You can know when someone " crosses the line".

This high tech device creates an invisible infrared beam up to 60 ft. long. When the beam is crossed a loud alarm or a pleasant chime sound will be emitted.

Can be used as an invisible playpen to alert you to your child or pet leaving a safe area. The alarm can also be used at night to scare off would-be intruders. In the chime mode, retail stores in businesses can use it to announce customers are alert them when someone enters an area that is "off limits".