Quote - "The path of light is symmetrical for the direction...because this ray always reach the camera."

That is what I first read of this, on a Metacreations page somewhere, back in the days of Poser3...I still do'nt think the Vector path of the light, backwards, is all too key though.  Sure you'll trace light rays that lose their charge, but then you've found a coordinate to map, so what if it's dark?

The light rays do not exist, they are only a geometrical construction. In the end you will have a path starting in the light source and ending in the camera, or starting in the camera or ending in the light source, direction of flow doesn't matter. Even you can exchange the camera with the light source and the result is the same. You only trace the path, calculate how each object that is in the ray path affects the light and the resulting intensity and color is what you display for every pixel of your rendering.
The rays are nothing more than mathematical abstractions that produce a real result.

Stupidity also evolves!