i'm going to assume you're just using Final Gather with that dome?
first make sure you disabled the Default Light in Render Globals (in the Common Tab).
if that doesn't solve what you need, try the following stuff:
just reduce the incadescence and irradiance of the dome material and you'll get a darker render. the higher those are, the brighter the render will be.
when i do renders like that i usually use the Ambient Color instead of incadescence and irradiance. its a lot simpler if you ask me.
the size of your dome will kinda determine the value of ambient to use. if you're dome is pretty close to the model, you won't have to use as high a value, but if its a very large dome you may have to increase the ambient.
hope that helps ya out
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