From LuxRender Wiki
Portals currently only works for infinite light, sun light and sky light.
Portals do not emit light and do not contain any visible geometry. A scene with and without portals should look identical, except that the one with portals renders faster.
To create a portal, just create a rectangle with its normal pointing inward in all window openings. Now you have to activate the checkbox "Exit Portal" in the mesh tab of the rectangle object (LuxBlend).
The process behind portals is quite complex (probability/sampling computations) and grows linearly with the number of portal shapes. This means that if a scene contains many windows, it may be more efficient to use a few big portals instead of individual portals for each window.
Adding portals to a scene may result in lower number of samples being calculated, but because the samples contribute more to the image the rendering process will be faster anyway.
Tip 1: If you have lots of windows and the space between the windows is small, it might be better to use one huge portal over all the windows.
Tip 2: The simpler the portal mesh is -- the better.
Tip 3: Use a portal to cover the visual part of the outdoor scene to get better lighting outdoor (counter effect is slow reduction in efficiency indoor).
How to use it in LuxRender scene file format