Understanding Unreal Engine 5’s lighting architecture: DirectionalLight, SkyLight, SkyAtmosphere, Lumen, Volumetric Fog, and Exponential Height Fog in sunset and night environments.
This project is a lighting study built in Unreal Engine 5, developed following this Udemy course. Rather than building gameplay systems, the entire focus is on understanding how Unreal’s lighting components work individually and how they compose into a coherent lighting rig. The exercise takes existing maps from the Unreal Marketplace — environments with placeholder or default lighting — and relights them in two distinct conditions: sunset and night. Both are technically demanding scenarios that stress different parts of the lighting system.
You can watch the demo here: YouTube
Unreal’s Lighting Architecture: A System of Complementary Components
A common misconception about Unreal’s lighting is that it’s controlled by a single system. In practice, believable outdoor lighting in UE5 requires at least six distinct components working together, each responsible for a different aspect of the illumination. Understanding what each one does — and what breaks when it’s missing or misconfigured — is the core lesson of this project.
Directional Light: The Sun
The DirectionalLight is the primary light source in any outdoor scene — it simulates the sun or moon, casting parallel rays from a fixed direction across the entire scene. Its rotation determines the time of day: pointing nearly horizontal produces sunrise or sunset light with long shadows; pointing straight down produces midday light with short shadows; pointing slightly upward below the horizon produces moonlight for night scenes.
For sunset, the Directional Light’s color shifts toward warm orange — physically, this is the effect of sunlight passing through more atmosphere at low angles, scattering the shorter blue wavelengths and leaving the longer red and orange ones. In Unreal, this color shift is partly automatic when using SkyAtmosphere (the atmosphere simulates the scattering) and partly manual (the light color can be adjusted to enhance or override the atmospheric result).
The Directional Light also carries the primary shadow budget — it casts the scene’s dominant shadows. Shadow quality, distance, and cascade count are all configured on this actor. For dynamic lighting with Lumen, the Directional Light works in conjunction with Lumen’s global illumination rather than baking shadows.
SkyAtmosphere: Physically Based Sky Color
The SkyAtmosphere component simulates Rayleigh and Mie scattering — the physical processes that make the sky blue during the day, orange at sunset, and dark at night. It’s driven by the Directional Light’s rotation: as the sun approaches the horizon, the atmosphere automatically computes the correct color gradient from zenith to horizon.
Without SkyAtmosphere, the sky is a flat color or a static texture. With it, the sky is a physically computed gradient that changes continuously with the sun angle. For sunset and night lighting, this component is responsible for the characteristic color banding — deep blue at the zenith transitioning to warm orange near the horizon at sunset, deep navy to near-black at night.
SkyAtmosphere also contributes aerial perspective — the haze that makes distant objects appear lighter and more blue-shifted than nearby ones. This depth cue is subtle but significant for large outdoor environments.
SkyLight: Ambient Illumination from the Sky
The SkyLight captures the current sky color and uses it as ambient light for the scene. Where the Directional Light provides direct illumination from one direction, the SkyLight provides the diffuse ambient fill that simulates light arriving from the entire sky hemisphere.
For sunset, the SkyLight captures the warm orange tones of the low sun sky, tinting shadows with the complementary cool-warm contrast that makes sunset scenes visually striking. For night, the SkyLight captures the dark blue of the night sky, providing minimal but correctly colored ambient fill that prevents shadows from going completely black.
The SkyLight must be recaptured when sky conditions change — either via real-time capture mode (expensive, runs every frame) or via periodic recapture triggered at key lighting transitions. For a static sunset or night scene, a single capture baked to the scene’s current sky is sufficient.
Lumen: Global Illumination and Reflections
Lumen is Unreal 5’s fully dynamic global illumination system — it computes how light bounces between surfaces and accumulates in indirect illumination without baking. For a lighting study, Lumen is what separates physically plausible results from flat, direct-illumination-only rendering.
At sunset, Lumen propagates the warm Directional Light bounce throughout the environment — surfaces facing the sun glow with reflected warm light; surfaces in shadow receive indirect fill from nearby surfaces that caught the direct light. This bounce is what makes sunset lighting feel volumetric and warm throughout the scene rather than just on directly lit surfaces.
At night, Lumen handles the local light bounces from artificial sources — a street lamp’s warm light bouncing off the wall beneath it, windows glowing and contributing indirect fill to the exterior. Without Lumen, night scenes feel theatrically lit with harsh light/shadow transitions. With it, the indirect fill softens the transitions and produces a more naturalistic result.
Volumetric Fog and Exponential Height Fog
These two fog components serve different purposes and are typically used together:
Exponential Height Fog adds a ground-level fog layer that fades exponentially with height — dense near the ground, thinning as altitude increases. For sunset scenes, it adds atmospheric haze to the mid-distance. For night scenes, it thickens the ambient darkness at ground level, making enclosed areas feel heavier. The fog color can be set independently from the sky color, allowing warm ground fog beneath a cool night sky.
Volumetric Fog enables light shafts — the visible beams of light (god rays) that appear when light passes through a participating medium like fog or dust. When the Directional Light shines through Volumetric Fog, it produces visible light shafts that radiate from the sun’s position. At sunset, these shafts are one of the most visually impactful atmospheric effects available — they make the light physically visible and give the scene a sense of weight and atmosphere that no other technique matches. Volumetric Fog is significantly more expensive than Exponential Height Fog, so its density and resolution need to be tuned to balance quality against performance cost.
Post-Process Volume: Final Look Development
The Post-Process Volume wraps everything together — it’s the final pass over the rendered image that establishes the scene’s look. For a lighting study, the most relevant post-process settings are:
Exposure — controlling how bright or dark the overall image appears. Manual exposure is preferable for a controlled lighting demo; automatic exposure (eye adaptation) adjusts continuously and can work against the intended look in dark night scenes.
Color grading — shifting the overall color balance via temperature, tint, and tone curve adjustments. Sunset scenes benefit from a slight warm push in the highlights and a cool-blue tint in the shadows, exaggerating the natural complement. Night scenes benefit from desaturation and a cool blue grade that reads as moonlit.
Bloom — the glow around bright lights. At night, bloom from light sources is one of the main contributors to the scene reading as nighttime — the spread of light around a lamp post or a window is a strong visual cue.
Ambient occlusion — darkening cavities and contact points. Both sunset and night scenes benefit from well-tuned AO: it grounds objects in the environment and prevents the flat, floating look that poorly lit scenes suffer from.
Reflection
The lighting study is unusual in this series because its output is purely visual — no gameplay, no interaction, no AI. Its value is entirely in building a systematic understanding of how Unreal’s lighting components relate to each other. Working through sunset and night as two distinct scenarios forces deliberate choices at each layer: which component drives the primary light, how the sky color propagates into the scene via SkyLight and Lumen, how fog and post-process complete the look.
Coming from a VFX background at DreamWorks and Sony, the lighting concepts — direct vs. indirect illumination, color temperature, atmospheric scattering, bounce light — are familiar from offline rendering. The transition to real-time lighting is a question of constraints: which effects can be computed dynamically, which need to be baked, and how to get the most out of a fixed frame budget. Lumen narrows the gap between offline and real-time GI significantly, but the fundamental lighting principles remain the same regardless of renderer.
Nicolás Bertoa 
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