Understand physical light units

HDRP uses Physical Light Units (PLU) for its lighting. These units are based on real-life light measurements, like those you see on light bulb packaging or a photographic light meter. Note that for lights to behave properly when using PLU, you need to respect HDRP unit convention (1 Unity unit equals 1 meter).

A room with lights.

Units

Candela:

The base unit of luminous intensity in the International System of Units. For reference, a common wax candle emits light with a luminous intensity of roughly 1 candela.

Lumen:

The unit of luminous flux. Describes the total amount of visible light that a light source emits in all directions. When you use this unit, the amount of visible light is independent of the source's size meaning the illumination level of a Scene does not change depending on the size of a light source. However, highlights that a light source produces dim as the area of the light source increases. This is because the same power is spread across a larger area.

A light source that emits 1 candela of luminous intensity from an area of 1 steradian has a luminous flux of 1 lumen.

Lux (lumen per square meter):

The unit of illuminance. A light source that emits 1 lumen of luminous flux onto an area of 1 square meter has an illuminance of 1 lux.

Nits (candela per square meter):

The unit of luminance. Describes the surface power of a visible light source. When you use this unit, the overall power of a light source depends the size of the light source, meaning the illumination level of a Scene changes depending on the size of the light source. Highlights that a light source produces conserve their intensity regardless of the size of the surface.

A light source that emits 1 candela of luminous intensity onto an area of 1 square meter has a luminance of 1 candela per square meter.

Exposure value (EV):

A value that represents a combination of a camera's shutter speed and f-number. It is essentially a measurement of exposure such that all combinations of shutter speed and f-number that yield the same level of exposure have the same EV. HDRP Lights can use EV100, which is EV with a 100 International Standards Organisation (ISO) film.

Light intensities

Natural

Light measurements from natural sources in different conditions:

Illuminance (lux) Natural light level
120 000 Very bright sunlight.
110 000 Bright sunlight.
20 000 Blue sky at midday.
1 000 - 2 000 Overcast sky at midday.
< 1 Moonlight with a clear night sky.
0.002 Starry night without moonlight. Includes airglow.

Artificial

Approximate light measurements from artificial sources:

Luminous flux (lumen) Source
12.57 Candle light.
< 100 Small decorative light, such as a small LED lamp.
200 - 300 Decorative lamp, such as a lamp that does not provide the main lighting for a bright room.
400 - 800 Ceiling lamp for a regular room.
800 - 1 200 Ceiling lamp for a large brightly lit room.
1 000 - 40 000 Bright street light.

Indoor

Architects use these approximate values as a guide when designing rooms and buildings for functional use:

Illuminance (lux) Room type
150 - 300 Bedroom.
300 - 500 Classroom.
300 - 750 Kitchen.
300 - 500 Kitchen Counter or Office.
100 - 300 Bathroom.
750 lux - 1 000 Supermarket.
30 City street at night.

For more examples of indoor light levels see Archtoolbox’s web page on Recommended Lighting Levels in Buildings.

Lighting and exposure diagram

The following cheat sheet contains the color temperature values and light intensities of common real-world Light sources. It also contains Exposure values for different illumination scenarios.

Unity Lighting & Exposure Cheat Sheet. A visual guide showing typical lighting values across color temperature, directional light intensities, local light intensities, and exposure value. Color temperature (Kelvin): Ranges from 1,900 K (candle) to 15,000 K (blue sky), with intermediate examples like incandescent (3,000 K), fluorescent (4,000 K), sunlight at noon (5,500 K), cloudy (6,500 K), and shade (8,000 K). Directional light intensities (Lux): Ranges from 0.001 lx (dark night) to 130,000 lx (sun with PBS sky), with intermediate examples for moonlight (0.5 lx), low sun (5,000 lx), cloudy (20,000 lx), and noon sun (100,000 lx). Local light intensities (Lumen): From 12 lm (candlelight) to 10,000 lm (exterior light), with examples like decorative lights (300 lm) and interior lights (1,000 lm). Exposure value (EV): From -2 EV (moonless) to 14 EV (sunlit), with stages like moonlit (1 EV), interior (4 EV), low sun (7 EV), and cloudy (10 EV).