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For the System of a Down song, see System of a Down (album).
For the computer compiler techniques, see peephole optimization.
A visitor approaches a door, as viewed through the peephole

A peephole or door viewer is a small opening through which one may look.

In a door, usually for apartments or hotel rooms, a peephole allows the person behind the door, the security of seeing outside without opening the door. Glass peepholes are often fitted with a fisheye lens to allow a wider field of view from the inside and little to no visibility from the outside.[1]


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