I think I get the concept of a machine sending pulses of light, the light bouncing back to a sensor and measuring that time delay (for speedometers and such). But how does it work with non reflective surfaces? How can the light speed be measured with tiny distances and short travel times? Or do I have it wrong and they’re really just cameras that only measure incoming light?
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You can use light to measure a non-reflective surface. In practice all material is reflective to some degree, even black material, just try to shine a flashlight on them, if you can see it changes the object it reflects some light.
That said you can detect if something that does not revert any light move in front of a sensor. Unless what is normal there also does not reflect any light you can notice that less light return to you.
You can do motion detection by sending you light pulses and measuring distance, that would be a LIDAR system but that is not how it commonly is done. It is very complicated and not really needed.
The most common way to deter the motion of humans is passive infrared, the sensor detects infrared light that a human body emits. We are in general warmer than the room it is in so you just need to detect something warm is there, you can do that with a single 1-pixel sensor that looks at all light from the room.
Another way is doped radar where you see conditions microwaves and a moving object will change the frequency of the reflected microwave and motion is detected. The same idea can be done with ultrasonic waves. Here too you look at the area in front of the sender as a whole and just detect that something move without caring where it is
Another way is cameras that can use visible light but also near-infrared so you can illuminate an area but humans can see it. The system will do image processing to detect changes in the image. Normals CMOS sensors that digital camera users can see a bit into the IR range, Filters are usually added to the sensor to keep IR light. You can test by looking at an IR remote through a camera sensor.
Multiple sensors can be combined like passive infrared and microwave and require both to trigger the alarm. That way something like a curtain that a wind moves can trigger the microwave but not the passive infrared. Heating by sunlight can make something warm to trigger the passive IR but not the microwaves. That results in fewer false alarms because in general, it is humans we try to detect.
https://en.wikipedia.org/wiki/Motion_detector
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