Door Viewer. Image Credit: Global Industrial Door Viewer with knocker. Image Credit: Gammy Hardware
Door viewers are security devices that allow the user visual access to the other side of the door. They are optical devices, also known as peepholes, which fit through a door. Traditional peepholes have several disadvantages including a limited viewing angle and weak material construction. Improved door viewers correct for these disadvantages.
Models differ in size and view range degrees.
Standard peepholes have two lenses that provide a limited view range of 160 degrees. They are inexpensive and easy to install but plastic lenses can become opaque and scratched reducing their effectiveness.
Wide angle (projection) viewers have larger lenses for easier identification. The viewer uses double prisms to project the image onto the rear of a large opaque lens. This allows users to see a132 degree "projected" image. The disadvantage of this device is that the image is reversed (like looking in a mirror) and requires a good light source. The advantage is that viewer can be much further away from the door to see the person or object on the other side.
The far and near viewer design compromises between size, resolution and price. This design uses three lenses and allows the user to see a more precise and sharper 200 degree view with minimal distortion as well as better functionality in all light conditions. The user can be several feet away from the door and still see through it.
View angle comparison. Image Credit: peepholesecurity.com
Digital viewers comprise of a small camera connected to a video screen. The wide angle image is displayed on a color monitor mounted to the door or nearby wall.
Door viewers are available in various decorative finishes with some having features such as lens covers and integration into door knockers.
Brief video on door viewer installation. Video Credit: Woodies DIYTV