Because a single beam is stepped over or crawled under, and because requiring two simultaneous interruptions removes most weather and wildlife false alarms.

Stacked beams at different heights make it impractical to pass without breaking at least two.

Requiring two to be broken together means a bird crossing one beam produces nothing.

Quad-beam units add redundancy as well, allowing the system to keep working if one beam is obscured.

The interruption time is adjustable, so a fast-moving object below the threshold is ignored while a person walking through is not.

Single beams remain useful indoors across a doorway or a corridor, where the environment is controlled.

Far less than the free-space figure - manufacturers publish an outdoor rating that already allows for weather, and installations should stay well inside it.

Fog, heavy rain and snow attenuate infrared, so the beam that works on a clear day fails on a bad one.

The convention is to install at a fraction of the maximum rated range so that degradation does not cause a fault.

Long shots also make alignment harder and more sensitive to structural movement.

Where a boundary is long, several shorter shots with intermediate towers are more reliable than one long one.

Check the manufacturer's derating guidance for the local climate rather than assuming a general figure.

Structural movement, thermal expansion and mounting on anything that is not rigid - and it presents as intermittent faults rather than an obvious failure.

Beam detectors need precise optical alignment, and the tolerance narrows as the shot gets longer.

Posts move in the ground, fences flex, and buildings settle. A gate post is a particularly poor mounting.

Thermal expansion moves a long shot measurably over a day, which is why some faults appear only in the morning or only in summer.

Most detectors provide an alignment aid - a signal strength meter or an audible tone - and the received signal level should be recorded at commissioning.

Comparing the current level against that record is the quickest way to diagnose a drifting installation.

Vegetation growing into the beam, wildlife, snow accumulation, and direct sunlight into the receiver.

Vegetation is the commonest and most predictable: a branch that clears the beam in spring interrupts it by late summer.

Larger wildlife breaking two beams at once will alarm, and there is little to be done about it beyond siting.

Snow building up beneath a low beam eventually reaches it, and drifting snow can obscure a lens.

Low sun directly into the receiver saturates it, which is why the shot direction should be chosen at survey with the sun path in mind.

Keep a maintained clear zone along the beam line, and include vegetation cutting in the site's grounds maintenance rather than in the alarm contract.

On perimeters, boundary walls, long facades and building approaches - anywhere the protection is a line rather than a volume.

A perimeter fence line is the classic application, giving warning before an intruder reaches the building.

Facades and roof lines are protected with beams where a volumetric detector would be impractical.

Internally they cover long corridors, atria and lines across glazing where a PIR cannot see through the glass.

They pair well with cameras: a beam activation directs an operator or an analytic to the right place.

They are not a substitute for detection inside the building, since a beam that has been crossed reports only that the line was broken.