 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.