A peephole, also commonly called an eye hole, spy hole, or door viewer is a small optical device installed through a door that allows someone inside to see outside without opening the door.
Why Were Peepholes Invented?
Peepholes were invented to improve personal safety and privacy. Before they became common, people often
had to open the door to identify visitors or rely on nearby windows.
Peepholes allowed occupants to discreetly see who was outside before opening the door. They became
especially common in apartments, hotels, and urban homes where unplanned visitors or shared hallways
increased safety concerns.
How Do Peepholes Work?
Peepholes work using a wide-angle optical lens system. The exterior lens captures a broad field of view, often between 160 and 220 degrees, allowing you to see more than just what is directly in front of the door.
Light enters through the outside lens, travels through the narrow tube in the door, and is focused by
the interior lens. This design makes visitors appear smaller and farther away while allowing you to see
a wider area around the doorway.
Are Peepholes One Way?
Peepholes are mostly one way, but given the right circumstances you can see through a peephole from the outside.
From inside the home, the viewer sees a clear, wide-angle image of the outside. From the outside, a
standard peephole usually appears dark, making it difficult to see inside.
However, visibility from the outside can increase if the interior is brightly lit, if the peephole has
no privacy cover, or if a reverse peephole viewer is used.
What is a Reverse Peephole Viewer?
A reverse peephole viewer, sometimes called a peephole magnifier, is a small optical
device designed to fit over the exterior side of a peephole.
These devices reduce lens distortion and can allow someone outside the door to see inside, especially
if the interior space is well lit. They are inexpensive, portable, and easy to use, which is why
privacy covers are often recommended.
Reverse Peephole Viewer App
Some smartphone apps claim to function as reverse peephole viewers by using the phone’s camera.
In practice, these apps are far less effective than physical magnifiers.
Apps cannot bypass the physical optics of a peephole. Their effectiveness depends entirely on lighting
conditions and camera quality, making them more of a novelty than a serious privacy risk.