A laser warning receiver is a warning system used as a passive military defence. It detects, analyzes, and locates directions of laser emissions^[1] from laser guidance systems and laser rangefinders. Then it alerts the crew and can start various countermeasures, like smoke screen, aerosol screen (e.g. Shtora), active laser self-defence weapon with laser dazzler (LSDW, used on the Chinese Type 99 main battle tank^[2]), laser jammer, etc.

Detectors used in LWR are usually based on a semiconductor photodetector array, which is typically cryogenically or thermal-electric cooled. Sometimes avalanche photodiodes (APD), photoconductivity, photoelectromagnetic, or photodiffusion devices are used even without cooling.^[3] Some devices detect only the main beam of foreign lasers while others detect even scattered rays.