A radar detector is an electronic device used by motorists to determine if their speed is being monitored. The term originates from early speed-detection technologies, in which police bounced a radio wave off a moving vehicle with a device called a radar gun that determined the vehicle's speed by the Doppler-effect-moderated change in the wave's frequency.
Most of today's radar detector work by detecting signals across a variety of wavelength bands usually X, K and Ka (as well as Ku, in Europe, also recently approved for use in the U.S).
These devices should not be confused with the ones that work by using the GPS technology and a database to warn the driver when he approaches a speed camera location, like it is done in the FoxyTag collaborative project.
Newer speed-detection technology uses pulsed laser light (LIDAR, commonly referred to as laser detection) rather than radio waves. Modern "radar detector" have been adapted to suit that technology, by detecting the infrared light emitted by these new detection methods.
LIDAR detection is not nearly as reliable as the detection of radar, since the light is much more focused and often aimed below the windscreen level, where the detector is usually mounted.
(The reflective coating of the vehicle licence plate is an excellent laser beam reflector.)
There are many products that are advertised to claim to "scramble" or "absorb" radar; many of these scrambler devices do not work at all, and if they do work, it is by emitting an active radar scrambling pulse, which makes the device illegal to operate under in many jurisdictions.
There are also LIDAR jammers that are currently legal throughout most of the US, some Canadian provinces and other countries. LIDAR jammers, like police lidar guns are regulated by the FDA for eye safety as Class I devices. They operate on the principle of destructive interference to "jam" most LIDAR guns in most circumstances.
Despite the advent of LIDAR speed detection, radar detector remains more prevalent for several reasons, not the least of which are the lower costs of radar (although some insurance companies supply laser guns to police departments for free) and total amount of radar detector equipment in historical service.
Popularity of LIDAR speed detector is on the rise, though, as costs decline and ease of operability approach radar; existing radar guns reach their end-of-service life; and they are rotated out of service replaced with newer technology.
LIDAR also has the advantage of being able to be used in higher traffic density areas (ie; crowded multi-lane highways of major cities) where conventional radar guns have trouble isolating a single vehicle's speed.
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