Quick answer: Lumen does light the scene from emissive surfaces, but only if the surface is bright enough after exposure, is opaque or masked, and is present in the Lumen Scene. Small or thin emissive meshes are dropped from the Lumen Scene, so their light flickers or vanishes when they leave the screen. Raise emissive intensity, raise Lumen Scene Detail, check Visualize > Lumen Scene, and use a real light for anything gameplay depends on.
One of the selling points of Lumen is that an emissive panel can light a room with no light actor. In practice many developers see a glowing mesh that lights nothing, or light that pulses and disappears as the camera turns. Both come from how Lumen sees emissive surfaces.
How Lumen sees emissive surfaces
Lumen gathers indirect light by tracing rays. Rays that hit the screen use screen traces; rays that miss the screen hit the Lumen Scene, a simplified representation built from mesh distance fields and surface cards. An emissive surface contributes light wherever those rays find it. If the mesh is not in the Lumen Scene, its light only exists while it is on screen, which is the flicker people describe.
Emissive surfaces contribute to diffuse global illumination. They do not cast shadows of their own, and they are noisier than real lights, because Lumen has to find them by tracing rather than sampling them directly.
Fix 1: make it bright enough
An emissive value of 1 is about as bright as a white surface lit normally, which is not enough to light anything. Multiply your emissive color by a scalar parameter and try values from 10 to 50 or more, depending on your exposure settings. Use a fixed exposure while tuning (in the Post Process Volume, set Exposure > Min EV100 and Max EV100 to the same value) so auto exposure does not hide the change.
Fix 2: make sure Lumen can see the mesh
Use Show > Visualize > Lumen Scene in the viewport. If your emissive mesh is missing, dark or blocky there, Lumen cannot see it off screen. Then:
- Check Project Settings > Rendering > Software Ray Tracing > Generate Mesh Distance Fields is on (the default), and the mesh has a distance field.
- Raise Lumen Scene Detail in the Post Process Volume's Lumen Global Illumination section. Higher values keep smaller objects in the Lumen Scene.
- Very thin emissive planes (a strip light, a screen) are the worst case. Give the mesh some thickness, or increase its distance field resolution scale in the Static Mesh Editor.
- On the component, leave Affect Dynamic Indirect Lighting enabled.
Fix 3: check the material type
- Translucent materials are not part of the Lumen Scene, so translucent emissive surfaces do not light anything. Use Opaque or Masked for light-emitting surfaces.
- Unlit opaque materials with emissive do contribute; you do not need a lit material.
- Emissive (Static Lighting) options on the mesh are for baked lighting with Lightmass. They do nothing for Lumen.
Fix 4: reduce noise and flicker
Small, bright emissive surfaces are hard for any tracer to sample. If the light is present but noisy:
- Make the emissive surface larger and less intense rather than tiny and extreme.
- Raise Final Gather Quality in the Post Process Volume.
- With hardware ray tracing enabled, Lumen's traces find emissive geometry more reliably than with software tracing.
When to use a real light instead
For anything gameplay depends on, such as a lamp that should cast a shadow or a light that has to read the same on every scalability level, place a point, spot or rect light and let the emissive mesh provide the visible glow. Lumen GI is disabled at lower scalability levels, so emissive-only lighting disappears completely for players on Medium or Low.
Lighting that differs on players’ machines
“It is pitch black in the second room” is the kind of report that only makes sense once you know the player's settings. Bugnet's Unreal SDK lets players file reports from your UMG menu with a screenshot and their GPU, OS and RAM attached, so a scalability-dependent lighting problem shows up with the evidence instead of a vague complaint.
Related problems
- Fix: Unreal Lumen Lighting Flickering and Artifacts
- How to Fix Lumen Looking Too Dark or Noisy in Unreal Engine 5
- How to Fix Objects Missing or Low-Detail in the Lumen Scene in Unreal
- Fix: Unreal Lumen Not Working in a Packaged Build
Frequently asked questions
Do emissive materials cast light with Lumen?
Yes. Lumen includes emissive surfaces in global illumination, as long as they are bright enough, opaque or masked, and present in the Lumen Scene. They do not cast shadows.
Why does my emissive light disappear when I look away?
The mesh is missing from the Lumen Scene, so its light only comes from screen traces while it is on screen. Raise Lumen Scene Detail and check Visualize, Lumen Scene.
Why don’t translucent emissive materials light anything in Lumen?
Translucent surfaces are not part of the Lumen Scene. Use an opaque or masked material for surfaces that should emit light.
Emissive is for glow and fill. If the player needs the light, place a light.