How to add Illumination (not Global Illumination) to composite?

[Daniel Bull]

I have rendered out a multipass from C4D, but I cannot figure out how to add the Illumination channel properly into the composite in Nuke. It’s a kind of greyscale version of the RGB, but it doesn’t seem to add properly using any blend mode. Does anyone know the key to adding the Illumination channel into a multipass composite?

Cheers