What you'll learn:
- Radiometry vs photometry — measuring light for physics vs for human perception
- Units of light — lumens, lux, candelas, and when to use each
- Inverse square law — why distance matters so much
- Physical units in UE5 — enabling accurate light values
- Real-world reference values — what lights actually output
Section 1: Radiometry vs Photometry (10 min)
Light can be measured two ways:
| Approach | What It Measures | Units | Used By |
|---|---|---|---|
| Radiometry | Total electromagnetic energy | Watts (W) | Physics, sensors, ML |
| Photometry | Light as perceived by humans | Lumens (lm) | Lighting design, photography |
Radiometry measures all electromagnetic radiation — UV, visible, infrared. It's physics-accurate but doesn't account for human perception.
Photometry weights light by human eye sensitivity. We're most sensitive to green (~555nm) and less sensitive to red and blue. Photometry accounts for this.
In Unreal Engine, lights use photometric units by default:
- Directional Light: Lux
- Point/Spot Light: Lumens or Candelas
- Rect Light: Lumens
When you set a light to 800 lumens, you're specifying how bright it appears to human vision — not its raw energy output.
The conversion between radiometry and photometry uses the luminosity function V(λ):
Luminous Flux (lumens) = 683 × ∫ Radiant Flux(λ) × V(λ) dλ
The 683 factor converts watts to lumens at peak sensitivity (555nm green).
Key insight: Two lights with equal wattage can have very different lumen outputs depending on their color spectrum.
Section 2: Units of Light (10 min)
| Unit | Symbol | What It Measures | Analogy |
|---|---|---|---|
| Luminous Flux | Lumens (lm) | Total light output | Water flow (gallons/min) |
| Luminous Intensity | Candelas (cd) | Light in a direction | Hose pressure |
| Illuminance | Lux (lx) | Light hitting a surface | Water on the ground |
| Luminance | cd/m² (nits) | Light leaving a surface | Puddle brightness |
Lumens → Total output from source
↓
Candelas → Output per solid angle (steradian)
↓
Lux → Candelas spread over distance (lux = cd/m²)
↓
Luminance → Lux reflected from surface
Point source at distance:
Lux = Candelas / Distance²
A 1000 cd light at 2m:
Lux = 1000 / 4 = 250 lux
Lumens to Candelas (uniform sphere):
Candelas = Lumens / (4π)
Candelas = Lumens / 12.57
An 800 lm bulb:
Candelas = 800 / 12.57 = 64 cd (average)
| Scenario | Use This Unit |
|---|---|
| Specifying a bulb | Lumens |
| Measuring at a surface | Lux |
| Directional output | Candelas |
| Display brightness | Nits (cd/m²) |
Section 3: Inverse Square Law (10 min)
Light intensity decreases with the square of distance:
Intensity ∝ 1 / Distance²
Double the distance → 1/4 the light Triple the distance → 1/9 the light
Light spreads in all directions. The surface area of a sphere is 4πr². As distance doubles, the same light covers 4× the area — so each point receives 1/4 the light.
Distance: 1m 2m 4m 8m
Relative: 100% 25% 6.25% 1.56%
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Close lights are critical. Moving a light from 1m to 2m away cuts its effect by 75%.
Fill lights need power. A fill light at 3m needs 9× the power of a key light at 1m to provide equal illumination.
The sun is special. At 150 million km, an extra few meters makes no difference — sunlight is effectively parallel and constant intensity across a scene.
UE5's lights follow inverse square falloff by default with Use Inverse Squared Falloff enabled. This is physically accurate.
Legacy behavior (disabled): Linear falloff — not physically accurate but sometimes useful for artistic control.
Section 4: Light in UE5 — Physical Units Mode (8 min)
By default, UE5 uses physical units for lighting. Verify in Project Settings:
Project Settings > Engine > Rendering > Default Settings:
[x] Use Physical Lights
With this enabled:
- Point/Spot lights use Lumens or Candelas
- Rect lights use Lumens
- Directional lights use Lux
| Light Type | Available Units |
|---|---|
| Directional | Lux (always) |
| Point | Lumens, Candelas |
| Spot | Lumens, Candelas |
| Rect | Lumens |
Point Light example:
Details > Light:
Intensity Units: Lumens
Intensity: 800
This creates a point light equivalent to a typical household LED bulb.
When using physical units, Source Radius affects soft shadows but NOT intensity. The light outputs the same lumens regardless of radius.
This matches reality — a bigger bulb with the same lumens is the same brightness, just with softer shadows.
Section 5: Common Light Values Reference (7 min)
| Environment | Illuminance |
|---|---|
| Direct sunlight | 100,000 lux |
| Overcast day | 1,000 - 10,000 lux |
| Sunrise/sunset | 400 lux |
| Office lighting | 300 - 500 lux |
| Living room | 50 - 150 lux |
| Street lighting | 10 - 20 lux |
| Full moon | 0.1 - 0.3 lux |
| Starlight | 0.001 lux |
| Source | Lumens |
|---|---|
| Candle | 12 lm |
| 40W incandescent equiv | 450 lm |
| 60W incandescent equiv | 800 lm |
| 100W incandescent equiv | 1600 lm |
| Fluorescent tube (4ft) | 2500-3500 lm |
| Car headlight (low beam) | 700-1200 lm |
| Car headlight (high beam) | 1200-2000 lm |
| Stadium light | 20,000-50,000 lm |
For outdoor scenes, the Directional Light (sun) uses Lux:
| Time of Day | Suggested Lux |
|---|---|
| Noon, clear | 100,000 - 120,000 |
| Noon, overcast | 10,000 - 20,000 |
| Golden hour | 10,000 - 40,000 |
| Sunset/sunrise | 1,000 - 10,000 |
| Twilight | 10 - 100 |
| Moonlight | 0.05 - 0.3 |
Create a reference document for your projects:
- Note the lighting conditions you commonly recreate
- Record real-world measurements or reference values
- Document your UE5 settings that achieve those looks
- Use consistent calibration (18% grey reference) to verify
Key Points:
- Radiometry measures physical energy; Photometry measures human-perceived brightness
- Lumens = total output, Lux = light at surface, Candelas = directional intensity
- Inverse square law — double distance = 1/4 light
- Enable Physical Light Units in Project Settings for accurate values
- Use real-world reference values for physically plausible lighting
In Part 2: Camera Science, we'll explore how cameras capture light — sensor sizes, the exposure triangle, and why depth of field depends on actual sensor dimensions, not full-frame equivalents.