If you are a Revit lighting fixtures power user, you have possibly come across this issue before while rendering in Revit (if you haven't yet, thank us later...):
This happens when rendering a scene with some lighting fixtures that use .ies files. At it's highest level, the issue is that the Mental Ray rendering engine found in Revit does not fully support the .ies file format at this point in time.
In this specific case, that rendering glitch happens because the .ies file uses "absolute" lighting values. Here's how to fix it:
1) Open the .ies file in question (which on Windows 7 should have been saved in C:\ProgramData\Autodesk\RVT 2013\IES\) in Notepad.
2) Scroll down to find the line right after the last line of the "header" that starts with a keyword in brackets (ie.: [OTHER] or [LAMP], or [_INFOLINK]) and then go down one more line past "TILT=NONE". The following screenshot describes this better:
If that second number I have higlighted is set to "-1", then you are using "absolute" lighting values.
3) To fix the bug, change the -1 to whatever the true lumen output of your lighting fixture is.
4) Reload all of this in Revit and hit render. I think you will be pleasantly surprised with the results:
Now keep in mind that you have just messed around with an .ies file... I wouldn't trust our modification to hold up in a realistic lighting analysis environment (such as a point by point lighting analysis), but from a rendering perspective it works great.