A LIDAR detector or laser detector is a passive device designed to detect the infrared

Infrared

Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 emissions of law enforcement agencies' LIDAR

LIDAR

LIDAR is an optical remote sensing technology that can measure the distance to, or other properties of a target by illuminating the target with light, often using pulses from a laser...

speed detection devices and warn motorists that their speed is being measured.

A limitation of LIDAR is that it cannot be used while a police car is in motion, because it requires the operator to actively target each vehicle, whereas traditional radar can be operated while the police officer is driving his car. Other restrictions include a precipitation free environment, as the laser can produce erratic readings from airborne moisture or smoke. Popularity of LIDAR speed detection is on the rise, though, as costs decline, ease of operation approaches radar, and existing radar equipment reaches its end-of-service life and is rotated out of service.

LIDAR detectors are generally less effective than radar detectors as the emissions they monitor are more brief, more concentrated and less easily scattered than radar; a motorist may therefore not have sufficient time to respond to the burst transmission of a LIDAR device, or the narrow beam might be focused on a specific part of a vehicle where the sensor cannot "see" it.