Huge noise in a night scene

[Levonto]

Hi team!

I've just faced an unexpected problem with lights in a night scene. All the light sources in the LightMix behave totally fine exept one layer with celling lights, which gives me lots of noise inside and outside the building. There is about 7 hours of rendering on 9700k, and the result is still pretty disappointing. What can be the cause?

AA is 16
Light sources on the problem layer are instanced corona spheric lights, visible only in reflections, other lights occlusion is turned off.
The total number of lights in the scene is under 250 (about 210)
The brightness multiplier of the layer in LightMix is about 35

And i have no idea. Thanks!

[TomG]

Is denoising on for that LightSelect layer?

35 is actually rather high for an amount in LightMix - better to raise the intensity of the light sources, and use less of a value shift in the LightMix (I'd say 10 times the brightness in LightMix is roughly a max to aim for).

Are you sure it's noise, and not just accurate reflections in the grass (if that's what you are referring to as noise)? If your grass has a high glossiness, high reflectivity, it is quite likely to (correctly) reflect lots of little points of light. Things such as lower glossiness, lower reflectivity, realistic curved geometry for the grass, anisotropy, sharpen and blur, bloom and glare, could help.

[Levonto]

Thanks for answering.

Well, even inside the building there were about 500 passes on the wooden surfaces, and it's still noisy. Yes, the grass isn't perfect here, you're right. But the wood renders well in a daytime, probably 3 times faster.
Thanks for the advice about LightMix multiplier. I will try to increase the power of lights instead of it. Maybe it could help.

Thanks again!