There's this message spreading virally over the internet:

Read This Message.....!!!!

How to Detect Hidden Camera in Trial Room?

In front of the trial room take your mobile and make sure that mobile can make calls........ Then enter into the trail room, take your mobile and make a call..... If u can't make a call......!!!! There is a hidden camera...... This is due to the interference of fiber optic cable during the signal transfer......

Please forward this to your friends to educate this issue to the public......To prevent our innocent ladies from HIDDEN CAMERA...........

Pinhole Cameras in Changing Rooms of Big Bazaar, Shoppers Stop?

A few days ago, I received this text message: Please don't use Trial room of BIG BAZAAR there are pinhole cameras to make MMS of young girls. So, please forward to all girls. Also forward to all boys who have sisters and girlfriends.

Don't be shy in forwarding this message. Because its about protecting the integrity of all girls & ladies.


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 or bathroom or bedrooms.

It is very difficult to positively identify the surface by just looking at it. So, how do we determine with any amount of certainty what type of Mirror we are looking at?


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, IT IS A 2-WAY MIRROR! (There may be someone seeing you from the other side). So remember, every time you see a mirror, do the "fingernail test." It doesn't cost you anything. It is simple to do.

This is a really good thing to do. The reason there is a gap on a real mirror, is because the silver is on the back of the mirror UNDER the glass.

Whereas with a two-way mirror, the silver is on the surface. Keep it in mind! Make sure and check every time you enter in hotel rooms.

Share this with your sisters, wife, daughters, friends, colleagues, etc.

Pass this message to all Ur friends in the Contacts.

Examples of web-sites that have picked up and publish the claims include:

The essence of the message is that, you can detect a hidden camera in trial rooms simply by attempting to make a call from inside the room. If the call fails, there probably is the camera in the room. They say this happens because of "interference of fiber optic cable during signal transfer". I am unable to understand this phenomenon. Does this test actually work? If yes, how?

  • 1
    From context, I assume "trial room" is the same as a "changing room", but I have never heard the expression before. Is that right?
    – Oddthinking
    Sep 1, 2011 at 14:09
  • 4
    @Oddthinking: Yes. In India, we call it trial room. Sep 1, 2011 at 14:28
  • 2
    Sounds like a scam to me. Install hidden cameras. Tell everybody about a method to detect cameras that doesn't work. Carry on filming unsuspecting people.
    – Simon B
    Dec 29, 2021 at 20:31
  • (1/2) The camera fiber optics is so hilariously wrong that I had to join this community to write an answer. The phenomena they are describing which would block or interfere with a cellphone is called Electromagnetic Interference (EMI). Electric current passing through a wire generates a magnetic field in a circle centred about the wire. The amount of EMI experienced is related to the strength of this field and the sensitivity of the device. Now, fiber optic cables are glass strands which transmit light, not electricity. Continued: Feb 29 at 0:38
  • (2/2) Fiber optics cannot generate EMI, and would not have any effect on a cellphone. A spy camera is likely going to be a small device, which even the post alludes to when it says "pinhole camera". This would operate on a very low voltage, and barely generate any field. Furthermore, most modern devices use EMI shielding to prevent influence of magnetic fields (including the phone and camera in the message). In order to interfere with a phone, you would need either a very powerful magnetic field, or a source that is practically touching the phone. This claim is outlandishly false. Feb 29 at 0:55

1 Answer 1


The trick for detecting a 2-way mirror is not 100% reliable, but is not entirely wrong either. Two-way mirrors intended for observation are made using dark ("smoked") glass (/acrylic/whatever), with a partially-reflective coating on one side. Critically, the reflective side is installed toward the subject (i.e. the one being spied on).

The reason for this is that the dark glass makes the observer's side appear even darker than it really is -- Pilkington's Mirropane Guidelines gives a recommended scenario where the subject's reflection is 50 times brighter than their view of the observer, and the observer's relection is 5.5 times dimmer than their view of the subject; this is a result of a combination of the lighting on the two sides (8x brighter on the subject side), reflectivity of the coating, and attenuation through the acrylic (compare R_f and R_g in the PDF). It also says:

Where an 8 to 1 light ratio cannot be achieved, an additional light [layer? -GD] of grey glass can be added, to the glass side of the transparent mirror, by either multiple glazing or lamination, to obtain a satisfactory Masking Ratio. This will however, reduce the brightness of the observer’s image of the subject.

Note that it's possible that someone could put another layer of glass in front of the reflective side, or use a partially-reflective mirror with normal glass (though I don't know if anyone makes this in large sizes -- it's normally small pieces used for optics work) and an even higher light ratio. But those don't seem terribly likely...

Normal mirrors, on the other hand, are generally second-surface mirrors -- that is, the reflective layer is behind the glass. There are exceptions, for example I've seen mirrors in freeway rest stops with no glass at all, just polished metal. But they're the exception, not the rule.

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