A thermal camera does not see the face. It sees that there is something warm behind the hedge.

An ordinary camera needs light: it collects the light that bounces off things. A thermal camera does not collect light, it collects the heat that bodies give off, and it draws an image out of the differences in temperature. That is why it works in total darkness, without a single floodlight, and that is why a person shows up clearly against a cold field even when hidden among bushes.

Two trades come out of that. One is detecting presence at a distance where there is no light: it sees that there is something shaped like a person or a vehicle and how far away it is. The other is seeing temperature where there should not be any —a hot spot in a pile of waste, a belt that is rubbing, an electrical panel, a battery— and the models that measure allow an alert by threshold before there is any smoke.

What it does not do, and this has to be said on the first page and not the last: it does not identify. In a thermal image there is no face, no colour of clothing and no number plate. It says there is somebody; it does not say who.

On long dark perimeters: a fence a kilometre long, an open compound, a substation, a site, a quay. The alternative is lighting all of that —light fittings, consumption, permits, neighbours— and often it cannot be done. A thermal camera detects without switching anything on.

In early heat detection, which in some places is the only way of getting there in time: waste and recycling plants, biomass yards, wood chip, battery stores, conveyor belts, silos. What burns there starts by heating up inside a heap where no ceiling detector is going to notice.

Anywhere somebody has to be identified, a number plate read or a person described: it is not for that, and no configuration changes it. Nor behind glass, because glass blocks heat: a thermal camera inside a window does not see the street. And not as a substitute for a fire detection installation to the regulations, which is another thing with its own rules and its own approval.

There are two more limits that are only learnt on site. Heavy rain and thick fog cut the real range below the catalogue one. And at dusk, with the ground warmed all day and almost at body temperature, the contrast drops and the scene goes flat.

And it costs: for the price of one thermal camera you can fit several ordinary ones. Where there is light, or light can be put in, the ordinary camera almost always comes out better. The thermal camera goes where it does something no other one does.

The honest way to use a thermal camera is in a pair: the thermal one detects and an image camera looks at the same place to verify and to document. The first says «there is somebody at metre one hundred and eighty of the fence»; the second says what it is and leaves an image that is good for something. With only one of the two, either there are alerts nobody can check or there is a camera that sees nothing at night.

On thermal the analytics get it right more often, and for a simple reason: a warm body separates from the cold background with no ambiguity, so telling a person from a branch is easier. A well adjusted thermal perimeter gives far fewer useless alerts than a conventional outdoor detector.

The limit is of a different kind from the other brands: the camera knows there is something warm and shaped like a person, and it knows nothing else. It does not tell a worker on shift from an intruder, and it does not know that the lorry that is warm is the one that unloads at that hour every day.

What comes now is a recommendation with an interest of our own, so it is declared: in the same house there is IRIS Neural, the NVMS from Infinity Neural, the other company in the group. It is our own product. We would rather you knew it by reading this line.

What it brings here is joining the two images into a single decision: the thermal one detects, the image camera confirms and the system decides whether that is worth waking somebody up for. It also detects smoke and fire by image, which reaches where a point detector does not — and it does not replace the approved control panel, it complements it. We always state that boundary.

On thermal the height is a sharp compromise. The flatter it looks, the further it reaches —the target takes up more pixels— and the easier it is for a car, some pallets or a change of level to hide twenty metres of fence. Steeper down, fewer obstructions and less range. It is settled by measuring the ground.

The angle and the height decide for good the distance at which that camera is going to detect. It is settled in the project, and it is checked at night and with somebody walking before calling the installation good.