How can i fix this reflection in LUMEN ??

Unfortunately? You cannot at the moment, I’m sorry to say.

I wish the forums had an ‘easy answers’ section as I have found myself answering this a couple times, but the essential behavior is this: Lumen reflections only handle the first bounce of reflections dynamically (minus screen-traces), and after the first bounce, you’ll be shown the lumen scene, a low-resolution version of the scene used to speed up ray-tracing evaluation.

Reflections have the option of evoking hit lighting, which means that anything in direct lighting will have accurate RT lighting and shadowing. That’s why the desk and the walls lit up by sunlight look so good, but the areas that lack direct lighting recieve that low-resolution cache.

There’s no way to resolve it short of adding enough dynamic lights to your scene such that there is no area lit by purely indirect light, but at that point a real-time GI solution is pointless. If this is for archvis, I’d advise using GPU lightmass and standalone RT reflections, which support multiple bounces and interact with lightmass in a more-or-less correct way. Best of luck to you.

Hi jblackwell … Thanks for the “bad news” … I Hope they fix this soon … It’s a very important point for us (Archviz Artist’s) … Thank you !

I am happy to. To be clear, you can’t resolve it, but you can make it better by increasing the ‘lumen scene detail’ and ‘lumen scene lighting quality’ in the post-process volume. If you crank those values to the max and add a slight increase in roughness for your mirror (pure specularity is nonphysical anyways), you might be able to get away with it.

Worst comes to worse, the highest real-time quality is GPU lightmass+RT shadows (for now), and the path-tracer for stills. I understand if that’s not available for your workflow, but I figured you should know the options.