Help with Lighting effect and layering

[Blazer07070]

Hi, I need help with lighting effect and layering. I've just rendered my 3D scene and I want to put lighting inside the street lamp but do not want the lighting to affect my scene as I've already got lighting going on in the background prior to rendering my scene.
I've fooled around with layering, but I am missing a step. If I create a layer for the just the head of the lamp and put lighting inside, well it changes the color of the background as well so that when I put it all together it doesn't look right. Can someone please tell me what I am missing? Is it masking? Thanks much, I appreciate any help you can offer.

I've attached a copy of my scene.

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