Peephole

The opening is typically no larger than the diameter of a dime (0.7 inches, 18 mm).

Glass peepholes are often fitted with a fisheye lens to allow a wider field of view from the inside.

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.