Physical light units

Physical light units are the real-world units of light (lumen, candela, lux and nits) used to set the brightness of lights in a game engine. Instead of an arbitrary intensity, you type the output printed on a bulb's box, or the illuminance of real sunlight. In Unity's HDRP manual, the lumen describes "the total amount of visible light that a light source emits in all directions", and lux is "the unit of illuminance" (High Definition Render Pipeline manual: Physical light units).

#Why studios switched

Before physically based rendering, lighting artists usually picked "an arbitrary value for the strongest light of a scene" and set every other light relative to that key light, as Sébastien Lagarde and Charles de Rousiers put it in their SIGGRAPH 2014 talk on moving EA's Frostbite engine to PBR (SIGGRAPH 2014 Physically Based Shading course: Moving Frostbite to Physically Based Rendering). Their argument was consistency: with "a consistent unit system for light intensities", rigs can be reused across scenes and real values looked up for any light (SIGGRAPH 2014 Physically Based Shading course: Moving Frostbite to Physically Based Rendering). Frostbite converted its whole pipeline: "Frostbite's entire lighting pipeline is in photometric units" (SIGGRAPH 2014 Physically Based Shading course: Moving Frostbite to Physically Based Rendering).

The team checked the result against measured bulbs and reported that "the physics is correct!" (SIGGRAPH 2014 Physically Based Shading course: Moving Frostbite to Physically Based Rendering). They also describe the cost: converting old levels automatically "wasn't a high-quality conversion", and artists had to be trained while the features were being built (SIGGRAPH 2014 Physically Based Shading course: Moving Frostbite to Physically Based Rendering).

#The four units

Unit	Measures	Used for	Source
Lumen (lm)	Total light a source emits	Bulbs, lamps (point, spot, rect lights)	(High Definition Render Pipeline manual; Unreal Engine Documentation)
Candela (cd)	Light emitted in one direction; 1 cd = 1 lm/sr	Point, spot and rect lights	(Unreal Engine Documentation)
Lux (lx)	Light arriving on a surface; 1 lm per m²	Sun and moon (directional lights)	(Unreal Engine Documentation)
Nits (cd/m²)	Brightness of a glowing surface	Sky light, emissive materials	(Unreal Engine Documentation; High Definition Render Pipeline manual)

Unreal's point, spot and rect lights can also use its old "Unitless" value; for converting old scenes, Epic gives "1 cd = 625 unitless" (Unreal Engine Documentation: Using Physical Lighting Units).

A logarithmic lux scale from 0.001 to one million. Markers show a starry night at 0.002, moonlight under 1, a city street at night at 30, bedrooms 150 to 300, offices 300 to 500, supermarkets 750 to 1,000, overcast midday 1,000 to 2,000, blue sky 20,000 and bright sunlight 110,000. — Real illuminance spans about eight orders of magnitude. The gap between a sunny street and an office is roughly 8 stops of exposure.Diagram by the handbook author; values from Unity's HDRP manual

#Reference values

Unity's HDRP manual lists real values you can type straight in (High Definition Render Pipeline manual: Physical light units):

Natural light	Lux	Artificial light	Lumen
Very bright sunlight	120,000	Candle	12.57
Bright sunlight	110,000	Small decorative LED lamp	under 100
Blue sky at midday	20,000	Ceiling lamp, regular room	400–800
Overcast sky at midday	1,000–2,000	Ceiling lamp, large bright room	800–1,200
Clear moonlit night	under 1	Bright street light	1,000–40,000

Frostbite's own sun measurements agree: "We found it to be above 100,000" lux (SIGGRAPH 2014 Physically Based Shading course: Moving Frostbite to Physically Based Rendering).

ExampleWorked numbers: one ceiling lamp in a bedroom

A regular ceiling lamp emits about 600 lm (High Definition Render Pipeline manual: Physical light units). A point light spreads that over a full sphere of 4π steradians, so its intensity is 600 ÷ 12.57 ≈ 48 cd (Epic's conversion: 1 cd ≈ 12.6 lm for a point light) (Unreal Engine Documentation: Using Physical Lighting Units). Illuminance falls with the square of the distance, and Epic notes that 1,000 cd "would measure 1000 lux at one meter" (Unreal Engine Documentation: Using Physical Lighting Units). So 48 cd gives 48 lux at 1 m and 48 ÷ 2² = 12 lux on a floor 2 m below.

HDRP lists a bedroom at 150–300 lux (High Definition Render Pipeline manual: Physical light units). One lamp's direct light falls far short; the rest comes from more fixtures, bounce light (global illumination) and daylight. If a room looks dark with a realistic lamp, check GI and exposure before turning the lamp up.

#Physical units and exposure

Bright sunlight is about 275 times an office, around 8 stops, so the game must choose an exposure for each. If you place a physically bright light and the image turns white, Epic's advice is that the auto exposure range may be too limited, and to consider raising Auto Exposure Max EV100 (Unreal Engine Documentation: Using Physical Lighting Units). Unity's manual places scenes on the same scale, from about -2 EV for a moonless night to 14 EV for sunlit scenes (High Definition Render Pipeline manual: Physical light units). See exposure and post-processing.

Different viewsPhysical units, or artist-relative values?

Physical units everywhere. Lagarde and de Rousiers argue that a shared unit system lets values be measured, looked up and reused between scenes (SIGGRAPH 2014 Physically Based Shading course: Moving Frostbite to Physically Based Rendering). Epic and Unity build their light settings around the same units (Unreal Engine Documentation: Using Physical Lighting Units; High Definition Render Pipeline manual: Physical light units).

Relative values still have a place. Unreal keeps a "Unitless" option for local lights (Unreal Engine Documentation: Using Physical Lighting Units), and Frostbite's automatic conversion of old levels "wasn't a high-quality conversion" (SIGGRAPH 2014 Physically Based Shading course: Moving Frostbite to Physically Based Rendering). Some teams compress the range on purpose: one Unreal user describes a sun of about 2,000 lux with "a deliberately limited exposure range", and finds that a realistic 4,000 lm fixture "may no longer appear strong enough", so lamps need a separate gameplay multiplier (Unreal Engine Forums: Benefits of Physically Based Lighting Values…). With game-scaled values, once one light departs from real units, the others have to be adjusted as well.

In practice: use physical units for the sun, sky and any light with a real counterpart, then make artistic changes with exposure and grading rather than by breaking the light values.

#Common mistakes

Fighting exposure with intensity. A sun turned down to 10 lux to suit a fixed exposure breaks every other light in the scene (Unreal Engine Documentation: Using Physical Lighting Units).
Emissive surfaces left unitless. Emissive materials use cd/m² and should be set to match the light their fixture casts; Frostbite's emissive surfaces "use physical light units" for that reason (SIGGRAPH 2014 Physically Based Shading course: Moving Frostbite to Physically Based Rendering).

#Further reading and watching

Lagarde and de Rousiers: Moving Frostbite to Physically Based Rendering (slides)

the SIGGRAPH 2014 talk that made the case for photometric units in a AAA engine. Advanced, free.

PaperAdvancedFree

Unity: Physical light units (HDRP)

definitions and tables of real-world lux and lumen values. Beginner, free.

DocsBeginnerFree

Epic: Using Physical Lighting Units

which unit each Unreal light uses, with conversion factors. Intermediate, free.

DocsIntermediateFree