I have a light bulb that I wanna use an emissive material on but the way UE5 works makes the material spill light onto the scene. is there a way to stop this from occurring?
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Another member pointed me to this thread, for those who might come across this in the future.
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Good looking out
This seems to be possible on a per-mesh basis by unchecking Affect Distance Field Lighting in the details panel (see Problem-Causing Meshes in Lumen Docs). The mesh with an emissive material will still be visible in the Screen Tracing reflections.
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Seems there is a very simple way: Just set the emissive material blend mode to “translucent”.
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bless you bro
Just a small note: Setting the material to translucent will not work if you are using TSR as your antialiasing method in 5.2 and below. In 5.3 and up it should work correctly, even with TSR.
Why does this work for translucent but not masked materials with all other settings the same? I have also tried turning off TSR. I am using 5.5.
Lumen’s screen traces use the scene depth, but this is a problem for translucent materials because they do not write depth and so Lumen has no way of knowing how far away the emissive translucent surface is. In earlier versions of Unreal 5 this resulted in lighting from emissive translucency being projected infinitely far into the distance no matter how close the lightsource was.
To fix this, Lumen’s screen tracing source was changed to use scene color without translucency by default. As a result, Lumen will no longer pick up lighting from emissive translucency (unless you change the screen tracing source back)
This was never an issue for masked materials, because they do write to scene depth and so Lumen doesn’t ignore these.
Thank you for the detailed explanation. My goal however, is to have a masked material with emissive not contribute to lighting. In the mesh’s settings I’ve turned off “Affect Distance Field Lighting” and in the material settings I’ve turned off “Emissive (Dynamic Area Light” but the material is still spilling light into my scene. Is it not possible to turn off this feature?
The reason this doesn’t work is because Lumen’s screen traces will pick up anything on screen, even if the mesh is set to not affect distance field lighting. That toggle only works as long as the mesh is not on screen. As noted previously, the only way to avoid this is by using something that Lumen skips screen traces for like translucency.
You can either turn off screen traces for Lumen or set the material to use one of the shading models that skips screen traces (two-sided foliage/hair/subsurface scattering). This may change the lighting response though so you’ll have to weigh that. It should be noted that this will also remove the light from reflections.
Disabling screen traces will reduce the quality of the GI (and potentially increase cost when using HW raytracing), but if you can’t change the blend mode, and you can’t change the shading model… well…
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Yes, emissive materials do not cast real light by default in — they just appear bright. But they can affect the scene lighting indirectly in two main ways:
1. Static Lighting (Baked GI)
If you’re using static or stationary lights with Lightmass (baked lighting):
- Emissive materials can cast light during the bake as emissive GI.
- To stop this:
Disable Emissive Lighting Contribution:
- Open the Material.
- Set “Use Emissive for Static Lighting” to unchecked.
OR
- Go to the mesh’s Lightmass settings and reduce:
- Emissive Boost to 0
- Diffuse Boost to 1 or lower
Thank you for the explanation.