A reverse peephole viewer is a device used to see into a room or home from the outside, using the peephole. The convex lenses of the peephole are designed to allow the person inside to see out, but not the other way around. This, incidentally, is why things look smaller and somewhat distorted through a peephole. The viewer contains a set of lenses that reverse the effect of the peephole lenses, allowing someone to see in from the outside. It is simple to use and is small enough to be carried in a person's pocket.
It is not uncommon for law enforcement to make use of a reverse peephole viewer or similar device to see into a home. This is done with the primary objective of being able to assess threats that may be inside. In a typical situation, an officer will place the reverse viewer over the outside of the peephole to assess any activity inside. Ideally, this can be done without alerting anyone inside to the fact that they are being watched.
These items were primarily developed for use by law enforcement, but have other applications as well. A property owner may need to be able to asses the condition of the interior of a property, and a reverse peephole viewer provides an excellent reconnaissance tool. It is also used by debt collectors in some cases, to see if anyone is home, without giving the occupant the ability to fool the collector into thinking he is not in. It can be used on most of the peepholes in the world, although there are a few specialized types that do not allow someone too see in, no matter what.
Fortunately or unfortunately, while a reverse peephole viewer can be very effective, it is also easily thwarted. It is, for instance, common in Europe for a peephole to come with a small metal cover that goes over the lens except when the occupant slides it back to see out. This, or even a small piece of paper, can make the peephole viewer essentially useless.