Explainer
No operator pressing start, but a room that notices someone walked in. Which sensors exist, what they do and do not know, and how to stop one stray reading from wrecking the experience.
Do not start with the sensor but with the question. Do you need to know that someone is there, or how many people? Whether they are standing still or moving through? Where exactly, or only in which zone?
Every sensor answers one of those well and the rest badly. Most failed installations come down to a sensor chosen to answer a different question from the one the experience asks.
Under the floor, invisible, and as reliable as a light switch: someone is standing there or nobody is. No light needed, no sightline, no calibration.
The limitation is that a mat is one spot. To follow a route you need several, and then the floor construction decides what is possible.
For a trigger that absolutely has to work, such as the start of a scene, this is usually the choice.
A PIR sensor sees movement within a cone. Cheap, easy to hide, but it also sees the curtain moving in the air conditioning and it stops reporting when someone stands still.
Distance sensors, laser or ultrasonic, measure how far away the nearest object is. That tells you whether someone is approaching an object, and at what distance. We also use them on machinery to confirm that something really is in position.
A light curtain or beam notices a line being crossed, which is exactly what you want at a doorway: someone goes in or out, and with two beams you even know which way.
A camera with image analysis knows more than all of the above combined: how many people, where in the room, which direction, how long already.
Against that stand three costs. There has to be light, and in a dark immersive room that is not a given. There has to be a sightline that does not ruin the set. And there is a privacy assessment: even if you keep no footage, you have to be able to explain what happens.
Our rule of thumb: use a camera when you genuinely need counts or positions, not because you can.
A pressure mat switching twice because someone shifts their weight. A PIR triggering on an airflow. A light curtain seeing a bag instead of a person. That is not a defect, that is normal behaviour.
The answer sits in the logic above it. Two triggers within a second is one visitor. A sensor sitting high for ten minutes is probably a coat over the mat, not someone standing still for ten minutes. And a zone reporting empty while the door has not opened does not add up.
We build those rules per project, because they depend on the room. What always holds: one sensor deciding one thing is one point at which the experience breaks.
For every moment that genuinely has to happen we build in a second way of recognising it, or a time limit behind it.
If the trigger comes, the story moves on. If it does not, it moves on anyway after a while. The audience never sees the difference and you see in the log that something needs checking.
That is the difference between an installation that occasionally stutters and an installation where nobody ever noticed it stuttered.
Frequently asked
A pressure mat, because it needs no light and no sightline and knows only two states. For triggers that absolutely have to work that is usually the choice; for counts or positions you need something else.
Only when we genuinely need counts or positions. There has to be light, there has to be a sightline that does not ruin the set, and there is a privacy assessment you have to be able to explain.
We catch it in the logic: two triggers within a second is one visitor, and a sensor sitting high for ten minutes is probably a coat over the mat. We build those rules per room, because they depend on how people move through it.
Further reading
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