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).
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 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.
