Two-way mirror
Encyclopedia
A one-way mirror, also known as a two-way mirror, one-way glass, or two-way glass is a mirror
which is partially reflective and partially transparent. When one side of the mirror is brightly lit and the other is dark, it allows viewing from the darkened side but not vice versa.
layer of metal (usually aluminium
). The result is a mirrored surface that reflects some light and is penetrated by the rest.
When one side is brightly lit and the other kept dark, the darker side becomes difficult to see from the brightly lit side because it is masked by the much brighter reflection of the lit side.
A two-way mirror is typically used as an apparently normal mirror in a brightly lit room, with a much darker room on the other side. People on the brightly lit side see their own reflection—it looks like a normal mirror. People on the dark side see through it—it looks like a transparent
window. The light from the bright room reflected from the mirror back into the room itself is much greater than the light transmitted from the dark room, overwhelming the small amount of light transmitted from the dark to the bright room; conversely, the light reflected back into the dark side is overwhelmed by the light transmitted from the bright side. This allows a viewer in the dark side to observe the bright room covertly.
When such mirrors are used for one-way observation, the viewing room is kept dark by a darkened curtain or a double door vestibule. These observation rooms have been used in:
Smaller versions are sometimes used in:
There is nowadays a trend for use of glass and even foil based setups of such mirrors in showcase, arts and staging setups, somewhat comparable to Pepper's ghost
, in very large dimensions. The second image blended to the scenery in this cases is then either a flat screen or a projection from one or more video beamers rasing the illusion that an image is there in free air somewhat behind the mirroring surface. That technique was used at a few public locations (e.g., for creating special effects in live fashion shows). Sometimes the term holographic display stands very close even though that sort of device is a totally different category. The image is indeed as flat as the projection that it is based upon. One such commercially available entertainment product is the Musion Eyeliner
.
The same type of mirror, when used in an optical instrument
, is called a half-silvered mirror or beam splitter
. Its purpose is quite different: to split a beam of light so that part, usually about half, passes straight through, while the other part is reflected. In a typical scientific application the two resulting beams are made to interfere
after traversing different paths. An unusual single-lens reflex camera
used a half-silvered mirror to create an image of the scene both in the film plane and in the viewfinder.
Mirror
A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...
which is partially reflective and partially transparent. When one side of the mirror is brightly lit and the other is dark, it allows viewing from the darkened side but not vice versa.
Design
The glass is coated with, or encases, a thin and almost-transparentTransparency and translucency
In the field of optics, transparency is the physical property of allowing light to pass through a material; translucency only allows light to pass through diffusely. The opposite property is opacity...
layer of metal (usually aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
). The result is a mirrored surface that reflects some light and is penetrated by the rest.
When one side is brightly lit and the other kept dark, the darker side becomes difficult to see from the brightly lit side because it is masked by the much brighter reflection of the lit side.
Use
Transparency and translucency
In the field of optics, transparency is the physical property of allowing light to pass through a material; translucency only allows light to pass through diffusely. The opposite property is opacity...
window. The light from the bright room reflected from the mirror back into the room itself is much greater than the light transmitted from the dark room, overwhelming the small amount of light transmitted from the dark to the bright room; conversely, the light reflected back into the dark side is overwhelmed by the light transmitted from the bright side. This allows a viewer in the dark side to observe the bright room covertly.
When such mirrors are used for one-way observation, the viewing room is kept dark by a darkened curtain or a double door vestibule. These observation rooms have been used in:
- Interrogation rooms
- Execution rooms
- Experimental research
- Security observation decks in public areas
- Market research
- Reality television, as in the series, Big BrotherBig Brother (TV series)Big Brother is a television show in which a group of people live together in a large house, isolated from the outside world but continuously watched by television cameras. Each series lasts for around three months, and there are usually fewer than 15 participants. The housemates try to win a cash...
, which makes extensive use of two-way mirrors throughout its set to allow cameramen in special black hallways to use movable cameras to film contestants without being seen. - Train conductor rooms of newer metroRapid transitA rapid transit, underground, subway, elevated railway, metro or metropolitan railway system is an electric passenger railway in an urban area with a high capacity and frequency, and grade separation from other traffic. Rapid transit systems are typically located either in underground tunnels or on...
trains, such as Bombardier TransportationBombardier TransportationBombardier Transportation is the rail equipment division of the Canadian firm, Bombardier Inc. Bombardier Transportation is one of the world's largest companies in the rail-equipment manufacturing and servicing industry. Its headquarters are in Berlin, Germany....
's MoviaMoviaMovia is a family of metro train built by Bombardier Transportation. The structure and body shell are fully customisable for the needs of each system that orders it. Unlike most traditional metro trains, they usually have full-width gangways between carriages, allowing passengers to walk the entire...
family of metro trains, including the Toronto Rocket
Smaller versions are sometimes used in:
- Security cameras, where the camera is hidden in a mirrored enclosure
- TeleprompterTeleprompterAn autocue is a display device that prompts the person speaking with an electronic visual text of a speech or script. Using a teleprompter is similar to the practice of using cue cards...
s, where they allow a presenter to read from text projected onto glass directly in front of a film or television camera - Stage effects (particularly Pepper's ghostPepper's ghostPepper's ghost is an illusionary technique used in theatre and in some magic tricks. Using a plate glass and special lighting techniques, it can make objects seem to appear or disappear, transparent, or make one object seem to morph into another...
) - Low-emissivityLow-emissivityLow emissivity - actually low thermal emissivity - is a quality of a surface that radiates, or emits, low levels of radiant thermal energy. All materials absorb, reflect and emit radiant energy...
windows on vehicles and housing
There is nowadays a trend for use of glass and even foil based setups of such mirrors in showcase, arts and staging setups, somewhat comparable to Pepper's ghost
Pepper's ghost
Pepper's ghost is an illusionary technique used in theatre and in some magic tricks. Using a plate glass and special lighting techniques, it can make objects seem to appear or disappear, transparent, or make one object seem to morph into another...
, in very large dimensions. The second image blended to the scenery in this cases is then either a flat screen or a projection from one or more video beamers rasing the illusion that an image is there in free air somewhat behind the mirroring surface. That technique was used at a few public locations (e.g., for creating special effects in live fashion shows). Sometimes the term holographic display stands very close even though that sort of device is a totally different category. The image is indeed as flat as the projection that it is based upon. One such commercially available entertainment product is the Musion Eyeliner
Musion Eyeliner
The Musion Eyeliner is a modern commercial variation of a stage illusion called Pepper's ghost, using high definition video projection or LED screens to allow moving images to appear within a live stage setting....
.
The same type of mirror, when used in an optical instrument
Optical instrument
An optical instrument either processes light waves to enhance an image for viewing, or analyzes light waves to determine one of a number of characteristic properties.-Image enhancement:...
, is called a half-silvered mirror or beam splitter
Beam splitter
A beam splitter is an optical device that splits a beam of light in two. It is the crucial part of most interferometers.In its most common form, a rectangle, it is made from two triangular glass prisms which are glued together at their base using Canada balsam...
. Its purpose is quite different: to split a beam of light so that part, usually about half, passes straight through, while the other part is reflected. In a typical scientific application the two resulting beams are made to interfere
Interferometry
Interferometry refers to a family of techniques in which electromagnetic waves are superimposed in order to extract information about the waves. An instrument used to interfere waves is called an interferometer. Interferometry is an important investigative technique in the fields of astronomy,...
after traversing different paths. An unusual single-lens reflex camera
Canon Pellix
In the mid 1960s through the lens exposure metering was the new technology entering the SLR camera market. The Pellix was Canon's answer to that challenge. This technology was first successfully realised in a 35mm SLR camera in the remarkable Tokyo Kogaku KK Topcon RE-Super, launched in 1963, but...
used a half-silvered mirror to create an image of the scene both in the film plane and in the viewfinder.