Lumen Light Update Speed

Using UE5.0.3, I already wondered how one can create fast light effects - as Lumen relies on caching and accumulating over multiple frames.
Doing a test shows, that a simple point light takes around a second to really switch off completely - that’s too long for effects like a switched off flash light, or lightning flicker effects as found in many environments.

The only settings I can find are those in the Post Process Volume: Lumen Scene Lighting Update Speed and Final Gather Lighting Update Speed. as shown here:
Lumen Global Illumination and Reflections in Unreal Engine | Unreal Engine 5.0 Documentation

Unfortunately, these do not have a noticeable effect, even when setting to really high values.

So, requiring these effects in a game looks like a no go decision for Lumen?

Thanks for some insights.

You can try using r.Lumen.RadianceCache.ForceFullUpdate 1 to flush the current lighting when a light is turned on/off.

1 Like

Thank you @ZacD for your answer. I did give this a test, but unfortunately it does not change anything.

On the other hand r.LumenScene.Lighting.ForceLightingUpdate 1 seems to work fine, but I’m not sure if it’s a one time event.