Door viewers are also described as peep holes or spy holes. They are a simple lens that is fitted inside a door to allow the person inside to see how is on the other side of the door.
The lense is the most important feature of a door viewer or peephole. It's fitted with a fish eye lense which allows it to view a wide spectrum of vision. From the inside looking out you can see a lot of information, meaning its perfect for home security. However, from the outside looking in it has minimal to zero visibility meaning you can safely open your front door after you have viewed whoever it is through the door viewer.