Outside there is wind, fog and rabbits. And all three set it off.

A Japanese manufacturer dedicated to detection. On perimeters it covers four different technologies: outdoor passive infrared, beam barriers, microwave and laser scanners —LiDAR— that draw a plane of detection.

One house having four ways of detecting the same thing sounds like a padded catalogue and it is not. Outdoors there is no such thing as the good technology: there is the one that holds up on THAT site. A manufacturer with only one is going to sell you that one every time.

It turns up on medium-sized perimeters and at points that have to be solved properly: a material yard, the face of a warehouse, a flat roof people get in over, a gap between two industrial units.

Passive infrared does not see: it feels differences in heat. It detects something at body temperature moving in front of a background at another temperature, so it works with no light and with nothing opposite it. The outdoor ones require the signal to appear at two points in the field, and that tells a person apart from a warm leaf.

The beam barrier is the opposite: transmitter, receiver and an alarm when the beam is broken. With several beams you can require two to be broken at once, and that stops a bird setting off a whole passage. Microwave creates a field between two heads: it goes through rain and fog far better than anything optical and it detects the very slow.

LiDAR sweeps a plane with a laser beam, measuring distances. That gives it what the others do not have: it knows how far away the thing that moved is and what size it is, so it can ignore something small and distant. It goes in vertically, like an invisible wall in front of a façade, or horizontally over a flat roof. And it works the same at night, because it brings its own light.

The height and the tilt decide what comes into the field. Too low takes in what passes at ground level: cats, rabbits, bags. Too high leaves a gap underneath. And every unit has a dead zone right below itself, which is not seen on the drawing and is seen at night.

The cleared zone is the condition most often broken: in front of a beam or a microwave field there can be no branches, no long grass and no tarpaulin. None of that was there on handover day — the vegetation grows in spring, which is when the false alarms appear on a perimeter that had been quiet for six months.

And the alignment is lost on its own: the ground settles, the metal expands, and a knock from a lorry shifts a post by one degree nobody sees. The overlap avoids the textbook hole —two stretches cross, they do not end butt to butt— and the slope rules: what on a drawing is a clean field is, in a dip, a gap. You see that by walking it, not on an aerial photograph.

When an outdoor detector fires at three in the morning, whoever is on duty has two bad options: sending somebody out blind every time —it costs money and it stops happening after three weeks— or sending nobody and trusting. Both end with the alert meaning nothing.

With an image, the alert arrives with the seconds of video of what set it off: thirty false alarms a month become thirty ten-second dismissals, and the one real one becomes an immediate phone call.

And there is something to declare here: in the same house there is IRIS Neural, the NVMS of Infinity Neural, the other company in this group. It is our own product, and we say so because recommending your own without warning throws away everything else. It adds the image of what set the alarm off; it does not replace the detector or the panel. With other video software it is done too. With nothing that lets you look, it is not.

«Which is better, infrared or microwave?» has no answer, just as «which is better, a screwdriver or a spanner?» has none. The one that does have an answer is «what holds up on this stretch?», and to answer it you have to know the ground, the vegetation, the orientation and what moves there every night.

And two are almost always combined. It is not selling more: two technologies that fail for different reasons, each requiring agreement from the other, give you a perimeter that does not go off with the fog or with the rabbit and does go off with the person. The price is a little sensitivity, and it is a decision per stretch.

The typical split: the fence with one technology, the gap between units with another, the flat roof with a scanner, the lorry gate with beams, and cameras over the lot. We are not going to say this brand is better than the other four; we will say what its catalogue is for: when a site calls for three technologies, having them from one house saves pieces and spares.

An outdoor detector is a zone on a panel. The EN 50131 standards classify systems into security grades, and each grade demands specific things of the equipment, the cabling, the power supply and the tamper detection. What grade each model supports is read in its documentation; the installation meeting that grade is another matter. We do not say here which one we hand over: it depends on the design, and it is asserted in writing by whoever signs.

Connecting to an alarm receiving centre is a separate decision, with its own regulations and its own contract. Outdoors you also have to write down what is done with an alert from outside, which is not handled the same way as one from inside. Who provides that service is settled in a quotation, not on a website.

And what comes after: the day-to-day belongs to the client, but they do not touch the sensitivity of an outdoor detector or its alignment. Touching it without knowing what you are touching is the fastest way for that stretch to stop detecting without anybody finding out. The same with isolations: the stretch that was isolated «one day» is exactly where they come in. Each one with an expiry date and with its date in the report.