Quote - ok, being a total newbie with PP2012, if i make the dome invisible to camera and raytracing but leave the light emitter box ticked, does Firefly still use the dome and its image in IDL calculation? 

ie i need the light effect, but want to overlay the render onto a BG of my choosing.

 

You need to have raytracing enabled for IDL calculation. So set invisible to camera and have the Emit light box ticked. As a texture you can use a texture or material for the environment dome and you can make it grayscale to eliminate color influence