benjaminriendeau , you're using line lights in your project, correct? Because with line lights this behavior is normal - could you kindly try to use emissive materials instead to see if you achieve better results through this? It should give you a more even output.
Thank you in advance. 
If you like, you can of course also gladly send me the project and I'll have a look at it myself.
benjaminriendeau , pardon, in this case, could you kindly send me the project so that I can have a look at it? It's a bit hard to see what's going on exactly here. Also, have you in this case tried to use our line lights instead? They can sometimes show as two light sources practically instead of one, but mostly if they're rather long, so please kindly try that as well. Finally, does this current behavior improve when you move closer to the source?
Demian Gutberlet it improves when I widen my emissive surface
Glad to hear that. Let me know in case you still run into any further problems!
...it improves when I widen my emissive surface
Hi - could you please share an SU model screenshot to give me / us a visual impression of how you have extended this emissive surface? thanks
benjaminriendeau That makes sense.