What are they?
Dark sky lamps are outdoor lamps which utilize shielding in order to minimize light pollution. They are known primarily for their use in that function.
How do they work?
Dark Sky units tend to be a typical lamp, often a spiral CFL, which is contained within a special housing. This housing prevents light from being emitted in any direction except the intended one. They are generally intended to keep light from shining upwards, although there are some units designed to stop side lighting as well.
Where are they used?
Dark sky lamps are used in a variety of situations where the reduction of light pollution is a primary consideration. They generally are designed to prevent light from traveling upward, but they are also used in some applications where light shining horizontally or outward needs to be controlled in order to prevent disruption of the migration patterns of birds and turtles, for example.