There are two types of speed camera detectors. The type, which simply alerts you as to the location of speed cameras, is legal. The other type which sends a laser signal to the laser speed detector preventing it from taking a reading are illegal and you could face prosecution for perverting the course of justice (among other things) which carries a maximum sentence of life imprisonment.
Gatso cameras are the most commonly used speed cameras on UK roads. They use radar technology to measure how fast a vehicle is travelling. If a vehicle is exceeding the speed limit for a given road, a camera and high powered flash take two photographs of the rear as it passes. The cameras only take rear facing photographs so the high powered flash doesn't blind drivers.
The Truvelo camera is usually forward facing (it can be rear facing) and uses four piezo sensors embedded in the road surface at set distances to measure the speed of passing vehicles.
SPECS cameras record a vehicle's average speed over a given distance. They are often used on motorways/dual carriageways or road works.
