A door viewer, also known as a peephole, is a small optical device installed in a door that allows the occupant to see who is outside without opening the door. Typically made of durable materials such as brass, stainless steel, or aluminum, door viewers offer enhanced security by providing a wide-angle view of the area outside the door. They are commonly used in residential and commercial buildings, particularly for entry doors, to ensure safety and privacy. Door viewers come in various designs and sizes, some offering features like adjustable angles or built-in lighting, providing clear visibility even in low-light conditions. Their simple yet effective design helps prevent unwanted visitors and adds an extra layer of security.