Parameter effects

Contents

• Light selection
• Core light properties
• Mobility
• Exposure
• Physical-unit anchors
• Lumen
• MegaLights

Light selection

Need	Light
Sun, moon, or effectively infinite source	Directional Light
Captured environment/sky contribution	Sky Light
Bare bulb or omnidirectional fixture	Point Light
Flashlight, headlamp, stage cone	Spot Light
Window, panel, fluorescent fixture, softbox	Rect Light

Core light properties

Property	Increase	Decrease / caution
Intensity	More direct contribution in the selected physical unit	Interpret only with exposure and distance
Temperature	Cooler/bluer at higher Kelvin	Warmer/oranger at lower Kelvin; avoid double-tinting a physically scattered sun unless stylized
Attenuation Radius	Affects more geometry and increases overlap	Not a brightness control; keep close to useful influence
Source Radius / Soft Source Radius	Larger apparent emitter; softer, broader shadows/reflections	Excessive size can erase contact definition
Source Length	Elongates emitter and shadow softness	Match tube/strip fixture shape
Indirect Lighting Intensity	Scales bounced contribution from this light	Nonphysical artistic override; debug GI before using
Volumetric Scattering Intensity	Stronger effect in participating media	Can create fog glow/noise and flatten contrast
Specular Scale	Changes specular response	Keep at default for physical work; use only as a deliberate art override

Spot Light

• Inner Cone Angle: fully lit core.
• Outer Cone Angle: full edge of influence.
• Larger gap between inner and outer angles creates a softer penumbra.
• Use IES profiles for fixture distribution when available instead of hand-shaping every falloff.

Rect Light

• Source Width/Height define emitter size and reflection shape.
• Use for windows and panels where emitter geometry matters.
• Face the emitting side toward the scene.

Mobility

Mobility	Use	Cost/constraint
Static	Never changes; baked Lightmass workflow	Requires precomputed lighting; not useful as a Lumen dynamic source
Stationary	Color/intensity may change, transform stays fixed	Mixed baked/dynamic constraints; overlapping Stationary limits apply
Movable	Transform and properties change at runtime	Fully dynamic shadows; profile shadow path

For Lumen-first fully dynamic projects, Movable lights are the normal baseline. Do not choose mobility from habit; choose it from the rendering path.

Exposure

Control	Effect
Metering Mode: Manual	Fixed exposure; camera/light comparison is stable
Histogram	64-bin adaptive exposure; default adaptive mode
Exposure Compensation	Offset in stops; +1 = 2× brighter, -1 = 2× darker
Min/Max EV100	Clamp adaptation range; equal values disable adaptation
Speed Up	Adaptation rate from dark environment/view to bright
Speed Down	Adaptation rate from bright environment/view to dark
Metering Mask	Weights screen regions; center weighting reduces edge-light pumping
Low/High Percent	Select histogram range used for target exposure; extreme values can destabilize or clip

Workflow:

1. Use Manual exposure while establishing physically plausible lights.
2. Enable HDR (Eye Adaptation) visualization.
3. If adaptation is required, set a purposeful Min/Max EV100 range.
4. Set speed from desired perceptual behavior.
5. Add a metering mask if peripheral bright objects cause pumping.
6. Use Local Exposure only after global exposure and light values are sound.

Physical-unit anchors

• UE documents a zenith sun around 120,000 lux with a 0.545° angular diameter.
• A perpendicular white diffuse surface under that sun may receive roughly 150,000 lux total.
• Point/Spot/Rect Lights can use Candela or Lumens when inverse-square falloff is on.
• 1000 cd produces 1000 lux at one meter for a perpendicular receiver in the documented example.

Use real fixture specifications as starting points. Do not compensate for incorrect exposure by multiplying every light arbitrarily.

Lumen

• Dynamic Global Illumination: Lumen.
• Reflection Method: Lumen.
• Software tracing requires Generate Mesh Distance Fields.
• Hardware ray tracing improves representation/quality in supported cases but costs more and requires compatible hardware.
• High scalability targets Lumen GI/reflections for 60 FPS on current-generation consoles; Epic targets cleaner indoor lighting at 30 FPS.
• Inspect Surface Cache/Card visualization when meshes appear black or fail to bounce light.

MegaLights

• Enable under Project Settings > Rendering > Direct Lighting.
• Hardware ray tracing support is recommended.
• All local lights are handled when enabled; disable per light with Allow MegaLights.
• Directional Light support is disabled by default. Enable it with r.MegaLights.DirectionalLights 1 only after testing: Epic recommends Deferred Lighting with VSM for a strong sun, while MegaLights may fit a dim, soft moonlight. Sky Light is not handled.
• Select Ray Tracing or Virtual Shadow Maps per light as shadow method.
• Disable by scalability/device profile with r.MegaLights.Allow 0.
• Avoid huge light bounds. Merge many tiny emitters when they create noise, blur, or ghosting at the same pixels.