Peephole

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View through a peephole

Barack Obama looking through the Oval Office door peephole

Door viewer in the gate of Vaxholm Fortress

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

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[]

Look up peephole in Wiktionary, the free dictionary.

Wikimedia Commons has media related to Peepholes.