At night police using spotter helicopters find it hard to tell cops from robbers on the ground. The simple answer, according to Richard Harvey of Newport in South Wales, UK, is for each officer on the ground to wear a small infrared transmitter on his or her head. Driven by an oscillator, this sends out a continual pulsing signal like Morse code, which shows up as a bright red spot on the helicopter's infrared night camera. The fugitives have no such markers and so are easily distinguished. The same system can be used in bad weather, or to track firefighters in smoke-filled buildings.
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