The system does not record all night. It records when the analytics see movement, and it writes the event with the size of the thing that moved in pixels. Camera 11 writes one every night, with a height that works out at about a metre and a half, and the clip is empty.
I am the network manager at a large secondary and the cameras are mine because everything with an address on it is mine.
How it works, because people picture a man watching screens.
Thirty one cameras onto a recorder. The recorder does not keep everything: it runs motion analytics, and when the analytics decide a region of the frame has changed in a way that looks like an object rather than a tree or a light, it writes an event.
The event is a row in a database. Time, camera, duration, a confidence figure, and a bounding box, which is the rectangle the software drew round what moved, in pixels.
Because we know the lens and where the camera is, a pixel height converts to a real height. That is how the system is able to ignore cats and flag people, and it is configured with a minimum object height so that we are not reviewing two hundred events a night off the car park.
Then it keeps ten seconds before the event and thirty after.
Camera 11 looks down the science corridor, from the double doors at the stairwell to the fire door at the far end. Forty one metres, no windows, three fire doors, emergency lighting only at night.
It writes an event at between one and ten past two, every night.
Duration is between four and nine seconds. The bounding box converts to a height of about a metre and a half and a width of about forty five centimetres.
The clip is an empty corridor.
I want to describe the clip accurately because that is the whole of this.
It is forty seconds of a dark corridor lit by two emergency fittings, the sort of green tinged light that makes a floor look wet, and because the floor is polished twice a week by a man who takes pride in it you can see both fittings reflected in it the whole length of the frame, along with the fire doors, the noticeboard, the bottom of the stairwell doors and a bin that has been in the same place since 2019.
Nothing enters the frame. Nothing leaves it. Nothing changes. I have watched about two hundred of these at normal speed and a dozen frame by frame.
What has been ruled out, some of it by me and the expensive part by the installer.
An insect on the lens, which is the classic. An insect gives you a huge bounding box because it is close, and it is visible.
A light changing. The emergency fittings are on all night and a flicker would show as a change across the whole frame, and the analytics would write a much bigger box.
A reflection off the floor when a car comes up the side road. There is no window in that corridor. I have stood in it at night to be sure, and the only light that gets in is under the doors at the stairwell end, and that door leads to another internal corridor.
Camera fault. Replaced in 2021 with a new unit, different model, and the recorder was replaced in 2023 when we moved to a new system entirely. Different manufacturer, different analytics engine, different everything.
The new system writes the event at the same time, with a box of the same size, and a confidence figure of between eighty one and ninety four percent.
What the second system does that the first one did not, and why I have stopped showing this to people.
It classifies. The analytics on the new recorder do not just find an object, they put it in a category: person, vehicle, animal, unknown.
Every one of those events is classified as a person.
Eighty seven of them since the install, and it has never once said unknown.
What I have done about it formally.
It is on the register of faults as a false positive on camera 11, reviewed annually.
I raised it with the installer, who came out twice and the second time sent a commissioning engineer who spent an afternoon in the corridor with a laptop and a tape measure.
His report says the camera is correctly configured, the analytics are within specification, and the events are consistent with a person walking from the stairwell doors towards the fire door at about one metre per second.
He wrote that. Then he added a paragraph saying that as the recorded footage shows no such person the events should be disregarded, and billed us for half a day.
What I do about it personally.
I do not review camera 11 any more. It writes what it writes and the events expire off the disk after thirty one days, and nothing about it needs a decision from me.
There is one thing I did change, and it is the only thing in four years I have done that I could not justify in a meeting.
The system can send an alert to a phone on a chosen camera and a chosen schedule, which we use on the gate and the bike sheds.
Camera 11 was on that list when I inherited the system, set to alert overnight, presumably by my predecessor for a good reason.
I took it off in 2022, after a night in February when my phone went at six minutes past two and I was awake anyway and I opened the app and watched it live.
The event had already been written by the time the app loaded, which is about four seconds.
What I saw was an empty corridor, and at the far end the fire door was standing open against its closer, which it cannot do, and it took about two seconds to swing shut.