Peephole

Simple peepholes may allow people outside to see inside. A fisheye lens offers little visibility from the outside, but that can be defeated using a peephole reverser. Some peepholes have a shutter that falls down on the hole when nobody inside is holding it. Digital peepholes have a camera outside and an LCD screen inside, without any information going from the inside to the outside.

Another technique to prevent someone on the outside from seeing inside that is used by manufacturers of peepholes is to design them so that the outside-facing lens actually projects an image onto a semi-opaque (frosted or ground-glass finish) piece of glass on the inside-facing side that acts as a screen so that the person inside can see, at arms length, who is on the other side. The frosted glass finish makes it impossible for someone to look through from the outside. There are possible drawbacks to the projection method. One drawback is that the area to be viewed must be well lit and yet another drawback might be that to install these types of viewers there needs to be a much larger hole made in the door than that which has to be made for the traditional style of peephole.