras and transmitters can now be made so small and unobtrusive that most of us wouldn’t be able to find them even if we knew where to look for them. Many would-be peeping toms still resort to less technologically-advanced methods such as peepholes and two-way mirrors, though, hence a warning which began circulating on the Internet in April 1999:

HOW TO DETECT A 2-WAY MIRROR When we visit toilets, 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? Well, once again, I am here to enlighten you. 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! You can further ascertain the nature of the suspect surface by hurling an object (e.g. a chair!!!) at it. The chair will go through. So remember, every time you see 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.

First off, we should clear up some confusing terminology. The subject this message addresses are panels which look like mirrors on one side but work like tinted windows from the other side — that is, if you’re on one side of such a panel, it appears to be mirror in which you can see your reflection; if you’re on the other side, it serves as a window through which you can see everything (and everyone) on the other side. These panels are confusingly referred to as both «one-way mirrors» and «two-ways mirrors,» so to avoid additional confusion we will henceforth call them «transparent mirrors.» The fingernail test described above is a method for determining the difference between first-surface mirrors and second-surface mirrors, both of which are ordinary mirrors glazed in a different fashion. With a first-surface mirror, any object you put up against it will touch its reflection because the reflective part of mirror is laid in right at the surface. With a second-surface mirror, a gap between the object and its reflection will be visible because a layer of clear glass has been incorporated over the reflective part of the mirror to better protect it. First-surface looking glasses are quite expensive and are generally used only for specialized applications such as fine optical instruments or lasers, in which a protective layer of glass would interfere with the path of light or diminish precision. As such, you’re unlikely to ever encounter one being used as an ordinary mirror in a home or a dressing room; for those applications much cheaper second-surface mirrors are nearly always used. Therefore, if you discover what appears to be a first-surface mirror (i.e., an object you place up against it touches its reflection) in a dressing room, the chances are good it’s a transparent mirror. However, the fingernail test isn’t necessarily a very good one, as judging whether an object placed up against such a mirror is touching its reflection can be affected by factors such as the size of the mirror, the angle of the mirror, the amount of lighting present, and the type of object used to conduct the test, and an unpracticed observer might easily mistake an ordinary mirror for a transparent one this way. (Ascertaining «the nature of the suspect surface by hurling an object at it» could also be quite embarrassing and costly should it prove to be an ordinary mirror.) There are better tests based on some of the more obvious physical features of transparent mirrors. What we call transparent mirrors are laminated coated glass products which have been treated with Mirropane on the first surface of the glass. They’re also known as «observation mirrors» and are commonly used in settings (such as police stations, schools, and businesses) where there is a need to conduct unobtrusive monitoring for safety and security purposes. (Think of the interrogation room as shown in the police station of the television drama Law & Order: suspects inside the room see a panel which looks like a mirror, but from the hallway outside the panel functions as a window through which detectives can monitor the proceedings unobserved.) Transparent mirrors are most obviously distinguishable from ordinary mirrors in that they’re not hung on walls as ordinary mirrors are, but they’re set into walls (or doors) as windows are. In other words, if there’s a wall behind a mirror, the mirror is almost certainly an ordinary one; a transparent mirror would be part of the wall itself. Also, the lighting in front of a transparent mirror must be much brighter than the lighting on the other side (where the hidden observers are) for it to work effectively, and some light leaks through from the brighter side to the dimmer side. According to the folks in Mirropane’s technical support group, you can use these factors to your advantage by pressing your eyes up against the mirror and cupping your hands around them (to block out the light from the room you’re in): if you’re truly dealing with a transparent mirror, you should be able to see at least a little something of the open area behind it. Also, rapping on the mirror should provide an aural clue: ordinary mirrors have backings and are usually placed against walls, so rapping on them will generally produce dull thuds; transparent mirrors are set into walls with open areas behind them, so rapping on them should produce much more open, hollow sounds. These methods of detection are more reliable than the fingernail test and should be preferred to taking a chance on getting arrested for property damage after tossing a chair through a perfectly normal mirror misjudged via less accurate means. Have you stood in front of a mirror and wondered if someone is spying on you? In this article, we’ll show you several easy techniques for detecting a two way mirror. A two way mirror is a transparent mirror that can look exactly like a regular mirror when the back side of it is dark. Don’t worry, there is more than one way to tell the difference!


We have a series of three tests to use to confirm that the mirror you see in front of you is not a two way mirror.

Test #1: The Fingernail Test

What is the fingernail test?

Place your fingernail to the front of the mirror. You can test to see which side the reflective coating is facing. For glass two way mirrors, the mirror coating has to be facing you. If your fingernail’s reflection touches itself, that is the coated side. If there is a gap between your finger and its reflection, go to Test #2.

The Fingernail Test Rules Out the Glass Two Way Mirror

The glass two way mirror is one-directional. You can only install it with the mirror coating facing forward so it is easy to spot using this test. The acrylic two way mirror and glass smart mirror are not one-directional. You must use the next two tests to be sure it is not a two way mirror in this case. fingernail-test-with-acrylic-and-glass-two-way-mirrors

More on the Glass Two Way Mirror

The glass two way mirror has a mirror coating that is 70% reflective. It only allows 11% of the light through the mirror. It is on a dark grey substrate, which makes it one-directional. The glass two-way mirror cannot cheat the fingernail test. The reflection coating must be facing the room that is being observed. That way the people being observed are only seeing their reflection. On the opposite side, the darker room is located where the observer will be watching. Notice the fingertip’s reflection touching itself. Very different from that of the standard mirror’s reflection. The light is even on both sides of the mirror in this picture. You are able to see through the semi-transparent glass at the moment. Glass two-way mirror cannot cheat the fingernail test. The reflection coating must be facing the room that is being observed. That way the people being observed are only seeing their reflection. On the opposite side, the darker room is located where the observer will be watching.

Test #2: Tint Comparison

How does a two way compare to a standard mirror?

All three mirror types we are looking at in this article have a 70% reflection. A standard mirror is 94% reflective. This means that it will have a darker tint than a standard mirror.

Bring a Small Makeup Mirror With You

To do this test, simply pull out a standard mirror from your purse or pocket. Put it up next to the mirror and check to see if the tint looks the same as your mirror. If it appears darker, it is likely a two way mirror.

More on the Acrylic Two Way Mirror

70% reflective with 25% light transmission on a grey substrate, acrylic two way mirror is reflective from both sides of the substrate. Meaning it can be installed either way. If installed backwards ( the coated side on the back ) it can cheat the fingernail test looking like the reflection of a standard mirror! Something to think about if you are testing a questionable mirror. Acrylic tends to have a different feel than glass. Rubbing your nail gently across the surface may help you determine if you have found an acrylic two way mirror, or a standard glass mirror.
Acrylic two way mirror will closer match the reflection tint of a standard mirror as well. Whereas glass two way mirror tends to have a bronze tint.

More on the Glass Smart Mirror

Smart mirror is similar to the acrylic two way mirror in properties. It has a 70% reflection with a 25% light transmission. Its reflective from both sides of the substrate. Smart mirror can be installed backwards as well ( the coated side on the back ) it can cheat the fingernail test looking like the reflection of a standard mirror!
If you are testing a questionable mirror, smart mirror tends to have a warmer tone on the reflective coating. Observe the smart mirror on the right compared to the standard mirror reflection of the mug on the left.

Test #3: Knock on the Mirror

Listen for a Hollow Sound

If you are worried about a two way mirror and the first two tests did not work for you, try knocking on the mirror gently. If there is a hollow sound going on instead of a more solid sound, it is possible that the area behind the mirror is open instead of the mirror being just mounted to the wall. This could reveal that there is an observation area or a camera behind the mirror. Be careful out there!

Did you know?

Two-way mirrors are dependent upon the lighting in the room. Both sides of the mirror cannot be evenly lit for the illusion to work. The observer side of the mirror must be at least half as dark for the reflection coating on the other side to properly reflect light back at those being observed.

Want to see samples of two way mirrors?

Visit our ORDER SAMPLES page.

CONCLUSION: Now that you are armed with knowledge, never be afraid of a sketchy looking mirrors again. Just remember the fingernail test & a makeup mirror.

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