Lighting and Lumen setup
Why this matters
Lumen replaced the old choice between fully baked lighting (fast, static, breaks the moment something moves) and fully dynamic ray-traced lighting (correct, expensive) with a single dynamic global illumination and reflections system that's meant to just work. It doesn't fully "just work" for free, though — the choice between hardware and software ray tracing, and a handful of quality knobs on the Post Process Volume, are the difference between a scene that holds 60fps and one that doesn't, and between reflections that look right and reflections that swim as the camera moves. Get the base setup wrong (no Sky Light, a Stationary Directional Light instead of Movable) and Lumen silently falls back to worse-looking defaults instead of failing loudly.
Mental model
Lumen isn't a light — it's a lighting solver that consumes the scene you already built (meshes, materials, lights) and traces against a representation of it to compute bounced light and reflections each frame. Which representation it traces against is the whole configuration story: real triangles via hardware ray tracing (accurate, expensive to keep updated as things move), or Distance Fields via software ray tracing (cheaper, coarser). Everything else — Surface Cache vs Hit Lighting, Screen Traces, quality sliders — is tuning how much of that trace's cost you're willing to pay in exchange for correctness, on a per-project or per-area basis via the Post Process Volume.
The mechanics
Base scene setup
Lumen needs a light to bounce and a sky to gather ambient contribution from:
- Directional Light — set to Movable. Lumen's dynamic GI expects a fully dynamic key light; a Stationary or Static Directional Light doesn't participate in Lumen the same way.
- Sky Light — provides the ambient/sky contribution Lumen gathers into scenes; use Real Time Capture for a sky that changes (time of day, dynamic weather) or a captured cubemap for a static one.
- Post Process Volume — Lumen's per-view quality settings (reflection quality, ray lighting mode, ambient occlusion) live here, either on an unbound volume for a global default or a bounded one for a specific area's override.
Hardware vs. software ray tracing
This is the single biggest Lumen decision, set in Project Settings → Engine → Rendering:
| Setting | What it does | Cost profile |
|---|---|---|
| Dynamic Global Illumination Method / Reflection Method | Selects Lumen as the GI/reflection method | — |
| Use Hardware Ray Tracing when available | Traces against actual scene geometry via RT hardware, falling back to software tracing when unavailable | Higher fidelity; scene update cost can be significant above roughly 100,000 instances |
| Ray Lighting Mode | Surface Cache (cheap, reuses cached lighting) vs Hit Lighting for Reflections (evaluates lighting at the actual ray hit, higher quality, higher cost) | Surface Cache for most content; Hit Lighting where mirror-quality reflections matter |
Software Ray Tracing (used when hardware RT is unavailable or disabled) traces against Distance Fields instead of real geometry, with two tiers: Detail Tracing against per-mesh Distance Fields for higher quality close up, and Global Tracing against a lower-detail Global Distance Field for cheaper, longer-range tracing. If your project has hardware ray tracing available and enabled, you generally don't want both systems paying their scene-update cost simultaneously — disable the software fallback's distance field support explicitly:
[/Script/Engine.RendererSettings]
r.DistanceFields.SupportEvenIfHardwareRayTracingSupported=0
Post Process Volume quality knobs
Per-view overrides that matter most for Lumen reflections specifically:
- Quality — trades noise/detail for GPU cost; raise it for hero areas, leave default elsewhere.
- Ray Lighting Mode (per-view override of the project default) — set to Hit Lighting for Reflections on a bounded volume around, say, a mirror-heavy room, without paying that cost everywhere.
- Screen Traces — uses scene depth/color for cheap, high-quality traces where available, but bypasses the Lumen Scene, meaning Lumen-Scene-only contributions (some emissive setups) won't show up in screen-traced results. This is a correctness tradeoff, not just a quality slider.
- High Quality Translucency Reflections — improves mirror-like reflections on translucent surfaces at extra GPU cost; enable selectively rather than project-wide.
Choosing per area, not per project
Because Ray Lighting Mode and reflection quality can be set on a bounded Post Process Volume, the practical pattern is: cheap Surface Cache lighting and default quality as the project-wide baseline, with bounded volumes raising quality (or switching to Hit Lighting) only around the specific rooms or set pieces where it's visually load-bearing — a mirror, a highly reflective floor, a cutscene camera angle.
Gotchas
If your key light isn't Movable, Lumen doesn't error — it just doesn't get the dynamic GI behavior you're expecting from it. This is one of the most common "why does my scene look flat/wrong compared to the Lumen demo" issues and it's a light property, not a Lumen setting.
Above roughly 100,000 instances, hardware ray tracing's scene update cost becomes significant regardless of how simple each instance is. Dense instanced foliage or PCG-scattered content is exactly the case where this bites — profile HWRT scene update cost on your actual dense scenes, not a test level.
Because screen traces bypass the Lumen Scene, effects that only exist there (certain emissive contributions to reflections/GI) can be missing from a screen-traced result even though the setting is purely a quality knob on paper. If something is invisible in reflections specifically, check whether Screen Traces is masking a Lumen Scene contribution rather than assuming the emissive setup is broken.
See also
- World Partition — how large streamed worlds interact with Lumen's scene update cost as content streams in and out.
- Landscape and foliage — dense foliage as a driver of Lumen/ray-tracing instance-count cost.
- Streaming and budgets — frame budget context for where Lumen quality tradeoffs fit.
- Epic — Lumen Global Illumination and Reflections