Use Emissive for Static Lighting
Where: select a mesh component and search Use Emissive for Static Lighting under Lighting > Lightmass Settings in Details. In a Blueprint, select the mesh component rather than the Blueprint root.
An emissive material can look bright without illuminating anything around it. Its visible glow and the light stored in a bake are different things. Bloom is a screen effect around bright pixels, not proof that nearby surfaces or probes received light.
What the checkbox means
In Unreal's CPU Lightmass workflow, this checkbox allows a mesh's material emission to contribute to static lighting. The material needs a nonzero Emissive Color; the checkbox does not make a non-emissive surface emit light.
GPU baking differs. Epic explicitly notes that this checkbox is not required by standard GPU Lightmass in its emissive material guide.
How Spectral handles emission
Spectral's GPU bake evaluates material emission and applies the mesh's Emissive Boost. Unlike the CPU Lightmass workflow, this checkbox does not switch that emission on or off. Do not rely on unchecking it to stop a mesh from emitting into the Spectral bake.
To keep a sign visibly bright but prevent its baked emission, set that mesh component's Emissive Boost to 0, then rebake. To use it as an emitter, start with a boost of 1 and adjust the material's emission and the size of the emitting surface.
Emission can contribute to baked lightmaps and probe lighting. Probe-lit fog can then use the recorded lighting too. This does not create a movable Point Light or make emission update dynamically after the bake.
Practical use
A broad luminous ceiling panel is a useful emitter. A tiny, extremely bright strip is harder to sample cleanly and can produce noisy or uneven baked illumination. First try a larger emitting area and reasonable brightness, then increase bake quality if needed.
If a sign changes color or switches off during gameplay, its baked light remains as it was when baked. Use a suitable runtime light for illumination that must follow those changes.
When to rebake
Rebake after changing the material's baked emission, the emitter's shape or placement, or its Emissive Boost. Save the material before starting the bake. Changing bloom alone does not require a bake and does not alter the lighting stored in probes or lightmaps.