Managing Emissive Materials in Lumen

Configure emissive materials to act as authentic light sources that contribute significantly to the scene's global illumination.

In this lesson, you will learn how to configure emissive materials so they function as dynamic light sources that contribute directly to your scene's Global Illumination (GI) via Lumen.

Emissive Lighting in Lumen

By default, emissive surfaces in Unreal Engine provide visual brightness, but they don't necessarily "light up" the surrounding geometry. To make an emissive material act as an authentic light source in Lumen, you must enable "Use Emissive for Static Lighting" (or simply rely on the Lumen scene update) and ensure your material’s intensity is high enough to register in the Lumen Surface Cache.

Configuring the Material

To convert an emissive surface into a functional light, you should multiply your base color by a scalar parameter. This allows you to push the intensity value well beyond 1.0 (HDR values).

// Inside your Material Graph:
// Multiply your Emissive Color by a High-Intensity Scalar Parameter
// To effectively light a room, intensities often range from 10 to 100+
Emissive_Output = BaseColor * EmissiveIntensityScalar;

Bridging the Surface Cache

Lumen relies on the Surface Cache to calculate indirect lighting. If your emissive mesh is too small or thin, it may not register correctly in the cache, resulting in no light bleed.

Increase Intensity: Because emissive lighting is calculated based on the energy hitting nearby surfaces, low-intensity emissive materials will be ignored by the Lumen solver to save performance.
Enable "Two-Sided": If your mesh is a single-sided plane, the light will only emit from the face with the normal. Tick Two-Sided in the Material Details panel if you need the light to cast from both sides.
Check Lumen Scene View: Use Show > Visualize > Lumen Scene in the viewport. If your emissive mesh does not appear as a bright object in this view, Lumen will not treat it as a light source. You may need to increase the Lumen Scene Lighting Quality in your Post Process Volume or check if your mesh is set to Visible in Ray Tracing (which, under the hood, helps Lumen capture the surface).

Key Takeaway: Emissive materials become true light sources when you use high HDR intensity values and verify their presence within the Lumen Surface Cache visualization.