Quote - You have Poser 8 and dependent parameters.

Connect the image to Ambient_Color. Make the Ambient_Value depend on your main light intensity. Done.

Not really done. This is another fudge, rather than an accurate solution. If I didn't care about the shadows, the backdrop would pick up all of the light colour emitted by lights in the scene, and would respond to the distance and angle from the source. Using your solution, I simply guess at the average ambient light across the entire surface and set that. It's not optically accurate.

But thanks for the suggestion. The whole point of this exercise was to get the actual light casting onto the background, but not shaows.

It occurs to me now, that the simplest way to do that, is simply to turn shadows off across the entuire scene in Poser's render settings - render that, then turn them on, and composite in Photoshop. Not ideal because I was hoping for a single rende solution, but more accurate than the other options,although I stil like the alternate diffuse channel solution a lot.

Thanks all.