News One: Security Door Viewer Principle

Metal door viewer principle

What is the principle of metal door viewer?

The door eye, like a telescope or a microscope, consists of a concave Lens (objective Lens) and a Convex Lens. The focal length of the objective lens is extremely short. It causes the outdoor person or object to form a virtual image, which falls exactly within the focus of the eyepiece, you can see outside the door clearly.

A peephole, peekhole, spyhole, doorhole or door viewer, is a small opening through a door allowing an individual to look from the inside to the outside. When viewed from the inside, all the views outside the door can be seen from a 120 degree angle, but from the outside the door can not be seen through the door viewer.