Managing Cycles Materials - Blender Cycles Materials and Textures - page 55

24. Set the Factor value for the Ambient Occlusion subpanel to 0.05 but let it remain
disabled. You can enable the Ambient Occlusion subpanel or not, depending on
your preferences, but remember that it adds light to the rendered image. I would
say that it's usually better not to have the Ambient Occlusion subpanel activated
by default but enabled only if really needed. In this case, the very low value can
compensate a bit for the darkened background of the World, which is shown in
the following screenshot for your reference:
Preparing the optional Ambient Occlusion setting
Optionally, other things that you can do include scaling the floor Plane bigger. In
the Outliner window, set the mode to Visible Layers and click on the arrow icon
to the side of the Plane item to make it nonselectable. Substitute Lamp, the
default Point item, with a different type (Sun or Spot) or with a mesh-light plane.