Active beam detectors. These alarm devices snd an active singe, dual or quad beam to a receiver. They require line of sight. When the beams are interupted or broken the detector will go into alarm. The are ideal for virtual fences or across driveways to detect intruders
Active Beams
Takex

EVE ER34615(D) 3.6V Lithium D Type Battery
Inovonics

Inovonics Wireless Repeater with Battery backup wireless iSeries
Optex

Optex 4 Channel Super AlignmentTwin Active Beam 200m
Optex

Optex 4 Channel Synchro Twin 30m Active Beam NO/NC Relay
Optex

Optex 4 Channel Synchro Twin 60m Active Beam No/NC Relay
Optex

Optex 40m Dual Active beam detector
Optex

Optex Basic Low Profile 30m Twin Active Beam Internal/External
Optex

Optex Battery Powered 4 Channel Synchro Twin 30m Active Beam
Optex

Optex Battery Powered 4 Channel Synchro Twin 60m Active Beam
Optex

Optex external synchronised twin beam 20m
Optex

Optex Wireless 4 Channel Synchro Twin 30m Active Beam
Optex

Optex Wireless 4 Channel Synchro Twin 60m Active Beam