Opening a door without knowing who is behind it can be dangerous. You can install a peephole in your front door to make sure you know who's knocking. It's relatively easy; just follow these steps.
Assess your door. Make sure it is of standard width, usually from 1 3/8 inch to 2 inches. (If your door is thinner or thicker, a standard peephole may not fit.) Know what your door is made out of, so you can decide what kind of tool you need to drill through it. Also check to make sure the lens on the peephole is not cracked or marred.
Decide where you want to peephole to go. It should be in the center of your door and about five feet from the ground. Keep in mind peepholes have 180 degree viewing lenses, which means they allow you to see just about everything within a 180-degree radius, and within a few feet of your door. Once you've found your spot, mark an "X" there with a pencil.
Drill a pilot hole. Using a power drill and a 1/8-inch drill bit, drill from the inside of the door. A pilot hole does not have to go all the way through the width of your door. A little over halfway will do.
Now drill a complete hole, all the way through the thickness of your door, using a 1/2-inch drill bit. Use a screwdriver to gently scrape the inside of the hole clean.
Insert the lens piece. Most peepholes come in two pieces. The lens piece typically has threads on the outside and has the large lens that will face the outside. Insert the lens piece from the outside of your door.
Insert the viewing scope piece. Thread the viewing scope from the inside of your door until it fits snuggly against the lens piece. It should not be too tight, but should not move around inside the door.
Place towels on the floor inside and outside of your door, for easy clean-up of sawdust. Be sure to use safety goggles to keep sawdust and other particles from your eyes. Don't drill any holes with the door closed. Prop it open or take it off of its hinges to make sure no one walks into a running drill.
Although many peepholes come with some type of warranty for manufacturing defects, they do not cover any damage you may do to your door.