Imagine sitting at home, concentrating on the TV. Out of the corner of your eye, you notice the dog. The glow from the cooker tells you that dinner is on. The security light comes on outside, and the shadow your Mum appears on the door as she comes home from work.
Seeing tells us a lot about the lived-in world. There is so much that you only notice part of it.
How do your eyes to detect all of these things? A source of illumination links to your eyes, which are detectors.
Cameras are made to detect light. So they can act as detectors.