spyhole

A spyhole, also known as a peephole or a door viewer, is a small optical device, typically a wide-angle lens, installed in a door. Its primary function is to allow someone inside a room or building to see who is on the other side without opening the door. This enhances security by enabling a person to identify visitors before granting them entry, thus preventing unwanted or potentially dangerous encounters.

Spyholes are commonly found in residential doors, hotel room doors, and other entryways where security is a concern. They are designed to provide a broad field of vision, often exceeding 180 degrees, ensuring that a significant portion of the area outside the door is visible. The lens itself is usually made of glass or plastic and is housed within a metal or plastic casing that is inserted through a pre-drilled hole in the door.

The typical installation involves drilling a hole of appropriate diameter through the door at eye level for an average adult. The spyhole unit is then inserted from one side and secured, often with a threaded barrel and a mounting flange on the other side. Some spyholes also include a rotating cover that can be closed from the inside to obscure the view from the outside, offering an additional layer of privacy. The effectiveness of a spyhole can be diminished in low light conditions or if the lens becomes dirty or damaged.