The first concept for an indirectly lit floor.
I'll draw a second one and at least one concept for an indirectly lit and a ceiling too.
The idea behind it is to not light up the objects in the room itself, but to light up the floor/walls/ceiling. A person then comes into a room that's well lit, but cannot see any light-source.
It can be seen in the test-chambers in Portal.
*The scale is in "Hammer-Units" and only estimated.