PS re the "detector" in ray-tracing
The "detector" means the computer screen, which is why it's represented as a 2-D cartesian grid. The point of ray-tracing is to trace BACK from the detector to the object to be visualized - the other way round from real life. That saves a huge amount of computer time since there's no point in tracing all the ray paths emitted by the object, as per real life - we're only only interested in the rays reaching the detector. The best way to capture those, is to reverse the process of ray tracing from the detector back to the object.