This has probably been requested before, but a simple override checkbox in a CoronaLight and CoronaLightMtl for "Visibility ignores directionality", to prevent lights rendering grey/black. Yes, it's physically incorrect, but it's needed in real-world projects. There's no such thing as a grey or black light source but then there's also no such thing as a virtual flat disk/rect/sphere light either, so this one's all about client expectations and realism ;)
