A two-way mirror, also called a one-way mirror, is mirror which is partially reflective and partially transparent. It is used with a darkened room on one side and a well-lit room on the other, allowing those in the darkened room to see into the lighted room but not vice versa.
The glass is coated with (or in some cases encases a layer of) a very thin almost transparent layer of metal (generally aluminum). The result is what appears to be a mirror from one side, and tinted glass from the other. A viewer in the brightly lit area has difficulty seeing into the darkened room, through what appears to be a mirror.
To take full advantage of the partially mirrored surface, the target side should be brightly lit, to obscure any hint of light coming through the glass from the viewer's side. The darkened room is only completely obscured when it is in complete darkness. Sometimes a darkened curtain or a double door type vestibule is used to keep the viewer's side darkened.
A flashlight held against the glass can be used to illuminate the darkened viewer's side, allowing someone on the lit side to see through.
Two-way mirrors are used for:
- providing security, through covert viewing of public spaces
- for the protection of covert cameras
- for some police interrogation rooms
- It is used during Focus groups and usability testing where participants on one side of the mirror do not see the observers
- The use of one-way glass is also apparent in the broadcast television where persons read from a script while seemingly looking directly at the camera. This effect is achieved where the camera is the viewer looking through the glass, upon which the prompt or script is also reflected for the subject to read.
- Techniques of camouflage can also be used with ordinary glass, for special effects such as Pepper's ghost, which projects the image of a person from one room to another as an apparition, as used in Anaheim California's Disneyland exhibit "Haunted Mansion". The effect can also be produced by a finely perforated white metal screen that is difficult to see through in one direction, depending on the relative light levels of the separated spaces.