Emissive materials

An emissive material gives off light of its own: a screen, a neon tube, a warning lamp, glowing lava, a sci-fi panel. glTF describes the emissive parameter as controlling "the color and intensity of the light being emitted by the material" (Khronos Group: glTF 2.0 Specification). It does not depend on the lights in the scene. glTF's own illustration of what is lost when a viewer ignores emission: "the headlights of a car model will be off instead of on" (Khronos Group: glTF 2.0 Specification).

Whether that glow also lights the world around it depends on the engine and its lighting setup. This decides whether a sign only looks lit or also lights the street.

#HDR values and bloom

Emissive values are not limited to 0–1. In Unreal, "inputting values higher than 1.0 into the Emissive Color input" pushes the material "into the HDR range, producing a bloom effect" (Unreal Engine documentation: Using the Emissive Material Input). Bloom is a post-process that spreads very bright pixels into a soft halo; see exposure and post-processing.

The usual setup is a texture or colour for the pattern and a scalar for the strength. Epic's own examples multiply colour parameters against a texture mask and expose colour and intensity as parameters for material instances (Unreal Engine documentation: Using the Emissive Material Input). The emissive texture itself is a colour map: glTF requires it to be sRGB encoded (Khronos Group: glTF 2.0 Specification). See colour space: sRGB and linear.

ExampleA neon sign in three numbers

A pink neon sign uses a black-and-white mask texture of the letters, a colour parameter of (1.0, 0.2, 0.6) and an intensity scalar. The values below are illustrative for a night scene; yours will depend on exposure.

Intensity 1: the letters are flat pink. Without values above 1 there is nothing for bloom to pick up (Unreal Engine documentation: Using the Emissive Material Input).
Intensity 20: the letters clip to near-white in the centre with a pink halo from bloom. This reads as neon.
Intensity 200: bloom swamps the wall, and if the scene has auto-exposure, the camera darkens everything else to compensate.

The right value depends on the scene's exposure, not on the material alone. A sign tuned in a dark alley will look dull in daylight, because the camera exposes for the sun.

#Physical units

Some engines let you set emissive brightness in real units, so it sits correctly next to physically based lights (see physical light units).

glTF: the product of emissive texture and factor is in "candela per square meter (cd/m²), sometimes called nits" (Khronos Group: glTF 2.0 Specification). It adds that some renderers simply treat an emissive factor of 1.0 as "a fully exposed pixel", which "may not match the authoring intent" (Khronos Group: glTF 2.0 Specification).
Unity HDRP: emission intensity can be set in Nits or EV100, and Exposure Weight controls "how much effect the exposure has on the emission power" (High Definition Render Pipeline manual 17.0: Lit material Inspector reference). Lowering exposure weight is how you keep a UI-like screen readable regardless of the camera's exposure.
Godot: with physical light units enabled, Emission Intensity is the "Luminance of emitted light, measured in nits", with a default "roughly equivalent to an indoor lightbulb" (Godot Engine: BaseMaterial3D class reference).

#Emissive surfaces as light sources

Engines differ most on this point.

Unreal with Lumen: "light from emissive Materials propagates through the scene automatically with no additional performance cost" (Unreal Engine documentation: Using the Emissive Material Input). But Lumen warns that "there's a limit to how small and bright emissive areas can be before they begin to cause noise artifacts", and that emissive light "is inherently much more difficult to solve than manually placed light sources" (Unreal Engine documentation: Lumen Global Illumination and Reflections).
Unreal with baked lighting: emissive surfaces only cast light into the lightmaps if you tick Use Emissive for Static Lighting; without it they "do not illuminate any of the objects around it" (Unreal Engine documentation: Using the Emissive Material Input). GPU Lightmass picks up emissive light automatically (Unreal Engine documentation: Using the Emissive Material Input).
Unity HDRP: a per-material toggle "Toggles whether emission affects global illumination" (High Definition Render Pipeline manual 17.0: Lit material Inspector reference).
Godot: emissive objects can light others "if a VoxelGI, SDFGI, or LightmapGI is used and this object is used in baked lighting" (Godot Engine: BaseMaterial3D class reference).

See global illumination for how each GI system gathers this light.

Different viewsEmissive as the light source, or real lights as well?

Let the emissive do it. With Lumen, emissive light propagates "with no additional performance cost" (Unreal Engine documentation: Using the Emissive Material Input), and large glowing surfaces such as screens, light panels and lava light their surroundings. That keeps the lighting tied to what the player sees.

Back it up with placed lights. Epic's own docs warn that small, bright emissive areas cause noise and are harder to solve than placed lights (Unreal Engine documentation: Lumen Global Illumination and Reflections; Unreal Engine documentation: Using the Emissive Material Input). Without dynamic GI or baked emissive, the glow lights nothing at all (Unreal Engine documentation: Using the Emissive Material Input). In an 80 Level breakdown of a UE5 cyberpunk scene, level designer and environment artist Jhosep Chevarria Capacoila paired the two: "The big neon lights complimented the rect lights", while "Lumen helped with the billboards and small elements" (80 Level: Setting Up Cyberpunk Lighting in Unreal…).

In practice: let large, moderately bright emissive surfaces contribute through GI, and place a real light (often a rect light matching the emissive shape) wherever a small, intense source must light the space or cast shadows. Treat bloom as the look of the source, and placed lights as the light it gives.

#In different tools

Unreal Engine: Emissive Color input on the material; values above 1 for bloom (Unreal Engine documentation: Using the Emissive Material Input); Use Emissive for Static Lighting for lightmaps (Unreal Engine documentation: Using the Emissive Material Input).
Unity HDRP: Emission Map and colour, intensity in Nits or EV100, Exposure Weight and the GI toggle (High Definition Render Pipeline manual 17.0: Lit material Inspector reference).
Godot: Emission, Emission Energy Multiplier, an add or multiply operator for the emission texture, and an option to read it from UV2 (Godot Engine: BaseMaterial3D class reference).
glTF: emissiveFactor and emissiveTexture; the default factor is zero, so the factor must be set for the texture to show (Khronos Group: glTF 2.0 Specification).

#Further reading and watching

Epic: Using the Emissive Material Input

bloom, Lumen and static lighting with emissive materials. Beginner, free.

DocsBeginnerFree

Epic: Lumen Global Illumination and Reflections

what Lumen can and cannot do with emissive light. Intermediate, free.

DocsIntermediateFree

80 Level: Setting Up Cyberpunk Lighting in Unreal Engine 5

a breakdown that combines neon emissives with rect lights. Intermediate, free.

ArticleIntermediateFree

glTF 2.0 Specification

the emissive section defines units and colour space for exchange files. Intermediate, free.

WebIntermediateFree