While the differences to shading alone made a big difference in outdoor areas and in space, the lighting system ran into more issues with interior spaces, dealing with things like the lights cast by computer monitors and the like. Fixing the lighting system thus required re-coding a variety of different pieces of the game to look more organic and realistic. Since the game’s construction is modular, though, the results could easily be spread throughout the game with only a few changes. Check out the full development diary to get an in-depth look at how just a little more light makes everything better.
Source: Official Site