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A modernpeephole,peekhole,spyhole,doorhole,magic eye,magic mirror ordoor viewer is a small, round opening through a door from which a viewer on the inside of a dwelling may "peek" to see directly outside the door. The lenses are made and arranged in such a way that viewing is only possible in one direction. The opening is typically no larger than the diameter of adime (0.7 inches, 18 mm).
In adoor, usually forapartments or hotel rooms, a peephole enables one to see outside without opening the door nor revealing one's presence. Glass peepholes are often fitted with afisheye lens to allow a widerfield of view from the inside.[1]
Simple peepholes may allow people outside to see inside. A fisheye lens offers little visibility from the outside, but that can be defeated using a peephole reverser. Some peepholes have a shutter that falls down on the hole when nobody inside is holding it. Digital peepholes have a camera outside and an LCD screen inside, without any information going from the inside to the outside.
Another design to prevent people outside from seeing in involves the outside-facing lens projecting an image onto a semi-opaquefrosted orground glass screen. An inside viewer can see the other side of the door from an arm's length away, rather than by peering through a small hole, while the frosted glass finish makes it impossible for someone to look through from the outside. There are drawbacks to the projection method: the area to be viewed must be well lit, and installation requires a much larger hole in the door than a traditional peephole.
