Lumen
Lumen is UE 5's dynamic global illumination and reflections system. Replaces baked Lightmass + Screen-Space Reflections for most projects.
Two backends
| Backend |
Strengths |
Weaknesses |
| Software RT (signed-distance-field tracing) |
Works on any DX12 GPU; cheaper |
Lower-detail GI, simplified mesh representation |
| Hardware RT (DXR / VRT) |
Higher fidelity; mirror-perfect reflections; supports translucency |
Needs RT-capable GPU; more expensive |
Toggle with r.Lumen.HardwareRayTracing 0|1.
What it does in one pass
- Screen Traces first (cheap)
- Falls back to Surface Cache sampling for off-screen GI (SW path) or mesh BVH rays (HW path)
- Reflections use the same machinery — Lumen Reflections are coherent with GI
- Final composite uses TSR to denoise and stabilize
Quality knobs (CVars)
| CVar |
Effect |
r.Lumen.DiffuseIndirect.Allow 1 |
Master switch |
r.LumenScene.SurfaceCache.AtlasSize 8192 |
Surface cache resolution |
r.Lumen.ScreenProbeGather.RadianceCache 1 |
Far-field caching |
r.Lumen.Reflections.Allow 1 |
Reflections master |
r.Lumen.Reflections.MaxRoughnessToTrace 0.4 |
Limit reflection rays to glossy surfaces |
r.Lumen.HardwareRayTracing.LightingMode 2 |
0=SurfaceCache 1=Hit 2=Hit+SurfaceCache |
When Lumen struggles
- Highly mirror-reflective surfaces in software mode — switch to HW RT for clean mirror reflections.
- Indoor scenes with small bright lights — increase
r.Lumen.ScreenProbeGather.AdaptiveProbeMinDownsampleFactor.
- Small thin geometry — Surface Cache can't represent it; bake static lighting for that region or use HW RT.
- Translucent GI — HW RT only.
Lumen vs static lighting
| Concern |
Lumen |
Lightmass (baked) |
| Dynamic time-of-day |
★ |
No |
| Build times |
None |
Hours |
| Storage |
Negligible |
Lightmaps can be GB |
| Quality of bounce |
Good |
Reference |
| Mobile / low-end |
No |
Yes |
Debugging
r.Lumen.Visualize 1
Show → Advanced → Lumen Scene
stat Lumen
ProfileGPU (capture one frame, see Lumen passes)
Links