The latest development diary explains that the game’s materials and geometry are solid and get regularly updated with new patches, but the lighting model was outdated and frequently led to too much contrast between light and dark spaces. The new model allows for far more lights to be used and more organic lighting displays, with the hopes that it won’t have an adverse effect upon players using older PCs. Check out all the details if you want to have a bit more light shone on the particulars.
Source: Official Site