Perhaps you have noticed that in Autodesk Revit Architecture 2013 with the Graphic Display Options popup (from the Visual Style icon) the Ray Trace option is grayed out with a 3D model loaded like this:
For an explanation, see the Known Issues PDF located here.
Here is the important context from the PDF.
Ray Trace Rendering functionality is disabled in 32-bit versions of Revit due to performance degradations. To use Ray Trace Rendering functionality, you need to install Revit on a 64-bit operating system.