Quick answer:
- In Settings > Engine Scalability Settings, set Global Illumination to High or Epic. Lumen is off at Low and Medium.
- Add a Post Process Volume, tick Infinite Extent (Unbound), then under Global Illumination > Lumen Global Illumination raise Final Gather Quality to 2 and Lumen Scene Detail to 2.
- Light leaking through walls? Make walls at least 10 cm thick, and don’t use single-sided planes for walls.
- Noise near small glowing objects? Light them with a real light instead of a bright emissive material.
Why Lumen gets noisy
Lumen traces a limited number of rays per pixel and smooths the result over several frames. When few rays find light (dark interiors, small bright sources), or when the meshes are hard for Lumen to see (very thin, missing distance fields), you get noise, blotches or flickering. It’s normal for the light to update over a fraction of a second.
Step 1: Check what Lumen sees
- In the viewport, open the Lit menu and choose Lumen > Lumen Scene.
- Your level should look like a simplified, coloured version of itself. Meshes that are missing, pink or flat in this view are where noise and leaks come from.
- For software Lumen, also check Show > Visualize > Mesh DistanceFields.
Software Lumen needs Generate Mesh Distance Fields, which is on by default, in Project Settings > Engine > Rendering. For a mesh with a blobby or missing distance field, open it and raise Distance Field Resolution Scale in Build Settings.
Step 2: Raise quality in a Post Process Volume
In Global Illumination > Lumen Global Illumination:
| Setting | What it helps | Try |
|---|---|---|
| Final Gather Quality | Noise and splotches in bounce light | 2 to 4 |
| Lumen Scene Detail | Small objects missing from GI | 2 |
| Lumen Scene Lighting Quality | Blotchy lighting on surfaces | 2 |
| Final Gather Lighting Update Speed | Lighting lags behind moving lights and objects | 2 |
| Lumen Scene Lighting Update Speed | Slow response to light changes | 2 |
Each one costs GPU time. Raise one at a time and keep the ones that make a visible difference.
Step 3: Stop light leaks
- Walls, floors and ceilings should be at least about 10 cm thick. Single planes leak.
- Close gaps at corners where walls meet.
- Thin meshes and complex interiors work better with hardware ray tracing (Step 4).
Step 4: Try hardware ray tracing
On a GPU with ray tracing support, enable Support Hardware Ray Tracing in Project Settings > Rendering and Use Hardware Ray Tracing when available under Lumen. Restart the editor. It handles thin objects and reflections more accurately, but costs more GPU time and needs DirectX 12.
Step 5: Emissive materials
Lumen picks up emissive surfaces, but small, very bright ones produce noisy spots and flicker. Use emissive for the glow you see, and add a point light or rect light for the light it casts.
FAQ
Why does my interior look darker with Lumen than with baked lighting?
Lumen limits how far rays travel and how much skylight gets inside. Add more light sources, or raise Skylight Leaking slightly in the same Post Process Volume section.
Why does Lumen flicker at the start of a Movie Render Queue render?
Lumen needs a few frames to settle. Raise Engine Warm Up Count in your render settings.
Why are reflections noisy?
Rough and medium-rough materials use Lumen’s traced reflections. Raise Quality under Lumen Reflections in the Post Process Volume, or lower the material roughness if the look allows it.