@ghostship

The Lightpath logic forces the render engine to use the diffuse component rather than the emitter when rendering the background. This allows the a certain amount of seperation between the background intensity and the emitter intensity. This is useful if the emitter intensity needs to be increased but you don't want the background to blow out.

It is possible to get noise when using high contrast HDR maps. If using the exact settings above the Light Control contrast would be best set to zero.