Peephole

A peephole, peekhole, spyhole, doorhole, magic eye, magic mirror or door viewer, is a small opening through a door allowing the viewer to look from the inside to the outside.

For the System of a Down song, see System of a Down (album).
For the computer compiler techniques, see peephole optimization.
View through a peephole
Barack Obama looking through the Oval Office door peephole
Door viewer in the gate of Vaxholm Fortress

In a door, usually for apartments or hotel rooms, a peephole enables to see outside without opening the door. Glass peepholes are often fitted with a fisheye lens to allow a wider field of view from the inside.[1]

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 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
  1. Peephole Is One Way Viewer Popular Science, July 1950, pg 153, right-side.
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