scottobear | 8214 - My original source link is gone, have a wiki entry.

The Active Denial System (ADS) is a non-lethal directed-energy weapon system under development by the U.S. military. It is a microwave laser.

The ADS works by directing electromagnetic radiation at a frequency of 95 GHz toward the subjects. This frequency means the radiation is in the microwave region of the electromagnetic spectrum. In comparison, a standard microwave oven cooks food with about 2.4 GHz waves, so the ADS's radiation is more energetic, but much less prone to penetrate skin — the military says the effect "penetrates the skin to a depth of less than 1/64 of an inch." The focused beam can be directed at targets at a range of one kilometer.

The energy in the waves turns to heat upon skin contact and immediately heats water molecules in the skin to around 130 degrees Fahrenheit, causing an intensely painful burning sensation. A spokesman for the Air Force Research Laboratory described his experience as a test subject for the system: "For the first millisecond, it just felt like the skin was warming up. Then it got warmer and warmer and you felt like it was on fire.... As soon as you're away from that beam your skin returns to normal and there is no pain."

Military researchers claim that the system causes no long-term damage and the Department of Defense hopes to use it to avoid killing rioters or insurgents. The leader of the Joint Non-Lethal Weapons Directorate currently administering the program says the system falls somewhere "between bullets and a bullhorn" — designed to force people to back down without the complications of killing them and bystanders. Proponents of the system hope to arm troops "with a nonlethal capability in military operations other than war" — citing, for instance, peacekeeping and crowd control operations. For example, the beam could be focused on a street or other critical area, and the pain would cause rioters to flee from that area.