Peephole
{{Short description|Small, round opening through a door}}
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File:Barack Obama looking through the Oval Office door peephole.jpg looking through the Oval Office door peephole]]
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A peephole, peekhole, spyhole, doorhole, magic eye, magic mirror or door 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 a dime ({{convert|0.7|in|mm|disp=comma}}).
In a door, usually for apartments or hotel rooms, a peephole enables to see outside without opening the door nor revealing one's presence. Glass peepholes are often fitted with a fisheye lens to allow a wider field of view from the inside. [https://books.google.com/books?id=Qi0DAAAAMBAJ&pg=RA2-PA53 Peephole Is One Way Viewer], Popular Science, July 1950, p. 153, right-side.
Preventing inside viewability
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-opaque frosted or ground 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.
References
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{{Wiktionary|peephole}}
{{commons category|Peepholes}}