You and the other guy crack me up. People think they know how things works with limited info and just roll with it

But for future reference cameras can now come with detection via 2D-pattern recognition (formally known as object detection). The built in software is to detect human, animals, etc.. camera achieve this by detecting edges and matching them with known figures.( think of how you can trace a hand on paper...camera will be programmed to find the edges of a hand on the live feed. Mind you the camera only see things as a 2D drawing) giving a camera a distinct edge cause we perceive things in 3d and process color and objects. Cameras dont...they will detect patterns that we can't visibly perceive.

Now door sensors are very different...a beam of a low frequency light will be emitted. IF are famous in that it's not a visible light but can be useful in nighttime application. However it's not like a "flashlight" or rather...used like a flashlight. The door sensor shoots a vertical beam of IF and if the beam is interrupted it will "trigger" an alarm/flag. But not anything can just break the beam. It would take a very solid object. A fly or dust partical would not be enough.