Peephole

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.

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

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 technique to prevent someone on the outside from seeing inside that is used by manufacturers of peepholes is to design them so that the outside-facing lens actually projects an image onto a semi-opaque (frosted or ground-glass finish) piece of glass on the inside-facing side that acts as a screen so that the person inside can see, at arms length, who is on the other side. The frosted glass finish makes it impossible for someone to look through from the outside. There are possible drawbacks to the projection method. One drawback is that the area to be viewed must be well lit and yet another drawback might be that to install these types of viewers there needs to be a much larger hole made in the door than that which has to be made for the traditional style of peephole.

References

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